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A Tapeworm - Echinococcus granulosus
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General Description
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Range Comments
Although Echinococcus granulosus can be found almost worldwide, adult E. granulosus has only been documented in Montana during the past few years.It is not known where the E. granulosus recently documented in Montana originated. Perhaps the parasite was maintained at a low level in canids such as coyotes and dogs in the absence of wolves prior to wolf reintroductions in 1995 and 1996, and the presence of wolves has amplified the parasite on the landscape. The possibility that E. granulosus was brought into Montana with transplantation of wolves from Canada into Yellowstone National Park cannot be ruled out. Transplanted wolves were treated with an anthelmintic drug effective against E. granulosus prior to release, however, it cannot be verified that treatment was 100% effective in all wolves.
Habitat
E. granulosus typically infects domestic dogs or wolves as a definitive host, and wild or domestic ungulates as the intermediate host.
Ecology
The domestic biotype of E. granulosus typically infects domestic sheep as it’s intermediate host. However, E. granulosus can occasionally infect domestic cattle and horses. Only a few cases have been reported in horses within the United States. In most cases, Echinococcus was an incidental finding during necropsy after death from another cause. Most of the infected horses had been imported from Europe, however the first documented case of Echinococcus in a horse that originated in the United States occurred in Maryland in 1993. Again, the horse died of unrelated causes, and the Echinococcus cyst was an incidental finding at necropsy. In mild infections, the intermediate host may show no signs of disease; however, severe infections could be fatal.
Reproductive Characteristics
E. granulosus requires two hosts to complete its life cycle. The adult tapeworm live in the intestine of the definitive host, which is typically a canine. Adult tapeworms lay eggs that are excreted with the feces of the definitive host. In many cases, the definitive host does not suffer adverse effects, even with a relatively heavy parasite burden.
The intermediate host becomes infected by ingesting eggs that were passed with the canine feces. The intermediate host is typically domestic or wild ungulate or occasionally a human. Once ingested, the eggs hatch in the digestive tract of the intermediate host, then enter the blood stream and are carried to organs, primarily the lung, liver, or brain, where they develop into a cyst that contains immature form of the parasite. The number of cysts that develop in an intermediate host ranges from 1 to many. Intermediate hosts with few cysts may not experience significant adverse effects whereas extremely heavy burdens may be fatal.
The parasite life cycle is completed when the intermediate host dies and another carnivore consumes the organs containing parasite cysts. Adult tapeworms again develop in the intestine of the canine definitive host, and begin laying more eggs.
References
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