Tapeworm tied to severe disease found in Washington coyotes

A parasitic tapeworm that can cause serious liver disease in people and dogs has been detected in coyotes in Washington state, according to University of Washington researchers. The finding matters because it is the first report of Echinococcus multilocularis in a wild host on the contiguous U.S. West Coast and suggests the parasite has reached the Pacific Northwest.

In a study published in PLOS Neglected Tropical Diseases, researchers reported that 37 of 100 coyotes sampled in the Puget Sound region carried the parasite. The University of Washington said the rate was unexpected for an area where the tapeworm had not previously been reported in the region until recently.

Lead author Yasmine Hentati, who recently earned a doctorate in environmental and forest science at the University of Washington, said the parasite has raised concern because it has been expanding across North America. She said infections have been reported in many dogs and in a small number of people.

How the parasite moves through wildlife

According to the University of Washington, E. multilocularis commonly uses canids such as coyotes and foxes as primary hosts. These animals can carry adult tapeworms in their intestines without appearing sick, then shed eggs into the environment through feces.

Rodents become part of the cycle when they consume food contaminated with those eggs, the university said. The parasite can then form cysts in the rodents’ livers, weakening or killing them; coyotes can acquire the parasite when they eat infected rodents.

People and domestic dogs are considered accidental hosts, according to the researchers. Infection can occur when eggs are swallowed, including through food contaminated with feces from infected coyotes or dogs.

The University of Washington said infection in humans can lead to alveolar echinococcosis, a disease involving slow-growing cysts that often affect the liver and can spread to other organs. Symptoms may take five to 15 years to appear, and untreated infection can be fatal, the researchers said.

Risk to pets and people

The researchers said dogs may carry the adult tapeworm and shed eggs without showing symptoms, depending on how they are exposed. Dogs that ingest the parasite’s eggs can also develop the cystic form of disease, according to the study team.

Guilherme Verocai, a co-author and associate professor at the Texas A&M University College of Veterinary Medicine and Biomedical Sciences, said dog owners can reduce risk by preventing pets from hunting rodents or eating rodent carcasses. He also recommended routine veterinary care, parasite screening and preventive medications for worms and ticks.

Despite the high share of infected coyotes in the Puget Sound sample, the University of Washington said there is limited evidence so far that the parasite is widely established in other hosts in the region. The researchers cited a separate study that found seven canine cases in Washington, Oregon and Idaho since 2023, including five in Washington.

Human cases remain rare in the United States, and no human infections have been reported on the West Coast, according to the University of Washington. Hentati said coyotes are more likely to carry the parasite because they frequently eat raw rodents, while most domestic dogs do not eat wild rodent livers.

A strain linked to wider spread

The University of Washington said E. multilocularis had previously been documented on remote islands in northwestern Alaska, but those earlier cases involved a tundra variant. Genetic work cited by the researchers links the current expansion to a more infectious strain of European origin.

The coyotes in the Washington study carried that newer variant, which the researchers said is now believed to be the dominant form in the United States and Canada. Scientists have not determined how it became established in North America, but the University of Washington said possible explanations include infected dogs entering the U.S. and Canada without deworming or red foxes imported for hunting about a century ago.

The study was funded by the National Science Foundation and the University of Washington Hall Conservation Genetics Fund.

This story draws on original reporting from ScienceDaily.