A deadly disease that affects cats big and small found in U.S.

On a cool May morning last year, Erica Rhinehart was walking around her Douglas County, Colorado, neighborhood with her Bengal cat, Mara when a neighbor called out with a warning: There was a mountain lion in her neighbor’s yard.

From inside the house, Rhinehart and her neighbor watched as the young female mountain lion, also called a cougar, struggled to move out from under patio furniture. Her hind legs and hips weren’t working correctly, so the feline hobbled on her two front legs, dragging the rest of her body behind her to a nearby pine tree. Rhinehart called several wildlife rehabilitation centers in hopes of saving the cougar somehow before alerting Colorado Parks and Wildlife staff, who came to euthanize the suffering creature shortly thereafter. The exact cause of the animal’s illness was an open question.

Finally, tissue test results published this month in Emerging Infectious Diseases show that the mountain lion had a fatal neurologic syndrome called staggering disease. The condition has been found in European domestic cats and a handful of zoo animals over the past five decades, but the Colorado cougar is the first confirmed case of the disease in North America among wildlife or domestic animals.

"We're not worried if this is an isolated case," says Karen Fox, a wildlife pathologist at Colorado State University who investigated the case. "But traditionally, when we find diseases, it means you're the tip of an iceberg, and there's a bunch more you've missed over time."

What is staggering disease?

The disease gets its name from the stumbling gait exhibited by the animals sickened with it, including the cougar. Felines lose their ability to use their back legs, and they can also experience a variety of other symptoms, from an inability to retract claws to increased affectionate behavior observed in domestic cats.

The disease was first identified in Sweden in 1974; later research found the virus had been present elsewhere in northern Europe too, with additional hotspots in Austria and Germany. The virus has been documented in a variety of zoo animals, including lions, marsupials, an otter and a donkey. While it’s most commonly found in domestic cats, no official statistics on how many cats die from the disease exist. 

The exact pathogen that causes the disease had been a mystery until recently. Last year, researchers made the connection between staggering disease and a mutated version of the rustrela virus, a relative of the rubella virus that infects humans and can be especially dangerous for pregnant people. 

Diagnosing the cougar case

Last year, Fox and her team began investigating the cougar’s illness for Colorado Parks and Wildlife, and she initially thought a pelvic fracture likely explained the odd movements. But x-rays and a necropsy turned up nothing, and a colleague convinced her to take a closer look.

The cougar's brain and spinal cord were inflamed and contained areas of damaged tissue called lesions. "There's so much inflammation that the spinal cord can't do its job," Fox said. That's why creatures lose movement of their back legs first. "The message is getting messed up somewhere between the brain and the spinal cord and the nerves that control the legs," she said.

But inflammation can also be a sign of many other diseases, so Fox looked deeper. She extracted RNA, a molecule that’s closely related to DNA and found in most living organisms and viruses, from a tissue sample, and had it sequenced. The genetic sequence in the sample matched RNA from the rustrela virus. 

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Finally, Fox needed to confirm the virus was in the cougar's brain lesions, so she sent a preserved tissue sample in a wax block to a lab in Germany that specializes in animal health called the Friedrich-Loeffler-Institut. Customs officers confiscated the first shipped sample, since cougars are a protected species internationally. But a second sample, complete with additional paperwork, made it overseas, where advanced diagnostic testing confirmed the case. 

Finding staggering disease in North America was a total surprise. "We expected it to be more widespread and to find more variants in other parts of Europe, but I personally wouldn't have expected to find such a close relative on a different continent," says Dennis Rubbenstroth, a veterinarian who leads the German lab’s diagnostic virology program.  

What does the virus mean for North American wildlife?

It's too soon to know if the rustrela virus will have major implications for wildlife in North America. "Disease is part of normal functioning ecosystems, and even healthy populations have disease," says Mary Wood, Colorado state wildlife veterinarian. 

As of August, Colorado Parks and Wildlife isn't receiving an increase in reports of sick or dead mountain lions, nor are they seeing an increase in dead collared mountain lions. The state will continue its existing surveillance programs, which could turn up sick cougars for more testing, if they exist.

Elizabeth Buckles, a wildlife pathologist at Cornell University who wasn’t involved in the diagnosis, notes that "almost anything" can happen to wildlife dealing with a new disease. "Do [the animals] have some level of immunity to it?" she said. "Or is it all new? If it's all new, like we saw with white nose syndrome or we're seeing with avian influenza virus, it's going to take a while for immunity to develop." How a population responds to a new disease can be influenced by a variety of factors, including how often animals reproduce, and if they gather in groups or travel long distances to new areas.

Now that the virus is officially confirmed in the U.S., Fox hopes to work with veterinarians who've saved samples from undiagnosable past cases to retroactively test the domestic cat population. This could help identify if the disease already exists in our pets. Another big question that Fox and her German colleagues want to tackle is how the disease spreads.

How does staggering disease spread?

The same 2023 study that found the rustrela virus link also identified rodents, including wood mice and yellow-necked field mice, as the virus’ European hosts. Any number of small mammals, including deer mice, squirrels or chipmunks, could be carrying the disease in Colorado with no side effects to their health — similar to the way mice transmit hantavirus but aren't sickened by it. Fox would like to trap these creatures and test their brain and spinal matter to find out more.

Eating mice is the most obvious way cats (and cougars) could pick up the virus. But the exact transmission mechanics, like how the virus might go from the stomach to the brain, are unclear. Researchers think cougars and other larger mammals with the disease are "dead end hosts," meaning they don't further spread the disease. The virus from the Colorado cougar is genetically different from several European strains of the virus, suggesting it may be a distinct version of the virus that's been in the U.S. for a while, undetected — not something that was recently introduced to the continent. 

So far, there are no documented cases of staggering disease in humans, but the wide variety of animals the virus can sicken makes it hard to rule out the possibility of animal to human transmission. "This pattern of very unrelated mammalian species certainly leaves room for our mammalian species on that list," Fox says. 

In Germany, Rubbenstroth and his colleagues continue to study the virus's behavior, buoyed by the knowledge that staggering disease isn't just sickening and killing cats in Europe — it's also found in wildlife thousands of miles away, too. "We have to keep our eyes open to find variants of this virus in South America, Africa or Asia," said Florian Pfaff, who heads the German laboratory. "I think they are out there, and they need to be found."