|Infected foxes spread Echinococcus|
multilocularis to dogs.
Image by Andy Potter; CC BY-SA 2.0.
Echinococcus multilocularis liver cysts in dogs
The liver cyst caused by E. multilocularis is a larval stage - a stage that multiplies asexually in the cyst. The adult stage of the parasite is found in canids, members of the dog family: arctic foxes, red foxes, jackals, coyotes, domestic dogs. Thus, the 2009 discovery of a liver cyst in a domestic dog in British Columbia, Canada (Jenkins et al.), is puzzling and alarming.
This scenario is not actually so bizarre. Taenia solium, or pork tapeworm, has a similar story: humans normally host the adult tapeworm in the intestine after consuming the larval cysticercus in undercooked pork. But if a human swallows the tapeworm egg instead, the eggs hatches and the larva moves into the tissues – sometimes the brain – and forms a cysticercus.
When humans have T. solium cysticerci in their tissues, the disease is called cysticercosis – or, in the brain, neurocysticercosis – and this can be much worse than having the tapeworm in the intestine. Clearly, in a dog, a liver cyst that can grow large enough to compromise liver function, and spread to other parts of the body, is worse than hosting a tapeworm in the intestine as well.
In cysticercosis in humans, and presumably alveolar echinococcosis in dogs, the tissue cysts often originate from the hosts own intestinal tapeworms. In humans, it’s poor hygiene and hand to mouth transmission. In dogs, it’s grooming – licking eggs off soiled fur. This raises the question: have dogs always frequently had the liver cysts when they had the worms or has something changed?
Echinococcus multilocularis spreading to new places
Jenkins et al. remark that “compared with native North American strains, European strains of Echinococcus multilocularis appear to have greater potential to cause alveolar hydatid disease (AHD) in humans.” The strain found in British Columbia was a European strain; perhaps they have greater potential to cause AHD in dogs as well. Do we know?
The British Columbia dog apparently did not have the adult tapeworm in its intestine and the authors speculate that the parasite may have been introduced by an imported infected dog. This, too, is alarming. It brings to mind my discussion of E. multilocularis in Parasites: Tales of Humanity’s Most Unwelcome Guests, in which I relate the identification of Echinococcus multilocularis in wild canids smuggled into the Eastern US for fox hunting.
If one imported dog can introduce the worm to British Columbia, what is the likelihood that many illegally translocated infected canids have not spread the worm as well? Is alveolar echinococcosis simmering in rodents, dogs, and people in the eastern United States?
Jenkins EJ, Peregrine AS, Hill JE, Somers C, Gesy K, Barnes B, et al. Detection of European strain of Echinococcus multilocularis in North America [letter]. Emerg Infect Dis [serial on the internet]. 2012 June.
Roberts, Larry S., and John Janovy Jr. Gerald D. Schmidt & Larry S. Roberts’ Foundations of Parasitology 8th ed. Boston: McGraw Hill, 2009. Pg 354-5.