Since August, Colorado Parks and Wildlife has not received an increase in reports of sick or dead mountain lions, nor has the number of dead collared mountain lions increased. The state will continue its existing surveillance programs, which, if any, could identify sick cougars for further testing.
Elizabeth Buckles, a veterinary pathologist at Cornell University who was not involved in the diagnosis, points out that “almost anything” can happen to wild animals fighting a new disease. “Do (the animals) have some immunity to it?” she asked. “Or is everything new? If everything is new, as we saw with white-nose syndrome or the bird flu virus, it will take a while for immunity to develop.” How a population responds to a new disease can be influenced by a number of factors, including how often animals reproduce and whether they congregate 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 have saved samples from undiagnosable cases from the past to retrospectively test the domestic cat population. This could help determine whether the disease is already present in our pets. Another big question Fox and her German colleagues hope to address is how the disease spreads.
How does a frightening disease spread?
The same 2023 study that showed the link to the Rustrela virus also identified rodents, including wood mice and yellow-necked mice, as European hosts of the virus. Numerous small mammals, including deer mice, squirrels or chipmunks, could transmit the disease in Colorado without causing adverse health effects – similar to how mice transmit the hantavirus but do not become sick from it. Fox would like to capture these animals and study their brains and spines to find out more.
Eating mice is the most obvious way cats (and cougars) can contract the virus, but the exact mechanisms of transmission, such as how the virus travels from the stomach to the brain, are unclear. Researchers believe that cougars and other larger mammals that have the disease are “dead-end hosts,” meaning they don’t spread the disease further. The Colorado cougar virus is genetically different from several European strains of the virus, suggesting it may be a distinct version of the virus that has been present undetected in the U.S. for some time—rather than something that was recently introduced to the continent.
