MEDICAL PARASITOLOGY TEXTBOOK.pdf

DIAGNOSING MEDICAL PARASITES: A Public Health Officers Guide to Assisting Laboratory and Medical Officers Compiled and edited by: Michael J. Cuomo, Maj, USAF, BSC (PH) Lawrence B. Noel, Maj, USAF, BSC (PH) Daryl B. White, Capt., USAF, BSC (Lab) Contents: Chapter 01. The Ameba Chapter 02. The Ciliates, Coccidia, and Microsporidia Chapter 03. The Flagellates Chapter 04. The Cestodes (Tapeworms) Chapter 05. The Nematodes (Roundworms) Chapter 06. The Trematodes (Flukes) Chapter 07. Unusual Tissue Dwelling Nematodes Chapter 08. Larval Cestodes and Nematodes which Infect Man Chapter 09. Malaria Chapter 10. The Blood Nematodes Chapter 11. Babesia, Trypanosomes, and Leishmania Chapter 12. Arthropod Vectors Chapter 13. Artifacts and Confounders -2- Chapter 1. The Ameba 1.1 An Introduction to Parasitology Parasitology is the study of parasites and as such does not include bacterial, fungal or viral parasites. Human parasites are separated into intestinal and blood-borne parasites. For a parasite to be defined as intestinal it must have an intestinal life-cycle stage, though it may have life-cycle stages in the heart, circulation, lung, tissue, other animals or the environment. Parasites found in the intestines can be categorized into two groups: Protozoa and Helminths. Protozoa are single celled organisms. There are four classes of Protozoa commonly found in concentrated fecal samples. These are differentiated by the method of motility. Protozoa include Entamoeba, Giardia, Trichomonas, Cryptosporidium, Isospora, Pneumocystis and Balantidium. There are two diagnostic life-cycle stages commonly seen in parasites - the cyst and the adult trophozoite stage. The trophozoite stage is analyzed directly on a slide without concentration. Cysts require concentration. The key diagnostic factor is that Protozoan cysts are typically 5-30µm (µm = microns or micrometers) in diameter, and as such are smaller than most Helminth eggs. Due to the size they are particularly difficult to see under the microscope if the sample clarity is bad. The medically important Helminths are nematodes (roundworms), cestodes (tapeworms) and trematodes (flukes). Genera include: Fasciola, Schistosoma, Ascaris, Hookworm, Trichuris, Taenia and Enterobius. The normal stage for examination is the egg stage, although larvae may develop in some organisms (Strongyloides); the diameter of the eggs range from 30µm - 150µm. The other major grouping of parasites is known as blood-borne parasites which are transmitted through an arthropod vector. By far the most important arthropod for transmitting parasitic infections is the mosquito. Mosquitoes are known to carry malaria and filarial nematodes. Different types of biting flies transmit African trypanosomiasis, leishmaniasis and several kinds of filariasis. Most protozoan and helminthic infections that are transmitted by arthropods can readily be diagnosed, on clinical grounds alone, but are usually identified by fairly simple techniques designed to present the presence of the causative parasite by microscopy. Sophisticated techniques are also being employed including highly sensitive and specific simple monoclonal antibody tests, DNA probes, and PCR primers. -3- 1.2 Infections Acquired Through the Gastrointestinal Tract Many of the infections of the gastrointestinal tract (GI) are caused by parasites that are cosmopolitan in distribution. Protozoa can be directly infectious for man when they are passed in the feces into the environment, but helminths require a period of maturation while in the soil, where they become infectious. Others such as Taenia saginata require the involvement of an intermediate host during their life cycle. Infections of the GI tract account for a high proportion of deaths in infants where the standards of hygiene and nutrition are low. Fecal-oral transmission of the pathogens is the most common mode of GI infections, whereby water, food and hands become contaminated with fecal material which then come in contact with the mouth. A number of GI infections can reach epidemic proportion, protozoal pathogen Cryptosporidium parvum, has been known to cause the severe water-borne epidemics, even in first-world countries such as the United States and the UK. Other infections such as amebiasis or enterobiasis can be more localized, infecting households or institutions. Some of the rarer, protozoal infections such as the microsporidia are only now being understood as they are appearing as concomitant infections in people with depressed immune responsiveness, e.g. AIDS. The Ameba Ameba (or Amoeba) are characterized by possessing clear protoplasm which form pseudopodia. These pseudopodia are the means by which these organisms move and engulf bacteria and red blood cells for feeding purposes. The most common amebas seen in the intestinal tract are Entamoeba histolytica/dispar, Entamoeba coli, Entamoeba hartmanni, Endolimax nana and Iodamoeba bütschlii. All but Entamoeba histolytica are thought to be non-pathogenic. The cysts can be identified in an ethyl acetate concentrate by the addition of iodine to reveal the characteristic inclusions and also by measuring the cyst using an eyepiece graticule. The trophozoites can be seen in a fresh saline preparation of the stool although accurate identification is on a permanently stained fecal smear. -4- Entamoeba histolytica Introduction There are a large number of species of ameba which parasitize the human intestinal tract. Of these, Entamoeba histolytica / dispar is the only species found to be associated with intestinal disease. Although many people harbor this organism world wide, only about 10% develop clinically invasive disease, thus the parasite has been shown to present as two very differing clinical presentations. 1. The commensal or non-invasive luminal form where the parasite causes no signs or symptoms of disease. 2. The pathogenic or invasive form where the parasite invades the intestinal mucosa and produces dysentery or amebiasis and may give rise to extra-intestinal lesions via the blood, mainly to the liver. Sargeaunt and Williams (1978) conclusively proved that invasive and non-invasive strains of E. histolytica could be differentiated by isoenzyme electrophoresis and the application of molecular biology has confirmed the presence of two distinct species with the same morphological features. The pathogenic or invasive species has retained the name E. histolytica and the non-pathogenic, non-invasive species has been named E. dispar. Illustration 1-1. Life Cycle of Entamoeba histolytica: A protozoan in which its life cycle consists of two stages; cysts and trophozoites (Cuomo) -5- Morphology of Trophozoites The trophozoites of E. histolytica / dispar recovered from dysenteric stools exhibit ingested red blood cells and clear pseudopodia. Those of E. dispar will have no ingested red blood cells. They can be up to 60µm in diameter and motility is rapid and unidirectional. On a permanently stained fecal smear e.g. Trichrome or Iron hematoxylin, the morphological features are more visible. When using Trichrome stain nuclei, chromidial bars, chromatin, red cells and bacteria stain red cytoplasm stains blue-green and background and yeasts stain green. The presence of a small centrally placed karyosome is clearly visible. With Iron hematoxylin, nuclear chromatin and the karyosome will be stained immensely black. The remainder will be varying shades of grey/black. Image 1-1. Entamoeba histolytica trophozoites (SOURCE: PHIL 336 -CDC / Dr. N.J. Wheeler, Jr.) Morphology of Cysts Cysts of E. histolytica / dispar are 10-15µm in diameter and contain one to four nuclei. Chromatoid bodies are usually present in young cysts as elongated bars with bluntly rounded ends. Glycogen is usually diffuse, but in young cysts it is often present as a concentrated mass, staining reddish brown with iodine. -6- Image 1-2. Entamoeba histolytica cyst. (SOURCE: PHIL 531 - CDC/ Dr. L.L.A. Moore, Jr.) Clinical Disease Amebiasis is an infection usually caused by the pathogenic Entamoeba histolytica / dispar, and is commonly an infection of the colon. It has a world wide distribution where environmental sanitation is poor. The parasite may behave as a commensal (causing no harm to the host) or it may act as a parasite (harming the host). It is a disease of human beings, although some monkeys can become infected and the infection is then transmissible to humans. Intestinal Disease Patients with intestinal disease may exhibit a number of symptoms including profuse diarrhea with blood and mucus, fever and dehydration. Amebic ulcers may develop in the large colon and can also be found in the rectal area. The ulcers are usually "flask shaped" with a small opening on the mucosal surface and a larger area below the surface. Image 1-3. illustrates E. histolytica trophozoites in the intestine, resulting in amebiasis. Image 1-3. Entamoeba histolytica, intestine. (SOURCE: PHIL 623 - CDC) -7- Hepatic Disease Trophozoites are transported from the intestine to the liver and liver disease is characterized with abdominal pain, fever, hepatomegaly and tenderness. If the abscess ruptures, there is spreading to the brain, pericardium and other sites. If left untouched the abscess will grow normally until it reaches a surface where it can discharge, e.g. the skin, the peritoneum, the pleural cavity or the pericardium. The stretching of the liver is presumably the main source of the pain. Laboratory Diagnosis Laboratory diagnosis is made by finding the characteristic cysts in an iodine stained, formol- ether concentration method or by detecting the characteristic trophozoites in a wet preparation or a permanent stained preparation. Microscopy Where amebic dysentery is suspected, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. On cooling the ameba stop moving which then become very difficult to identify. Direct microscopy should be done by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of motile trophozoites of Entamoeba histolytica / dispar and can also provide information on the content of the stool (i.e., the presence of leucocytes and red blood cells). On search e.g. primarily for cysts, not for ameba, several stool samples are required to be examined, by direct microscopy and a sensitive concentration technique. Three negative stool samples are required before it can be accepted that there is no amebic infection. Microscopic examination of an amebic abscess aspirate e.g. in the liver or lungs, may reveal hematophagous trophozoites. It must be examined immediately by mixing a drop of warm saline with some aspirated pus on a microscope slide. Serology If visceral or hepatic amebiasis is suspected, serological tests should be done as microscopic methods do not always reveal the characteristic trophozoites. The tests of choice are indirect fluorescent antibody test (IFAT), counter immunoelectrophoresis (CIEP) enzyme linked immunosorbent assay (ELISA), and Rapid Antigen testing by enzyme immunoassay. The search for E. histolytica / dispar is mainly carried out because there is a natural concern to ensure that patients, even in the absence of symptoms, are not harboring parasites that may lead to serious complications later on. -8- Entamoeba coli Introduction Entamoeba coli are a non-pathogenic ameba with world wide distribution. Its life cycle is similar to that of E. histolytica but it does not have an invasive stage and does not ingest red blood cells. Morphology of Trophozoite The trophozoite is larger than that of E. histolytica ranging from 15-50µm in diameter. It exhibits blunt pseudopodia with sluggish movement. A permanently stained preparation shows a nucleus with a moderately large eccentric karyosome with the chromatin clumped on the nuclear membrane. The cytoplasm appears granular containing vacuoles with ingested bacteria and other food particles. Image 1-4. Entamoeba coli trophozoite with ingested bacteria. (SOURCE: PHIL 605 - CDC) Morphology of Cysts Cysts of E. coli are 15-30µm in diameter and contain one to eight nuclei with irregular peripheral chromatin: karyosomes not central. Chromatoid bodies are not frequently seen but when present they are usually splinter-like with pointed ends. Glycogen is usually diffuse but in young cysts is occasionally found as a well-defined mass, which stains reddish brown with iodine. -9- Image 1-5. Entamoeba coli (larger) and Entamoeba histolytica (smaller) cysts. (SOURCE: PHIL 442 - CDC/Dr. Mae Melvin) Laboratory Diagnosis Laboratory diagnosis is made by finding the characteristic cysts in an iodine stained, formol- ether concentration method or by detecting the characteristic trophozoites in a wet preparation or a permanent stained preparation. Microscopy Where amebic dysentery is suspected, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. On cooling the ameba stop moving which then become very difficult to identify. Direct microscopy should be done by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of motile trophozoites of Entamoeba coli and can also provide information on the content of the stool (i.e., the presence of leucocytes and red blood cells). On search e.g. primarily for cysts, not for ameba, several stool samples are required to be examined, by direct microscopy and a sensitive concentration technique. Three negative stool samples are required before it can be accepted that there is no amebic infection. Microscopic examination of an amebic abscess aspirate (e.g. in the liver or lungs), may reveal hematophagous trophozoites. It must be examined immediately by mixing a drop of warm saline with some aspirated pus on a microscope slide. - 10 - Entamoeba hartmanni Introduction Entamoeba hartmanni is a non-pathogenic amoeba with world wide distribution. Its life cycle is similar to that of E. histolytica but it does not have an invasive stage and does not ingest red blood cells. Morphology of Trophozoites Morphology of the trophozoites is similar to those of E. histolytica / dispar but they do not contain ingested red blood cells and the motility is less rapid. Image 1-6. Entamoeba hartmanni trophozoite (SOURCE: PHIL 528 - CDC/Dr. L.L.A. Moore, Jr.) Morphology of Cysts Cysts of E. hartmanni 7-9µm in diameter and contain one to four nuclei. Chromatoid bodies are usually present in young cysts as elongated bars with bluntly rounded ends. Glycogen is usually diffuse, but in young cysts it is often present as a concentrated mass, staining reddish brown with iodine. - 11 - Image 1-7. Entamoeba hartmanni cyst (SOURCE: PHIL 533 - CDC/Dr. L.L.A. Moore, Jr.) Laboratory Diagnosis Laboratory diagnosis is made by finding the characteristic cysts in an iodine stained, formol- ether concentration method or by detecting the characteristic trophozoites in a wet preparation or a permanent stained preparation. Microscopy Where amebic dysentery is suspected, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. On cooling the ameba stop moving which then become very difficult to identify. Direct microscopy should be done by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of motile trophozoites of Entamoeba hartmanni and can also provide information on the content of the stool (i.e., the presence of leucocytes and red blood cells). On search e.g. primarily for cysts, not for ameba, several stool samples are required to be examined, by direct microscopy and a sensitive concentration technique. Three negative stool samples are required before it can be accepted that there is no amebic infection. Microscopic examination of an amebic abscess aspirate (e.g. in the liver or lungs), may reveal hematophagous trophozoites. It must be examined immediately by mixing a drop of warm saline with some aspirated pus on a microscope slide. - 12 - Endolimax nana Introduction Endolimax nana is a small non-pathogenic amoeba with world wide distribution. Its life cycle is similar to that of E. histolytica but is non-invasive. Morphology of Trophozoite Trophozoites of E. nana measures from 6-12µm. Motility is sluggish with blunt hyalin pseudopodia. In a permanently stained preparation, the nucleus exhibits a large karyosome with no peripheral chromatin on the nuclear membrane. Figure 1-8. Endolimax nana cyst. Iodine stain. (SOURCE: PHIL 530 - CDC/Dr. L.L.A. Moore, Jr.) Morphology of Cysts Cysts of E. nana are 6-9µm in diameter. They can be spherical or ovoid in shape and contain four pinpoint nuclei, which are highlighted by the addition of iodine. Chromatoid bodies are not found and glycogen is diffuse. - 13 - Image 1-9. Endolimax nana trophozoite (SOURCE: PHIL 1463 - CDC/Dr. Mae Melvin) Laboratory Diagnosis Laboratory diagnosis is made by finding the characteristic cysts in an iodine stained, formol- ether concentration method or by detecting the characteristic trophozoites in a wet preparation or a permanent stained preparation. Microscopy Where amebic dysentery is suspected, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. On cooling the ameba stop moving which then become very difficult to identify. Direct microscopy should be done by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of motile trophozoites of Endolimax nana and can also provide information on the content of the stool (i.e., the presence of leucocytes and red blood cells). On search e.g. primarily for cysts, not for ameba, several stool samples are required to be examined, by direct microscopy and a sensitive concentration technique. Three negative stool samples are required before it can be accepted that there is no amebic infection. Microscopic examination of an amebic abscess aspirate (e.g. in the liver or lungs), may reveal hematophagous trophozoites. It must be examined immediately by mixing a drop of warm saline with some aspirated pus on a microscope slide. - 14 - Iodamoeba bütschlii Introduction Iodamoeba bütschlii is a non-pathogenic amoeba with world wide distribution although not as common as E. coli or E. nana. Its life cycle is similar to that of E. histolytica but is non-invasive. Morphology of Trophozoites Trophozoites of I. bütschlii are 8-20µm and are actively motile. On a permanently stained fecal smear, a nucleus with a large karyosome is evident. Chromatin bodies form striations around the karyosome. The cytoplasm appears granular containing vacuoles with ingested bacteria and debris. Image 1-10. Histopathology of amebiasis of brain due to Iodamoeba bütschlii. (SOURCE: PHIL 323 - CDC/Dr. Martin D. Hicklin) Morphology of cysts Cysts of I. bütschlii are 9-15µm in diameter and have one nucleus in mature cysts usually eccentrically placed. Chromatoid bodies are not present. Glycogen is present as a compact well defined mass staining dark brown with iodine. - 15 - Image 1-11. Iodamoeba bütschlii cyst. (SOURCE: PHIL 587 - CDC/Dr. L.L.A. Moore, Jr.) Laboratory Diagnosis Laboratory diagnosis is made by finding the characteristic cysts in an iodine stained, formol- ether concentration method. Trophozoites are difficult to detect in a wet preparation. Microscopy Where amebic dysentery is suspected, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. On cooling the ameba stop moving which then become very difficult to identify. Direct microscopy should be done by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of motile trophozoites of Iodamoeba bütschlii and can also provide information on the content of the stool (i.e., the presence of leucocytes and red blood cells). On search e.g. primarily for cysts, not for ameba, several stool samples are required to be examined, by direct microscopy and a sensitive concentration technique. Three negative stool samples are required before it can be accepted that there is no amebic infection. Microscopic examination of an amebic abscess aspirate e.g. in the liver or lungs, may reveal hematophagous trophozoites. It must be examined immediately by mixing a drop of warm saline with some aspirated pus on a microscope slide. - 16 - Blastocystis hominis Introduction B. hominis is an inhabitant of the human intestinal tract. It is capable of both sexual and asexual reproduction by binary fission and of pseudopod extension and retraction. Morphology The classic form that is usually seen in stool specimens varies in size from 6-40µm and is characterized by a large membrane bound central body which occupies 90% of the cell. It has no internal nuclear structure and a rim of peripheral granules the function of which is not known. Image 1-12. Blastocystis hominis cyst-like forms in a wet mount stained in iodine. (SOURCE: CDC) Clinical Disease The pathogenic potential of B. hominis is unclear, though this organism has been associated with nausea, fever, vomiting, diarrhea and abdominal pain. Laboratory Diagnosis Permanently stained preparations of fecal smears is the procedure of choice for identification, although the organism can be seen in wet preparations. The recommended stains are Fields’ and Giemsa. - 17 - 1.3 Direct Fecal Parasite Concentration Methods Diagnosis of intestinal parasites is confirmed by the recovery of protozoan trophozoites and cysts, helminth eggs and larvae in the clinical parasitology laboratory. Microscopic examination of feces is essential for the recognition and identification of intestinal parasites. Due to the low density of the parasites in the feces, direct microscopy is useful for the observation of motile protozoan trophozoites and the examination of cellular exudate, is not recommended solely for the routine examination of suspected parasitic infections. It is essential to increase the probability of finding the parasites in fecal samples to allow for an accurate diagnosis. Therefore, a concentration method is employed. (Direct wet mount examination should not be entirely excluded as the trophozoites are usually destroyed during the concentration procedure and therefore, microscopic examination of wet mounts should be performed). Conventional Methods Ridley-Allen Method The concentration procedure used in hospitals requires the use of ether or ethyl acetate as a lipid removing agent and formalin as a fixative. The process involves the use of either expensive brass sieves or the use of tea strainers as the filter element. Tea strainers have a very open pore structure of at least 600 micron and due to the shape of the strainer it is a non-linear pore size. The fecal matter is filtered directly through these meshes in a dead stop manner, and hence there is the tendency for occlusion of the filter. There is also a formation of a secondary filter layer, which retains eggs and allows the extrusion of particles (particularly fibers) into the sediment. The net result is a reduction in egg yield and in sample clarity. a. Using orange sticks, select a quantity of feces (approx. 1g) to include external and internal portions. b. Place in a centrifuge containing 7ml of 10% formalin. c. Emulsify the feces in the formalin and filter through the brass/plastic filter into the dish. d. Wash the filter and discard any lumpy residue. e. Transfer the filtrate to a boiling tube-add 3ml of ether and mix well on a vortex mixer for 15 seconds or mix by hand for 1 minute. f. Transfer back to the centrifuge tube and centrifuge at 3,000 rpm for 1 min. g. Loosen the fatty plug with an orange stick and pour the supernatant away by quickly inverting the tube. h. Allow the fluid on the side of the tube to drain onto the deposit – mix well and transfer a drop to a slide for examination under a coverslip. (WHO Basic lab methods in Medical Parasitology) The advantages of this method are that it will recover most ova, cysts and larvae and retain their morphology (thus facilitating identification). It has the disadvantage of destroying trophozoite stages and distorting cellular exudate. Liquid feces do not concentrate well, thus it - 18 - is necessary in these cases to examine the stool by direct microscopy. Since the sieves are not disposable there is a problem with cleaning for re-use. The system is also open so there is a biohazard and odor issue. Flotation Method This technique is predominantly used in veterinary laboratories. By exploiting the density of the parasites, particularly eggs, it allows the parasites to float to the top of a dense solution (final specific gravity of about 1.20) and can then be skimmed from the top of the tube. The most commonly used reagent is zinc sulphate. Operculated eggs as well as schistosome and infertile Ascaris eggs are not easily recovered by this method. Also trophozoites are killed due to the high specific gravity and certain other fragile eggs such as Hymenolepis nana become distorted. 1. Crush 10-20g (about 1 teaspoon) of feces with applicator sticks and mix well with 10- 12ml of saline. Filter the mixture through two layers of dampened surgical gauze into a 15ml conical centrifuge tube. 2. Centrifuge the suspension at 1500 rpm for five minutes. Decant the supernatant into disinfectant. Resuspend the sediment and recentrifuge in saline if there is excessive debris in the sample. 3. Resuspend and thoroughly mix the sediment in 12ml of zinc sulphate solution (specific gravity, 1.18 to 1.20, as verified with a hydrometer). 4. Centrifuge for one minute at 2500 rpm. Place tube in a rack in a vertical position and slowly add enough zinc sulphate with a dropper pipette to fill the tube so that an inverted meniscus forms. 5. Without shaking the tube, carefully place a 22 x 22µm coverslip on top of the tube so that its underside rests on the meniscus. The meniscus should not be so high that fluid runs down the side of the tube carrying parasites away from the cover glass. 6. Allow the tube to stand vertically in a rack with the coverslip suspended on top for ten minutes. 7. Carefully lift the coverslip with its hanging drop containing parasite eggs and cysts on the underside and mount on a clean slide, liquid side down. A small drop of iodine stain may be placed on the slide prior to adding the coverslip. The slide is gently rotated after adding the coverslip to ensure a uniform mixture. The slide is then thoroughly examined microscopically. (Clinical Laboratory procedures) - 19 - Chapter 2. The Ciliates, Coccidia, and Microsporidia 2.1 Infections Acquired Through the Gastrointestinal Tract In vertebrates, by far the most favorable sites for intestinal parasites are the duodenum, ileum, cecum and large intestine. To survive to reproduce in the gastrointestinal tract the parasites have to adapt to continuous physiological changes relative to the feeding habits of the host, the battery of protein, fat and carbohydrate-splitting enzymes, pH changes and the almost oxygen-free environment. Despite these features many parasites, both protozoa and helminths are capable of colonizing the gastrointestinal tract successfully. Protozoa are single-celled animals which resemble a single cell of a higher organism. However, the protozoan cell is capable of carrying out vital functions such as reproduction, feeding, and locomotion. Intestinal protozoa include species which can live in the lumen of the intestine and others which additionally live and reproduce in the cells of the intestinal walls. The protozoa make up a wide spectrum of organisms which have different life cycles and variable characteristics. The Ciliates 2.2 The Parasites The ciliates belong to the family Ciliophora. They possess simple cilia or compound ciliary organelles, two types of nuclei and a large contractile vacuole. The only member of the ciliate family to cause human disease is Balantidium coli. - 20 - Balantidium coli Introduction Balantidium coli is widely distributed in warmer climates, which is where human infections most commonly occur. The organisms inhabit the large intestine, cecum and terminal ileum where they feed on bacteria. The most common hosts being humans, pigs and rodents. Human infection is usually from pigs and is rare. Illustration 2-1. Life cycle of Balantidium coli. (SOURCE: PHIL 3380 - CDC/Alexander J. da Silva, PhD/Melanie Moser) - 21 - Morphology of the Cyst The cyst is spherical or ellipsoid and measures from 30-200µm by 20-120µm. It contains 1 macro and 1 micronucleus. The cilia are present in young cysts and may be seen slowly rotating, but after prolonged encystment, the cilia disappear. Cysts form when diarrhea subsides and the rectal contents become formed. The cyst, ingested by a fresh host, excysts to liberate the trophozoite. Image 2-1. Balantidium coli cyst. (SOURCE: PHIL 584 - CDC/Dr. L.L.A. Moore, Jr.) Morphology of the Trophozoite Trophozoites of B. coli measure approximately 30-150µm in length x 25-120µm in width but have been known to attain lengths of up to 200µm. They are oval in shape and covered in short cilia. A funnel shaped cytosome can be seen near the anterior end. Multiplication is by binary fission in the trophozoite stage. In an unstained preparation, the organisms are easily recognized because of their size and rapid revolving rotation. In a stained preparation, the characteristic macro and micronuclei may be observed. - 22 - Image 2-2. Balantidium coli cyst with Trichuris egg inside. Unstained wet mount. (SOURCE: PHIL 1460 - CDC/Dr. Mae Melvin) Clinical Disease Severe B. coli infections may resemble amebiasis. Symptoms include diarrhea, nausea, vomiting, and anorexia. The diarrhea may persist for long periods of time resulting in acute fluid loss. Balantidium coli also has the potential to penetrate the mucosa resulting in ulceration just as those of Entamoeba histolytica, but perforation is more common. Metastatic lesions do not occur. Extra-intestinal disease has also been reported, but is rare. Laboratory Diagnosis Wet preparations of fresh and concentrated stool samples reveal the characteristic cysts and motile trophozoites. They are easier to identify in direct-smear saline preparations than permanently stained fecal smears. - 23 - The Coccidia The Coccidia are a group of organisms which parasitize the epithelial cells of the intestinal tract. This group includes Cryptosporidium parvum, Cyclospora cayetanensis and Isospora belli. Most of the coccidian infections in man are zoonoses (having the potential to infect animals or arise from animals). In immunocompetent individuals, they usually produce mild, self-limiting infections. Cryptosporidium parvum Introduction Cryptosporidium species, are coccidian protozoa, which are cosmopolitan in distribution, occurring in both developed and underdeveloped countries and causing infection in both humans and their livestock. Cryptosporidium parvum is the species responsible for human infection. Development of Cryptosporidia occurs in a parasitophorous vacuole located on the microvillous surface of the epithelial cells. Life Cycle and Morphology - 24 - Illustration 2-2. Life cycle of Cryptosporidium sp. Sporulated oocysts, containing 4 sporozoites, are excreted by the infected host through feces and possibly other routes such as respiratory secretions. Following ingestion (and possibly inhalation) by a suitable host, excystation occurs. The sporozoites are released and parasitize epithelial cells ( , ) of the gastrointestinal tract or other tissues such as the respiratory tract. In these cells, the parasites undergo asexual multiplication (schizogony or merogony) ( , , ) and then sexual multiplication (gametogony) producing microgamonts (male) and macrogamonts (female). Upon fertilization of the macrogamonts by the microgametes ( ), oocysts ( , ) develop that sporulate in the infected host. Two different types of oocysts are produced, the thick-walled, which is commonly excreted from the host , and the thin- walled oocyst , which is primarily involved in autoinfection. Oocysts (measuring 4-5µm in diameter and containing 4 sporozoites) are infective upon excretion, thus permitting direct and immediate fecal-oral transmission. (SOURCE: PHIL 3386 - CDC/Alexander J. da Silva, PhD/Melanie Moser) Clinical Disease C. parvum is now widely recognized as a cause of acute gastro-enteritis, particularly in children. The infection produces a persistent, watery, offensive diarrhea often accompanied by abdominal pain, nausea, vomiting and anorexia. In immunocompetent persons, symptoms are usually short lived (one to two weeks). The small intestine is the site most commonly affected, symptomatic Cryptosporidium infections have also been found in other organs including other digestive tract organs, the lungs, and possibly conjunctiva. Image 2-3. Cryptosporidium parvum oocysts. This photomicrograph revealed the morphologic details of Cryptosporidium parvum oocysts, i.e., encapsulated zygotes, which had been stained using the modified acid-fast method. These oocysts exhibit a bright red coloration when using this staining technique, and in this case, you’ll note the sporozoites that were made visible inside the two oocysts on the right. Sporozoites are the nucleated, motile stage of development through which many protozoans pass such as C. parvum, on their way to becoming adults, and represent a very infectious form of these organisms. The sporozoites will be released from these C. parvum oocysts. (SOURCE: PHIL 7829 - CDC/ DPDx - Melanie Moser) - 25 - Cryptosporidiosis in immunocompromised individuals, especially in HIV patients, can be life threatening, as many as 10% may pass oocysts of C. parvum. Infections are characterized by the production of frequent, large volume watery stools and sometimes there is invasion of the pancreas, biliary or respiratory tract. Oocyst excretion and symptoms may fluctuate during the course of infection. Asymptomatic infections are commonly found in developing countries with poor hygiene, where there is close contact with livestock. Laboratory Diagnosis Definitive diagnosis of cryptosporidiosis is by finding the characteristic spherical oocysts in fecal samples. They do not concentrate well using standard concentration techniques and are identified using various staining techniques. Using the modified Ziehl-Neelsen staining method (fuschin followed by methylene blue), the oocysts are acid fast. However, staining results within a smear and between specimens are diverse, varying from unstained to partial red staining and complete staining. Fully sporulated forms can be seen in which the red staining sporozoites are within an unstained oocyst wall. When staining the fecal smear with phenol-auramine/carbol-fuchsin, the oocysts appear as bright yellow discs with an "erythrocyte" pattern of staining against a dark red background. Detection of the oocysts can also be achieved by using specific polyclonal or monoclonal antibodies conjugated to fluorescein. These tests are now commercially available and offer a high degree of sensitivity. However, caution must be exercised when they are used to detect oocysts in the fecal smears distributed by NEQAS parasitology. Such specimens are preserved in formalin, which interferes with the fluorescent staining of the parasites, and are thus difficult to detect. Oocysts in stool specimens (fresh or in storage media) remain infective for extended periods. Thus stool specimens should be preserved in 10% buffered formalin or sodium acetate-acetic acid-formalin (SAF) to render oocysts non-viable. (Contact time with formalin necessary to kill oocysts is not clear; we suggest at least 18 to 24 hours). - 26 - Image 2-4. Staining of Cryptosporidium parvum oocysts in a stool smear with monoclonal antibodies conjugated to fluorescein. The Cryptosporidium oocysts appear with a peripheral green fluorescence. This technique could be of interest in epidemiological inquiries. (x 670) (SOURCE: Unknown) Detection of the oocysts can also be achieved by using rapid antigen detection testing kits that specifically detect antigen released by Cryptosporidium parvu. These tests are now commercially available and offer a high degree of sensitivity even on specimens that are preserved. Isospora belli Introduction Isospora belli is a coccidian protozoan of cosmopolitan distribution, occurring especially in warm regions of the world infecting both humans and animals. Life Cycle The life cycle of I. belli involves an asexual (schizogonic stage) and a sexual (sporogonic stage). - 27 - Illustration 2-3. Isospora belli life cycle. At time of excretion, the immature oocyst contains usually one sporoblast (more rarely two). In further maturation after excretion, the sporoblast divides in two (the oocyst now contains two sporoblasts); the sporoblasts secrete a cyst wall, thus becoming sporocysts; and the sporocysts divide twice to produce four sporozoites each. Infection occurs by ingestion of sporocysts-containing oocysts: the sporocysts excyst in the small intestine and release their sporozoites, which invade the epithelial cells and initiate schizogony. Upon rupture of the schizonts, the merozoites are released, invade new epithelial cells, and continue the cycle of asexual multiplication. Trophozoites develop into schizonts which contain multiple merozoites. After a minimum of one week, the sexual stage begins with the development of male and female gametocytes. Fertilization results in the development of oocysts that are excreted in the stool. Isospora belli infects both humans and animals. (SOURCE: PHIL 3398 - CDC/Alexander J. da Silva, PhD/Melanie Moser) - 28 - Morphology Infection with Isospora belli occurs in both immunocompetent and immunocompromised patients and begins when the mature oocyst is ingested in water or food. Morphology of oocysts. The mature oocyst contains 2 sporocysts, each containing 4 sporozoites measure on average 35 x 9µm. Figures 6. and 7. demonstrate fecal smears of oocysts. The sporulated oocyst is the infective stage of the parasite and they excyst in the small intestine releasing sporozoites which penetrate the epithelial cells, thus initiating the asexual stage of the life cycle. The sporozoite develops in the epithelial cell to form a schizont, which ruptures the epithelial cell containing it, liberating merozoites into the lumen. These merozoites will then infect new epithelial cells and the process of asexual reproduction in the intestine proceeds. Some of the merozoites form macrogametes and microgametes (sexual stages) which fuse to form a zygote maturing finally to form an oocyst. A B C Image 2-5. A, B, C: Oocysts of Isospora belli. The oocysts are large (25 to 30 µm) and have a typical ellipsoidal shape. When excreted, they are immature and contain one sporoblast (A, B). The oocyst matures after excretion: the single sporoblast divides in two sporoblasts (C), which develop cyst walls, becoming sporocysts, which eventually contain four sporozoites each. (Images contributed by Georgia Division of Public Health. (CDC)) Clinical Disease In the immunocompetent, infection is generally asymptomatic or a self-limiting gastro-enteritis. However, in chronic infections, severe non-bloody diarrhea with cramp-like abdominal pain can last for weeks and result in fat malabsorption and weight loss. Eosinophilia may be present (atypical of other protozoal infections). In immunocompromised individuals, infants and children, infection ranges from self-limiting enteritis to severe diarrheal illness resembling that of cryptosporidiosis. - 29 - Laboratory Diagnosis Oocysts are thin walled, transparent and ovoid in shape. They can be demonstrated in feces after a formal ether concentration where they appear as translucent, oval structures. Alternatively, oocysts can be seen in a fecal smear stained by a modified Ziehl-Neelsen method, where they stain a granular red color against a green background, or by phenol- auramine. Cyclospora cayetanensis Introduction Cyclospora cayetanensis, a coccidian protozoan, has been described in association with diarrheal illness in various countries, in particular Nepal, Pakistan and India. Infection results in a disease with non-specific symptoms. Quite often the disease is the cause of unexplained summer diarrheal illness and similar illness following travel to tropical areas. Life Cycle and Morphology The life cycle of this organism is unknown, however environmental data suggest that Cyclospora, like Cryptosporidium species, is a water-borne parasite. The oocysts of C. cayetanensis are spherical, measuring 8-10µm in diameter and the mature oocyst contains 2 sporocysts. Oocysts of Cyclospora cayetanensis, are twice as large in comparison with C. parvum and are not sporulated (do not contain sporocysts - upon excretion). - 30 - Some of the elements in this figure were created based on an illustration by Ortega et al. Cyclospora cayetanensis. In: Advances in Parasitology: opportunistic protozoa in humans. San Diego: Academic Press; 1998. p. 399-418. Illustration 2-4. When freshly passed in stools, the oocyst is not infective (thus, direct fecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium). In the environment , sporulation occurs after days or weeks at temperatures between 22°C to 32°C, resulting in division of the sporont into two sporocysts, each containing two elongate sporozoites. Fresh produce and water can serve as vehicles for transmission and the sporulated oocysts are ingested (in contaminated food or water). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine. Inside the cells they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools. The potential mechanisms of contamination of food and water are still under investigation. (SOURCE: CDC) - 31 - Image 2-6. This photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with modified acid-fast stain, revealed the presence of four Cyclospora cayetanensis oocysts in the field of view. Compared to wet mount preparations, the oocysts are less perfectly round and have a wrinkled appearance due to this method of fixation. Most importantly, the staining is variable among the four oocysts. (SOURCE: PHIL 7827 - CDC/ DPDx - Melanie Moser) Image 2-7. This photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with safranin, revealed the presence of three uniformly stained Cyclospora cayetanensis oocysts in the field of view. (SOURCE: PHIL 7828 - CDC/ DPDx - Melanie Moser) - 32 - Clinical Disease Patients from whose stools the organism has been isolated have reported nausea, vomiting, weight loss and explosive watery diarrhea. Flatulence and bloatedness, nausea and vomiting, myalgia, low-grade fever, and fatigue are associated symptoms. The site of infection is the small bowel. The disease is usually self-limiting to three to four days but untreated infections can last from several days to a month or longer, and may follow a relapsing course. Some infections are asymptomatic. Image 2-8. Four Cyclospora oocysts from fresh stool fixed in 10% formalin and stained with modified acid-fast stain. Compared to wet mount preparations, the oocysts are less perfectly round and have a wrinkled appearance. Most importantly, the staining is variable among the four oocysts. (SOURCE: CDC) Laboratory Diagnosis The oocysts of C. cayetanensis are spherical as can be seen in formol-ether concentrated stool samples by light microscopy. They are refractile spheres which exhibit blue autofluorescence under ultraviolet light. It is important to note that UV microscopes set up for FITC and auramine microscopy only (450-500nm) will fail to detect the autofluorescence of the oocyst. Iodine-quartz microscopes do not produce UV wavelength below 400nm, while both mercury vapor and xenon vapor microscopes must be fitted with a 340-380nm excitation filter to demonstrate autofluorescence. The oocysts are variably acid-fast when stained by the modified Ziehl-Neelsen method. Some cysts are acid-fast whereas others appear as round holes against a green background. They do not stain well with phenol-auramine. - 33 - Microsporidia Species Introduction The term microsporidia is also used as a general nomenclature for the obligate intracellular protozoan parasites belonging to the phylum Microsporidia. To date, more than 100 genera and 1,000 species have been described as parasites infecting a wide range of vertebrate and invertebrate hosts. There are at least seven microsporidian species that are well characterized as human pathogens. (Table 2-1.) Microsporidia are characterized by the production of resistant spores and the polar tubule (or polar filament) which is coiled inside the spore as demonstrated by its ultrastructure. They have recently come to medical attention as opportunistic pathogens in humans with Acquired Immune Deficiency Syndrome (AIDS) and have been implicated in conditions ranging from enteritis to keratoconjunctivitis. Life Cycle and Morphology Microsporidia are primitive organisms. They possess no mitochondria and have prokaryotic like ribosomes. Classification is based on the ultrastructural features, which include the number of coils in the polar tubes, the configuration of nuclei and the spore size 1-4µm, depending on the species. - 34 - Illustration 2-5. Life cycle of Microsporidia sp. The infective form of microsporidia is the resistant spore and it can survive for a long time in the environment. The spore extrudes its polar tubule and infects the host cell. The spore injects the infective sporoplasm into the eukaryotic host cell through the polar tubule. Inside the cell, the sporoplasm undergoes extensive multiplication either by merogony (binary fission) or schizogony (multiple fission). This development can occur either in direct contact with the host cell cytoplasm (e.g., E. bieneusi) or inside a vacuole termed parasitophorous vacuole (e.g., E. intestinalis). Either free in the cytoplasm or inside a parasitophorous vacuole, microsporidia develop by sporogony to mature spores. During sporogony, a thick wall is formed around the spore, which provides resistance to adverse environmental conditions. When the spores increase in number and completely fill the host cell cytoplasm, the cell membrane is disrupted and releases the spores to the surroundings. These free mature spores can infect new cells thus continuing the cycle. (SOURCE: CDC) - 35 - Microsporidia Size Associated Disease Gastrointestinal and biliary Enterocytozoon bieneusi 1µm x 1.5µm tract infections Gastrointestinal tract and Encephalitozoon intestinalis 1.5µm x 2.5µm systemic infections Keratopathy (corneal edema), Encephalitozoon hellem 1.5µm x 1µm respiratory tract infection Encephalitozoon cuniculi 1.5µm x 1µm Central nervous system disease Nosema connori 2µm x 4µm Systemic infections Nosema corneum 2µm x 4µm Keratopathy Pleistophora species 2.8µm x 3.4µm Myositis Table 2-1. Microsporidia found in humans and their associated disease. (Cuomo) In addition to the species in Table 2-1., above, there are other, not well-characterized microsporidian human pathogens. These are designated as Microsporidum, a collective taxon that includes Microsporidum africanum and Microsporidium ceylonensis. Clinical Disease The most common microsporeans found in patients with AIDS are Enterocytozoon bieneusi, Encephalitozoon intestinalis and Encephalitozoon hellem. Patients with these infections tend to be severely immunodeficient with a CD4 count less than 100 x 106/L. Additionally, cases of microsporidiosis have been reported in immunocompromised persons not infected with HIV and in immunocompetent individuals. The clinical manifestations of microsporidiosis are very diverse, varying according to the causal species, with diarrhea being the most common. Enterocytozoon bieneusi Infections with E. bieneusi are restricted to the enterocytes of the small intestine, resulting in villous atrophy and malabsorption. Clinical symptoms include chronic watery, non-bloody diarrhea, malaise and weight loss. - 36 - Image 2-9. Electron micrograph of an Enterocytozoon bieneusi spore. Arrows indicate the double rows of polar tubule coils in cross section which characterize a mature E. bieneusi spore. (SOURCE: CDC) Encephalitozoon intestinalis Infection with Encephalitozoon intestinalis occurs in the enterocytes of the small intestine but is more widely disseminated than E. bieneusi and has been found in the colon, liver and kidney. Image 2-10. Electron micrograph of an eukaryotic cell with Encephalitozoon intestinalis spores and developing forms inside a septated parasitophorous vacuole. The vacuole is a characteristic feature of this microsporidian species. (SOURCE: CDC) - 37 - Encephalitozoon hellem and Encephalitozoon cuniculi These organisms have also been found in disseminated microsporidiosis. Clinical symptoms may include sinusitis, nephritis, hepatitis, keratoconjunctivitis and peritonitis. Nosema corneum This organism has been detected in AIDS patients with keratoconjunctivitis. Laboratory Diagnosis Initially, the diagnosis of intestinal microsporidiosis depended on tissue biopsies which were stained with Gram’s stain and examined by light microscopy. However, in order that ill patients were not subjected to unnecessary invasive procedures, non-invasive diagnostic procedures were developed. The modified Trichrome stain and the Fungiqual fluorescent stain are the stains of choice. Immunofluorescence assays (IFA) using monoclonal and/or polyclonal antibodies are being developed for the identification of microsporidia in clinical samples. 2.3 Examination of fecal specimens for Parasites Many intestinal disorders are due to intestinal parasites which cannot be diagnosed symptomatically. Laboratory investigation is therefore required and the staff responsible should have adequate expertise in examining fecal specimens for parasitic organisms. 2.4 Relevant information required The request form should always state the patients clinical symptoms and signs and whether the patient had recent overseas travel. If the patient has had no recent history of overseas travel, examination for Cryptosporidium, Giardia and Microsporidia, if immunocompromised should be considered. If overseas travel has been undertaken, it is important to note is the patient ill or whether a routine post-tropical screen is requested. The geographical location is also important as it may indicate these parasites which could be present. - 38 - 2.5 Collection of samples If a fecal sample is not properly collected and taken care of before examination, it will be of little or no value for accurate diagnosis. This is especially true if protozoa are present. Amoebic trophozoites begin to degenerate one to two hours after passage, as do flagellate trophozoites. Cysts will deteriorate if the fecal specimen is left standing for many hours or overnight, especially at high temperatures. Helminth eggs and larvae are less affected by the age of the specimen than are protozoa. Nevertheless, changes may occur that could affect their identification; hookworm larvae may become embryonated and larvae may hatch from the eggs risking confusion with Strongyloides larvae. Larvae themselves may disintegrate thus making their identification difficult. To ensure that good specimens are provided for examination, it is important to note the following points. 1. A clean dry container must be used for the collection of fecal samples. Urine and water will destroy trophozoites, if present, and the presence of dirt also causes identification problems. 2. Ideally the specimen should be brought to the lab as soon as it is passed, to avoid deterioration of protozoa and alterations of the morphology of protozoa and helminths. 3. The specimen container should be clearly labeled with the patient’s name, date, and time of passage of the specimen. 4. An amount of stool adequate for parasite examination should be collected and a repeat sample requested if too little is supplied. The smallest quantity that should be accepted is about the size of a pigeon’s egg. 5. Diarrheal specimens, or those containing blood and mucus, should be examined promptly on arrival in the laboratory. The specimens may contain motile amoebic or flagellate trophozoites and may round up and thus be missed if examination is delayed. Where amoebic dysentery is suggested, the laboratory should be informed that a "hot stool" is being supplied so that it can be examined within twenty minutes of being passed. 6. With the exception of "hot stools" if specimens cannot be examined as soon as they arrive, they should be put in the refrigerator. 2.6 Visual observation of the fecal sample It is important to observe the macroscopic appearance of the stool as this can give a clue to the type of organisms present. Therefore the consistency; formed, unformed or liquid; the color and the presence or absence of the exudate are reported. The presence of adult worms can also be seen in a freshly passed stool e.g. adult stage of Ascaris lumbricoides and Enterobius vermicularis. Proglottids of Taenia species can also be seen. - 39 - 2.7Routine procedure for the microscopic examination of fecal samples for parasites 1. Direct microscopy should be done on all unformed and liquid samples by mixing a small amount of the specimen in 0.9% sodium chloride solution. This permits detection of trophozoites of Entamoeba histolytica and Giardia lamblia. It can also provide information on the content of the stool i.e. the presence of leucocytes and red blood cells. 2. A formol-ether concentrate should be done on all fecal samples examined for parasites. This reveals the presence of most protozoan cysts, eggs of nematodes, cestodes and trematodes and also the larval stages of some nematodes. 3. A permanently stained direct fecal smear should be used for all bloody, liquid or semi- formed stools. The smear can reveal the presence of intestinal parasites that can be either destroyed or missed by the formol-ether concentration method e.g. Dientamoeba fragilis. 4. Specimens from patients with HIV should be left in 10% formalin for one hour before proceeding with parasite examination. 2.8Principals of Diagnostic Methods for the Identification of Parasites The principal of the successful identification of fecal parasites is based upon, 1. Measurement - The use of an eyepiece graticule is of the utmost importance, especially for cyst identification. 2. Morphology - In protozoan cysts, the number of nuclei and the presence of inclusions e.g. glycogen mass and chromidial bar, aid the identification of protozoa. In trophozoites, the presence of red cells in amoebae is diagnostic of Entamoeba histolytica and flagella also aid identification of some protozoan trophozoites. 3. Appearance - In helminth eggs, the shape of the egg, the thickness of the shell, the color of the ovum and the presence or absence of features such as an operculum, spine or hooklets are diagnostic pointers to the identity of the parasite. 4. Stains also aid in identification of the parasite. The addition of iodine to formol ether concentrates highlights the internal structure of cysts and helps distinguish between vegetable matter and cysts. Permanently stained fecal smears are useful in demonstrating the nuclear pattern of cysts. - 40 - 2.9 Problems of identification Many things in stool specimens look like parasites but are not. Epithelial cells and macrophages can be confused with amoebic trophozoites, especially macrophages that show slight amoeboid movement and may contain red blood cells. Pus cells can be confused with amoebic cysts. The nuclei appear as 3 or 4 rings and usually stain heavily. The cytoplasm is ragged and the cell membrane is often not seen. Amoebic cysts have a distinct cell wall. Hair and fibers may be confused with larvae, but they do not have the same internal structure as larvae. Plant cells can be confused with cysts or eggs. Though plant cells usually have a thick wall and cysts have a thin wall. 2.10 Reporting of Parasites Ideally, the presence of all parasites should be reported, whether they are pathogens or non- pathogens. This particularly applies to the presence of cysts. However, if it is laboratory practice to report all cysts, the report should state whether they are pathogenic or non- pathogenic. The stage of the parasite should always be reported. For the protozoa, whether cysts or trophozoites are present; the stage of larvae as in Strongyloides; and whether adult stages or eggs of helminths are present. - 41 - Chapter 3. The Flagellates 3.1 Infections Acquired Through the Gastrointestinal Tract The Flagellates belong to the Magistophora and possess more than one flagellum. Beating these flagella enable them to move. A cytosome may be present which helps in the identification of the species. Flagellates possess one advantage over their ameboid relatives in that they can swim. Therefore, enabling them to invade and adapt to a wider range of environments unsuitable for other amoebae. They are able to change from a flagellated free-swimming environment to a non-flagellated tissue dwelling stage and vice versa. Flagellates are known to inhabit the reproductive tract, alimentary canal, tissue sites and also the blood stream, lymph vessels and cerebrospinal canal. There are pathogenic and commensal species of flagellates. The flagellates which are encountered in the intestinal tract are Giardia lamblia, Dientamoeba fragilis, Chilomastix mesnili, Trichomonas hominis, Retortamonas intestinalis and Enteromonas hominis (the latter two being less common). The trophozoites are easily recognized in saline preparations by their motility. However, accurate identification is done on a permanently stained fecal smear. Cysts are more commonly seen than the trophozoite. Giardia lamblia Introduction Giardia lamblia is a flagellate of world-wide distribution. It is more common in warm climates than temporal climates. It is the most common flagellate of the intestinal tract, causing Giardiasis. Humans are the only important reservoir of the infection. The infection is most common in parts of the world where sanitation is at its lowest. Giardiasis is an infection of the upper small bowel, which may cause diarrhea. Only Giardia spreads disease. Morphology of the Trophozoites The trophozoites of G. lamblia are flattened pear shaped and are an average size of 15μm long, 9μm wide and 3μm thick. When stained, the trophozoite is seen to have two nuclei, two slender median rods (axostyles), and eight flagella arising from the anterior end. They have - 42 - been described as looking like tennis rackets without the handle (they are often seen has having a comical face-like appearance when looking at the front view). The movement of the trophozoites is described as tumbling leaf motility, using their four pairs of flagella for locomotion. They attach themselves to the surface of the jejunal or duodenal mucosa by their disc-like suckers which are found on their ventral surface. They multiply in the gut by binary fission. Once the trophozoites drop off the mucosal surface they are normally carried in the intestinal contents down the gut where they usually encyst. Illustration 3-1. Life cycle of Giardia lamblia. Cysts are resistant forms and are responsible for transmission of Giardiasis. Both cysts and trophozoites can be found in the feces (diagnostic stages). The cysts are hardy and can survive several months in cold water. Infection occurs by the ingestion of cysts in contaminated water, food, or by the fecal-oral route (hands or fomites). In the small intestine, excystation releases trophozoites (each cyst produces two trophozoites). Trophozoites multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral sucking disk. Encystation occurs as the parasites transit toward the colon. The cyst is the stage found most commonly in nondiarrheal feces. Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. While animals are infected with Giardia, their importance as a reservoir is unclear. (SOURCE: PHIL 3394 - CDC/Alexander J. da Silva, PhD/Melanie Moser) - 43 - Morphology of Cysts The cysts of G. lamblia are 8-12μm in length and are ellipsoid in shape. They contain four nuclei which tend not to be obvious. Longitudinal fibrils consisting of the remains of axonemes and parabasal bodies may also be seen. Cysts may appear to shrink from the cell wall. The cysts are infective as soon as they are passed. Image 3-1. Giardia lamblia cyst iodine stained. (SOURCE: PHIL 3741 – CDC/ Dr. Mae Melvin) Image 3-2. Giardia lamblia cyst iodine stained. (SOURCE: PHIL 3742 – CDC/ Dr. Mae Melvin) - 44 - A B C Image 3-3. A, B, C Three trophozoites of Giardia intestinalis, stained with trichrome (A) and stained with iron hematoxylin (B and C). Each cell has two nuclei with a large, central karyosome. Cell size: 9 to 21 µm in length. (SOURCE: CDC) Image 3-4. This scanning electron micrograph (SEM) revealed some of the external ultrastructural details displayed by a flagellated Giardia lamblia protozoan parasite. G. lamblia is the organism responsible for causing the diarrheal disease "giardiasis". Once an animal or person has been infected with this protozoan, the parasite lives in the intestine, and is passed in the stool. Because the parasite is protected by an outer shell, it can survive outside the body, and in the environment for long periods of time. (SOURCE: PHIL 8698 - CDC / Janice Carr) - 45 - Clinical Disease Giardia lamblia colonizes the small intestine where the trophozoites adhere to the mucosal surface by means of their sucking disc. Cysts are produced as the parasites descend the intestinal tract although trophozoites can be passed in the feces in severe infections. G. lamblia is transmitted through ingestion of cysts in contaminated water or food. Cysts can survive outside the body for several weeks under favorable conditions. The main symptoms are abdominal pain, flatulence, and episodic diarrhea with steatorrhea and periodical soreness in severe cases. No blood or mucus is normally seen. However 50% of G. lamblia infections are symptomless, although severe infections may develop in immunocompromised hosts. What determines susceptibility is poorly understood. After swallowing cysts for the first time, symptoms commonly develop 2-6 weeks later. Laboratory Diagnosis Cysts can be found by examination of the deposit of a formol-ether concentrate of a stool preparation. The oval cysts with thick walls serve as characteristic features for these organisms. (Keys 3-1. & 3-2.) The flagella disintegrate and form a central ‘streak’ which becomes visible when stained with iodine or MIF (merthiolate-iodine-formaldehyde). Cysts may be excreted intermittently; therefore it is important to examine more than one stool. Stools are usually passed 3-8 times / day and are usually pale, offensive, rather bulky and accompanied by much flatus. Trophozoites are found by examination of saline wet preparations of fresh, diarrheic stool, duodenal or jejunal aspirate or in a permanently stained fecal preparation. Trophozoites can also be found in the jejunal aspirate. These can be recovered by the String Test or Enterotest capsule and the material examined microscopically for motile trophozoites. Trophozoites and cysts can be found to be scarce in chronic infections. Serological methods of diagnosis are proving to be useful as means of diagnosis. An ELISA to detect IgM in serum provides evidence of a current infection. A polyclonal antigen-capture ELISA can be used to demonstrate submicroscopic infections in feces and an IgA-based ELISA will detect specific antibodies in saliva. Table 3-1. details useful morphological features that are similar between species of flagellate and are used in laboratory diagnosis. Detection of the trophozoites and cysts can also be achieved by using rapid antigen detection testing kits that specifically detect antigen released by Giardia lamblia. These tests are now commercially available and offer a high degree of sensitivity even on specimens that are preserved and can be detected days before any trophozoites and cysts are shed. Dientamoeba fragilis Introduction Dientamoeba fragilis is an amoeba-flagellate with a cosmopolitan distribution. The life cycle is not completely known. - 46 - Morphology of Trophozoites and Lifecycle D. fragilis are relatively small, varying from 3-22μm in diameter and there can be considerable variation in size among organisms in the same fecal sample. The organisms have only a trophozoite stage and in a permanently stained preparation, one, two or rarely three nuclei can be seen, two being the most common. The nuclear chromatin is usually fragmented into three to five granules but these have not been visualized by Giemsa Stain, and there is normally no peripheral chromatin on the nuclear membrane. The cytoplasm is usually vacuolated and may contain ingested debris as well as some large uniform granules. The cytoplasm can also appear uniform and clean with a few inclusions. D. fragilis live in the lumen of the cecum and upper colon. Illustration 3-2. This is an illustration of the assumed life cycle of Dientamoeba fragilis, the cause of a protozoan parasitic infection. (SOURCE: PHIL 3389 - CDC/Alexander J. da Silva, PhD/Melanie Moser) - 47 - Image 3-5. Dientamoeba fragilis. Trichrome stain. (SOURCE: PHIL 548 – CDC) Pathogenesis This is a controversial area. The organism has been reported in association with mucous diarrhea, abdominal pain and tenderness. Nausea, vomiting and low-grade fever have also been reported in a number of cases. The precise role of this organism as a cause of disease remains to be determined. Laboratory Diagnosis Diagnosis is dependent on examination of the fresh direct wet preparation or permanently stained smears prepared from unformed or formed stools with mucus. It is particularly important that permanently stained smears of stool preparations should be examined, because survival times of the organism in terms of morphology, is very limited and specimens must be examined immediately or preserved in a suitable fixative as soon as possible after defecation. The recommended stains are Fields’ and Giemsa stain (trophozoites are destroyed in a formol- ether concentrate). Table 3-1. details useful morphological features that are similar between species of flagellate and are used in laboratory diagnosis. Trichomonas hominis Introduction This flagellate is cosmopolitan in its distribution. It is thought to be non-pathogenic although it has been associated with diarrheic stools. It is the most commonly found flagellate next to G. - 48 - lamblia and D. fragilis. Found in a wide host range including non-human primates, cats, dogs and various rodents. Diagram 3-3. This is an illustration of the life cycle of Trichomonas vaginalis, the causal agent of Trichomoniasis. (SOURCE: PHIL 3423 - CDC/Alexander J. da Silva, PhD/Melanie Moser) Morphology of Trophozoites Trichomonas hominis do not have a cystic stage. The trophozoites measure from 5-15μm in length by 7-10μm in width. The shape is pyriform and has an axostyle which runs from the nucleus down the centre of the body and extends from the end of the body. They also possess an undulating membrane which extends the entire length of the body and projects from the body like a free flagellum (this feature distinguishes it from other trichomonads). The characteristic number of flagella is five; there is some deviation from this number. They also - 49 - have a single nucleus at the anterior end. Trichomonads swim with a characteristic wobbly movement, which makes them unmistakable during diagnosis. Image 3-6. Two trophozoites of Trichomonas vaginalis obtained from in vitro culture. Smear was stained with Giemsa. (SOURCE: CDC) Laboratory Diagnosis In a fresh stool, the flagellates move very rapidly in a jerky, non-directional manner. The axostyle and undulating membrane are diagnostic. The flagellates are difficult to stain; however, the axostyle can be seen on a stained preparation and is diagnostic. Table 3-1 details useful morphological features that are similar between species of flagellate and are used in laboratory diagnosis. Chilomastix mesnili Introduction Chilomastix mesnili is cosmopolitan in distribution although found more frequently in warm climates. It is thought to be non-pathogenic although the trophozoite has been associated with diarrheic stool. This is the largest flagellate found in man with an incidence of 1-10% being in the large intestine. - 50 - Illustration 3-4. The life cycle of C. mesnili (Cuomo) Morphology of the Trophozoite The trophozoites of C. mesnili are pear shaped and measure 6-20μm in length. They have one large nucleus with a small karyosome and three flagella that extend from the nucleus at the anterior end of the parasite. A distinct oral groove or cytosome can be seen near the nucleus with its sides being supported by two filaments. They are known to move in a directional manner. Image 3-7. Chilomastix mesnili cyst.(SOURCE: PHIL 426 - CDC /Dr. Mae Melvin) - 51 - Morphology of cysts The cysts are 6-9μm; they have a large single nucleus with a large karyosome. They also have a prominent side knob giving it a characteristic lemon shape. The cytosome is evident with a curved shepherds crook fibril. It also has a characteristically coiled filament which when stained is darker in color. Image 3-8. Chilomastix mesnili cysts are excreted with feces and constitute the transmission form of the micro-organism. The uninucleated lemon shaped cysts are seen with a little proturberance at one end and a prominent cytostome. (Iodine stained). (6µm) (CDC) Laboratory Diagnosis The characteristic lemon shaped cysts can be seen in a formol-ether concentrate. Motile organisms can be seen in a wet preparation of a fresh stool however the characteristic morphology is evident in a permanently stained preparation. Table 3-1 details useful morphological features that are similar between species of flagellate and are used in laboratory diagnosis. Enteromonas hominis Introduction Enteromonas hominis is a small flagellate and is rarely encountered in man. It is found in both warm and temperate climates and is considered to be non-pathogenic. - 52 - Illustration 3-5. The life cycle of E. hominis (Cuomo) Morphology of the Trophozoite The trophozoites are oval and 4-10μm in length. They have four flagella, three anterior flagella and one adheres to the body ending in a tail, producing a jerky rotational movement. They have one nucleus with a large karyosome that is evident in a stained preparation. Morphology of the Cyst The cysts are oval and range between 6-8μm in length. They have up to four nuclei with a bipolar tendency. Laboratory Diagnosis The cysts are seen in a formol-ether concentrate. The cysts have no distinguishing characteristics and thus can be confused with E. nana or even yeasts. The characteristic trophozoites can be seen in a permanently stained fecal smear. Table 3-1. details useful morphological feature’s that are similar between species of flagellate and are used in laboratory diagnosis. - 53 - Retortamonas intestinalis Introduction Retortamonas intestinalis like Enteromonas hominis is a small flagellate and is rarely encountered. It is found in both warm and temperate climates and is considered to be non- pathogenic. Illustration 3-6. Life cycle of Retortamonas intestinalis (Cuomo) Morphology of the Trophozoite The trophozoite is small, measuring between 4μm and 9μm. Its movement is jerky and rotational and has two anterior flagella and a prominent cytosome that can be seen in an unstained preparation. It has a relatively large nucleus at the anterior end with a small compact karyosome. Morphology of the Cyst The cysts are small and pear shaped. They range in size between 4-7μm with one large nucleus frequently near the centre. The fibril arrangement from the nucleus is suggestive of a birds beak. This is characteristic of R. intestinalis cysts. Laboratory Diagnosis The small pear shaped cysts are uncharacteristic in an unstained formol-ether preparation. However, the addition of iodine reveals the characteristic bird beak fibrillar arrangement in the - 54 - pear shaped cyst. In a fresh stool preparation, the two anterior flagella and cytosome can be seen in the trophozoite. In a permanently stained preparation, the large nucleus with small central karyosome is diagnostic. The below Tables detail useful morphological features which are similar between species of flagellate and are used in laboratory diagnosis. 3.2 Identifying Flagellates It is important to know and understand the morphological features which differentiate each species of flagellate from one other. The below Tables details the important features that are used when identifying flagellates found in human stool samples. Trophozoites and cysts can be seen in saline mounts of fresh feces. On occasions, species identification may require stained preparations. Number Number of Species Size (Length) Shape Motility of Other Features Nuclei Flagella* Trichomonas 8-20 μm. Pear Nervous, jerky. 1 3-5 Undulating membrane hominis Usual range. 11- shaped. Not visible in anterior. extending length of body. 12 μm. unstained 1 posterior. mounts. Chilomastix 6-24 μm. Pear Stiff, rotary. 1 3 anterior. Prominent cytostome mesnili Usual range, 10- shaped. Not visible in 1 in extending 1/3-1/2 length 15 μm. unstained cytosome. of body. Spiral groove mounts. across ventral surface. Giardia 10-20 μm. Pear “Falling leaf.” 2 4 lateral. Sucking disk occupying intestinalis Usual range, 12- shaped. Not visible in 2 ventral. 1/2-3/4 of ventral surface. 15 μm. unstained 2 caudal. Median bodies lying mounts. horizontally or obliquely in lower part of body. Enteromonas 4-10 μm. Oval. Jerky. 1 3 anterior. One side of body flattened. hominis Usual range, Not visible in 1 posterior. Posterior flagellum extends 8-9 μm. unstained free posteriorly or laterally. mounts. Retortamonas 4-9 μm. Pear Jerky. 1 1 anterior. Prominent cytostome intestinalis Usual range, 6-7 shaped or Not visible in 1 posterior. extending approximately μm. oval. unstained 1/2 length of body. mounts. * Not a normal feature for identifying species in routine stool samples Table 3-1. Differential Morphology of Protozoa Found in Stool Specimens of Humans: Flagellates- Trophozoites (SOURCE: CDC) - 55 - Species Size (Length) Shape Number of Nuclei Other Features Trichomonas No cyst. hominis Chilomastix 6-10 μm. Lemon shaped with 1. Not visible in unstained Cytostome with supporting mesnili Usual range, 8-9 μm. anterior hyaline knob. preparations. fibrils. Usually visible in stained preparations. Giardia 8-19 μm. Oval or ellipsoidal. Usually 4. Not distinct in Fibrils or flagella longitudinally intestinalis Usual range. 11-12 unstained preparations. in unstained cysts. Deep μm. Usually located at one staining fibers or fibrils may be end. seen lying laterally or obliquely across fibrils in lower part of cyst. Cytoplasm often retracts from a portion of cell wall. Enteromonas 4-10 μm. Elongated or oval. 1-4, usually 2 lying at Resembles E. nana cyst. Fibrils hominis Usual range, 6-8 μm. opposite ends of cyst. or flagella are usually not seen. Not visible in unstained mounts. Retortamonas 4-9 μm. Pear shaped or slightly 1. Not visible in unstained Resembles Chilomastix cyst. intestinalis Usual range, 4-7 μm. lemon shaped. mounts. Shadow outline of cytostome with supporting fibrils extends above nucleus. Table 3-2. Differential Morphology of Protozoa Found in Stool Specimens of Humans: Flagellates- Cysts (SOURCE: CDC) Flagellate trophozoites are best identified in fresh saline mounts, allowing you to observe the way that they move. Use Key 3-1. to help to identify stained flagellate trophozoites. Iodine solutions are used primarily to stain flagellate cysts, this makes it possible to see the structure of the nuclei. Use Key 3-2. to help to identify amebic and flagellate cysts: Key 3-1. Flagellates (Adapted and redrawn, WHO, 1991) - 56 - - 57 - Chapter 4. The Cestodes 4.1 Identifying Intestinal Helminths Helminth Parasites - The word "worm" is used loosely to describe organisms with elongated bodies and a more or less creeping habit. Although the word "Helminth" does mean "worm," in zoological terms it is more restricted to members of the phyla Platyhelminths, Nematoda, and Acanthocephala. There are three groups of medically important helminths; Cestodes (tapeworms), Nematodes (roundworms) and Trematodes (flukes). These parasites live in both the body spaces (gut lumen, bile ducts, lungs, oral cavity, etc.) and in tissues (blood, muscles and skin). 4.2 Infections Acquired Through the Gastrointestinal Tract The cestodes (or tapeworms) form a group of worms, exhibiting two unmistakable morphological features; they all possess flat, ribbon like bodies and lack an alimentary canal. Adult tapeworms usually inhabit the alimentary canal of their hosts (though they occasionally are found in the bile or pancreatic ducts) and attach themselves to the mucosa by means of a scolex. Despite the lack of a digestive system they do absorb food from the hosts intestine; thereby providing the tapeworms a habitat that is associated with high nutritional levels, feeding the tapeworms high growth rate. Larvae on the other hand show a wide range of habitat preferences, being found in almost any organ of both vertebrate and invertebrate hosts. Though most larval species show a preference for a particular site. This lack of an alimentary canal markedly separates tapeworms from nematodes and trematodes. The outer tegument of the body must serve not only as a protective coating but also as a metabolically active layer through which nutritive material can be absorbed, along with secretions and waste material to be transported out of the body. The body consists of a chain of segments or proglottids, which can be immature, mature or gravid; the latter of which contain a fully developed uterus packed with eggs. Therefore, each tapeworm is made up of a ‘string of individuals’ having a complete set of reproductive organs - 58 - in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex. Except for Hymenolepis nana, which can develop directly in the same host, the lifecycle of tapeworms involves both an intermediate and definitive host. Stage of Specific Species Size Shape Color Development Features and When Passed Variations Cestodes Taenia saginata 35 μm. Spherical with thick Walnut brown. Embryonated. 6- Thick, striated Taenia solium Range, 31-43 striated shell. hooked oncosphere shell. Eggs of T. μm. present inside a solium and T. thick shell. saginata are indistinguishable and species identification should be made from proglottids or scoleces. "Taenia" spp. should be reported if only eggs are found. Hymenolepis 47 μm x 37 Oval. Shell consists of 2 Colorless, almost Embryonated. 6- Polar filaments. nana μm. Range, distinct membranes. On transparent. hooked oncosphere 40-60 μm x inner membrane are two inside shell. 30-50 μm. small "knobs" or poles from which 4 to 8 filaments arise and spread out between the two membranes. Hymenolepis 72 μm. Round or slightly oval. Yellow. Embryonated. 6- Resembles H. diminuta* Range, 70-86 Striated outer membrane hooked oncosphere nana but lacks μm x 60-80 and thin inner membrane inside shell. polar filaments. μm. with slight poles. Space Poles are between membranes may rudimentary and appear smooth or faintly often hard to granular. see. Dipylidium 35-40 μm. Spherical or oval. 5-15 Colorless. Embryonated. 6- Eggs are caninum* Range, 31-50 eggs (or more) are hooked oncosphere contained in a μm x 27-48 enclosed in a sac or inside shell. sac or capsule μm. capsule. which ranges in size from 58 μm to 60 μm x 170 μm. Occasionally capsules are ruptured and eggs are free. Diphyllobothrium 66 μm x 44 Oval or ellipsoidal with an Yellow to brown. Unembryonated. Egg resembles latum μm. Range, inconspicuous operculum Germinal cell is hookworm egg 58-76 μm x at one end and a small surrounded by a but has a thicker 40-51 μm. "knob" at the other end. mass of yolk cells shell and an which completely operculum. fills inner area of shell. Germinal cell is usually not visible. Table 4-1. Differential Morphology of the Diagnostic Stages of Helminths Found in Humans: Eggs (Cestodes) (SOURCE: CDC) - 59 - Taenia species Introduction Taenia species are the most common cestode parasites of humans. More than 60 million people are infected with T. saginata (beef tapeworm) world wide and about four million are infected with T. solium (pork tapeworm). The life cycle of a Taenia species can be seen in Illustration 4-1. T. saginata has a cosmopolitan distribution, but is more common in developing countries where hygiene is poor and the inhabitants have a tendency of eating raw or insufficiently cooked meat. T saginata is the most common adult tapeworm found in man. T solium is virtually extinct in Europe and the USA. Illustration 4-1. This is the life cycle of Taenia spp., the causal agents of Cysticercosis. Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium. This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier. Pigs - 60 - and humans become infected by ingesting eggs or gravid proglottids ,. Humans are infected either by ingestion of food contaminated with feces, or by autoinfection. In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis. Once eggs are ingested, oncospheres hatch in the intestine , invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci. In humans, cysts can cause serious sequellae if they localize in the brain, resulting in neurocysticercosis. The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci. Cysts evaginate and attach to the small intestine by their scolex. Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years. (SOURCE: PHIL 3387 - CDC/Alexander J. da Silva, PhD/Melanie Moser) Both humans and cattle or pigs are necessary to the complete life cycle of Taenia species. (In Europe and the USA cattle are the normal intermediate hosts, but in the tropics several other ruminants, e.g. goat, sheep llama and giraffe, may serve as the intermediate hosts.) Eggs ingested by the intermediate hosts usually contain oncospheres. The oncospheres then hatch out in the duodenum, pass into the intestine where they penetrate the intestinal wall and are then carried by the circulation to be deposited in tissues (usually muscle). There they develop into cysticerci larva which are white and ovoid, measuring approximately 8 x 5µm. Humans become infected by ingesting inadequately, cooked beef or pork with cysticerci, containing an invaginated protoscolex. The protoscolexes evaginate and pass into the small intestine where they attach themselves to the mucosa and develop into adult worms. Eggs and proglottids are passed out in the feces, and are then eaten by the intermediate host, thus, perpetuating the life cycle. Morphology Ova of Taenia species are spherical, yellowish brown and measure 31-34µm in diameter. The shell is thick and radially striated. Within the shell, the oncosphere has 3 pairs of hooklets. However, the microscopic appearance of the ova of T. saginata and T. solium are identical. (Table 4-1. highlights some of the differences between the two species) - 61 - Image 4-1. The eggs Taenia saginata and Taenia solium are rounded or subspherical, with a thick radially striated brown shell. The diameter is 31 - 43 µm. Inside each shell is an embryonated oncosphere with 6 hooks. (SOURCE: PHIL 4832 – CDC) The length of the adult T. saginata is 4-8 meters long and that of T. solium is 3-5 meters long. Image 4-2. This is an adult Taenia saginata tapeworm. (SOURCE: PHIL 5260 – CDC) - 62 - The proglottids of Taenia species can be identified by the number of uterine branches; 7-13 for T. solium and 15-20 for T. saginata. Image 4-3. This micrograph reveals the organ morphology within Taenia solium tapeworm proglottids. (SOURCE: PHIL 5261 – CDC) Image 4-4. This micrograph reveals the morphology of a gravid proglottid from the cestode Taenia saginata, a tapeworm. (SOURCE: PHIL 5259 – CDC) If the scolex is recovered, the four suckers and rostellum of hooklets of T. solium will distinguish it from T. saginata, which has four suckers but no hooklets. - 63 - Image 4-5. This is an adult Taenia solium taperworm scolex. (SOURCE: PHIL 5262 – CDC) Clinical Disease The presence of the adult worm rarely causes symptoms apart from slight abdominal irritation with diarrhea, constipation or indigestion. The accidental ingestion of the embryonated ova of T. solium may result in cysticercosis in man. An infection due to an adult Taenia, in man or animals, is referred to as taeniasis. T. saginata (the beef tapeworm) does not cause human cysticercosis. When the embryonated eggs are ingested, the embryos hatch out, migrate through the intestinal wall and are carried around the body in the circulation and deposited in various tissues. Muscle and subcutaneous tissues are usually infected, but cysticerci can infect most organs and tissues. Human cysticercosis is usually asymptomatic unless the infection is particularly heavy or cysticerci are formed in some vital area e.g. the brain, resulting in neurological sequelae. - 64 - Table 4-2. Some characteristics differentiating T. saginata from T. solium. Characteristic Taenia saginata Taenia solium Intermediate Host Cattle, reindeer Pig, wild boar Site of Development Muscle, viscera Brain, skin, muscle Scolex: adult worm No hooks Hooks Scolex: cysticercus No rostellum Rostellum & hooks Proglottids: uterine 23 (14 – 32) * 8 (7 –11) * branches Passing of proglottids Single, spontaneous In groups, passively Ovary

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