Department of Natural Resources
(Tapeworm cysts, blisters in rabbits and hares)
Cysticercosis is a parasitic infection common to cottontail rabbits and snowshoe hares in Michigan. The parasites are conspicuous and raise many inquiries from hunters who discover them when dressing out rabbits. They appear as sacs of clear fluid containing small white objects. Hunters see the parasites as "spots on the liver" and misinterpret their presence as evidence the rabbit is infected with tularemia. The parasites are the larval stage of tape worms of dogs and wild carnivores, principally foxes and coyotes.
Two species of tapeworms are involved. Larvae (tapeworm cysts) of Taenia pisiformis are most commonly seen in cottontails. They occur less frequently in snowshoe hares. The fluid-filled cysts are about 5 to 7 mm in diameter, occur singly and are attached to the surface of the liver, intestines and occasionally the lungs. Each cyst holds a white floating object which is the head (called the scolex) of an immature tapeworm.
A second tapeworm cyst primarily seen in snowshoe hares, but also reported to occur occasionally in cottontails, is the larval stage of Multiceps serialis. These fluid-filled cysts differ from those of T. pisiformis in that they are larger, elongate and each contains many white floating objects (tapeworm heads). They are located under the skin or between muscle layers rather than inside the body cavity. Clusters of cysts resemble water blisters, hence the term "blisters" which the condition is sometimes called.
T. pisiformis is one of the most common cestode of wild carnivores. It is common where rabbits and hares serve as a source of food for either domesticated or wild carnivores, and has been reported from practically all areas of the United States except the western states and Hawaii.
M. serialis is also common where wild rabbits, hares, and rodents serve as a source of food for canids; it has been reported from many areas of the United States and Canada.
Dogs and related carnivores are host to the adult tapeworm. Eggs are passed out of the body in the feces. Rabbits are the major intermediate host. Rabbits become infected when the eat vegetation contaminated with the eggs. In the digestive tract of the new host, the eggs develop into tiny free-moving parasites that penetrate the gut wall and enter the blood stream where they are carried to the liver. After traveling through the liver tissue for a number of days, they break through the liver wall and, in the case of T. pisiformis, enter the abdominal cavity. There they attach to the surface of various abdominal organs and complete their development into cysts. Larvae of M. serialis somehow manage to travel to the subcutaneous areas. The cyst stage is as far as these tapeworms develop in rabbits. If the rabbit (or infected viscera) is eaten by a dog or other suitable carnivore, the tapeworms will then continue their development to maturity.
The incidence and intensity of cysticercosis in rabbits are highly variable. In heavy infections, cysts of T. pisiformis nearly fill the abdominal cavity. Usually, infections are more moderate, ranging from 2 to 20 cysticerci in a cottontail (see illustration).
Infections are rarely clinically apparent. It may be possible to detect M. serialis infections in snowshoe hares by feeling for swellings (the cystic masses) under the skin.
We do not recognize moderate infections being pathologic to rabbits. Heavy infections are presumed pathologic, though the mechanism by which damage is inflicted has not been determined.
Infections are diagnosed through necropsy. T. pisiformis is characterized by an individual "pea-size" cyst containing one scolex and located within the body cavity. M. serialis tends to develop in massive clusters of cysts, each cyst containing multiple heads (scolices) and generally located just under the skin. Few lesions are seen, however tapeworm larvae may migrate through the liver and produce large white scars or fibrotic nodules.
Treatment is out of the question for wild rabbit and hare populations. Surgical removal of M. serialis from experimental snowshoe hares should be possible if ever desirable. Dogs should be wormed to rid them of adult tapeworms, though the effect of tapeworm control in dogs may have no appreciable effect on the incidence of infections in wild rabbits, since wild carnivores also are involved in the transmission of the parasite. Obviously, discarded rabbits and viscera should not be fed to dogs.
There is no evidence that cysticercosis is an important cause of decline in rabbit or hare populations.
Cysticercosis does not harm the meat of rabbits and hares, or make it unfit for human consumption. Adult tapeworms of these species do not occur in humans. Cysts are usually removed when rabbits are dressed out; any that might be overlooked are destroyed in cooking the meat.
Undoubtedly, many rabbits are unnecessarily discarded because hunters confuse cysticercosis with "white spots on the liver," so greatly publicized as evidence of a tularemia infection. However, the two are distinctly different, and the cysts can be readily differentiated from tularemia lesions.
There are records of human cysticercosis involving M. serialis. Such infections can only arise from swallowing the eggs passed in the feces by infected carnivores. Wildlife personnel working with foxes, coyotes, wolves, and other canids, especially if they are handling scat samples, should be careful to not expose themselves to or contaminate laboratories with tapeworm eggs.