First U.S. Case of Fatal Brain Virus Found in Colorado Mountain Lion

A devastating and only recently discovered virus has now been found in America’s wild cats. In a new case report, scientists and wildlife officials in Colorado document the first known U.S. infection of rustrela virus in a mountain lion. The germ causes an usually fatal neurological illness in cats called staggering disease.

In May 2023, Colorado Parks and Wildlife officials were notified of a sickly wild mountain lion that had found its way to a residential area in Douglas County, Colorado. The mountain lion’s hind legs had mostly gone limp, meaning that the cat moved mainly by dragging itself with its front legs. Officials first tranquilized and then euthanized the animal with a gunshot to the chest in order to keep its brain intact for further study.

Early tests failed to find other known germs that can cause severe brain infections in cats, such as the rabies virus or Toxoplasma gondii. But a detailed genetic analysis of the cat’s brain and spinal tissue eventually revealed the presence of rustrela virus, formally known as Rubivirus strelense.

Rustrela was only discovered several years ago. In October 2020, researchers detailed finding it in animals residing at a German zoo that had come down with neurological problems. Months before the lion death, in February 2023, scientists elsewhere published research appearing to confirm rustrela as the cause of staggering disease.

While rare, staggering disease has been known to sicken domesticated and wild felines throughout Europe since the 1970s. This new case, detailed Thursday in the journal Emerging Infectious Diseases, is the first confirmed report of rustrela virus and staggering disease among cats in the U.S. 

At this point, there is much we don’t know about the danger it poses to cats and other animals. The virus isolated from the mountain lion is fairly genetically distinct from the variants seen in Europe, for instance, leaving open the possibility that it’s already endemic in parts of the U.S. Rustrela also seems capable of infecting a wide variety of mammals, but it’s unknown whether it could potentially make people sick. It is a close cousin of the virus that causes rubella in humans, which can rarely cause neurological illness, particularly in babies who are infected in the womb. Rubella has been eliminated in the U.S. since 2004 thanks to a highly effective vaccine and widespread childhood vaccination program.

“Considering its wide host range in Europe, rustrela virus should be considered as a cause for neurologic diseases among mammal species in North America,” the researchers wrote.

Worrying as this discovery is, it also means that scientists can start to dig further and answer these important questions.

“Now that we know what we’re looking for, it should be easier to find new cases if they are out there,” said lead report author Karen Fox, a former pathologist with Colorado Parks and Wildlife, in a statement from the agency. “With continued collaboration, we plan to learn what we can from our colleagues in Europe while we continue to look for new cases of staggering disease here in Colorado.”

As part of this effort, residents who come across similarly sick mountain lions in the area are being asked to report their sightings to their local wildlife officers.