Study: Dangerous tapeworm never before seen in PNW live in Seattle-area coyotes

A tiny tapeworm never previously detected in the Pacific Northwest has been discovered in more than a third of urban coyotes tested by researchers in the Seattle area, according to a new University of Washington study that is raising alarms among veterinarians and public health experts.

It’s a parasite which researchers say can make a dog sick, but can kill a person many years after exposure.

Researchers discovered the parasite — known as Echinococcus multilocularis, or the “fox tapeworm” — while examining 100 coyote carcasses collected from the region. Thirty-seven of the animals were found to be carrying the parasite, some harboring tens of thousands of the microscopic worms in their intestines.

“Our reaction when we found this parasite was honestly shock,” said Dr. Yasmine Hentati, a postdoctoral scholar at UW and UC Berkeley in environmental and forest science who led the study. Her team had not been looking for the worm when they began their research.

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The worm itself — just three millimeters long, roughly the width of a staple — causes no harm to coyotes or foxes. But the microscopic eggs it sheds in animal droppings pose a serious threat to dogs that sniff or eat the scat and can be deadly in humans.

“It’s not hard to get rid of this parasite in a dog. It’s just incredibly problematic if a person gets it,” said Dr. Gary Richter, veterinarian and author

“This disease is considered one of the most important foodborne illnesses globally,” Hentati told KIRO-7, noting roughly a million people are infected worldwide. “However, it’s extremely rare here in North America,” — which is precisely what made the Seattle find so alarming to researchers.

In humans, the parasite can grow undetected for years. Eggs ingested accidentally — through contact with feces, contaminated soil, unwashed vegetables, or an infected pet — can develop into slow-growing cysts in the liver that mimic cancer.

Cassidy Armstrong, who lives in Alberta, Canada, learned that firsthand. By the time she sought medical care, doctors had already told her she appeared to have terminal liver cancer — a tumor the size of a grapefruit visible on scans. It was only after surgeons removed most of her liver that they realized the mass was not malignant, and it was not cancer.

“They said, ‘We have good news for you — we don’t think this is cancer, we think this is a parasite,’” Armstrong recalled. She is one of 15 people in her region known to have been infected. How she contracted it remains unknown.

Her Canadian doctors referred to the disease caused by the parasite as a ‘time bomb.’ “It can take up to 15 years to develop symptoms for people to be sick enough to go to the doctor,” Hentati said, “which is part of why it’s a concerning disease.”

The coyotes examined in the study were already dead, collected from roadsides and other sources. Hentati’s team gathered the carcasses themselves — including roadkill along Interstate 5. Hentati said the reason coyotes are the primary carriers of the parasite is that they primarily eat rodents infected with E. multilocularis.

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Veterinarian and author Dr. Gary Richter told KIRO 7 that dogs can be treated relatively easily if infected with deworming medications, but he stressed the danger of human exposure. “If people get exposed to it, it can cause all kinds of major medical issues, up to and including fatalities,” he said.

His advice for dog owners is straightforward: wash hands thoroughly after handling pets that may have been near wildlife, rinse vegetables well, and clean off any dog that has gotten into animal droppings. “None of this is rocket science,” Richter said, “but it is really, really important.”

At Discovery Park — one of Seattle’s largest urban green spaces, where coyote sightings and interactions with dogs are common — dog owners said the findings were unsettling. Alex and Angelica Tachiyama, who walk their dog Shuri there regularly, said they had seen no warning signs posted about wildlife hazards. “Signs would be great,” Alex Tachiyama said. “There’s been no signage in the park regarding that.” The couple said they would now be paying much closer attention to what their dog sniffs along the trail.

Key findings from the UW research

• 37 of 100 Seattle-area coyotes infected
• 3 mm: Length of adult tapeworm
• 15 yrs: Max time before symptoms appear in humans
• 1M+ people infected globally

How to protect yourself

• Keep dogs away from coyote and wild animal droppings
• Wash your hands thoroughly after handling pets that have been outdoors
• Rinse vegetables grown in areas with wildlife access
• Ask your vet about regular deworming if your dog frequents parks or trails

The parasite

Echinococcus multilocularis — also called the fox tapeworm — lives harmlessly in the intestines of foxes and coyotes. Its eggs, shed in droppings, can cause life-threatening cysts in the livers of humans and other mammals. It is common in scattered parts of Europe and Asia; previously unknown to exist in the Pacific Northwest.