By George Ongkojoyo 
A resilient and potentially lethal parasite that has been migrating across the North American continent for over a decade has officially reached the Pacific Northwest, marking a significant shift in the region’s public health landscape. New research led by the University of Washington has confirmed the presence of the tapeworm Echinococcus multilocularis in wild coyotes within the Puget Sound region. This discovery represents the first time the pathogen has been identified in a wild host along the contiguous United States’ West Coast, signaling an expansion that experts say warrants immediate public awareness and enhanced veterinary vigilance.
The findings, published in the journal PLOS Neglected Tropical Diseases, highlight a growing biological threat that has largely flown under the radar of the general public. While the parasite is common in parts of Europe and Asia, its presence in the United States was historically restricted to specific, remote regions. However, the new data suggests that a highly infectious European strain of the tapeworm is now firmly established in the local wildlife of Washington State, creating a new transmission corridor between wild canids, domestic pets, and humans.
The Scope of the Puget Sound Discovery
The study was spearheaded by Yasmine Hentati, a recent doctoral graduate in environmental and forest science at the University of Washington. To understand the prevalence of the parasite, researchers surveyed 100 coyotes across the Puget Sound region, a densely populated area where the urban-wildlife interface is increasingly blurred. The results were startling: 37 of the 100 coyotes tested positive for E. multilocularis.
"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," Hentati stated. "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 detection of the parasite in such a high percentage of the sampled population suggests that the tapeworm is not merely passing through the region but has become endemic to the local ecosystem. This high prevalence is attributed to the dietary habits of coyotes, which frequently prey on small rodents—the primary intermediate hosts for the parasite’s larval stage.
A Complex and Stealthy Life Cycle
Understanding the danger posed by E. multilocularis requires a look at its complex, multi-stage life cycle. The parasite relies on two types of hosts to survive and reproduce: definitive hosts and intermediate hosts.
Definitive hosts are typically wild canids, such as coyotes and foxes, though domestic dogs can also fulfill this role. In these animals, the adult tapeworms live in the small intestine. A single coyote can carry thousands of these microscopic worms without showing any outward symptoms of illness. As the worms reproduce, they release eggs that are shed in the coyote’s feces, contaminating the surrounding soil, water, and vegetation.
The intermediate hosts are usually small rodents, such as voles, mice, and rats. These animals become infected after inadvertently consuming the tapeworm eggs from the environment. Once inside the rodent, the eggs hatch into larvae that migrate to the liver, where they form slow-growing, invasive cysts. These cysts eventually weaken or kill the rodent, making it easy prey for a coyote. When the coyote eats the infected liver, the cycle begins anew.
Humans and domestic dogs enter this cycle as "accidental hosts." For humans, infection occurs through the ingestion of microscopic eggs. This can happen through the consumption of contaminated wild berries or garden vegetables, or through hand-to-mouth contact after touching soil or a pet whose fur is contaminated with egg-laden feces.
Alveolar Echinococcosis: The "Silent" Killer
In humans, the infection leads to a condition known as alveolar echinococcosis (AE). Unlike many other parasitic infections that manifest quickly, AE is characterized by an exceptionally long incubation period, often lasting between five and 15 years. During this time, the parasite behaves much like a malignant tumor, forming multi-chambered cysts primarily in the liver.
These cysts have the ability to metastasize, spreading to other organs such as the lungs or brain. Because the symptoms—which can include abdominal pain, jaundice, and weight loss—take years to develop, the disease is frequently misdiagnosed as liver cancer in its later stages. If left untreated, AE is nearly always fatal. Even with modern medical intervention, treatment often requires aggressive surgery to remove the cysts followed by years, or even a lifetime, of antiparasitic medication.
The World Health Organization (WHO) classifies alveolar echinococcosis as one of the top 20 "neglected tropical diseases" and the third most important food-borne illness globally. While human cases in the United States remain rare, the increasing prevalence of the parasite in wildlife near urban centers has raised the statistical probability of human exposure.
Historical Context and the Rise of a More Virulent Strain
The presence of E. multilocularis in North America is not entirely new, but the strain currently circulating is. For much of the 20th century, the parasite was known to exist in the "tundra variant" found in northwestern Alaska and parts of the Canadian Arctic. This variant was relatively stable and did not show significant movement into the lower 48 states.
However, the landscape changed approximately 15 years ago. Infections began to spike in domestic dogs and humans in the Canadian Prairies and the American Midwest. Genetic analysis revealed that these new cases were not caused by the native North American tundra variant, but rather by a more infectious strain of European origin.
Scientists believe this European strain is more adept at jumping between species and surviving in varied climates. The coyotes identified in the Puget Sound study were found to be carrying this newer, more aggressive variant. The exact mechanism of its arrival remains a subject of scientific debate. One leading theory suggests the parasite may have been introduced via red foxes imported from Europe for hunting purposes roughly a century ago. A more contemporary possibility is the movement of infected domestic dogs across international borders without mandatory deworming treatments.
Risks to Domestic Dogs and Preventive Measures
Domestic dogs occupy a unique and dangerous position in the E. multilocularis life cycle. They can act as both definitive hosts (carrying adult worms and shedding eggs) and intermediate hosts (developing liver cysts).
Dogs that roam off-leash in wooded or rural areas and prey on rodents are at the highest risk. If a dog consumes an infected rodent, it can become a carrier, shedding eggs into the household environment and posing a direct risk to its owners. Conversely, if a dog ingests the eggs from coyote feces, it can develop the same life-threatening liver cysts seen in humans.
Guilherme Verocai, a co-author of the study and director of the Parasitology Diagnostic Laboratory at Texas A&M University, emphasized the importance of proactive pet management. "To minimize the risk of dogs getting infected with E. multilocularis, owners should not let them prey on rodents or scavenge their carcasses," Verocai advised.
Veterinarians are now recommending that dog owners in the Pacific Northwest incorporate routine parasite testing into their pet’s healthcare regimen. Preventative medications, particularly those containing praziquantel, are effective at clearing adult tapeworms from a dog’s system, thereby stopping the shedding of eggs.
Broader Implications for Public Health and Urban Ecology
The discovery of E. multilocularis in Washington State underscores the complexities of managing wildlife in urbanizing environments. Coyotes are highly adaptable and have become permanent fixtures in many American cities. As they move through suburban backyards and city parks, they bring the parasite into close proximity with human populations.
The University of Washington study serves as a critical baseline for future monitoring. While there have been seven documented canine cases of the infection in Washington, Oregon, and Idaho since 2023, no human cases have yet been reported on the West Coast. However, given the long incubation period of the disease, experts warn that human infections contracted today might not be diagnosed until the late 2030s.
"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 research was a collaborative effort involving experts from several institutions, including the University of Washington, Texas A&M, the College of William and Mary, and UC Berkeley. Funding was provided by the National Science Foundation and the University of Washington Hall Conservation Genetics Fund.
As the parasite continues to establish itself in the Pacific Northwest, health officials are calling for a "One Health" approach—an integrated strategy that recognizes the interconnection between the health of people, animals, and the environment. Increased surveillance of wildlife, better diagnostic tools for veterinarians, and public education regarding hand hygiene and pet safety are viewed as the primary defenses against this silent but formidable biological invader.
