Shope papilloma is a viral disease that primarily affects rabbits, characterized by the growth of distinctive skin tumors. These growths often resemble horns, typically appearing on or around the animal’s head. It is a naturally occurring disease in wild rabbit populations.
The Virus and Its Hosts
The virus responsible for Shope papilloma is known as Cottontail Rabbit Papillomavirus, abbreviated as CRPV, or sometimes referred to as Kappapapillomavirus 2. This DNA virus belongs to the Papillomaviridae family. CRPV predominantly infects wild rabbit species, particularly the Eastern cottontail rabbit (Sylvilagus floridanus), which is its major natural host in the western United States.
The virus also infects other leporids, including brush rabbits, black-tailed jackrabbits, snowshoe hares, European rabbits, and domestic rabbits. When infected, a rabbit’s follicular cells, often in areas like the ears, nose, eyelids, and anus, develop papillomas on hair-bearing skin. These growths develop into circular warts and eventually horn-like structures.
The warts are composed of densely packed, vertical tissue strands. Their outer surface often appears black or grayish, with the dark coloration due to melanin pigment within the growths. These tumors can grow large enough to interfere with the rabbit’s ability to eat, potentially leading to starvation.
Transmission and Disease Progression
Shope papilloma virus is transmitted among rabbits primarily through the bites of arthropod vectors, such as mosquitoes and ticks. Outbreaks of the disease are more commonly observed during summer and fall months, when insect populations are higher. Direct contact or contaminated environments may also contribute to the spread of the virus.
After infection, the disease begins with the appearance of raised lesions. These lesions are frequently found on the upper half of the animal’s body, including the neck, shoulders, eyelids, and ears. The infection progresses from initial papillomas to larger, more developed warts.
While the tumors themselves do not contain the infectious virus, transmission between rabbits remains high. In some instances, the benign papillomas can undergo malignant transformation, progressing into squamous cell carcinomas. Approximately 25% of papilloma infections in cottontail rabbits can become malignant, and this progression to metastasizing carcinomas has been observed in up to 75% of infected domestic rabbits. These cancerous growths can spread to the lungs and lymph nodes, and in advanced stages, may develop in the kidneys and liver.
Significance in Medical Research
The study of Shope papilloma has had a significant impact on understanding viruses and their connection to cancer. Richard E. Shope’s investigation of these horned growths in 1933 led to the discovery of the virus, marking it as the first mammalian tumor virus identified. This provided the first mammalian model of a virus-induced cancer.
Researchers were able to isolate virus particles from tumors in wild rabbits and use them to induce similar growths in domestic rabbits. This demonstrated that a filterable infectious agent could cause transmissible tumor-like diseases. The Shope papillomavirus genome was sequenced in 1984, revealing significant similarities to Human Papillomavirus type 1a (HPV1a).
The rabbit papillomavirus model became a valuable tool for studying viral-host interactions and understanding how papillomaviruses cause disease. Its study laid groundwork for the later discovery of Human Papillomaviruses (HPV) and their role in human cancers, particularly cervical cancer. The insights gained from Shope papilloma research contributed to the development of prophylactic virus-like particle vaccines against HPV, demonstrating its lasting influence on human medicine.