Lecture Notes on Medical Parasitology II PDF

**LECTURE NOTES ON PARASITOLOGY II- HELMINTHOLOGY** A parasite is an organism which lives in/on the body of a host. A host is that which harbors the parasite. There is usually some association such as mutualism, commensalisms, or parasitism between the parasite and the host. This association may pr...

**LECTURE NOTES ON PARASITOLOGY II- HELMINTHOLOGY** A parasite is an organism which lives in/on the body of a host. A host is that which harbors the parasite. There is usually some association such as mutualism, commensalisms, or parasitism between the parasite and the host. This association may produce a variety of effects and the host usually tends to react to it. Medical helminthology is concerned with the study of helminthes or parasitic worms. Helminthes are trophoblastic metazoa (multi-cellular organisms). Helminthes are among the common parasitic causes of human suffering. They are the cause of high morbidity and mortality of people worldwide. They cause different diseases in humans, but few helminthic infections cause life-threatening diseases. They cause anemia and malnutrition. In children they cause a reduction in academic performance. Helminthes also cause economic loss as a result of infections of domestic animals. There is age dependent distribution of infections from geo-helminthes and schistosomes. As a result of predisposing behavioral and immunological status, children disproportionately carry the burden of schistosomes and geo-helminthes. The sources of the parasites are different. Exposure of humans to the parasites may occur in one of the following ways: 1\. Contaminated soil (Geo-helminthes), water (cercariae of blood flukes) and food (Taenia in raw meat). 2\. Blood sucking insects or arthropods (as in filarial worms). 3\. Domestic or wild animals harboring the parasite (as in echinococcus in dogs). 4\. Person to person (as in *Enterobius vermicularis, Hymenolepis nana*). *5.* Oneself (auto-infection) as in *Enterobius vermicularis.* The parasites enter the body through different routes including: mouth, skin and the respiratory tract by means of inhalation of airborne eggs. The helminthes are classified into three major groups. These are: 1\. Nematodes (Round worms) 2\. Trematodes (Flukes) 3\. Cestodes (Tape worms) The Trematodes and Cestodes are groups of flat worms. The major objective of this lecture note is to provide good understanding of the most common helminthes prevalent in the tropics in general. **HELMINTHES** The helminthic parasites are multicellular, bilaterally symmetrical animals having three germ layers (trophoblastic). The helminthes of importance to human beings are divided into three main groups with the peculiarities of the different categories as described below: **Shape** Tape like, segmented Leaf like, Unsegmented Elongated, Cylindrical unsegmented **Sexes** Not separate (monoecious) Not separate (monoecious) Separate (dioecious) **Head End** Suckers: with hooks Suckers: no hooks No suckers and Hooks **Alimentary** Absent Present Present and **Canal** but incomplete complete **Body cavity** Absent Absent Present **NEMATODES (ROUND WORMS)** All the important human parasites of the Phylum Nemathelminthes (Aschelminthes) belong to the Class Nematoda. **GENERAL CHARACTERISTICS OF NEMATODES** \(i) They are unsegmented, elongated and cylindrical. (ii) They have separate sexes with separate appearances. (iii) They have a tough protective covering or cuticle. (iv) They have a complete digestive tract with both oral and anal openings. (v) The nematodes are free living (majority) or parasites of humans, plants or animals. (vi) The nematodes are generally light cream-white colored. **The Parasitic Nematodes:** Their life cycle includes: egg, larvae and adult. **The parasitic nematodes are divided into:** **1. Intestinal nematodes** **(I). Intestinal nematodes with tissue stage** **(**a) *Ascaris lumbricoides* (b) Hookworm (c) *Strongyloides stercoralis* **(II). Intestinal nematodes without tissue stage** (a). *Enterobius vermicularis* (b). *Trichuris trichuria* **2. Tissue and blood dwelling nematodes** a\. Filarial worms (*Wuchereria bancrofti* amd *Brugia malayi*) b\. Guinea worms (*Dracunculus medinensis*) c\. *Trichinella spiralis* d\. Larva migrans **INTESTINAL NEMATODES WITH TISSUE STAGE** ***(a). ASCARIS LUMBRICOIDES*** Description: These are common roundworms infecting more than 700 million people worldwide. **Morphology:** Male adult worm measures 15-20 cm in length. The posterior end is curved ventrally. The female worm measures 20-40 cm in length. Its posterior end is straight. **Infective stage and modes of infection:** The egg containing larva when ingested with contaminated raw vegetables causes ascariasis. **Life cycle:** In warm, moist conditions, infective larvae develop within fertile eggs, but do not hatch. Such eggs can survive for long periods in soil. Ingested eggs hatch in the duodenum. The larvae penetrate the intestinal wall and circulate in the blood. From the heart they migrate to the lungs, ascend to the trachea, descend to the esophagus and finally reach the small intestine to become adult and live for several years. The female pass immature eggs which is passed to the soil and mature in 2 weeks. Life cycle of *Ascaris lumbricoides* **Eggs of** *Ascaris lumbricoides* **Pathogenicity and clinical features** Adult worms in the intestine cause abdominal pain and may cause intestinal obstruction especially in children. Larvae in the lungs may cause inflammation of the lungs (Loeffler's syndrome) -- pneumonia-like symptoms. **Diagnosis** 1\. Examination of stool for eggs by direct saline smear method. The egg is ovoidal, 75x60 microns, covered by albuminous mamillatins. 2\. Demonstration of adult worms **Treatment** Mebendazole, Albendazole and Piperazine ***HOOK WORMS*** There are two species of hookworm: *1. Ancylostoma duodenale* 2. *Necator americanus* The adults are found in the small intestines of man. Mixed infection is common. Both of the species are found in Ethiopia, but *N. americanus* is more common. **Ancylostoma duodenale:** Description: Grayish-white in color. The body is slightly ventrally curved. The anterior end follows the body curvature. The buccal cavity is provided ventrally with pairs of teeth and dorsally with a notched dental plate. Distribution: This species is found in the northern part of the world including China, Japan, Europe, North Africa and Ethiopia. **Morphology** **Male:** The male measures 10 cm in length. The posterior end is broadened into a membraneous copulatory bursa that is provided with two long spicules. **Female:** The female measures 12 cm in length. The posterior end is straight. **Life cycle** Adult male and female worms live in the small intestine. The female lays eggs (oval, 60x40 microns), which contain immature embryo in the 4 cell stage. When the eggs pass in the stool to the soil and under favorable conditions of temperature, moisture and oxygen, they hatch into larvae, which molt twice and become infective. When the filariform larvae penetrate the skin, they circulate in the blood, reach the lungs, ascend to the trachea, descend to esophagus to reach the small intestine and become adults. Life cycle of Hookworms **Clinical Features** Adult worms in the intestine feed on blood causing iron deficiency anemia. The larvae may cause inflammation of the lungs. **Diagnosis:** Examination of stool by direct saline smear to detect the eggs. Egg of Hookworm **Treatment** Mebendazole: 1 tab 2x daily for 3 days. **INTESTINAL NEMATODES WITHOUT TISSUE STAGE** **ENTEROBIUS VERMICULARIS (PIN WORM OR THREAD WORM)** ***Description**: Enterobius vermicularis* is a small white worm with thread-like appearance. The worm causes enterobiasis. Infection is common in children. **Morphology** **Male:** The male measures 5 cm in length. The posterior end is curved and carries a single copulatory spicule. **Female:** The female measures 13 cm in length. The posterior end is straight. **Infective stage** Infection is by ingestion of eggs containing larvae with contaminated raw vegetables. **Mode of infection** By direct infection from a patient (Fecal-oral route). Autoinfection: the eggs are infective as soon as they are passed by the female worm. If the hands of the patient get contaminated with these eggs, he/she will infect him/herself again and again. Aerosol inhalation from contaminated sheets and dust. **Life cycle** Adult worm lives in the large intestine. After fertilization, the male dies and the female moves out through the anus to glue its eggs on the peri-anal skin. This takes place by night. The egg is 50x25 microns, plano-convex and contains larva. When the eggs are swallowed, they hatch in the small intestine and the larvae migrate to the large intestine to become adult. Life cycle of *E. vermicularis* **Clinical presentation** The migration of the worms causes allergic reactions around the anus and during night it causes nocturnal itching (pruritus ani) and enuresis (involuntary passing of urine, as a result of urinary tract infection). The worms may obstruct the appendix causing appendicitis. **Diagnosis** ♦ Eggs in stool: Examination of the stool by direct saline smear to detect the egg: this is positive in about 5% of cases because the eggs are glued to the peri-anal skin. ♦ Peri-anal swab: The peri-anal region is swabbed with a piece of adhesive tape (cellotape) hold over a tongue depressor. The adhesive tape is placed on a glass slide and examined for eggs. The swab should be done in the early morning before bathing and defecation. Egg of *E. vermicularis* **Treatment** Mebendazole; Piperazine. **TRICHURIS TRICHIURA (WHIP WORM)** Description: The worm is divided into a thin whip-like anterior part measuring 3/5 of the worm and a thick fleshy posterior part of 2/5 the length. **Male:** The male measures 3 - 4.5cm in length. Its posterior end is coiled and possesses a single cubicle. **Female:** The female measures 4 - 5cm in length. Its posterior end is straight **Infective stage and mode of infection:** Infection is by ingestion of eggs containing larvae with contaminated raw vegetables. **Life cycle:** Ingested eggs hatch in the small intestine and the larvae migrate to the large intestine to become adult. After mating, the female lays immature eggs, which pass with the stool to the soil and mature in 2 weeks. Life cycle of *Trichuris trichiura* **Clinical Features (Symptoms)** The patient complains of dysentery (blood and mucus in stool together with tenesmus- a sensation of the desire to defecate, which is continuous or recurs frequently without the production of significant amount of faeces- often small amounts of mucus or blood alone are passed). Rectal prolapse is also possible. **Diagnosis** Finding of characteristic eggs. The egg of trichuris is barrel-shaped, 50x25microns. The shell is thick with a one mucoid plug at each pole. Egg of *Trichuris trichiura* **Treatment** Mebendazole: 1 tablet twice daily for 2 days. **TISSUE NEMATODES** This group includes the filarial worms, the guinea worm (*Dranculuculus medinensis)* and *Trichinella spiralis.* **FILARIAL WORMS** The filarial worms have complex life cycles involving a developmental stage in an insect vector. They require an arthropod vector for their transmission. The worms inhabit either the lymphatic system or the subcutaneous tissues of man. The female worm gives rise to a young worm called microfilaria. The microfilariae, when taken by the arthropod intermediate host during biting, develop into filariform larvae, which are the infective stages. Humans get infected when bitten by the infected arthropod intermediate host. **Wuchereria bancrofti** This is a parasite of lymph nodes and lymphatic vessels- causing lymphatic filariasis. This filarial worm is transmitted by the bite of various species of mosquitoes. It is believed that over 100 million people are infected. The microfilariae are nocturnal -- seen in greatest numbers in peripheral blood in the night between 10pm - 2am. The physiological basis of this nocturnal periodicity is not understood. Life cycle of W. bancrofti **Mode of transmission and pathogenesis** The filariform larvae are introduced through the skin by the bite of the arthropod intermediate host. The larvae invade the lymphatics, usually the lower limb, where they develop into adult worms. The microfilariae are librated into the blood stream. They remain in the pulmonary circulation during day, emerging into the peripheral circulation only during night, to coincide with the biting habit of the vector. Presence of the adult worms causes lymphatic blockage and gross lymphedema, which sometimes lead to elephantiasis. **Epidemiology:** W*. bancrofti* infection is not reported in higher altitudes of Ethiopia, but limited to lowlands of Gambella. The epidemic area covers a long distance along the Baro River. **Pathogenicity and clinical features:** ♦ The adult worm obstructs the flow of lymph in the lymph nodes and the lymphatic vessels draining the lower limbs and the external genitalia. ♦ The lower limbs and external genitalia become swollen. The skin becomes thick and fissured. The disease is called bancroftian elephantiasis. ♦ The major symptoms and findings include: lymphangitis, lymphedema, fever, headache, myalgia, hydrocele and chyluria. **Diagnosis** ♦ Blood film examination after staining by Giemsa or Leishman stain to detect microfilaria. The film should be taken by night. Microfilaria of W*. bancrofti* in blood smear **Treatment -** Diethyl carbamazine (DEC): 2 mg/kg 3x daily for 2 weeks. ***Onchocerca volvulus*** Infection by this filarial worm is common in Ethiopia. Endemic foci are found in Bebeka, Gojeb valley, Dedessa valley, Agaro, Metekel, and in Northwestern Ethiopia around Gondar. **Morphology:** **Male:** Similar to that of *Wuchereria bancrofti*. **Female:** The female measures 30-50 cm in length. It is present inside of a fibrous nodule (onchocercomata or onchocerca tumor). **Intermediate Host and vector** Female Simulium, (*Simulium damnosum*), Black fly, found around plantations following rivers or river basins. **Microfilaria** Measures 300 microns in length. It is non-sheathed microfilaria. It is present in the subcutaneous tissue fluids and not in blood. Life cycle of O. volvulus **Infective stage and mode of infection** is similar to that of *Wuchereria bancrofti*. **Pathogenicity and clinical manifestations:** The disease, onchocerciasis or river blindness includes: Skin fibrous nodules (onchocercomata) enclosing female worms. The nodules are common in neck, iliac crest and the coccyx. Skin hypo- or hyper- pigmentation. Dermatitis is present. In advanced cases, the skin becomes thickened and wrinkled, showing lizard or leopard skin appearance. Elephantiasis of the external genitalia and corneal opacity and optic atrophy may finally cause blindness. **Diagnosis** Superficial biopsy (skin snip) is taken from the skin using sharp razor blade. The specimen is allowed to stand for 30 minutes in saline before it is examined microscopically for microfilariae. Microfilaria of *O. volvulus* **Treatment** Ivermectin: 50 mg/kg bodyweight, given every 6 or 12 months. Because it kills microfilariae but not adult worms, retreatment is necessary over a period of years. **Prevention** Vector control Mass treatment Establishment of villages away from Simulium breeding places. Use of repellents Protective clothing **Loa loa** Description: The eye worm, *Loa loa,* causes Loiasis. The insect vectors include mango flies of Chrysops - *Chrysops silacea, Chrysops dimidiata*. Loiasis is endemic in Central and West Equatorial Africa. The abundant rubber plantations provide a favorable environment for the vector to transmit the disease. **Morphology** Adult male worms: 30-34 mm in length; Adult female worms: 40-70 mm in length **Pathogenesis** The microfilaria has a sheath. Their diurnal periodicity corresponds to the feeding pattern of the insect vector, which bites humans from 10:00am to 4:00pm. **Clinical Features** Incubation period is about one year. It causes calabar swelling beneath the skin due to parasites. There is fever, pain, pruritus, urticaria, allergic reactions, retinopathy, glomerulonephritis, meningo-encephalitis etc. **Laboratory diagnosis** Detection of microfilaria in peripheral blood, urine, sputum, CSF - stained with Giemsa or unstained Eosinophilia **Treatment** DEC, 6 to 10 mg per kilogram per day for 2 to 3 weeks: but has side effects - allergic reactions **Guinea worm or Medina worm** *Dracunculus medinensis* causes dracunculiasis. The infection is endemic to Asia and Africa: India, Nile Valley, central, western and equatorial Africa, lowlands of Ethiopia and Eritrea. **Morphology** Gravid female worms measure 70-120 cm in length. Their body cavity is almost fully occupied by a uterus greatly distended with rhabditiform larvae (250-750 μm in length). A digestive tube and cuticular annulations distinguish the larvae from microfilariae. Larvae of *D. medinensis* **Pathogenicity and life cycle** Infection is acquired by drinking unfiltered or not boiled water that contains Cyclops species. The larvae are released in the stomach, penetrate the intestinal wall and find their way to the subcutaneous tissue. Mating takes place in the axillary or inguinal regions 3 months after infection. The male worms then die in the tissue and the female worms move down to the limbs within 10 months. In about 1 year, female worms in the subcutaneous tissue provoke the formation of a burning blister in the skin of the legs. When in water, the blister bursts, and about 5 cm of the worm is extruded from the resulting ulcer - thus releasing many thousands of first stage larvae. The larvae swim in water and are ingested by the intermediate host - Cyclops species- within about 4 days. Inside the Cyclops, the larvae molt twice and become infective in 2 weeks. Life cycle of *Drancunculus medinensis* **Clinical feature** The female parasites in the subcutaneous tissue release toxic byproducts of histamine-like nature, which cause systemic allergic reactions, like erythema, urticaria, pruritus, fainting, asthma, dyspnea, etc. This is followed by the appearance of a blister on the legs, which ruptures on contact with water releasing larvae into the water by the female worm. The wound may ulcerate. The worms migrate into other tissues and may cause arthritis, pericarditis, abscesses etc. It occasionally penetrates the eyeball and causes loss of the eye. **Diagnosis** 1\. Clinical: Observation of blister, worm or larvae 2\. Histologic features of subcutaneous sinus tract 3\. Eosinophilia and radiographic evidence **Treatment** 1\. Surgical excision when the worm is in the leg 2. Niridazole (Ambilhar) or DEC **Prevention** Health education on: ♦ Boiling or filtering of drinking water ♦ Treating of patients and educating them not to enter water bodies ♦ Using insect larvicides to kill Cyclops in water. **CESTODES (TAPEWORMS)** **INTRODUCTION** The tapeworms are hermaphroditic and require an intermediate host. The adult tapeworms found in humans have flat body, white or grayish in color. They consist of an anterior attachment organ or scolex and a chain of segments (proglottids) also called strobilla. The strobilla is the entire body except the scolex. The scolex has suckers or grooves. It has rosetellum, which has 1 or 2 rows of hooks situated on the center of the scolex. Adult tapeworms inhabit the small intestine, where they live attached to the mucosa. Tapeworms do not have a digestive system. Their food is absorbed from the host's intestine. **HYMENOLEPIS NANA (DWARF TAPEWORM)** **Morphology** Adult worm measures 1-3 cm in length. It is made up of head (scolex), neck and segmented body. The head carries four suckers and a rostellum armed with one row of hooks. The segments of the body are divided into mature and gravid segments. In the mature segment, there are three testes in the middle. **Infective stage and mode of infection** The egg, which is immediately infective when passed by the patient, is rounded, about 40 microns in diameter. It contains a six- hooked oncosphere within a rigid membrane (the embryosphere). This embryosphere has two polar thickening or knobs from which project 4-8 long, thin filaments called polar filaments. Infection takes place by: 1\. Ingestion of egg with contaminated raw vegetables. 2\. Direct infection from a patient 3\. Auto infection: the eggs of H. nana are infective as soon as they are passed with feces by the patient. If the hands of the patient are contaminated by these eggs, she/he infects herself/himself again and again. **Pathogenicity** Light infections produce no symptoms. In fairly heavy infections, children may show lack of appetite, abdominal pain and diarrhea. **Treatment -** Niclosamide: 4 tablets chewed in a single dose daily for 5 days. **TAENIA SAGINATA (BEEF TAPEWORM)** In adult stage, *T. saginata* inhabits the upper jejunum where it may survive for as long as 25 years. It causes intestinal infection, Taeniasis. It has worldwide distribution. These are one of the true and segmented tapeworms. Their body is divided into three regions: 1\. Scolex: the hold fast organ 2\. Neck: posterior to the scolex 3\. Stobilla: the main bulk, made up of proglottids. **Morphology**: Adult worm measures 5-10 meters in length. The pyriform scolex has 4 suckers but no rostellum. The mature segments have irregularly alternate lateral genital pores. Each of the terminal segments contains only a uterus made up of a median stem with 15-30 lateral branches. **Life cycle** The adult worm lives in the small intestine of man. Gravid segments pass out in the stool and become disintegrated and eggs come out to the soil. The gravid proglottid uterus contains about 100,000 eggs. The egg of *T. saginata* is round, about 40 microns in diameter. The 6-hooked embryo is enclosed in a radially striated embryophore. Eggs are ingested by an intermediate host, cattle. The 6-hooked embryo escapes from its shell, penetrates through the intestinal wall into the blood vessels and is carried to the muscles where it develops into a larval stage, **cysticercus bovis** (made up of an invaginated /inverted head and spherical body). Infection to man takes place by the ingestion of raw or insufficiently cooked beef. In the small intestine of man, the head of the cysticercus gets invaginated and the body becomes segmented. Life cycle of *Taenia saginata* **Pathogenicity** Infected persons may complain of epigastric pain, abdominal discomfort, diarrhea, weight loss, hunger sensation, vomiting, etc. **Diagnosis** Recovery of the gravid segments or the eggs from the stool Egg of *Taenia spp.* **Treatment:** Niclosamide: Four tablets chewed in a single dose. Mebendazole 100mg twice daily for three days **Prevention**: ♦ Thorough cooking of meat (above 57^0^C). ♦ Proper disposal of human excreta **TAENIA SOLIUM (PORK TAPEWORM)** The adult worms of *T. solium* reside or inhabit the upper jejunum. Infection has worldwide distribution. **Morphology:** Adult worm measures about 3 meters in length. The globular scolex has rostellum with 2 rows of hooklets. There are \