ECHINOCOCCUS GRANULOSUS - HYDATID DISEASE

   In the normal life cycle of the tapeworm, Echinococcus granulosus, a carnivore serves as the final host and a herbivore as the intermediate host.  Freeman et al. (1961) reported E. granulosus in 20% of 520 timber wolves but in only 0.5% of 339 coyotes from Ontario.  The mature tapeworm is located in the small intestine of the final host, is about one centimetre in length and is not considered a serious pathogen.  Dogs have been reported to harbour up to 5,000-6,000 worms without signs of damage.

   Herbivores become infected by eating vegetation or faeces contaminated with eggs.  From  the eggs, cysts containing immature tapeworms, develop in the intermediate hosts.  The intermediate or larval stage of E. granulosus is known as a hydatid cyst and the occurrence of these cysts is called hydatid disease.  The shape of the cyst is usually spherical and ranges in size from that of a pinhead to that of a football.  Each cyst contains a fluid and the heads (microscopic in size) of thousands of immature tapeworms.  Carnivores become infected by eating viscera of herbivores containing cysts.

   Among wild herbivores of North America, hydatid disease has been recorded in wapiti, mule deer, white-tailed deer, moose, caribou and mountain goat.  In Ontario, the disease is more common in moose than in deer.  Thirty-six of 54 moose from the Chapleau area which were examined during 1963-1965 were infected with hydatid cysts.  Of 1,154 hydatid cysts recovered from these moose 95.3% were from the lungs, 3.6% were from the liver, 0.9% were from the spleen and there was one cyst (0.1%) from each of heart and kidney.  Hydatid cysts have also been reported from intercostal muscles of moose.

Older moose are more frequently infected with hydatid cysts than are younger moose and the number of cysts per moose increases with age.  On several occasions, more than 100 cysts have been found in the lungs of old moose from Ontario.

Man may accidentally become an intermediate host in this cycle by swallowing eggs passed by an infected carnivore.  Hydatid disease in man can be highly pathogenic.  It is not common in North America and is seldom acquired from wild carnivores.  Dogs are generally considered to be the principal source of human infection.  Thirty-five cases of hydatid disease in humans were diagnosed from 1932 to 1963 in Toronto hospitals (Finlayson and Fergus, 1963).  Only six of the 35 people affected acquired the infection in Canada and all of these came from northern Ontario where there is a greater chance of contact with infected dogs.  Man is not capable of harbouring the adult tapeworm and, consequently, cannot become infected either by handling or eating hydatid cysts.

Efforts should be made to prevent the infection in dogs and wolves since these species are the main disseminators of the disease to all intermediate hosts, including man.  Wolf carcasses and droppings may be a source of infection to wildlife biologists and technicians and perhaps also to zoo employees.  Careful consideration should be given to personal hygiene while handling such specimens.  Viscera from moose and deer found to be infected with hydatid cysts should be destroyed by burning.  Infected viscera should never be left available to wild predators and should never be fed to dogs.

For aesthetic reasons, organs containing cysts are not used for human consumption but the rest of the carcass is considered edible.

Selected References:

Finlayson, D.M. and A. Fergus. 1963.  Hydatid disease in Toronto.  Can.  Med.  Assoc.  J.  88:10041009.

Freeman, R.S., A. Adorjan and D.H. Pimlott. 1961. Cestodes of wolves, coyotes, and coyote-dog hybrids in Ontario.  Can.  J. Zool. 39:527-532.

Sweatman, G.K. 1952.  Distribution and incidence of Echinococcus granulosus in man and other animals with special reference to Canada, Can.  J. Public Health 43:480-486.