Echinococcus multilocularis found in Washington coyotes, study finds

A study led by researchers at the University of Washington (UW) was the first to detect Echinococcus multilocularis in a wild host on the West Coast of the contiguous US. The study’s findings suggest that the tapeworm has been spreading in coyotes, foxes, and other canid species. E multilocularis can cause severe disease if passed to domestic canines or even humans and has been identified as a public health threat in parts of the northern hemisphere, including Asia and Europe. However, it was considered extremely rare in North America until approximately 15 years ago, when cases in dogs and humans began appearing in Canada and the Midwestern US.

“This parasite is concerning because it has been spreading across North America. There have been numerous cases of dogs getting sick, and a handful of people have also picked up the tapeworm," said lead author Yasmine Hentati, PhD, who recently graduated from UW with a doctorate in environmental and forest science. "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.”

Related: Tracking the spread of a deadly parasite in North America

The researchers surveyed 100 coyotes in the Puget Sound region and found E multilocularis in 37 of them, with 18 from Whidbey Island, 8 from Everett, 4 from Burlington, 2 from Seattle, and 1 each from Sammamish, Issaquah, Auburn, Bothell, and Anacortes, according to their results published in PLOS Neglected Tropical Diseases. When an animal or person is infected, it causes the formation of cancer-like cysts in the liver, and sometimes other organs, and can be fatal if untreated.

However, not all carriers become sick because it has a complex life cycle that includes multiple hosts. Canids can support thousands of worms in their intestines without becoming sick, and the worms then shed eggs that are passed in the canid’s feces. Another host then becomes infected by eating food contaminated by coyote feces. The parasite eggs then migrate to the liver and form cysts, which weaken and kill the host, and the parasite’s life cycle then begins again when the coyotes eventually prey on the infected rodents.¹

Humans and domestic dogs are categorized as accidental hosts. Humans can pick up the parasite by consuming tapeworm eggs in food contaminated with canine or coyote feces, according to the release.¹ They can then develop alveolar echinococcosis, but symptoms may not appear for 5 to 15 years after exposure, complicating diagnosis and treatment.

Domestic dogs exposed to E multilocularis may or may not become sick, as it depends on the parasite’s life cycle at exposure; it is more common for dogs to carry the parasite and shed eggs without developing disease. However, they are still at risk of developing the same cancer-like cysts as other infected animals. Preventive medicine for worms and ticks can protect dogs, along with routine veterinary care.

“The reason that it's so high in coyotes is because they are regularly eating raw rodents, and that is the primary way for them to get infected. Most domestic dogs are not eating the raw livers of wild rodents,” Hentati said.

Although found in a third of local coyotes testing positive for E multilocularis, there is little evidence of the infection spreading to other hosts. In fact, a 2023 study found 7 cases of the parasite in dogs located in Oregon, Idaho, and Washington, where 5 of the 7 were found.³ Few human cases have been reported in the US, and none have been on the West Coast.¹

Prior to the uptick that began in the 2010s, the remote islands in northwestern Alaska reported several cases that were caused by a parasite with different origins than the current outbreak, the study researchers discovered. The coyotes in this study carried the newer variant and are now thought to be the predominant variant within Canada and the US. At this time, neither the US nor Canada requires dogs to be dewormed upon arrival, which could be contributing to the spread. According to the release, past research suggests that the tapeworm could have come to the US in red foxes that were imported for hunting 100 years ago, but truly, no one knows for sure.

“The main takeaway is that E multilocularis is here. It's pretty prevalent in the local coyote population, and people should be aware of potential risks,” Hentati said.

References

1. Parasitic tapeworm — a risk to domestic dogs and humans — found in Washington coyotes. News release. University of Washington. April 6, 2026. Accessed May 4, 2026. https://www.eurekalert.org/news-releases/1122921
2. Hentati Y, Reese E, Curran CC, et al. Detection of Echinococcus multilocularis in coyotes in Washington state, USA highlights need for increased wildlife surveillance. PLoS Negl Trop Dis. 2026;20(3):e0013502. doi:10.1371/journal.pntd.0013502
3. Evason MD, Peregrine AS, Jenkins EJ, et al. Emerging Echinococcus tapeworms: fecal PCR detection of Echinococcus multilocularis in 26 dogs from the United States and Canada (2022–2024). J Am Vet Med Assoc. 2024;263(2):1-5. doi:10.2460/javma.24.07.0471