Fascioliasis is a parasitic infection typically caused by Fasciola hepatica, which is also known as "the common liver fluke" or "the sheep liver fluke." A related parasite, Fasciola gigantica, also can infect people. Fascioliasis is found in all 5 continents, in over 50 countries, especially where sheep or cattle are reared. People usually become infected by eating raw watercress or other water plants contaminated with immature parasite larvae. The immature larval flukes migrate through the intestinal wall, the abdominal cavity, and the liver tissue, into the bile ducts, where they develop into mature adult flukes, which produce eggs. The pathology typically is most pronounced in the bile ducts and liver. Fasciola infection is both treatable and preventable.
In hypo- to hyperendemic areas of Central and South America, Europe, Africa and Asia, human fascioliasis presents a range of epidemiological characteristics related to a wide diversity of environments. Fasciola hepatica has succeeded in expanding from its European original geographical area to colonize five continents, despite theoretical restrictions related to its biology and in turn dependent upon environmental and human activities.
Immature eggs are discharged in the biliary ducts and in the stool. Eggs become embryonated in water, eggs release miracidia, which invade a suitable snail intermediate host including the genera Galba, Fossaria and Pseudosuccinea. 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. 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. The adult 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. Fasciola hepatica infects various animal species, mostly herbivores.
Both the acute and chronic phases of infection can be symptomatic and symptom free. Nonspecific clinical features of both phases can include fever, which can be intermittent; malaise, abdominal pain, in the right upper quadrant, epigastrium, other abdominal symptoms such as anorexia, nausea, vomiting, diarrhea, change in bowel habits, and weight loss and signs such as hepatomegaly and jaundice, eosinophilia, anemia, especially in children and transaminitis. After ingestion, Fasciola cysts open in the small intestine. The acute stage of the disease occurs as the worms migrate through the lining of the small intestine into the liver and bile duct. While often asymptomatic, the onset of this stage can produce gastrointestinal bleeding, inflammation, abdominal pain, and diarrhea. The chronic phase of the disease occurs when the worms reach the bile duct. The long-term presence of the worms causes progressive inflammation from scar tissue and debris that can lead to fibrosis and obstruction of the ducts.
Treatment is with triclabendazole (10 mg/kg po once after meals or, for severe infections, twice 12 h apart); it is available from the Centers for Disease Control and Prevention (CDC) as an investigational drug. Alternatives include nitazoxanide 500 mg bid po for 7 days and bithionol 30 to 50 mg/kg po every other day for 10 to 15 doses. Treatment failures are common with praziquantel.