A parasitic tapeworm that has silently crept across the continent over the past 15 years has now arrived in the Pacific Northwest, where scientists discovered it thriving in local wildlife in numbers that caught researchers off guard.
The parasite, Echinococcus multilocularis, was found in 37 of 100 coyotes surveyed in the Puget Sound region, according to research published in PLOS Neglected Tropical Diseases. The discovery marks the first time the pathogen has been identified in wild animals along the contiguous U.S. West Coast, signaling another frontier in the parasite's expanding range across North America.
University of Washington researchers led by Yasmine Hentati were surprised by the prevalence. "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," Hentati said.
The tapeworm poses a genuine threat to human and animal health. When it infects a person or domestic dog, it can produce cancer-like cysts that grow in the liver and sometimes spread to other organs. Without treatment, the disease can be fatal. Yet many infected animals show no signs of illness, making the parasite particularly difficult to detect before damage occurs.
Coyotes and foxes are the natural reservoir for the adult tapeworm. These animals can harbor thousands of worms in their intestines without becoming sick. The worms reproduce in the host's gut and release eggs that contaminate the environment through feces. Rodents then become infected when they eat contaminated food or soil, allowing the parasite to develop cysts in their livers. When coyotes hunt and eat those infected rodents, the cycle continues.
Humans and domestic dogs are accidental hosts. A person can contract the infection by swallowing tapeworm eggs through contaminated food or water, or by handling infected animals or their feces. Once inside a human body, the disease, called alveolar echinococcosis, develops slowly, often producing no symptoms for five to 15 years after exposure. This long incubation period makes early diagnosis nearly impossible and treatment extremely challenging.
The World Health Organization ranks alveolar echinococcosis as one of the top 20 neglected tropical diseases and identifies it as the third most important food-borne illness globally.
Dogs present a particular vulnerability. They can acquire the infection from eating infected rodents or their remains, though many remain asymptomatic carriers shedding infectious eggs in their feces. Guilherme Verocai, an associate professor and director of the Parasitology Diagnostic Laboratory at Texas A&M University College of Veterinary Medicine and Biomedical Sciences, recommends pet owners prevent their dogs from hunting or scavenging rodents. He also advises routine veterinary care, parasite testing, and preventative medications.
The tapeworm's recent expansion into North America represents a dramatic shift. For decades, E. multilocularis was recognized as a serious public health threat only in parts of Europe and Asia. About 15 years ago, infections began appearing in dogs and people across Canada and the Midwest, indicating the parasite was establishing itself on the continent. Today, a more infectious European strain is driving the current outbreak, distinct from an older Alaskan tundra variant documented decades earlier.
How the parasite arrived remains uncertain. One theory suggests infected dogs entering the country without required deworming treatment may have introduced it. Another hypothesis, proposed in earlier research, points to red foxes imported for hunting roughly a century ago.
While more than one-third of surveyed coyotes carried the parasite, researchers found limited evidence that it has become widespread in other animal populations. Seven canine cases have been documented in Washington, Oregon, and Idaho since 2023, with five occurring in Washington. Human infections remain rare across the United States, and the West Coast has seen none to date. Hentati explained the discrepancy: coyotes regularly consume raw rodents, their primary infection route, whereas most domestic dogs do not eat raw rodent livers.
The research emphasizes the need for continued surveillance. "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.
Author Jessica Williams: "This is a parasite that operates in shadows, invisible in animals that show no symptoms, and the fact that it's now established on the West Coast means vigilance and pet owners' awareness just became critical tools for preventing a public health crisis before it starts."
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