Parasitic Threat to Dogs and Humans Arrives in Western Washington

New research from the University of Washington and Washington State University has found a harmful parasitic tapeworm spreading across the U.S. and Canada is now in Washington.

The tapeworm, called Echinococcus multilocularis, lives as a parasite in coyotes, foxes, and other canid species and can cause severe disease if passed to domestic dogs or humans.

A study led by UW scientists found 37 infected coyotes out of 100 tested in the Puget Sound region. The study was published last March in the Public Library of Science journal Neglected Tropical Diseases.

“This parasite is concerning because it has been spreading across North America,” said lead author Yasmine Hentati, PhD.

“The fact that we found it here in one-third of our coyotes was surprising, because it wasn’t found anywhere in the Pacific Northwest until earlier this year.”

When the parasite infects an animal or person, it causes cancer-like cysts in the liver and sometimes other organs. If untreated, infection can be fatal.

However, coyotes and foxes can host tapeworms without becoming ill. The worms shed eggs that are passed in feces. Rodents like mice and squirrels can become infected after eating food contaminated by that feces. The eggs migrate to the liver, ultimately weakening or killing them.

Domestic dogs can be exposed by eating rodents or scavenging in areas contaminated by their feces. A dog may carry the parasite and shed eggs without being affected, but it can instead develop the same cancer-like cysts as other infected animals.

“Humans get infected the same way rodents do, through fecal-oral contamination,” said Dr. Kyle Taylor, a veterinarian and pathologist at the Washington Animal Disease Diagnostic Laboratory at WSU. The tapeworm was detected there for the first time in May during a necropsy of a fox found on the Pullman campus.

“If you’re handling soil or anything contaminated with fox or coyote feces and then inadvertently ingest those microscopic eggs, that’s how infection can occur,” Taylor said.

Infected people can develop a disease called alveolar echinococcosis, characterized by slow-growing metastatic cysts that can mimic liver cancer. Symptoms may take years to emerge, and then not until significant liver damage has occurred.

The disease is considered the third most important food-borne illness globally by the World Health Organization.

There is little evidence of the infection spreading from wild to domestic animals for now. One study published in a journal of the American Veterinary Association reported seven cases in dogs in Washington, Oregon, and Idaho since 2023, five of which were in Washington. Few human cases have been reported in the U.S., and none on the West Coast.

Owners can give their dogs preventive medication for worms and ticks and ensure routine veterinary care, which should include diagnostic tests for parasites.

“There’s likely more of it in the environment than people realize,” Taylor said. “Awareness and basic precautions are the best prevention.”