The strange virus that is making Colorado rabbits grow horns and tentacles

Bizarre images of wild rabbits in Colorado, with black, horn-like nodules sprouting from their heads have been widely shared on social media in recent weeks.

The stumpy growths are caused by Shope Papilloma Virus (SPV) which affects certain wild rabbit species (cases in domestic rabbits are rare). The horn-like tentacles called papillomas can appear on various parts of the rabbit’s body, including the head, neck, and legs - and sometimes can restrict or prevent eating.

The virus affects wild California brush rabbits and the various species of cottontails - abundant in Colorado’s diverse habitats. It is mechanically transmitted by insect bites, usually by ticks but also by any biting arthropod (arachnids, millipedes, centipedes).

What are the black growths, are they cancerous?

The dark coloration of the nodules is due to abundant melanin pigment. Once infected, rabbits can transmit the virus to other rabbits. The tumors are benign and can be manually or surgically removed, but in some species, including domestic rabbits, removal may lead to malignant transformation to squamous cell carcinoma - ie. becoming cancerous.

SPV was first discovered by American virologist Dr. Richard E. Shope in 1933. The SPV tumor in rabbits was the first with malignant potentialities in a mammal to be proven to have been caused by a virus.

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