Wildlife officials in Colorado say the cluster of cottontail rabbits seen around Fort Collins with black, horn- or tentacle-like growths on their heads is the result of a long-known infection, not a new mutation. Colorado Parks and Wildlife spokesperson Kara Van Hoose said the animals are carrying Shope papillomavirus, also known as cottontail rabbit papillomavirus, which causes keratinised wart-like tumours that can elongate into spines. The virus spreads only between rabbits, typically via biting insects such as fleas and ticks, and is not known to infect humans, dogs or other wildlife. In most cases the growths are benign and regress once the rabbit’s immune system clears the infection; they become life-threatening only if they obstruct the animal’s eyes or mouth. There is no treatment, so authorities advise residents to keep their distance and prevent pets from interacting with the animals. First described in the 1930s by virologist Richard E. Shope, the pathogen helped scientists establish links between viruses and cancer and is thought to have inspired North American folklore about horned “jackalopes.” Colorado Parks and Wildlife says sightings rise in summer when insect vectors peak, but the agency sees no need for large-scale intervention beyond routine public cautions against handling wildlife.