What Is Causing the “Horns” on Rabbits in Colorado?

Unsettling photographs of wild cottontail rabbits near Fort Collins, Colorado, sporting hornlike or tentacle-like tumors on the face and head surfaced in news reports in August 2025. Although these images caused some shock and confusion, the tumors are the product of a relatively common viral disease in rabbits spread by mosquitoes and ticks, according to researchers and wildlife officials. The virus—called cottontail rabbit papillomavirus (CRPV), Shope papillomavirus, or Shope virus—was first identified by American physician Richard Edwin Shope in 1933. CRPV affects cottontail rabbits (genus Sylvilagus) and European rabbits (Oryctolagus cuniculus and its domesticated form, O. cuniculus domesticus), presenting as lesions or hornlike growths.

Although some of these tumors can become malignant, most are benign. However, even benign tumors can threaten the rabbit’s life by growing so large that they prevent the rabbit from seeing or eating. CRPV is reminiscent of a condition found in Tasmanian devils called devil facial tumor disease, which also causes large growths around the head and mouth that can interfere with the animal’s ability to eat.

There are treatments for CRPV that can eliminate the lesions and reduce the chances of their recurrence. Some of these involve injecting antiviral drugs directly into the growth or into areas of healthy skin, and others make use of topical medications.