Colorado wildlife officials say a rash of wild rabbits seen around Fort Collins with black, horn-like growths on their heads is the result of an infection known as cottontail rabbit papillomavirus, or Shope papilloma virus. The wart-forming virus causes keratinised nodules that can elongate into tentacle-shaped projections but generally does not harm the animals unless the lesions block their eyes or mouths. The virus is spread among rabbits—both wild and domestic—primarily by biting insects such as fleas and ticks, according to Colorado Parks and Wildlife. It is species-specific and cannot be transmitted to humans, dogs or other pets, the agency said. Outbreaks are most common in the warmer months when insect activity peaks. While there is no treatment or vaccine, most infected wild rabbits survive and eventually shed the growths. CPW advises residents and pet owners not to handle or feed affected animals and to keep domestic rabbits indoors or screened from contact with wildlife. Euthanasia is recommended only in rare cases where the lesions prevent an animal from eating or drinking. The virus, first identified in the 1930s, periodically surfaces in the Midwest and Mountain West but typically poses no broader ecological or public-health threat. CPW is monitoring the situation and has not announced additional control measures.