F magna is up to 100 mm long, 2–4.5 mm thick, 11–26 mm wide, and oval; it is distinguished from Fasciola spp by the lack of an anterior projecting cone. It is found in domestic and wild ruminants; deer are the normal hosts. The life cycle resembles that of Fasciola spp.

The life cycle is not completed in cattle. In this host, pathogenicity is low, and losses are confined primarily to liver condemnations. In sheep and goats, a few parasites can cause death due to extensive fluke migration in the liver parenchyma. In deer, there is little tissue reaction, and the parasites are enclosed in thin, fibrous cysts that communicate with bile ducts. In cattle, Fascioloides magna causes severe tissue reaction, resulting in thick-walled encapsulations that do not communicate with bile ducts. In sheep, encapsulations do not develop, and the parasites migrate in the liver and other organs, causing tremendous damage. Histologically, infected livers of cattle, sheep, and deer show black, tortuous tracts formed by migrations of young flukes.

While the eggs of resemble those of Fasciola hepatica, this is of limited use; eggs usually are not passed in cattle and sheep. Recovery of the parasites at necropsy as well as differentiation of Fasciola hepatica and is necessary for definite diagnosis. When domestic ruminants and deer share the same grazing, the presence of disease due to Fascioloides magna should be kept in mind. Mixed infections with Fasciola hepatica are seen in cattle.

Oxyclozanide has been reported to be effective against in white-tailed deer, and rafoxanide has been used successfully against natural infections in cattle. Albendazole (7.5 mg/kg), clorsulon (15 mg/kg), and closantel (15 mg/kg) have shown efficacy against this fluke in sheep. Currently no products are approved for use against this fluke in the USA. Deer are required for completion of the life cycle; if they can be excluded from the areas grazed by cattle and sheep, control may be effected. Control of the intermediate host (lymnaeid snails) may be possible once it has been identified in a region and the nature of its habitat examined.

Last full review/revision March 2012 by Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP