A lone mule deer that was found blind and emaciated in Custer County, Idaho, this summer had the plague, the Washington State University College of Veterinary Medicine announced Sept. 18.
The Idaho Department of Fish and Game was notified of the sick mule deer on June 9, when a member of the public saw it stumbling around on private land in Custer County. After locating the animal the next day, an IDFG officer euthanized it. The agency sent a variety of tissue samples, including the deer’s eyeballs, to the Washington Animal Disease Diagnostic Laboratory at the WSU College of Veterinary Medicine for disease testing. Those test results show that the deer was carrying plague in its eyes, which caused the blindness. The lab did not specify whether the sick deer was a doe or a buck.
Plague, also known as black plague, is caused by the bacterium Yersinia pestis, which occurs naturally in the western U.S. and passes through the food chain in various vector species like rodents and fleas. Only five other cases of deer contracting plague have been recorded by disease pathologists in the U.S., including in Wyoming and Oregon. This is Idaho’s first confirmed case of plague in deer.
“Deer and other ruminants don’t appear to be particularly susceptible to plague, but in the few published cases, it has specifically been found in the eyes,” WADDL pathology resident Dr. Elis Fisk said in the press release. “The disease doesn’t necessarily kill the deer on its own, but it causes blindness, likely making them vulnerable to predators and significantly reducing their chances of survival.”
The deer’s eyes were “highly abnormal,” Fisk explained. Eyeballs are usually full of clear fluids that help maintain their structure and internal pressure. But when pathologists cut into this deer’s eyes, they were instead filled with “thick, reddish-brown debris.” Fisk also described finding detached retinas, severe inflammation, and necrosis (an accumulation of dead tissue).
Because plague is so rare in deer, the team at WADDL didn’t automatically assume it was to blame, pathologist Dr. Kyle Taylor said in the press release. A variety of different bacteria could have caused the same issue.
“But it seemed prudent to test for plague to rule it out, especially since the samples could still have contained live bacteria, posing a safety risk to anyone handling them,” Taylor said. “The chances were small since proper safety protocols were followed, but it’s not something you want to overlook because plague is a potentially fatal disease, yet treatable if caught early.”
Plague is usually transmitted to humans through bites from infected fleas, according to the World Health Organization. But it can also be transmitted via inhalation of airborne “droplets” or contact with infected tissue. In humans, it manifests as either bubonic plague in the lymphatic system or pneumonic plague in the respiratory system. Both can be fatal without proper antibiotic treatment. Lab testing is the only way to confirm a plague diagnosis.
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Wildlife officers found the deer on private land, and everyone who came in close contact with it tested negative for the disease, IDFG public information supervisor Roger Phillips tells KREM2 News. Even the landowner’s dog tested negative, and there weren’t any other signs of animals with plague in the vicinity.
As Tyler points out, most deer that contract ocular plague die of predation or starvation before anyone spots them, making this discovery an outlier.
“It’s probably just a super rare finding that anyone came across this animal before it died in the wild,” Taylor said.
