FASCIOLOIDES MAGNA - LARGE AMERICAN LIVER FLUKE

The parasite, Fasciolaides magna, is a large, oblong fluke found in the livers of wild and domestic herbivores.  F. magna has been reported in deer, elk, moose and bison in Ontario.  Adult liver flukes in wild Cervidae average about five centimetres in length, are reddishbrown in colour and in some instances resemble blood clots.

White-tailed deer and wapiti (elk) are normal hosts of F. magna.  Clinical disease associated with F. magna is uncommon in whitetailed deer.  However, "liver rot", which is characteristic of the severe reaction of cattle to infections of F. magna, has been observed occasionally in deer from Ontario.  Until recently the moose was considered as a natural host of liver fluke.  However, Lankester (1974) observed liver damage from F. magna in moose comparable to "liver rot" in cattle.  He also noted that the wall of cysts which encapsulated flukes in the liver of moose were much thicker than in cysts normally seen in white-tailed deer.  He suggested that liver flukes rarely mature in moose and that moose are not suitable natural hosts of this parasite.  The effects of F. magna on the health of moose require further examination.

In white-tailed deer and wapiti, eggs are produced by adult flukes in the liver and they pass by way of the bile duct out of the liver and into the intestinal tract.  The eggs leave the body of the final host in the faeces and those eggs reaching water hatch into larval or immature flukes called miracidia.  To achieve further development the young flukes must penetrate the body of appropriate species of snails.  After passing through several stages of growth and multiplication within the snail host, the parasites are released and at this stage are known as cercariae.  The cercariae encyst upon aquatic vegetation or terrestrial vegetation in very moist areas and are then infective to susceptible hosts feeding on this vegetation.  In the final host, the parasites escape from their cysts, penetrate the intestinal wall, migrate to and enter the liver and in about five months reach maturity.

Within liver tissue of the final host, F. magna becomes enclosed in a fibrous capsule which prevents extensive destruction of both liver tissue and parasite and which permits the escape of the eggs of the fluke.  Characteristic black pigment, thought to be the excreta of the fluke, is frequently observed in the liver of infected animals.

Except for the liver, the meat of an animal lightly or moderately infected with F. magna is suitable for human consumption.  Carcasses that are emaciated as a result of heavy fluke infestation should, however, not be used for human consumption.

Selected References:

Anderson, R.C. and M.W. Lankester. 1974.  Infectious and parasitic diseases and arthropod pests of moose in North America.  Nat.  Can. (Que.) 101: 23-50.

Kingscote, A.A. 1950.  Liver rot (Fascioloidiasis) in ruminants.  Can.  J. Comp.  Med. 14:203-208.

Lankester, M.W. 1974.  Parelaphostrongylus tenuis (Nematoda) and Fascioloides magna (Trematoda) in moose of southeastern Manitoba.  Can.  J. Zool. 52: 235-239.

Swales, W.E. 1935.  The life cycle of Fascioloides inagna (Bassi, 1875), the large liver fluke of ruminants, in Canada with observations on the bionomics of the larval stages and the intermediate hosts, pathology of Fascioloidiasis magna, and control measures.  Can.  J. Res. 12:177-215.