Dicrocoelium dendriticum

Dicrocoelium dendriticum, the lancet liver fluke, is a parasite fluke that tends to live in cattle or other grazing mammals. Much of what is presently known about Dicrocoelium dendriticum is the result of the work of the naturalist Wendell Krull. While D. dendriticum was discovered by Rudolphi in 1819 and D. hospes was discovered by Loos in 1899, the full life cycle was not known until Krull and C.R. Mapes published a series of papers from 1951-1953 detailing their observations and experiments. It was known that D. dendriticum affected sheep, but everything else was a mystery. The first link in the chain was the discovery of the first intermediate host, the land snail Cochlicopa lubrica (synonym: Cionella lubrica). Next came the discovery that the slime balls coughed up by the snails could be a potential method of transfer of the parasite. Shortly thereafter, the ant Formica fusca was found to be the second intermediate host by which sheep were infected. Their work is the foundation of modern understanding of the parasite. Dicrocoelium dendriticum along with Dicrocoelium hospes are part of a group of flukes that can infect the bile ducts of humans. Because the bodies of these parasites are long and narrow, infections are generally confined to the more distal parts of the bile ducts. As a result, most Dicrocoelium dendriticum infections of the biliary tree produce only mild symptoms. These symptoms can include biliary colic and general digestive disturbances, including bloating and diarrhea. However, in heavier infections, bile ducts and the biliary epithelium may become enlarged in addition to the generation of fibrous tissue surrounding the ducts, and as a result, causing an enlarged liver (hepatomegaly) or inflammation of the liver (cirrhosis). In one unique case, an infection with Dicrocoelium dendriticum was associated with a skin rash urticaria. Due to the highly specific nature of this parasite’s life cycle, human infections are generally rare. Ruminants such as cows and sheep are usually the definitive host, but other herbivorous mammals and humans can also serve as definitive hosts through ingestion of infected ants. One definitive case involved a man who ingested bottled water contaminated by infected ants. The main reservoirs for Dicrocoelium dendriticum are sheep, cows, land snails and ants. However, Dicrocoelium dendriticum has also been found in goats, pigs and even llamas and alpacas. Ruminants are the main definitive host of this fluke but other herbivorous animals, carnivores, and humans can be accidental definitive host. Most infections, especially in cows, are asymptomatic but the effect on the liver depends on the number of flukes and the length of infection. Since the fluke migrates up the biliary duct — but does not penetrate the gut wall or liver tissue — long infections may cause hypertrophy of the bile duct and liver lesion, even in the absence of symptoms. While infections with D. dendriticum are usually symptom free, some animals may show anemia, edema, emaciation, and liver cirrhosis. However, many of the symptoms of dicroceliosis are similar to those of other gastro-, intestinal-, and lung-nematode infections. The diagnosis of D. dendriticum flukes is mainly from the recovery of adults in liver during necropsy or detecting eggs in animal feces. There is some evidence connecting decreased liver function from the trematode infection with pregnancy toxaemia and mastitis in ewes when combined with other risk factors.