The Deer Liver Fluke

The liver fluke of deer, Fascioloides magna, is a parasitic flatworm. Adult flukes are found in the liver of deer from scattered locations in North America, including Minnesota (see Fig. 1) and other Great Lakes States. They are liver-colored, and will get as large as an elm leaf in deer. Adults are usually found in pairs in a ping-pong-ball-sized, hollow, whitish-appearing capsule, filled with black fluid. This fluid is made up of digested liver and blood and fluke eggs. Biologists and hunters can find the fluke by slicing the liver and looking for the capsules or black fluid. Larval fluke stages are found in certain species of aquatic snails (see Fig. 2). Deer can tolerate large numbers of flukes with few apparent ill effects, and infection does not affect the meat. However, cattle, sheep, and moose are not normal hosts and do not accommodate this parasite as well as deer. A single fluke can kill a sheep; however, infection in moose or cattle is usually not lethal (deadly).

Life Cycle: (See Fig.2)
The life cycle of the liver fluke is complex. Deer are the definitive or final host for this parasite. This means the liver fluke will mature in the host and successfully complete its life cycle. Adult flukes are found in thin-walled cavities, cysts, in the liver. The cavities are drained by bile ducts through which fluke eggs pass into the intestine and are removed with fecal material. Moisture and suitable temperatures are needed for the eggs to develop into free-swimming larva called miracidia, which hatch from the eggs in 14 or more days. The miracidia must locate and penetrate an intermediate host snail within 24 hours. Inside the snail, the miracidium changes into a "tailed" cercariae. This cercaria leaves the snail and when suitable substrate (preferably vegetation) is found, it attaches to it, loses its tail, and secretes a protective mucus coat (cyst) which can withstand environmental extremes. These cysts stages are the infective stages for ungulates.That means the host deer, cattle, moose - must eat the affected vegetation. Once the host ingest the metacercaria (immature stage), it migrates to the liver where the adult fluke develops. F.magna can probably remain in the immature migrating state for an indefinite period, perhaps years. Maturity is probably dependent upon meeting another immature fluke, and only then do pairing and capsule formation take place.

Effects on Deer: Deer have evolved with this parasite and impacts are negligible.

Effects on Moose: Moose have not evolved with this parasite. This parasite was first reported in moose in 1933. In moose the flukes migrate through the liver and create tissue damage. The flukes are unable to complete their life cycles. This migration can affect the liver in a variety of ways depending on the number of flukes, the condition of the moose, and the amount of time the flukes have parasitized the host. Some livers become enlarged; one was three times the normal size. Livers may become soft and pulpy. Flukes can reach maturity but usually are encapsulated in thick cysts from which eggs do not pass. Thus, moose are considered dead-end hosts. Flukes found in moose are never as large as those found in deer. Disease due to F. Magna has not been reported in moose and never has there been a documented case of death as a result of infection.

Where do flukes occur? F. Magna is native to North America; however, it was first found in deer and cattle in Italy in 1875, probably as a direct result of importation of elk from the United States. In North America white-tailed deer and elk are the usual definitive hosts.

Environmental Factors: The relationships of temperature and rainfall to fluke development are primary factors that regulate the speed of fluke development within the snail and rate of snail reproduction. Moisture is essential for all stages of development of the liver fluke outside the mammal host.

•  Do you think the drought years of the late 1980's followed by the wet years in the early 1990's affected liver fluke infestations on moose in northwest Minnesota? If so how?
•  Were white-tailed deer historically common in this part of Minnesota?
•  What was the historical range of moose and white-tailed deer in North America?
•  Why did deer expand into many parts of North America where they had never been found?
•  Between 1989 - 1995, we have had elevated populations of white-tailed deer on Agassiz NWR. How could the numbers of deer affect liver fluke transmission?

Selected References for more information on parasites of moose:

Lankester, M.W. " Diseases of Moose in Southeastern Manitoba." 1972, 8th N. American Moose Conference Workshop (Proc), pp. 42-59.

Diseases and Parasites of White-tailed Deer, 1981, edited by Davidson, W.R. with Hayes, F.A., Nettles, V.F. and Kellog, F.E., Southeastern Cooperative Wildlife Disease Study, Dept. of Parasitology, College of Vet. Medicine, University of GA., Athens, GA.

Moose Home Page	Moose History	Study Area Map	Study Area	Research Design
Research Summary	Adopt-A-Moose Program	Research Cooperators	Sponsors	Media Index

   U.S. Fish and Wildlife Service

URL: http://midwest.fws.gov/agassiz/moose.html
Last updated on: October 19, 1999
Minnesota Moose Mystery managers:
Margaret Anderson, U.S. Fish and Wildlife Service
Gretchen Mehmel, Minnesota DNR - Wildlife
Questions and comments on this web site: Mike Caucutt