Colorado finds first mountain lion with staggering disease

In May 2023, a homeowner in Douglas County was astonished to find a partially paralyzed mountain lion taking cover between her basement window well and a spruce tree, dragging itself forward with its front paws. The year-old female couldn’t stand up, a phenomenon clear in the video the owner took while safely inside the house.

Wildlife officers tranquilized the debilitated lion, then euthanized her with a gunshot to the chest to protect brain cells for a necropsy. 

After a year of studying the animal, researchers are declaring her the first North American case of “staggering disease” in a mountain lion, according to Colorado State University veterinarian and former Colorado Parks and Wildlife pathologist Karen Fox.

The disease, caused by variants of the rustrela virus, makes a virulent attack on muscles and the nervous system and is more frequent in Europe. It was known best in European cats, especially in Sweden, but has now been found to attack mammals including cats, horses and possums. 

The CSU-led paper announcing the find, to be published in August in the “Emerging Infectious Diseases” journal of the Centers for Disease Control, recommends more screening of Colorado mammals exhibiting symptoms. Fox also wants to research mice or other rodent populations that appear to be reservoirs for the virus. 

“We really are at the stage where, now that we know what it is, and we have a test for it, we can start looking for it,” Fox said in email responses to questions on the study. “We aren’t able to comment on how common this might be since we’ve only had one case, but hopefully we can start to answer that question soon.”

“This is the first case of rustrela virus-associated staggering disease that has been identified outside of Europe, so it is the first case identified in North America,” said Fox, who collaborated in pinpointing the unusual cause with experts at the Friedrich-Loeffler-Institut in Germany. “There have been reports of similar disease syndromes in domestic cats previously in North America but we are not sure whether or not those may have been associated with rustrela virus.” 

Colorado Parks and Wildlife officials and Fox emphasized that scientists “don’t yet know how widespread the virus is in either domestic cats or wildlife populations. In Europe, rustrela virus causes staggering disease in cats, but also has been found in a wide range of other species, including rodents, a donkey and marsupials.”

The virus as the cause of staggering disease was identified “only recently,” in Europe, Fox said.

Despite the recent advances, scientists still aren’t sure what the rustrela virus threat may be to humans, or whether humans have already contracted variants of it. “Future research should include the evaluation of a possible zoonotic potential of RusV,” according to the definitive paper on the virus and staggering disease connection last year. 

Fox said the university’s involvement from now on “will depend on funding, but one thing CSU would like to do is work with feline practitioners to identify and test suspect cases both prospectively and retrospectively if possible.” 

“Since the virus can occur in species other than cats, we’re also hoping that the publication of our findings will bring awareness to the virus so that other diagnosticians will consider rustrela virus as a possible cause of neurologic disease in any mammalian species,” Fox said. 

“I’d also like to do some testing of small mammal populations to see if we might be able to find a reservoir host for the virus as has been found in Europe.”

State Parks and Wildlife urged Colorado residents who observe mountain lions that are staggering, having trouble walking or behaving abnormally to contact their local Colorado Parks and Wildlife office.