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Description 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.
Transmission and Development 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 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.
Treatment and Control In recent years, fasciolicide drugs have been developed which are effective against mature F. magna. Captive deer have been successfully treated. 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.
Significance 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.
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