Parasites

2 Pages 403 Words

The trematodes Fasciola hepatica (the sheep liver fluke) and Fasciola gigantica, parasites of herbivores that can infect humans accidentally. The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75 mm) reside in the large biliary ducts of the mammalian host. Immature eggs are discharged in the biliary ducts and in the stool. After development in water, each egg releases a miracidium which invades a suitable snail intermediate host. In the snail the parasites undergo several developmental stages (sporocysts, rediae, and cercariae). The cercariae are released from the snail and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals acquire the infection by eating vegetation containing metacercariae. 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. Fasciola hepatica infect various animal species, mostly herbivores. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress. In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months. Fascioliasis occurs worldwide. Human infections with F. hepatica are found in areas where sheep and cattle are raised, and where humans consume raw watercress, including Europe, the Middle East, and Asia. Infections with F. gigantica have been reported, more rarely, in Asia, Africa, and Hawaii. During the acute phase (caused by the migration of the immature fluke), manifestations include abdominal pain, hepatomegaly, fever, vomiting, diarrhea, urticaria and eosinophilia, and can last for months. In the chronic phase (caused by the adult fluke), the symptoms are more discrete and reflect intermittent biliary obstruction and inflammation. Occasionally, ectopic locations of infection (such as intestinal wall, lungs, subcutaneous tissue, an...