A parasitic tapeworm capable of producing tumor-like cysts in human organs has turned up in more than a third of coyotes surveyed near Puget Sound, Washington, the first time the pathogen has been detected anywhere in the Pacific Northwest.
University of Washington researchers reported finding the tapeworm Echinococcus multilocularis, sometimes called the "fox tapeworm", in 37 of 100 coyotes sampled in the region. The study was published in March in the peer-reviewed journal PLOS Neglected Tropical Diseases.
The finding matters because the parasite has been quietly spreading across the United States and Canada for the past 15 years, moving westward from its traditional stronghold in the upper Midwest. Now it has reached the coast, and it brought a more dangerous strain with it.
Lead author Yasmine Hentati, a University of Washington researcher, said the prevalence caught her team off guard.
"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."
That 37 percent infection rate is not a marginal signal. It suggests the tapeworm is already well-established in the local wildlife cycle, coyotes eating infected rodents, shedding microscopic eggs in their feces, and contaminating the landscape where pets and people walk, garden, and forage.
The worm's life cycle is grimly efficient. Infected coyotes and foxes can harbor thousands of worms in their intestines without showing symptoms, silently seeding the environment. Rodents pick up the eggs from contaminated food or soil. When a coyote eats an infected rodent, the cycle restarts. Humans and domestic dogs are accidental hosts, but in those hosts, the consequences can be severe.
In humans, ingested tapeworm eggs don't produce adult worms. Instead, the larvae migrate to the liver and form slow-growing cysts that mimic cancer on imaging scans. The disease, alveolar echinococcosis, can spread to other organs. Left untreated, it can be fatal.
Hentati described the broader risk plainly:
"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."
The human case count remains small. But the disease is difficult to diagnose because its cysts look like malignant tumors, and many physicians outside endemic areas have never seen it. A parasite that was once confined to arctic and sub-arctic zones is now turning up in suburban wildlife corridors, places where joggers run, children play, and dogs roam off-leash.
The United States has faced a string of emerging parasite threats in recent months. A flesh-eating screwworm parasite was detected just 30 miles from the Texas border, prompting federal concern about a pest that hadn't been seen in the continental U.S. in decades.
The New York Post reported that the coyotes near Puget Sound carried a newer, more infectious European variant of the tapeworm, a strain that has become the predominant form circulating in the U.S. and Canada. That detail adds urgency to a finding that might otherwise read as a routine wildlife survey.
The European variant has been linked to increasing dog and human infections across the Midwestern U.S. and Canada. Its arrival on the West Coast suggests the parasite's geographic expansion is accelerating, not slowing.
Dr. Linda Yancey, an infectious disease specialist at Memorial Hermann, told the Post that the simplest defense is also the most obvious: "The most important way to protect yourself from the fox tapeworm is also the simplest: good hand hygiene."
Yancey also urged dog owners to take specific precautions: "For dog owners, prevent your animals from eating wild rodents and make sure all your pets get routine veterinary care."
Coyotes are the ideal vehicle for this parasite. They eat rodents constantly, they range widely through suburban and urban-edge habitats, and their populations have expanded dramatically across North America over the past several decades. Unlike wolves, coyotes thrive alongside human development.
Hentati explained the transmission dynamic:
"The reason it's so common 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."
That last point offers some reassurance for pet owners. But "most" is doing heavy lifting in that sentence. Dogs that roam freely, hunt rodents, or scavenge in areas with coyote activity face elevated risk. And humans who garden, forage wild plants, or handle soil in contaminated areas can ingest eggs without knowing it.
The situation echoes other recent parasite scares that have forced federal and state agencies to respond. Texas activated its emergency operations center after the screwworm resurfaced near the southern border, a reminder that biological threats don't respect political boundaries or bureaucratic timelines.
Veterinarians recommend routine preventive treatments to reduce the risk of infection in pets, though no specific veterinary organization or treatment protocol has been publicly named in connection with the Puget Sound findings.
That gap matters. The study identifies a 37 percent infection rate in local wildlife. It names the parasite. It flags the public health risk. But no state or federal health agency has yet issued formal guidance to residents of the Puget Sound region, at least none that has surfaced publicly alongside the research.
The USDA moved quickly when the first screwworm case in Texas since 1966 was confirmed. Whether Washington state or federal public health agencies will treat the fox tapeworm's arrival with similar urgency remains an open question.
Several basic facts are still missing from the public record. How many confirmed human cases of Echinococcus multilocularis have occurred in the Pacific Northwest? What is the precise geographic scope of the survey area? How did the European variant reach the West Coast? None of these questions have been publicly answered.
This is not an isolated event. The fox tapeworm's westward march is part of a broader pattern of parasitic and vector-borne threats gaining ground across the United States. From brain-eating amoeba warnings in freshwater to screwworm outbreaks along the southern border, Americans are encountering biological hazards that public health infrastructure was not designed, or funded, to track in real time.
Hentati's summary was measured but direct:
"The main takeaway is that Echinococcus multilocularis is here, it's pretty prevalent in the local coyote population, and people should be aware of the potential risks."
"People should be aware" is a polite way of saying that the burden of protection is falling on individuals, not institutions. Wash your hands. Watch your dogs. Don't eat unwashed wild greens. Hope your doctor knows what a parasitic liver cyst looks like on a CT scan.
When a third of the coyotes in your backyard carry a parasite that mimics cancer, "awareness" is the floor, not the ceiling.
