Spotlight on Nasty Parasites: Echinococcus granulosus

This is the fifth of 16 student posts, guest-authored by E. Jane Kelley.

Did you know that some dogs might have a tapeworm in their small intestine that can cause the development of large cysts in people’s livers, lungs, and brains? This is not very common in the United States currently, though there are cases reported periodically (2), but in some areas of the world it is a huge problem. An infection that can spread from animals to humans or vice-versa is called a zoonotic infection.

The tapeworm is called Echinococcus granulosus and the cystic disease it causes is called hydatid disease (http://www.cdc.gov/parasites/echinococcosis/) .  Human beings are considered a “dead-end” host because they are not usually the host of the parasite. The most common life cycle of the tapeworm involves dogs as the definitive host (which means that the adult tapeworm lives in their intestine) and sheep as the intermediate host (which means that they have the immature stage of the parasite, called a metacestode, in their bodies). As you can imagine, this means that this infection is more common in areas of the world where there is extensive sheep-raising. There are other definitive hosts (such as foxes, wolves, coyotes, wolves and cats) and there are other intermediate hosts (cattle, goats, deer, rabbits, and rodents for instance), but the dog-sheep cycle is globally the most important as far as human disease.

Back to the life-cycle: the adult tapeworm is small and lives quite innocuously in the small intestine of the dog, not usually causing any problem for the dog. Tapeworm eggs are shed in the dog’s feces and, in the usual scheme of things, are ingested by sheep as they graze. The eggs develop into an immature stage of the parasite, which travels in the bloodstream from the small intestine to other organs such as liver and lung. After a while, the immature stage develops into a cyst in the liver and/or lung and the life cycle is complete when a dog eats organs from a dead sheep. In most cases, multiple small cysts (3-4 cm in diameter and they are fluid-filled) develop in the liver or lung of the sheep, but, remarkably, affected sheep do not usually show any clinical signs and the cysts are found at slaughter.

Sometimes humans ingest the eggs accidentally (this parasite is spread by what is called fecal-oral transmission from the dog to the human, use your imagination, enough said) and the cysts can form in the liver, lung, brain or other places in the human. In human beings, the infection tends to result in a single large cyst rather than multiple small cysts like the sheep. These can take years to develop and can become very large (20 cm in diameter or more). Some cysts in the liver do not cause problems, but cysts in the lung may cause respiratory problems such as coughing and shortness of breath and cysts in the brain are not good for obvious reasons. The treatment is surgical removal of the cyst.

Again, we don’t have much problem with hydatid disease in this country currently but there is potential for it to become more prevalent. It was believed to have been introduced in the early 1900s with infected livestock. Interestingly, the infection initially established itself in a domestic dog/domestic pig cycle (not sheep) in the east (8) and by the 1920s was in Mississippi, Louisiana, Tennessee, and Arkansas. By the 1970s, it was fairly prevalent in the sheep-raising areas of the west (Utah, Arizona, California, and New Mexico)(4) and there is a cervid (elk, moose, deer) cycle in wildlife in Canada, Alaska, and Minnesota (6). Recently in Montana and Idaho, approximately 60% of gray wolves sampled had adult tapeworms in their intestine and cysts were found in organs of elk, mule deer, and a mountain goat (5).

Let’s focus on Utah for a moment. Echinococcus granuosus was believed to have been introduced into the state in 1938 in sheep dogs from Australia (3). Sanpete County in central Utah is the area where most of the sheep-raising went on (and still does). In the 1970s Echinococcus granulosus was endemic in sheep and dogs in central Utah. Endemic means that the disease is constantly present in a certain area. About 25-30% of the dogs were infected and 13% of sheep carcasses examined had cysts (1,7). In addition, during this time period, there were several human cases. The close contact between sheep-herders and their families and the sheep dogs and sheep kept the cycle going. Also, the sheep were range sheep with common grazing allotments and contact with other dogs and sheep. Control measures such as proper disposal of dead sheep, hand-washing, regular deworming of the dogs and educating the families were implemented and greatly decreased the incidence of infection in dogs, sheep and people (1,3,7). However, there are occasional cases still in Utah so either control measures were not 100% effective and/or there is a wildlife reservoir of infection (such as coyotes). There have been 3 cases in humans reported to the Utah Department of Health since the 1970s and multiple liver and lung cysts were found in a slaughtered sheep in 2008.

In some areas of the world, such as China (11), Echinococcus granulosus is a serious and increasing public health concern. In China, it is considered an emerging or re-emerging disease (a disease that was at a low level but is recently causing many more infections). Eradication of hydatid disease has been achieved in some island countries such as Iceland, New Zealand and Tasmania (10). It is certainly an easier task to control infection on a relatively small island with a single government than in other areas. Control of infection involves measures previously mentioned such as  regular deworming of dogs, proper disposal of sheep and cattle carcasses, stray dog control, education about the importance of hand-washing, and elimination of wildlife reservoirs if present. This is not as easy as it sounds and requires long term commitment which may be a problem in countries with economic and political unrest. For example, in Kazakhstan (a country in central Asia), after independence from the Soviet Union in 1991, social and economic changes have brought about changes in animal husbandry (from large collective farms to smaller farms with closer contact between people, dogs, and livestock) such that incidence of Echinococcosis has been increasing in people, dogs, and livestock (9).

In the United States, hydatid disease is less likely to become a serious problem because we have a relatively stable government and the infrastructure for the handling of carcasses and stray dog control. However, there are parts of the country where this still exists and, I don’t know about you, but I don’t relish the thought of having a huge cyst in my liver or brain. For those of you who kiss your dogs, have you seen what dogs eat?  Add this disease to the list of reasons that it’s not a good idea.  Don’t let your dogs defecate in children’s playgrounds because small children are not very discerning about what goes in their mouth.

References:

1. Barbour AG, Everett JR, Andersen FL, et al. Hydatid disease screening: Sanpete County, Utah, 1971-1976. Am J Trop Med Hyg 1978 Jan; 27: 94-100.
2. Bistow BN, Lee S, Shafir S, et al. Human echinococcosis mortality in the United States, 1990-2007. PLos Negl Trop Dis 2012 Feb; 6: e1524.
3. Crellin JR, Andersen FL, Schantz PM, et al. Possible factors influencing distribution and prevalence of Echinococcus granulosus in Utah. Am J Epidemiol 1982 Sep; 116: 463-474.
4. Donovan SM, Mickiewicz N, Myer RD, et al. Imported echinococcosis in southern California. Am J Trop Med Hyg 1995 Dec; 53: 668-671.
5. Foreyt WJ, Drew ML, Atkinson M, et al. Echinococcus granulosus in gray wolves and ungulates in Idaho and Montana, USA. J Wildl Dis 2009 Oct; 45: 1208-1212.
6. Himsworth CG, Jenkins E, Hill JE, et al. Emergence of sylvatic Echinococcus granulosus as a parasitic zoonosis of public health concern in an indigenous community in Canada. Am J Trop Med Hyg 2010 Apr; 82: 643-645.
7. Loveless RM, Andersen FL, Ramsay MJ, et al. Echinococcus granulosus in dogs and sheep in central Utah, 1971-1976. Am J Vet Res 1978 Mar; 39: 499-502.
8. Pappaioanou M, SchwabeCW, Sard DM. An evolving pattern of human hydatid disease transmission in the United States. Am J Trop Med Hyg 1977 Jul; 26: 732-742.
9. Shaikenov BS, Torgerson PR, Usenbayer AE, et al. The changing epidemiology of echinococcosis in Kazakhstan due to transforming of farming practices. Acta Trop 2003 Feb; 85: 287-293.
10. Torgerson PR, Budke CM. Echinococcosis- an international public health challenge. Res Vet Sci 2003 Jun; 74: 191-202.
11. Yang YR, McManus DP, Huang Y, et al. Echinococcus granulosus infection and options for control of cystic echinococcosis in Tibetan communities of Western Sichuan Province, China. PLos Negl Trop Dis 2009 April; 3: e426.