A Deadly Tapeworm That Grows Like Liver Cancer Has Reached the Pacific Northwest
Echinococcus multilocularis is a tapeworm so small, one to four millimeters as an adult, that it takes a microscope to see. What it does inside the human body is anything but small. The larval stage grows like a slow, infiltrative liver cancer, forming metastatic cysts that destroy organ tissue over years. Untreated, the fatality rate exceeds 90% within 10 to 15 years of diagnosis.
For decades, the parasite was confined to the north central United States, Iowa, Minnesota, the Dakotas. A new study published in PLOS Neglected Tropical Diseases has found it in a place it has never been detected before in wild animals: the US West Coast.
Researchers at the University of Washington tested 100 coyotes in the Puget Sound region, from roadkill carcasses and field-collected scat collected between 2018 and 2023. They found E. multilocularis in 37% of them.
“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,” said Yasmine Hentati, the study’s lead author and a recent PhD graduate in environmental and forest science at UW.
E. multilocularis has a complex, two-host life cycle. Adult tapeworms live in the intestines of canids, coyotes, foxes, and domestic dogs, where they can number in the thousands without causing illness. The worms shed eggs that pass into the environment through feces. Rodents, particularly voles and mice, ingest the eggs while foraging. In the rodent’s liver, the eggs develop into cysts, the larval stage, a mass of parasite tissue that grows and infiltrates like a tumor.
The cycle completes when a canid eats an infected rodent. The cysts develop into adult worms in the canid’s gut. The canid is unharmed. The rodent is not.
Humans and domestic dogs are accidental hosts. They become infected by ingesting eggs from contaminated soil, food, or water, or, in the case of dogs, by preying on infected rodents. Once inside a human, the eggs follow the same path as in a rodent: they migrate to the liver and develop into alveolar echinococcosis (AE), a disease that mimics slow-growing liver cancer and can metastasize to the lungs and brain.
“To minimize the risk of dogs getting infected with E. multilocularis, owners should not let them prey on rodents or scavenge their carcasses,” said Dr. Guilherme Verocai, a co-author and associate professor at Texas A&M’s College of Veterinary Medicine.
The European Strain
The Washington State coyotes carried a specific genetic variant, a European-origin haplotype previously documented in British Columbia, Canada. This is distinct from the older North American tundra variant found in Alaska. The European strain appears to be expanding faster and has been moving south and west from Canada across the border.
Seven cases in domestic dogs have been reported in Washington, Oregon, and Idaho since 2023. British Columbia has detected the strain in its southern regions. No human cases have been reported yet on the US West Coast, but the parasite is now established in a wild reservoir, coyotes, within reach of urban and suburban areas.
“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.
The Disease
Alveolar echinococcosis has a uniquely insidious course. The incubation period ranges from 5 to 15 years, sometimes up to 50. By the time symptoms appear (jaundice, abdominal pain, weight loss), the liver is often extensively damaged.
Treatment has improved significantly. Surgical resection of the liver lesions can be curative, with 15-year survival rates above 90% when complete removal is possible. Long-term therapy with albendazole, a benzimidazole drug taken for years or sometimes for life, reduces 10-year mortality to below 20%. Liver transplantation is an option for advanced cases.
But the disease requires a high index of clinical suspicion. Most doctors in the Pacific Northwest have never seen a case of alveolar echinococcosis, and its symptoms overlap with far more common liver diseases.
What This Means
The discovery has immediate practical implications. Neither the United States nor Canada requires deworming of dogs crossing the border, a policy gap that may have contributed to the parasite’s spread. The study’s authors recommend increased surveillance of wild canids and routine veterinary screening for dogs that roam or hunt rodents.
The study also revealed a methodological insight: intestinal examination of coyote carcasses detected the parasite far more reliably than fecal scat sampling. Non-invasive DNA testing of scat, the authors warn, may underestimate true prevalence in wildlife populations.
“This parasite is concerning because it has been spreading across North America,” Hentati said. “There have been numerous cases of dogs getting sick, and a handful of people have also picked up the tapeworm.”
Source: Hentati, Y., Reese, E., Curran, C.C., Miller, E.M., Díaz-Morales, D.M., Kreling, S.E.S., Verocai, G.G., Prugh, L.R., Schell, C.J. & Wood, C.L. (2026). “Detection of Echinococcus multilocularis in coyotes in Washington State, USA highlights need for increased wildlife surveillance.” PLOS Neglected Tropical Diseases. DOI: 10.1371/journal.pntd.0013502.