In parts of the United States, wild rabbits have been spotted carrying an unsettling feature: black, horn-like projections sprouting from their heads. Residents who encounter them call them “Frankenstein” rabbits, but biologists know them by another name: eastern cottontails infected with Shope papillomavirus.
The virus, first identified in the 1930s by virologist Richard Shope, causes skin tumors made of keratin, the same material as hair and nails. These growths, often on the face or neck, can be small or several inches long, twisting into shapes that have fueled centuries-old legends of horned hares. Scientists believe such sightings helped give rise to the American jackalope myth.
A virus with a strange history
Shope’s discovery was groundbreaking, revealing the first virus proven to cause cancer in mammals. It became a model for studying papillomaviruses, later informing the fight against human cancers caused by HPV. While its scientific legacy is profound, the virus remains a part of the landscape for wild rabbits, transmitted mainly by biting insects like fleas and mosquitoes during warmer months.
For most wild cottontails, the growths are temporary. The immune system often clears the infection within months, and the warts dry up and fall off. But when tumors develop near the mouth or eyes, they can make it hard for the rabbit to eat or see, sometimes leading to starvation. A smaller number progress to squamous cell carcinoma, a malignant form that can spread internally.
From folklore to Facebook feeds
Viral photographs from Colorado this summer reignited public fascination. One image showed a rabbit with a crown of black spines, earning it nicknames like “zombie bunny” and “demon rabbit” online. Wildlife officials stressed there’s nothing new about the condition. “It’s not an unusual sight,” said Kara Van Hoose of Colorado Parks and Wildlife, noting they receive calls about such cases almost every summer.
The phenomenon has been documented well beyond Colorado, from the Midwest to the Southwest. Despite its unsettling look, the virus doesn’t affect humans or other animals. Wildlife agencies advise leaving infected rabbits alone, both for their safety and to avoid unnecessary stress.
For pet owners, though, the stakes are higher — domestic rabbits are more likely to develop invasive cancers from the virus, and veterinarians recommend surgical removal of any growths.
Coexisting with a strange piece of nature
There’s no cure for Shope papillomavirus in wild populations, and no need for eradication. The virus has likely coexisted with North American rabbits for centuries, flaring in individuals but rarely threatening whole populations.
The horned silhouettes hopping through fields are a reminder of how strange and interconnected nature can be, linking backyard wildlife to Nobel-worthy science and campfire folklore.
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