Residents of Fort Collins, Colorado, have been startled by an unusual sight: rabbits with black, tentacle-like growths sprouting from their heads and faces. These disturbing protrusions have earned the infected animals the nickname “zombie rabbits,” and has some wondering if they represent a risk to humans.
What is Causing the Bizarre Growths?
Fortunately, a zombie apocalypse has not fallen on the rabbit population in Colorado. Instead, the rabbits are infected with Shope papillomavirus, which is also known as cottontail rabbit papillomavirus (CPRV). This virus was first discovered in the 1930s by researcher Richard Shope, who observed growths – often described as horns or tentacles – on wild rabbits. The virus is now known to be a DNA virus that is genetically related to human papillomavirus (HPV), which can cause benign warts or certain types of cancer in humans. Shope virus is spread primarily through the bite of an arthropod, such as ticks and mosquitoes, and can lead to the development of wart-like tumors on an infected animal’s head, face and around the mouth.
Are These Rabbits a Threat to Humans?
Despite their alarming appearance, these infected rabbits pose no threat to humans or their pets. Colorado Parks & Wildlife has indicated that Shope virus is not contagious to humans, dogs and cats. The virus is species-specific, meaning it can infect rabbits but it cannot crossover to infect humans, dogs, cats or most other animals. Wildlife and public health officials generally recommend not touching or handling visibly infected or sick wild animals; however, Shope virus itself represents no direct health risks to people or their pets.
What Happens to the Infected Rabbits?
Other than disqualifying the rabbits from a beauty contest, the growths that result from the infection are generally not fatal. If the growths become large enough, they can interfere with the rabbits’ vision or their ability to eat and drink. Most of the tumors are benign, but in some cases, they can progress to malignant carcinomas (i.e., cancer). Most infected rabbits will survive their condition, but quality of life may be impacted depending on the size and location of the growths.
Shope Virus’ Connection to Humans and Cancer Research
Although Shope papillomavirus is not able to cause disease in humans, its discovery has contributed to medical research and our understanding of human papillomavirus (HPV) and its relationship to cancer, specifically cervical cancer. Most humans are infected with an HPV virus at some point in their life, which can lead to the development of benign genital or skin warts, or in some cases, cancer of the cervix, head or neck. Human papillomaviruses integrate into the host cell’s DNA and disrupt normal cell death pathways, which can result in overgrowth of cells and subsequent warts or tumors.
The similarities between Shope virus and HPV are quite striking. Both are DNA viruses that can cause benign warts that may progress to malignant tumors. In 1984, the genome of Shope virus was sequenced, which revealed significant homology with HPV type 1a. Because of these similarities, Shope virus and the cottontail rabbit provided the first animal model to study viral-induced cancers, as well as their treatment and prevention. The identification of Shope virus laid the foundation for the subsequent discovery of HPV, and has contributed to research that ultimately led to development of vaccines against HPV.
So if you happen to see a “Zombie Rabbit” in the wild, remember there’s no reason for concern. Instead, you can reflect on the lasting impact these animals and the virus causing their condition have had on medical research and human health.