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(Thin-necked Bladderworm, Cysticercosis)
The thin-necked bladderworm (formerly known as Cysticercus tenuicollis) is the larval stage of the canine and feline tapeworm Taenia hydatigena. The larval stage of T. hydatigena is a common parasite of Michigan deer and other ruminant species and is generally seen attached to the omenta, mesenteries, or liver surface. Bladderworms are seen more often and in greater numbers in older animals. The adult tapeworm is found living in the small intestine of the canine and feline definitive hosts.
The cysticerci, or larval stage, of T. hydatigena have been reported in all wild and domestic ruminants in North America, while the adult tapeworms have been reported from dogs, coyotes, foxes, wolves and bobcats in the United States and Canada. A small percentage of animals are infected with the parasite, with wildlife populations in the north and west having a higher level of infection.
Carnivores infected with T. hydatigena adults pass gravid or mature proglottids or segments in their fecal material. The detached proglottid is motile and crawls away from the feces, shedding eggs as it leaves. The proglottid and eggs attach to blades of grass or enter a source of water. While eating grass or drinking, a ruminant accidentally ingests the eggs which pass into the small intestine. Each egg contains an oncosphere with three sets of hooks, which are used to penetrate the wall of the intestine after the eggshell is digested away. After penetrating the wall of the intestine, the larvae migrate to the liver. The larvae may remain in the liver or migrate to the mesenteries or omenta where they attach themselves. The larvae begin development and form a bladder. A head and neck develop within the bladder and the larvae are ready to infect a definitive carnivore host.
When the bladder is eaten by a canine or feline, the cyst wall digests, the scolex evaginates, and the hooks and suckers attach to the wall of the small intestine. The scolex and neck grow a strobila (body segment) and segments continue to grow posterior to the neck. The adult tapeworm matures, reaching a length of three feet, and begins to liberate egg-bearing segments within 10 to 12 weeks.
There are no clinical signs seen with a bladderworm infection.
There are few pathological changes which occur with a T. hydatigena cysticerci infection, but severe infections can be hazardous to the animal's health. Cysts, varying in number from 1 to 75, can be present in the omenta and in the mesenteries. In extremely heavy infections the viscera may become knotted together and organ function may be impaired. Degenerate cysts are replaced by caseous and calcareous debris and may damage tissue. Meandering streaks may be present in the liver due to larval parasite migration. If large numbers of larvae migrate through the liver parenchyma, tissue will be damaged and acute and fatal hepatitis may be the result.
Diagnosis of a T. hydatigena larval infection is based on finding one or several cysticerci in or on the liver or on the mesenteries during postmortem examination. Serpentine markings in the liver tissue may also be indicative of larval migration. T. hydatigena cysticercus identification may be confirmed by the size of the cyst and due to the presence of only one scolex in the bladderworm.
There are no known treatment or control measures for this parasite; nor are any considered necessary.
Infection of ruminants with cysticerci of T. hydatigena is usually of little significance. Severe infections may kill the host but generally damage to the viscera is confined to the liver, is mild and localized, and has little significance to the overall health of the host.
The presence of cysticerci in ruminants is an indicator of the incidence of T. hydatigena among wild and domestic carnivores. Ruminant entrails should be disposed of properly and not fed to dogs or left for wild carnivores.
The parasite is not transmissible to humans; therefore, the meat of the ruminant is safe for human consumption.
