At first glance, they appear to be creatures out of a nightmare—cottontail rabbits in northern Colorado with dark spikes and twisted horns protruding from their heads. Shocked locals have nicknamed them “zombie bunnies” and “Frankenstein rabbits,” sharing eerie photos online. However, the truth is stranger than myth and far more important to science. 

These are infected with a virus that has been known for nearly a century, the cottontail rabbit papillomavirus, often called Shope papillomavirus after the scientist who discovered it in the 1930s. This virus spreads mainly through insect bites from mosquitoes, ticks, and fleas, which are abundant in summer.

Interestingly, this is the same virus that once helped researchers uncover how some viruses can trigger cancer in humans. At the same time, historians believe this infection also served as the inspiration behind the old jackalope legend, which imagined a rabbit with antlers roaming North America’s plains.

What’s going on with the rabbits?

Once a rabbit is infected, the virus can make skin cells multiply uncontrollably. The result is hard, keratin-based lumps, similar to human warts, that sometimes stretch into horn-like spikes.

“It exhibits as these wart-like growths on their face—sometimes they’re smaller, like little nodules, and sometimes they’re bigger, like tentacles or horns,” Kara Van Hoose, a Colorado Parks and Wildlife representative, told Time.

For many wild rabbits, the condition is temporary. Their immune systems can eventually clear the infection, and once that happens, the horns fall off. Research suggests that about one in three infected rabbits recover naturally within six months.

However, not every animal is so lucky. Growths near the eyes or mouth can block vision or make eating impossible, which may lead to starvation. In a smaller number of cases, the virus can trigger squamous cell cancer, a type of tumor that can spread through the body. 

Moreover, pet rabbits tend to fare worse than wild ones. They often develop aggressive cancers if they catch the virus.

“In experimentally infected domestic rabbits, 75% developed carcinomas if kept longer than six months. Thus, regression of papillomas occurred less frequently in domestic rabbits than in cottontail rabbits,” a research paper notes. 

The connection between rabbit papillomavirus and cancer is not new. In the 1930s, Richard E. Shope, who first described this infection, showed for the first time that a virus could drive cancer-like growths. 

His work opened the door to studying human papillomaviruses (HPV), which are now known to cause cervical and several other cancers. This research work eventually led to the creation of the HPV vaccine, one of the biggest public health successes of recent decades. 

Is there any risk to humans?

Colorado’s horned rabbits have become an online sensation this summer, but wildlife officials stress that the animals pose no risk to humans or pets. The virus affects only rabbits and hares, and the growths themselves aren’t contagious. 

Still, people are advised to keep their distance. Handling wild rabbits can spread other diseases, even if the horns are harmless. However, for scientists, every outbreak is another chance to observe how the virus behaves in the wild.