Deadly tapeworm species found in Pullman red fox. What you need to know to keep pets safe

The death of a red fox kit in Pullman this May confirms what scientists have long suspected: A species of deadly tapeworm has made itself at home in Washington.

"We've known it's been around," said Laura Williams, veterinary parasitologist at Washington State University. "We just hadn't confirmed its presence in the wildlife cycle that it lives in."

Echinococcus multilocularis, colloquially known as the fox tapeworm, is a small parasite about 1 to 2 millimeter s long that requires two mammals to complete its life cycle: typically a rodent and a wild canine. In recent years, it has been detected in squirrels and domestic dogs in the region but not its usual fox or coyote hosts.

"For us to naturally find it in a fox and naturally find it in a squirrel sort of confirms that that is the true source around here," she said. "And that helps us determine what the risk is for the general person or dog in the region."

The risk to humans and their furry friends is as low as it has ever been, as far as Williams can tell.

In most cases, a mouse or other rodent eats food contaminated with the tapeworms' eggs. The eggs hatch inside the rodent and as the parasites grow into larvae they form aggressive hydatid cysts on the rodent's organs.

After the rodent is eaten by a larger animal - often a canine species such as a fox or coyote - the tapeworm larvae mature into microscopic adults and live in the new host's intestines. Latched onto an intestinal wall, the parasites shed their eggs, which the host in turn spreads in their feces. In this version of the infection cycle, the canine is typically asymptomatic, Williams said.

In the case of the unfortunate baby fox in May, WSU anatomic pathologist Kyle Taylor said the kit was battling another parasite and sickly for unknown reasons before being euthanized. While the hundreds of E. multilocularis likely contributed to a nutrient deficiency, Taylor remains uncertain exactly why the fox was doing as poorly as it was.

If a fox, coyote, domestic dog or even human eats the tapeworm eggs directly, however, they can develop the same hydatid cyst disease that the rodents do, called alveolar echinococcosis.

Alveolar echinococcosis is characterized by the formation of aggressive fluid sacs, typically on the liver. The cysts grow into the surrounding organ tissue "almost like a cancer," Williams said. Though clarifying that she can only speak to the process in animals, the spread throughout the liver can take years before triggering symptoms of lethargy, abdominal distention and listlessness.

If caught early enough, the affected tissues can be surgically removed, and the once-parasitized animals can go on to live a normal life. Otherwise, medication is the only real option.

"And the medications that we use for that just aren't very good," Williams said. "There are some that could potentially prevent them from expanding and infiltrating more, but they don't usually cure the disease at that point."

Ultimately, the cysts can lead to organ failure and kill the host.

"Thankfully, it's pretty rare, though," Williams said. "For a long time, people didn't think dogs could even get hydatid cysts."

It is likely that the tapeworm species has been in the area far longer than the recent detections suggest - there has been no historic push for wild canid necropsies, after all.

"This fox just happened to be found by the public and be brought to the (veterinary teaching hospital)," Taylor said. "We've seen increased numbers of cases over the last few years in pets and things, and so we just wanted people to be aware."

While Williams suggested that the recent detections could be due to a greater fox population in the area, local Washington Department of Fish and Wildlife spokeswoman Staci Lehman said the state doesn't specifically track fox populations and there haven't been enough individual reports to track any increases over time. While there were no formal reports of the parasite in Washington prior to 2023, a study published in PLOS Neglected Tropical Diseases earlier this year found that 37 out of 100 coyote samples in the Puget Sound were infected with the parasite, suggesting that it has been established in the state for some time.

"Nothing has changed, really, but knowing that a fox could potentially be a source of the eggs is helpful," Williams said. "So if you have a fox pooping in your yard, probably don't let your dog eat that poop, and if you are going to be picking it up yourself, make sure that you're washing your hands."

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