The parasite commonly infects coyotes, foxes, and other canids. While these animals often show no signs of illness, the tapeworm can cause severe disease in domestic dogs and humans if transmission occurs.

For decades, E. multilocularis has been recognized as a significant public health concern across parts of Europe and Asia. In North America, however, it was once considered exceptionally rare. That changed roughly 15 years ago when infections began appearing in dogs and people in Canada and the Midwest, signaling that the parasite was expanding its range.

Tapeworm Found in Pacific Northwest Coyotes

Researchers from the University of Washington surveyed 100 coyotes in the Puget Sound region and discovered that 37 carried the parasite. Their findings were published in PLOS Neglected Tropical Diseases.

"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, who recently graduated from the 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."

When E. multilocularis infects a person or animal, it can produce cancer-like cysts in the liver and, in some cases, other organs. Without treatment, the infection can be fatal.

How the Parasite Spreads

Despite the danger it poses, many infected animals never become ill. The parasite relies on a complex life cycle involving several different hosts.

Coyotes and other canids serve as the primary hosts for adult tapeworms. These animals can carry thousands of worms in their intestines without becoming sick. The worms release eggs that pass into the environment through feces.

Rodents are another key part of the cycle. After consuming food contaminated with coyote feces, they can become infected. The parasite eggs migrate to the rodents' livers and develop into cysts, eventually weakening or killing the animals. Coyotes then become infected when they eat those rodents, continuing the cycle.

Humans and domestic dogs are considered accidental hosts. People can become infected by swallowing tapeworm eggs, such as through food contaminated with coyote or dog feces. Infection can lead to alveolar echinococcosis, a disease marked by slow-growing metastatic cysts. Symptoms may not appear until five to 15 years after exposure, making diagnosis and treatment particularly challenging.

Alveolar echinococcosis is considered the third most important food-borne illness globally and is listed by the World Health Organization among the top 20 neglected tropical diseases. Many countries have established extensive monitoring programs to track the disease.

Risks for Dogs and People

Dogs exposed to E. multilocularis do not always become sick. The outcome depends largely on which stage of the parasite they encounter. In many cases, dogs carry the parasite and shed eggs without developing symptoms. However, dogs exposed to parasite eggs can develop the same cancer-like cysts seen in other infected animals.

"To minimize the risk of dogs getting infected with E. multilocularis, owners should not let them prey on rodents or scavenge their carcasses," said co-author Guilherme Verocai, an associate professor and director of the Parasitology Diagnostic Laboratory at the Texas A&M University College of Veterinary Medicine and Biomedical Sciences.

Verocai also recommends routine veterinary care, including parasite testing, as well as preventative medications for worms and ticks.

Although more than one-third of the coyotes examined in the study carried the parasite, researchers found little evidence that it has become widespread in other hosts. One study documented seven canine cases in Washington, Oregon, and Idaho since 2023, including five in Washington. Human infections remain rare in the United States, and no cases have been reported on the West Coast.

"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.

A More Infectious Variant

Reports of E. multilocularis have surfaced before in North America. Prior to the rise in cases seen during the 2010s, the parasite had been documented on remote islands in northwestern Alaska.

Researchers say those earlier cases involved a different strain than the one driving the current spread. Genetic analyses indicate the older infections were linked to a tundra variant, while today's outbreak is associated with a more infectious strain of European origin. The coyotes examined in this study carried the newer variant, which is now believed to be the dominant form circulating in both the United States and Canada.

Scientists are still unsure how the parasite became established in North America. One possibility is that infected dogs entering the U.S. and Canada were not required to undergo deworming treatment. Another theory, proposed in earlier studies, suggests the parasite may have arrived in red foxes imported for hunting about a century ago.

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

Co-authors include Ellie Reese, lab manager at UW; Samantha Kreling, UW doctoral graduate in environmental and forest science; Laura Prugh, a UW professor of environmental and forest science; Chelsea Wood, a UW associate professor of aquatic and fishery science; Claire Curran of the College of William and Mary; Erika Miller of Sound Data Management; Dakeishla M. Díaz-Morales of DePaul University; and Christopher J. Schell of UC Berkeley.

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