A dangerous and potentially fatal parasite that has been steadily expanding its geographic footprint across North America has now been officially identified on the contiguous U.S. West Coast for the first time. Recent scientific surveillance conducted in the Puget Sound region of Washington state has revealed that Echinococcus multilocularis—a small tapeworm capable of causing severe, cancer-like cyst formations in humans and domestic animals—is present in local coyote populations at surprisingly high rates. The discovery has prompted renewed vigilance among wildlife biologists, veterinarians, and public health officials who track zoonotic diseases.
The findings, published in the peer-reviewed journal PLOS Neglected Tropical Diseases, highlight a significant shift in the ecology of a pathogen that was once thought to be largely confined to remote northern regions or distant overseas continents. While researchers emphasize that human infections on the West Coast remain undocumented, the high prevalence of the parasite among urban and rural canids underscores a growing intersection between wildlife habitats and human residential zones.
Groundbreaking Discovery in the Puget Sound Region
The recent investigation was spearheaded by researchers at the University of Washington (UW), who set out to determine the local status of E. multilocularis. The team collected and analyzed biological samples from 100 coyotes across the Puget Sound landscape. To the astonishment of the research team, 37 of those animals—more than one-third—tested positive for the tapeworm.
"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 earned her doctorate in environmental and forest science from UW. "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."
The parasite’s primary hosts are wild canids, including coyotes, foxes, and wolves. In these animals, the adult tapeworm resides in the intestines, often numbering in the thousands. Remarkably, the host animals typically exhibit no clinical signs of illness, acting as asymptomatic carriers while continuously shedding microscopic parasite eggs into the environment via their feces.
The life cycle of E. multilocularis relies heavily on a predator-prey dynamic. Once eggs are deposited into the soil or vegetation, small rodents consume them while foraging. Inside the rodent, the eggs hatch into larvae that migrate to the liver, where they develop into destructive cysts. When a coyote or other wild canid preys upon an infected rodent, the cycle completes as the larvae mature into adult tapeworms inside the predator’s digestive tract.
A Complex and Hazardous Life Cycle
While canids tolerate the parasite well, accidental hosts—such as humans and domestic dogs—face significantly graver health consequences. When humans or dogs inadvertently ingest tapeworm eggs, typically through contaminated food, soil, or contact with infected animal feces, the parasite behaves differently.
Instead of passing harmlessly through the digestive tract, the larvae migrate to internal organs, most frequently the liver. There, they form slow-growing, infiltrative, tumor-like lesions known as alveolar echinococcosis (AE). These cysts can metastasize to other organs, such as the lungs or brain, mimicking malignant cancer.
The clinical latency period for AE is notoriously long, with symptoms often taking anywhere from five to fifteen years to manifest. This prolonged asymptomatic phase makes early diagnosis extraordinarily difficult. If left untreated, alveolar echinococcosis can be fatal, causing extensive liver destruction and multi-organ failure. Consequently, the World Health Organization classifies AE among the top 20 neglected tropical diseases and ranks it globally as the third most important food-borne parasitic illness.
Historical Context and Geographic Spread
For decades, E. multilocularis was recognized primarily as a severe public health hazard in endemic regions of Central Europe, parts of Russia, and northern Asia. In North America, the parasite was historically considered exceedingly rare, documented only occasionally on remote islands in northwestern Alaska and select northern Canadian territories.
However, a marked epidemiological shift occurred approximately fifteen years ago. The parasite began appearing with increasing frequency in domestic dogs and human patients across central Canada and the American Midwest, signaling an aggressive expansion of its continental range.
Genetic analyses conducted by parasitologists and molecular biologists have revealed a critical detail regarding this modern expansion: the strain driving the contemporary outbreak differs significantly from the historical tundra variant found decades ago in Alaska. Modern outbreaks across the United States and Canada are overwhelmingly linked to a more infectious strain of European origin. The coyotes sampled in the recent Puget Sound study carried this newer, highly transmissible variant.
The precise mechanism by which the European strain established itself in North America remains a subject of ongoing scientific debate. One leading hypothesis suggests that infected domestic dogs transported across international borders without rigorous mandatory deworming protocols may have introduced the parasite. An alternative theory, proposed in historical evaluations, points to the potential importation of red foxes for hunting purposes roughly a century ago, which may have planted a dormant reservoir that only recently found favorable ecological conditions for explosive spread.
Risks and Recommendations for Domestic Dogs
Domestic dogs occupy a unique position in the transmission chain of E. multilocularis. Because dogs can share living spaces closely with humans, their potential exposure to the parasite represents a direct public health bridge.
Veterinary experts emphasize that a dog’s risk of infection correlates directly with its behavior, specifically its propensity for hunting or scavenging.
"To minimize the risk of dogs getting infected with E. multilocularis, owners should not let them prey on rodents or scavenge their carcasses," advised 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.
Canines that consume raw wild rodents can acquire the adult tapeworm, subsequently shedding eggs in their feces and placing their human owners at risk. Alternatively, dogs that ingest parasite eggs can develop the same destructive, cyst-forming disease seen in human patients.
To mitigate these risks, veterinary professionals recommend stringent preventive measures. These include routine wellness examinations, regular diagnostic parasite screening, and the consistent administration of broad-spectrum deworming medications prescribed by licensed veterinarians. Furthermore, keeping dogs on leashes during walks in wooded or rural environments prevents them from digging up rodent burrows or consuming wildlife carcasses.
Epidemiological Status on the West Coast
Despite the high infection rate documented among Puget Sound coyotes, researchers stress that the parasite has not yet established a widespread foothold among other local animal populations or domestic pets in the region. Retrospective data tracking canine infections indicate that only a small number of clinical cases—just seven total across Washington, Oregon, and Idaho since 2023, including five within Washington state—have been formally diagnosed.
Furthermore, no human cases of alveolar echinococcosis have been reported along the U.S. West Coast. The concentration of the parasite in coyotes rather than domestic pets or other urban wildlife is largely a matter of dietary habits.
"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 explained.
Broader Implications and Ongoing Surveillance
The confirmation of E. multilocularis in West Coast wildlife marks a new chapter in American wildlife parasitology. As urban development continues to encroach upon natural habitats, the overlap between human populations, domestic pets, and urban-adapted wildlife such as coyotes is intensifying.
Public health analysts note that while the immediate risk to the general human population remains low, the establishment of a robust wildlife reservoir necessitates heightened awareness among primary care physicians, veterinarians, and public health agencies. Early recognition of symptoms and proper diagnostic protocols will be vital if human cases eventually emerge in the region.
The research team behind the Puget Sound study underscores that public alarm is unwarranted, but proactive education is essential. Basic hygiene practices—such as washing hands after gardening, thoroughly washing wild-harvested produce, preventing pets from hunting rodents, and maintaining regular veterinary care—remain the most effective defenses against exposure.
"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 concluded.
Funding for the foundational research was provided by the National Science Foundation and the University of Washington Hall Conservation Genetics Fund. The study’s broader author collective included Ellie Reese, Samantha Kreling, Laura Prugh, and Chelsea Wood from the University of Washington; Claire Curran from the College of William and Mary; Erika Miller from Sound Data Management; Dakeishla M. Díaz-Morales from DePaul University; and Christopher J. Schell from the University of California, Berkeley.
