Researchers have confirmed a worrying first: the fox tapeworm, Echinococcus multilocularis, has turned up in West Coast wildlife. The discovery raises questions about how far this parasite has spread and what that means for people, pets, and local wildlife.
A University of Washington study tested coyotes around Puget Sound and found the parasite in 37 of 100 animals sampled. The work was published in PLOS Neglected Tropical Diseases and points to a new chapter in the parasite’s westward move across North America.
Genetic analysis in the study showed the coyotes carried a European-origin strain that appears more infectious and has become the dominant type in parts of the U.S. and Canada. That detail matters because different strains behave differently in hosts and can change the landscape of risk.
Before the 2010s, this tapeworm was nearly unknown on the continent outside some remote Alaskan islands. Over the last decade and a half, however, cases in animals and occasional human infections have climbed in the Midwest and Canada, and now evidence shows the parasite reaching coastal Washington.
“The reason that [cases are] so high in coyotes is because they are regularly eating raw rodents, and that is the primary way for them to get infected,” said lead author Yasmine Hentati. That predator-prey cycle is the engine that keeps the parasite moving through wildlife populations.
The parasite cycles mainly among canids and small mammals: foxes, coyotes, stray dogs, and various rodents. When eggs are passed in canid feces they can contaminate soil, water, or vegetation, creating an infection risk for other animals and, rarely, humans.
Human disease is uncommon but dangerous when it happens. Infections form fluid-filled cysts that most often develop in the liver and can take years to grow large enough to cause symptoms, making diagnosis slow and tricky.
Doctors differentiate two main forms: cystic echinococcosis, which produces distinct cysts that the body walls off, and alveolar echinococcosis, a rarer and more aggressive form that invades tissue like a tumor. The aggressive form can destroy liver tissue, spread to other organs, and carry a high mortality risk if untreated.
“Symptoms of liver failure include fatigue, yellowing of the skin and eyeballs, and abdominal pain,” Yancey cautioned. Because the infection can mimic tumors or other chronic diseases, clinicians must think broadly when patients show these signs and have relevant exposure histories.
People typically become infected by accidentally ingesting eggs from contaminated soil or food, or by handling pets that have been exposed and then touching their mouth. The latent, slow-growing nature of the cysts means infections are often discovered long after exposure occurred.
“The most important way to protect yourself from the fox tapeworm is also the simplest: good hand hygiene,” Yancey advised. For dog owners the advice is practical and direct: “For dog owners, prevent your animals from eating wild rodents and make sure all your pets get routine veterinary care,” Yancey added.
The Centers for Disease Control and other experts recommend avoiding contact with wild canids, keeping pets from scavenging, and maintaining routine parasite prevention for domestic animals. These basic steps dramatically lower the already small risk and help keep both people and pets safer as the parasite’s range changes.
