Rabbits with hornlike growths spotted in northern Wisconsin

Rabbits with horn-like growths shooting out of their heads and faces have been spotted in northern Wisconsin and other states, inspiring an internet frenzy over the “mutant” bunnies.

Sightings of the unfortunate furry critters occur every year, but they’ve sparked national attention after photos of the bunnies in Colorado went viral this month. On social media, people have dubbed the rabbits “Frankenstein” bunnies or likened them to zombies in the HBO series “The Last of Us.”

While they may look like something out of a horror flick, disease experts say the virus causing the wart-like growths isn’t a sign of the end times. The rabbits are infected with the Shope papillomavirus, according to Nancy Businga, wildlife disease specialist with the Wisconsin Department of Natural Resources.

“That can, of course, be very eye-catching and very strange-looking to people that haven’t seen it before. And we do, every year, get a few random reports of these rabbits with these lesions,” Businga said. 

Businga said there’s been a slight uptick in sightings this year in northwestern Wisconsin. The agency has received eight reports of 23 rabbits in St. Croix, Dunn, Pierce and Polk counties. Last year, the agency only received one report of the virus affecting rabbits.

Paul Lambert is an oncology professor and director of the McArdle Lab for Cancer Research at the University of Wisconsin School of Medicine and Public Health. Lambert, who studies human papillomavirus, said the Shope virus was the first tumor virus ever discovered. American physician and animal pathologist Richard E. Shope first identified the virus in the 1930s.

“This is not a bloodborne pathogen,” Lambert said. “This virus, papillomaviruses, is transmitted by exposure on the skin.”

Lambert said the virus is mostly benign and the warts usually go away as the animal’s immune system fights off the infection. In a small percentage of humans, infections may not resolve from sexually transmitted human papillomaviruses and could pose a risk of cancer.  More than 300 papillomaviruses have been identified, including more than 200 human papillomaviruses.

“This is not new. It’s just that pictures of these rabbits draw interest because they look kind of bizarre,” Lambert said. “The jackalope has been around for as long as I can remember and as a mystical creature that was to reflect the taxidermist version of these rabbits with horns.”

Kristen Bernard, a virology professor at the University of Wisconsin School of Veterinary Medicine, said the Shope virus cannot be transmitted to humans. However, she said, the virus can be transmitted by insects and ticks among wildlife.

“It’s because when the insect bites that area of an animal to blood feed like a tick does, like a mosquito does, it gets the virus on its mouth parts, and then goes and feeds on another animal and transmits the virus mechanically that way,” Bernard said.

Businga said an increase in sightings of the virus may be tied to warmer weather for a longer portion of the year, which is favorable for insects like mosquitoes and ticks. While some have speculated that climate change may play a role in increased sightings, Lambert said evidence is lacking.

“I believe in climate change, but I’m not aware of any epidemiological study that has demonstrated that climate change is increasing the incidence of papillomavirus infections in animals or in humans,” Lambert said.

Bernard said there is evidence of diseases from ticks and mosquitoes spreading due to climate change, but it’s anyone’s guess whether it’s playing a role in the spread of the Shope virus among rabbits.

“I wouldn’t say there’s strong evidence, but I wouldn’t be surprised if climate change increased the amount of this in the summer months due to that mechanical transmission by insects,” Bernard said.

Rabbits aren’t the only animals in Wisconsin that have been spotted with growths due to papillomaviruses. People have also reported a few sightings of papillomaviruses  in deer and a couple affecting squirrels this year.

If people spot animals with papillomaviruses, Bernard urged them to use caution when handling sick wildlife. While the virus can’t be transmitted to humans or pets, rabbits may have other diseases that can spread to humans including tularemia or rabbit fever. They can also infect domestic rabbits with the virus.

People can report sick wildlife to the DNR by filling out a form on the agency’s website or by calling 608-267-0866.

Editor’s note: The story has been updated to more accurately reflect comments from the DNR’s Nancy Businga on warmer weather possibly affecting the spread of the Shope papillomavirus.