Medical Parasitology 2nd Stage PDF

Medical Parasitology 2nd stage Dr. Abbas M. Faraj Lec. (6), 2/3/2020 Phylum Platyhelminthes Trematodes (Flukes) The Trematoda, or flukes, constitute one class of the Phylum Platyhelminthes. Adult trematodes are unsegmented organisms. Some are flattened dorso-ventrally, others are rounded and fleshy,...

Medical Parasitology 2nd stage Dr. Abbas M. Faraj Lec. (6), 2/3/2020 Phylum Platyhelminthes Trematodes (Flukes) The Trematoda, or flukes, constitute one class of the Phylum Platyhelminthes. Adult trematodes are unsegmented organisms. Some are flattened dorso-ventrally, others are rounded and fleshy, and few are long and nematode-like. All digenean trematodes are hermophrodites except the members of the Family Schistosomatidae, where the sexes are separated. The most important trematodes from a clinical point of view are blood flukes, Schistosoma mansoni, S. japonicum and S. hematobium. Other trematodes of medical importance are intestinal fluke, Fasciolopsis buski, liver fluke, Fasciola hepatica, Clonorchis sinensis, and lung fluke, Paragonimus westermani. It is estimated that more than 980 million around the world are at risk of acquiring a foodborne infection by Clonorchis, Fasciola, and Paragonimus. When humans eat uncooked or improperly cooked food items from endemic areas, they can acquire Clonorchis by eating the metacercariae encysted in freshwater fishes (e.g. carp), Fasciola by eating metacercariae encysted on aquatic vegetation (e.g. watercress), and Paragonimus by eating crustacean hosts such as a crayfish or freshwater crab (often as crushed crab (often as crushed crab in salad dressing). Liver trematodes (flukes) The liver trematodes 1. Dicrocoelim dendriticum (Dicrocoeliasis) It is the parasite of biliary ducts of Dicrocoelim dendriticum infection in humans is quite unusual, and many suspected cases are simply the result of the person herbivores in many parts of the world. having eaten the liver of an infected animal (typical eggs from flukes in the liver migrate through the intestine and are passed in the stool without real infection). True cases of human infection with this fluke occur when people accidentally or deliberately- ingest infected ants. This fluke is similar to C. sinensis regarding both localization and the clinical symptoms it produces. Morphology (for practical part). 1 Figure 1: Dicrocoelium dendriticum. The drawing illustrates the adult worm. The microphotograph shows stained adult worm. Life cycle (Figure 2) The life cycle of D. dendriticum is exceptional to the general pattern of life cycles of trematodes in that it involves land snails and ants as intermediate hosts. The eggs are fully embryonated when passed in the feces and when ingested by land snail, they develop into cercariae that liberated from the snails during rainy periods and become massed in “slime balls” shed on vegetation as the snail crawls. Ants collect the slime balls and carry them back to the nest and when these balls are ingested by ants, the cercariae become encysted to form metacercariae. The humans must ingest infected ants (with food or water) to be infected. Metacercariae in ants that have been ingested migrate up the bile duct, after the liberation of the young worms from the cyst, into the liver and mature to adult flukes within two months and begin producing eggs. Figure 2: The life cycle of Dicrocoelium dendriticum. 2 Diagnosis The final diagnosis depends on finding the eggs in feces. The eggs are dark brown in colour, thick shelled, have a large operculum, and measure 38-45 μm x 22-30 μm. Symptoms Human infection is mostly light and asymptomatic. In heavy infections, biliary colic and digestive disturbances have been reported. Treatment Praziquantel is effective. 2. Fasciola hepatica (sheep liver fluke infection/ Fascioliasis) Fasciola hepatica (the sheep liver fluke) is a parasite of herbivores that can infect humans accidentally. Fascioliasis occurs worldwide. Human infections with F. hepatica are found in areas where sheep and cattle are raised, and where humans consume raw watercress. Many human cases have been recorded in Iraq. Recently, Hawramy et al. (2012) retrospectively reviewed and analyzed all cases that had been admitted to the Sulaimani Teaching Hospital and Kurdistan Centre for Gastroenterology and Hepatology and diagnosed with Fascioloiasis (18 cases). Fourteen patients (77.7%) had a history of raw watercress ingestion. The authors concluded that Fascioliasis is an emerging disease, but it is underestimated and ignored in Iraq. Morphology (for practical part). Figure 3: Line drawing illustrates the adult worm of Fasciola hepatica. 3 Life cycle The life cycle of F. hepatica is illustrated in Figure 4. Immature eggs are discharged in the biliary ducts and then in the stool. Eggs become embryonated in water, hatch and release miracidia, which invade a suitable snail intermediate host. Development of the larval stages only occurs in snails which belong to the Family Lymnaedae which are freshwater snails. Several developmental stages (sporocysts, rediae, and cercariae occur in snail. The cercariae are released from the snail and encyst as metacercariae mainly on aquatic vegetation. Mammals acquire the infection by eating vegetation containing metacercariae. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress. After ingestion, the metacercariae excyst in the duodenum and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults. In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months and the adult worms may live for 9 years in the bile ducts. Figure 4: Life cycle of Fasciola hepatica. Symptoms The damage caused by F. hepatica or F. gigantica depends mainly on the number of worms. The toxic products of the larvae may cause hyperplasia of the bile ducts. Although some infected persons are asymptomatic, symptoms associated with the larval stage include fever and gastric pain. In addition, mild to moderate anemia, eosinophilia and leukocytosis occur in many patients. The adult worms cause considerable damage mainly due to the mechanical irritation and metabolic byproducts as well as obstruction. Diagnosis Diagnosis depends mainly on the recovery and identification of eggs in stool specimens. In heavy infections, adult worms can be detected in the stool, especially during treatment. 4 Treatment and control Bithinol is recommended for treatment of Fascioliasis. Main control measures include adequate washing of contaminated plant materials, and adequate disposal of faecal waste. Watercress and other water vegetation should not be grown for human use in water to which herbivores have access. 3. Fasciola gigantica (the giant liver fluke) Fasciola gigantica (Figure 5) is a parasite of herbivores (mainly camels, cattle, and water buffalo) that can infect humans accidentally. Infections with F. gigantica have been reported, more rarely, in Asia and Africa. It is larger than F. hepatica and may attain a length of 75 mm. Accordingly, it is easy to distinguish between the two worms. The life cycle and the clinical symptoms are very similar to that of F. hepatica. The eggs of F. gigantica are larger than those of F. hepatica. Figure 5: A microphotograph shows stained adult worm. Blood trematodes (Schistosomiasis/Bilharziasis) Schistosomes are digenetic trematodes belonging to the Phylum Platyhelminthes. Although more than 18 species of the genus Schistosoma have been recognized, the majority are parasites of animals. The three main species of Schistosoma infecting humans have different geographic distributions. Schistosoma haematobium is prevalent in Africa and Asia, S. mansoni is found in Africa and America and S. japonicum is common in the Far East. With the increasing use of irrigation in tropical regions, the incidence of human infection (known as Schistosomiasis or Bilharzia) is rising alarmingly. Schistosomiasis is second only to malaria in importance as a cause of human disease. Approximately 250 million people are infected with schistosomes in 77 countries in the tropics and subtropics and 600 million are at risk. Infections with S. haematobium have been reported from Iraq. Although S. mansoni does not exist in Iraq, it has been found in some Egyptian farmers working in Iraq. Morphology (for practical part). 5 Life cycle The life cycle of schistosomes is illustrated in the diagram (Figure 6). Both male and female cercariae must infect the human for the continuity of the life cycle. When humans make contact with fresh water infested with cercariae, infection occurs by penetration of skin. Cercariae are attracted to the warmth of a body and skin lipids and begin to burrow into exposed skin. Within 30 minutes, The cercariae have penetrated the epidermis and transformed into schistosomules, which enter the peripheral circulation, where they eventually become adults in the hepatoportal system or venous plexus surrounding the bladder, heart, lungs by the venous circulation. In about 3 weeks, they mature and reach the mesenteric (S. japonicum and S. mansoni) or the bladder (S. haematobium) vessels where they live and lay eggs. Males usually migrate against the blood flow, each carrying a female in the gynaecophoral canal, to their preferred site around the bladder or intestine. When the couple (male and female worms) reach very narrow blood vessel, the female leaves the male and starts lying eggs and then returns back to the gynaecophoral canal. Each mature female produces 300 to 3,000 eggs per day depending on the species. Eggs develop as they pass through the vessel wall into the intestine or bladder and are excreted in feces (S. japonicum and S. mansoni) or urine (S. haematobium). In fresh water, the miracidium hatches out of the egg and swims about until it finds an appropriate snail. After two generations of multiplication in the snail, the fork-tailed cercariae emerge into the water and infect another human. Figure 6: The diagram illustrates the life cycle of schistosome flukes. Symptoms Dermatitis (swimmers’ itch/ itchy rash) that occurs within an hour after cercariae penetrate the skin followed by headach, chills, fever, diarrhea, and sosinophilia (known as snail fever or Katayama fever). The symptoms of Schistosomiasis are mainly due to a reaction against the eggs and include splenomegaly, lymphadenopathy and diarrhea. In the bladder, the eggs produce granulomatous lesions and hematuria, urethral increased urinary frequency, dysuria, and bladder obstruction leading to secondary bacterial infections. In endemic areas, most bladder cancers are associated with chronic infection. In the intestine, the eggs cause polyp formation which, in severe cases, may result in life threatening dysentery. In the liver, they cause periportal fibrosis and portal 6 hypertension resulting in hepatomegaly and splenomegaly. S. japonicum eggs are sometimes carried to the central nervous system and cause headache, disorientation, amnesia and coma. Diagnosis Diagnosis is mainly based on finding eggs in urine or feces which are very characteristic and confirm diagnosis. S. haematobium eggs in urine and have a terminal spine. S. mansoni eggs in feces and have a lateral spine while those of S. japonicum are more round with a knob-like spine on one side. Treatment and control Praziquantel is effective against all species of schistosomes. Control measures include adequate disposal of fecal and urine waste, sanitary disposal of sewage and snail control. In addition, swimming in still water should be avoided. 7