New evidence suggests that a disease-causing tapeworm that has been spreading across the United States and Canada has arrived in the Pacific Northwest. 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.
E. multilocularis has long been recognized as a public health threat in parts of the Northern hemisphere, including Europe and Asia, but was considered extremely rare in North America until approximately 15 years ago, when cases in humans and dogs began cropping up in Canada and the midwestern U.S., indicating that the parasite was spreading.
This study, led by University of Washington researchers, is the first to detect E. multilocularis in a wild host on the west coast of the contiguous U.S. Researchers surveyed 100 coyotes in the Puget Sound region, and found E. multilocularis in 37 of them. The results were published March 24 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 an animal or person, it causes cancer-like cysts to form in the liver and sometimes other organs. If untreated, infection can be fatal.
However, not all carriers become sick. E. multilocularis has a complex life cycle that involves multiple hosts. Canids, which host adult parasites, can support thousands of worms in their intestines without becoming sick. The worms shed eggs that are then passed in their feces.
Rodents - another host - become infected by eating food contaminated with coyote feces. Once consumed, the parasite eggs migrate to the liver and form cysts, ultimately weakening or killing the rodents. The parasite's life cycle begins again when coyotes prey upon infected rodents.
Humans and domestic dogs are categorized as accidental hosts. Humans may pick up the parasite by consuming tapeworm eggs - in food that is contaminated with coyote or dog feces, for example - and can develop a disease called alveolar echinococcosis, characterized by slow-growing metastatic cysts. Symptoms may not appear for five to 15 years after exposure, which complicates diagnosis and treatment.
Alveolar echinococcosis is considered the third most important food-borne illness globally, and one of the top 20 neglected tropical diseases by the World Health Organization. Many countries have developed robust protocols for tracking it.
Domestic dogs that are exposed to E. multilocularis may or may not become sick, depending on where the parasite is in its life cycle at exposure. It is more common for dogs to carry the parasite and shed eggs without developing disease, but dogs that are exposed to parasite eggs may develop the same cancer-like cysts as 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.
Owners can also give dogs preventative medication for worms and ticks and ensure routine veterinary care, which should include diagnostic tests for parasites, Verocai said.
Although the researchers found E. multilocularis in more than one-third of local coyotes tested, there is little evidence of the infection spreading to other hosts. One study reported seven cases of the parasite 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.
"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.
Before the uptick in the 2010s, there were several reports of E. multilocularis on remote islands in northwestern Alaska. Those cases were caused by a parasite with different origins than the current outbreak. Genetic analysis pins the earlier cases to a tundra variant while these recent cases are driven by a more infectious variant with European origins. The coyotes in this study carried the newer variant, now thought to be the predominant variant in the U.S. and Canada.
Neither Canada nor the U.S. require dogs to undergo deworming upon arrival, which may explain the spread. Previous studies also proposed that E. multilocularis could have come over in red foxes imported for hunting 100 years ago, but no one knows for sure.
"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. This study was funded by The National Science Foundation and the University of Washington Hall Conservation Genetics Fund.
