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Professional Version

Fascioloides magna in Ruminants

(Large American Liver Fluke, Giant Liver Fluke)


Lora Rickard Ballweber

, DVM, DACVM, DEVPC, Department of Microbiology, Immunology, and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University

Reviewed/Revised Nov 2021 | Modified Oct 2022

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 its large size and lack of an anterior projecting cone. It is found in domestic and wild ruminants; white-tailed deer, elk, and caribou are the reservoir hosts in the US and Canada; red deer and fallow deer are the reservoir hosts in Europe where it has been introduced. The life cycle resembles that of Fasciola spp, except the prepatent period is 30 weeks, and the entire life cycle takes ~7 months.

Although flukes will mature in cattle, the intense encapsulation response forms a closed cyst, so eggs rarely pass out of the animal. Pathogenicity is low, and losses are confined primarily to liver condemnation; death, although rare, can occur. In sheep and goats, encapsulations do not occur, and the parasites migrate in the liver and other organs, causing tremendous damage. A small number of parasites can cause death because of extensive migrations. Infection with F magna appears to be rare in alpacas and llamas, in which the response mirrors that observed in cattle. In cervids, there is little tissue reaction, and the parasites are enclosed in thin, fibrous cysts that communicate with bile ducts. Histologically, infected livers of all species show black, tortuous tracts formed by the migration of young flukes.

Eggs of F magna resemble those of Fasciola hepatica; however, fecal sedimentation is of limited use because eggs usually are not passed by cattle and sheep and probably are not passed by alpacas and llamas. Recovery of the parasites at necropsy is necessary for definitive diagnosis. When domestic ruminants and cervids share the same grazing area, the presence of disease due to F magna should be kept in mind. Mixed infections with F hepatica occur in cattle.

Oxyclozanide has been reported to be effective against F magna in white-tailed deer, and triclabendazole has been used in captive and free-ranging red deer. Rafoxanide has been used successfully against natural infections in cattle. Albendazole (7.5 mg/kg), clorsulon (15 mg/kg), triclabendazole (20 mg/kg), and closantel (15 mg/kg) have shown partial efficacy against this fluke in sheep. Albendazole is currently the only product approved for use (in sheep) in the US. Treatment should be given 8–10 weeks after peak snail activity; however, because none of these drugs is completely effective and a single parasite can result in death, preventing infections is best. Cervids are required for completion of the life cycle; if they can be excluded from the areas grazed by cattle and sheep, control may be achieved. Other control methods are the same as those for F hepatica.

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