TORONTO - Animal health researchers are watching what appears to be mounting evidence of the spread of a potentially dangerous parasite in coyotes, foxes and other animals in Canada.
That's a concern, they suggest, because the parasite, a tapeworm, can on occasion spill over from its wild animal hosts to infect dogs and humans.
And while people aren't the tapeworm's preferred hosts, a growing number of human cases are being seen in Europe and parts of the world where the parasite is more established.
The researchers are keen not to incite undue alarm. It would appear, they say, that the risk to Canadians is low. But they say the situation bears continued scrutiny, especially in light of the fact that urban sprawl in cities like Calgary is leading to closer contact between coyotes and domestic animals.
"It seems like there is a real route for coyotes to bring this into the cities and into dog parks," says Dr. Emily Jenkins, a professor in the University of Saskatchewan's department of veterinary microbiology and school of public health.
"I think there's a need for a risk assessment. And I think there's a need for enhanced surveillance — if only to get baselines to say, 'Oh, yeah, this is the prevalence now' and 'Oh, look, it's not changing' or 'Oh, look, it is.'
"But getting that on the radar of a particular public or animal health agency is challenging."
The parasite is called Echinococcus multilocularis. Scientists sometimes call it E. multi for short. When it infects people the parasite causes a condition called alveolar echinococcosis, with tumour-like cysts forming in the liver, lungs or sometimes the brain.
A study published in 2010 in the journal PLoS Neglected Tropical Diseases estimated that about 18,200 human cases occur globally every year, with 90 per cent of them recorded in China.
But the range of the parasite — which is a Northern Hemisphere phenomenon — is expanding.
In the past decade, nine new countries in Europe have reported finding it. Germany, France and Switzerland see about 100 human cases a year among them, the PLoS article says.
In North America, there have only ever been two human cases recorded — a Minnesota woman in 1977 and a Canadian man in 1928.
Humans are an accidental host. The parasite's definitive hosts — the place the adult tapeworm likes to live — are foxes and coyotes.
But its life cycle involves other animals. Infected foxes and coyotes shed the tapeworm's eggs in their feces, which are eaten by small rodents such as deer mice and voles.
In those animals, which are considered intermediate hosts, the eggs become larvae that form in large cysts. The cysts eventually kill the rodent or render it vulnerable to being preyed upon. If those preying animals are coyotes or foxes, the larvae they ingest become adult tapeworms, closing the circle.
Dogs too can carry the adult tapeworms. But recently, to the surprise of parasitologists, occasional cases have been seen where dogs were found to have the cysts in which the larvae develop.
In a report published this summer in the journal Emerging Infectious Diseases, Jenkins and several other researchers reported finding a dog in which E. multi cysts had developed. The dog lived in Quesnel, B.C.
The case raised a red flag for a couple of reasons. E. multilocularis had never been seen in British Columbia. Up until then, it had only been found in foxes in the Arctic and in the south of the three Prairie provinces.
(It's also been seen in about 14 states in the U.S. Most recently it was reported in Ohio and Michigan, which suggests it may be moving eastward, Jenkins says.)
More startling was the fact the dog was infected with the E. multi strain that is seen in Europe. It was not welcome news.
The European strain of E. multi had never been reported in North America. And while it's only a theory at this point, Jenkins and others believe the European strain is more likely to infect humans than the North American strain of the parasite.
The dog had never been out of the country — or out of B.C. for that matter. So researchers tested foxes and coyotes found in a 10-kilometre radius of Quesnel and discovered some carrying the European strain of the parasite. They don't know how the strain got there or how long it's been there.
"In light of these findings, it now looks like it's pretty well established in British Columbia, in that region — which is not thought to be one of the places this parasite should be," Jenkins says.
The North American strain of the parasite also appears to be pretty well established in and around Calgary.
Dr. Alessandro Massolo, a wildlife health ecologist with the University of Calgary's faculty of veterinary medicine, has been examining coyotes found dead in the city (plus a few from Edmonton) to see if they were carrying the parasite.
In a study coming out in the October issue of Emerging Infectious Diseases, he and his team report that 23 of 91 coyotes (25 per cent) that they checked had the tapeworms in their intestines.
Since the article was submitted for publication, Massolo's team has continued the work. They have now studied about 250 coyotes. In 2012, roughly half of the animals studied were carrying the parasite.
Massolo says there aren't good numbers on how many coyotes roam through Calgary, though the estimate of 600 to 700 is sometimes used.
When you combine that number with the city's estimated dog population — 120,000 in the most recent census — and the number of hectares of city park land dedicated to off-leash space — 1,250 — you can see why Massolo thinks this is an important situation to monitor.
"The big problem, to me, is that let's say that the likelihood of being infected by the North American strain of Echinococcus multilocularis for people is very low," he says.
"But when you increase the degree of interaction between the parasite and people, the likelihood that these unlikely events are happening is increasing."
Are people at risk? "My point is: We don't know yet," Massolo says.
Dogs acquire the adult tapeworms by eating infected rodents. In rare cases, as the Quesnel dog incident suggests, they may also be able to pick up eggs by eating the feces of an infected dog or a coyote.
The eggs are microscopic; an owner would not know if his or her dog is shedding them. It's not hard to imagine how a shedding dog would pass the eggs on to humans.
In people, the cysts can take years to develop, often looking and acting like liver cancer when they do. In rural China, people who become infected often die. But in places with sophisticated medical care, people can be treated.
Dr. Andrew Peregrine, a clinical parasitologist at the Ontario Veterinary College in Guelph, Ont., has been collaborating with Jenkins on some E. multi research. He can understand why people might wonder why scientists would make a fuss over something that causes so few human cases.
The risks are probably low. But the consequences of a human infection are high.
Treating a person infected with E. multi is a complicated, lengthy and costly undertaking, Peregrine explains. After surgery to remove the cyst, people need to go through years of drug therapy and undergo multiple scans.
"It's incredibly expensive. It's in the hundreds of thousands of dollars," Peregrine says.
So what are dog owners to do? The experts suggest the behaviours associated with responsible dog ownership — scooping poop, keeping dogs from preying on rodents, washing hands after handling dogs or their feces — should help protect animals and the people they live with.
Deworming drugs will kill the tapeworms before they start shedding eggs, though Jenkins notes the drugs used to prevent heartworm will not work against E. multi.
"What I wouldn't like to send is the message that it's dangerous to walk your dog in the park," Massolo stress.
"There are a lot of health components in walking your dog in parks, for you and for your dogs. And we don't want to send the message of risk because there are so many other risks (in life) that are higher than Echinococcus."
Note to readers: This is a corrected story. An earlier version said half of coyotes tested in Calgary contained the parasite. It should have read that half tested this year carried the parasite.