Department of Natural Resources
(Fascioloides magna, Fascioloidiasis)
The trematode frequently found in the liver of deer is known as the large American liver fluke. Its scientific name is Fascioloides magna (from Latin: fasciola = band, magna = large), and is in the class Trematoda (flukes), phylum Platyhelminthes (flatworms). The worms are flat, elongate, oval, and look like ‘bloodsuckers' or ‘leeches'. They are purple-gray in color, and when found while cutting open or slicing deer liver, resemble a blood clot. They are frequently surrounded by a fibrous capsule, bathed in a dark, muddy-appearing fluid. The flukes vary in size from 15-30mm wide by 30-100mm long by 2-5mm thick.
The natural host of F. magna is the deer, which is also a reservoir host. A survey in Michigan indicated 20-35% of the deer in the Upper Peninsula, and 7-15% of those in the northern half of the Lower Peninsula are infected. The flukes are rarely encountered in deer from the southern half of the Lower Peninsula. Liver flukes have been found in moose in the Upper Peninsula and in captive elk, but have not been reported in free-ranging elk. Some other states and Canadian provinces report the parasite in moose, elk, white-tailed deer, mule deer, bison, and yak. It has also been reported in red deer, fallow deer, and sambar deer in Europe. The fluke may infect cattle, sheep, and llama.
Adult flukes deposit eggs in the ducts and cavities of the liver of the host. From there, the eggs pass to the intestinal tract and are eliminated in the feces. The eggs need moisture for development, and will hatch in moist feces or shallow water. Low-lying marshy areas, well suited for snail propagation, are ideal sites for fluke development. It takes about 25 days for eggs to hatch into the next stage, miracidia.
Miracidia enter the proper snail host, in Michigan usually Fossaria parva or Stagnicola palustria nuttaliana. In the snail, development proceeds to a sporoscyst form which can produce redia and daughter redia; these in turn produce cercaria, the final intramolluscan form. Cercaria leave the redia while immature, and emerge from the snail after about four days.
Once outside the snail, cercaria encyst on vegetation; these encysted forms are called metacercaria. They represent infective larvae, or young flukes, which are quite resistant to the elements. These are ingested by the definitive host (in this case, deer); the larvae then penetrate the wall of the intestine, and migrate to the liver. The flukes develop to maturity in about three months. If all conditions are favorable, the entire cycle can be completed in five months.
In sheep these parasites do great damage to the liver. There is little tissue response or encapsulation, and uninterrupted migration throughout the liver causes massive hemorrhage and a peritonitis may develop. According to some parasitologists, sheep are the only host in which the fluke causes such damage; two or three of these parasites can cause death in a sheep.
Cattle, bison, yak, and deer will encapsulate mature flukes in the liver, restricting their migration, and hence, damage. In deer, as with other cervidae, there is a favorable balance between the host and the parasite; resulting in minimal evidence of disease. However, under experimental conditions, mule deer fawns and elk calves have died from fluke infection. Mule deer may be as susceptible as domestic sheep to this parasite.
A diagnosis of a fluke infestation is frequently made at necropsy by making slices at 0.5 inch intervals through the entire liver, examining visually for the parasites. Fibrous capsules may be visible on the lung surface. Adult flukes have also been found free in pleural and peritoneal spaces in experimental trials. It is also possible to diagnose fascioloidiasis by microscopic examination of the feces for eggs of the parasite. There is a DNA extraction test that can determine the presence of Fascioloides sp. in field collected snails.
In recent years, fasciolicide drugs have been developed which are effective against mature F. magna. Although captive deer have been successfully treated, administration of the drug to a wild population would be a major problem. Triclabendazole in a medicated corn bait has been given to white-tailed deer in a wildlife refuge in Texas resulting in the prevalence of liver flukes being significantly lower. Control of the disease in livestock could be effected by preventing grazing on snail-infested areas, especially those occupied by infected deer. Snail-infested pastures can be treated with copper sulfate but control is almost impossible to maintain. Restrictions should be implemented against feeding hay harvested in enzootic areas.
There is no indication that deer suffer any major ill effects due to infection with F. magna. It is true that individual animals could develop liver damage with a heavy infection, but the deer herd in general tolerates the parasite well. Recent findings indicate the liver fluke is pathogenic for moose.
The problem in cattle and sheep is more acute. As has been mentioned, liver flukes can readily kill sheep by causing extensive liver damage. Areas endemic for flukes would be hazardous for sheep ranching. Where liver flukes are common in cattle-raising areas, they may present an economic burden to the farmer. At slaughter, fluke-infected cattle liver is condemned, and is not marketable, thus representing a loss of income.
This parasite is not infective for humans and presents no public health menace in this regard. The main prohibition against human consumption of cooked ‘flukey' deer liver would be an aesthetic one. Consumption of venison from an infected deer poses no risk to humans.