The discovery of a parasitic tapeworm in Canadian wildlife can affect humans

Published: September 10, 2019

coyote-on-the-hunt Photo by Walter Eastland/Shutterstock

The eggs of a parasitic tapeworm, so tiny that you can’t see them with the naked eye, can cause serious health problems for people and dogs. Echinococcus multilocularis (E. multilocularis) is a tapeworm that causes cysts to form in the liver and other organs of infected hosts. Researchers are working to understand how the distribution and risk associated with this tapeworm is changing after detecting that the parasite is established in Ontario and Alberta wildlife.

Multilocularis needs to pass through two host animals to complete its life cycle. Adult tapeworms live in the small intestine of what’s called a definitive host, a role typically played by foxes and coyotes. The adult tapeworms shed eggs into the definitive host animal’s feces. The infectious eggs are then ingested by an intermediate rodent host. Once the eggs hatch, the parasite develops into cyst-like larvae in the host’s organs. The cycle is complete when the parasite is consumed alongside its rodent host by a carnivore like a fox or coyote.

People can become accidental intermediate hosts of E. multilocularis by ingesting the parasite’s eggs. The larvae can cause alveolar echinococcosis, an infection characterized by tumour-like growths in the liver that can be fatal if left untreated. The larvae develop slowly, creating a lag time of between five to fifteen years between the moment of infection and the start of symptoms. This makes the infection difficult to detect and track.

Advancements in genetics have allowed researchers to determine that different strains of E. multilocularis are found in North America, Europe, and Asia. Dr. Alessandro Massolo, associate professor at the University of Pisa in Italy, says that though the parasite was known to exist in North America, until the 2000s there had only been two human cases of locally acquired alveolar echinococcosis: one in 1928 in Manitoba and another in 1977 in Minnesota. In contrast, Dr. Massolo says that the European strains cause around one hundred to two hundred new cases of alveolar echinococcosis every year.

In 2012, a European-type strain was detected in Canadian wildlife. After the detection, Dr. Massolo and his research team began collecting coyote carcasses found by city crews along with coyote feces. Their samples came from Edmonton, Calgary, and other parts of Alberta. When checked for E. multilocularis, “Forty to fifty of these coyotes were positive,” says Dr. Massolo. “So we began strain typing the worms that we could find in the intestinal tracts of these animals.” Surprisingly, the team found that the animals were infected with a strain similar to the tapeworms found in Europe, with some small genetic modifications. “We couldn’t find any North American [type]. It was a crazy thing,” says Dr. Massolo.

This discovery prompted the team to reach out to Alberta Health Services and coordinate a search for human cases of alveolar echinococcosis. They were able to find an infected patient, and while they determined the infectious parasite was a European-type, they weren’t able to extract enough DNA to identify the strain. They asked surgeons to start collecting liver biopsies from patients. “This was the changing point,” says Dr. Massolo.

They found seven cases of the infection in humans, including the previously mentioned patient. One was a man who had never travelled outside Alberta. The other five were infected by a European-type strain. “Not only all of them were European, but with a slight snip mutation that was present in our wildlife samples but not present in Europe. This was the smoking gun that this European strain is now local and has slightly mutated.” Alberta now has 13 confirmed cases of alveolar echinococcosis, indicating an increase in detection of the disease.

Dogs can act as both definitive and intermediate hosts for the parasite. If they hunt and eat rodents, they can harbour the adult tapeworm in their intestines. If they eat coyote and fox feces contaminated with eggs, they can contract liver disease from the developing larvae. In southern Ontario, since 2012 alveolar echinococcosis has been detected in six dogs, three lemurs from a wildlife rehabilitation centre, and one chipmunk. Five out of the six dogs lived in the Golden Horseshoe Area, with the sixth dog living just north-west of Ottawa. “There was clear evidence that the parasite appeared to have established in a high human populated area,” says Dr. Andrew Peregrine, associated professor with the Ontario Veterinary College at the University of Guelph.

To aid the public health community, Dr. Peregrine worked with hunters and trappers across southern Ontario to track E. multilocularis in coyotes and foxes. “Overall about twenty-three percent of all foxes and coyotes we sampled were infected, but the risk of infection in foxes and coyotes is much higher just to the west end of Lake Ontario,” says Dr. Peregrine. “What’s surprising is that we didn’t just find the infection in foxes and coyotes in the Golden Horseshoe Area, we found it all the way to Windsor and the Quebec border.”

Dr. Peregrine says, “One of the problems with this parasite is suddenly everyone panics. But the important thing to realize is there’s only one way people get infected, and that’s by ingesting eggs.” Washing your hands after being outside or cleaning up dog poo should already be a part of your daily hygiene routine, as it keeps you from contracting far more common diseases like salmonella, but it will also protect you from accidentally eating any eggs. “The take-home message always is: if you wash your hands before you eat or drink you will get rid of any eggs, and so eliminate the risk of infection.”

Another possible route of infection is from contaminated food: berries and vegetables that have been defecated on by wild canids. “In Germany, they suggest not to pick up berries that are below seventy centimetres because they might be contaminated by feces of foxes that could spread eggs,” says Dr. Massolo. The key element again is hygiene: don’t eat wild berries off the vine without thoroughly washing them.

If you have a dog that you know is a mouser, you can help protect Fido from infection by talking to your vet about a regular deworming schedule. And don’t give your dog the opportunity to eat coyote and fox feces in the first place by keeping them on-leash and supervised while outside.

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