Cysticercosis
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 cottontail rabbits. 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 cottontail rabbits, 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.
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.
