Fascioliasis is a parasitic disease of cattle and sheep caused by the liver fluke, Fasciola hepatica.
The life cycle involves a snail intermediate host, Lymnaea trunculata. The adult fluke lives in the bile ducts of the liver of ruminants and its eggs are passed onto the pasture in the faeces. When temperature and moisture levels are sufficient, the eggs hatch into free-swimming miracidia, which infect and develop further in the snail. Around five to eight weeks later, cercariae emerge and encyst on the herbage. The life cycle is completed when the cercariae are eaten by cattle or sheep. The snails prefer swampy areas or land with small streams or springs, so cattle and sheep grazing such areas are at most risk of becoming infected with the parasite.
The timing of the life cycle is dependent on climatic conditions. In Great Britain, miracidia hatch in spring and develop in the snail over the summer. Cattle and sheep become infected over the late summer and autumn and clinical fascioliasis occurs three to four months later when the adult flukes reach the bile ducts, that is, from December through to March. The flukes start producing eggs about 10 to 12 weeks after infestation. Adult ruminants may remain carriers of F. hepatica for many years.
Cattle are more resistant to infection with F. hepatica than sheep and develop some immunity to the parasite. Therefore, the disease in cattle is more apparent in young stock and is usually chronic. Adult flukes in the bile ducts cause inflammation, biliary obstruction, destruction of liver tissue and anaemia. This affects the growth rate and feed conversion of young animals. In dairy cows, there may be a drop in milk production and a reduction in conception and pregnancy rates. Livers affected with liver flukes are condemned at the abattoirs. Acute hepatic fascioliasis is mainly a condition of sheep but it has been described in calves exposed to large numbers of cercariae. Deaths may ensue.
Fascioliasis is treated and controlled through the strategic use of flukicidal drugs. Control is aided by preventing cattle grazing snail habitat, by removal of snail habitat and by using molluscicides to reduce snail numbers. Climatic data are used to forecast when fluke levels are likely to be high.
Humans may become infected with F. hepatica through the ingestion of marsh plants such as watercress contaminated with cercariae.
Products are available to treat and control liver fluke in both sheep and cattle:
Supaverm, Flukiver, Combinex, Fasinex, Tribex and Virbamec Super are probably the most widely used but contact us for advice on which product to use on your farm.