Causative agent: Fasciola hepatica
F. hepatica has been isolated from fecal samples taken from bison in Utah (42). The life cycle of F. hepatica requires the passage of immature stages of the fluke through snails. Immature stages of the fluke leave the snail and encyst on vegetation. Bison, cattle and sheep are infected when they consume infected snails or cysts. Once consumed, the immature flukes leave the intestinal tract of the new host and migrate through the peritoneal cavity to enter the liver, where they travel around causing damage to the liver. After 4 to 5 weeks the flukes enter the bile ducts of the liver and begin to lay eggs. These are passed into the intestinal tract, where they hatch into immature forms of the fluke. The immature flukes pass out of the host in the feces. Once on the ground the immature flukes can infect snails.
In cattle and sheep, flukes can cause acute hepatitis while they are migrating through the liver. Alternatively, chronic hepatitis may be associated with damage to the biliary system, caused by flukes residing in the bile ducts (9).
Clinical signs:
Clinical signs associated with liver flukes in bison have not been reported. In cattle, liver fluke infection produces clinical signs associated with acute and chronic hepatitis. These include sudden death, anemia, hypoalbuminemia, edema, and ascites (9).
Postmortem findings:
Pathological changes associated with liver fluke infection in bison have not been reported. Cattle that have acute hepatitis caused by fluke infection may have a large swollen liver with perforations of the capsule and subcapsular hemorrhages. There may be damage to the liver parenchyma caused by the passage of liver flukes. Cattle with chronic disease may have enlarged bile ducts with adult liver flukes in them (9).
Diagnosis:
In chronic cases fluke eggs can be observed in fecal samples taken from bison. In cattle, acute cases are most commonly diagnosed on postmortem examination (9).
Treatment:
There have been no treatment protocols reported for bison. In cattle, trichlabendazole is commonly used. Neither the efficacy nor safety of this drug has been established for bison.
Control:
Control should be aimed at reducing the levels of immature flukes found in snails and in vegetation and at preventing bison from consuming encysted flukes and infected snails. Encysted flukes do not survive freezing, and severe die offs of snails often happen during the winter, especially among snails that are infected with flukes. In the spring, cattle and sheep that carry adult flukes in their bile ducts often contaminate pastures. If bison were to be treated with flukicides, the treatment should be administered early in the spring, to reduce the number of flukes that infected bison pass out onto pasture during the summer.
Immature liver flukes do not survive well in dry pasture conditions. Snails are most likely to be found in marshy conditions. If liver flukes were a problem on a pasture, it would be wise to fence any wet or marshy areas to prevent bison from gaining access to them.