The Shope Papilloma Virus (SPV), also known as Cottontail Rabbit Papillomavirus (CRPV), is a naturally occurring DNA virus that primarily affects rabbits. Discovered in the 1930s by Richard E. Shope, it was one of the first viruses recognized to cause tumors in mammals. The virus is native to the wild cottontail rabbit, which serves as its natural host. This infection is a classic example of an oncogenic virus, meaning it has the potential to induce the formation of abnormal growths.
Recognising the Signs
The most distinguishing feature of an SPV infection is the appearance of papillomas, which are wart-like growths on the skin. These lesions are raised, rough, and typically dark in color, often described as “cutaneous horns” or “rabbit horns.” The growths are composed of keratinous masses, giving them a hard, horny texture. These characteristic papillomas often appear on the head, neck, shoulders, and around the muzzle or eyelids. While initially benign, they can grow quite large, potentially interfering with the rabbit’s ability to see or eat, which can lead to secondary health problems like starvation.
How the Virus Spreads
The transmission of Shope Papilloma Virus relies heavily on biting arthropods, such as ticks and mosquitoes, which act as vectors. These insects pick up the virus from an infected rabbit and transmit it during a subsequent bite, especially in wild populations. Transmission can also occur through direct contact between rabbits, particularly if there are breaks or abrasions in the skin. The virus exhibits a difference in disease progression between its primary host and others. In wild cottontail rabbits, the papillomas typically regress spontaneously, meaning the immune system clears the infection within a few months. Conversely, in domestic rabbits, the lesions are more likely to persist. SPV is species-specific and is not a zoonotic risk, meaning it does not spread to humans, cats, or dogs.
Veterinary Assessment and Management
A veterinarian will often suspect SPV infection based on the clinical appearance of the characteristic, horn-like lesions. To confirm the diagnosis and rule out other conditions, a biopsy of the growth is typically recommended. Histopathology—the microscopic examination of the tissue—will confirm the presence of a papilloma and determine if any malignant changes have occurred. Molecular techniques, such as Polymerase Chain Reaction (PCR), can be used to identify the viral DNA, but this is less common in a standard clinical setting. Management of SPV depends on the size and location of the papillomas. Since the growths often regress spontaneously in the wild host, a benign case in a domestic rabbit may initially be managed with monitoring. Surgical removal is the treatment of choice if the growths interfere with the rabbit’s quality of life, such as obstructing vision or ability to eat. The prognosis for a benign papilloma is generally favorable, especially after surgical excision.
The Viral Link to Cancer
The Shope Papilloma Virus is a significant model in oncology because of its ability to cause malignant transformation. While the initial papillomas are benign, they can progress into aggressive, invasive tumors called squamous cell carcinomas (SCC). This progression is observed in a small percentage of wild cottontails, but the risk is significantly higher in domestic rabbits where the lesions are less likely to regress. The transformation is driven by specific viral genes, primarily E6 and E7. These viral oncoproteins interfere with the rabbit’s normal cell cycle regulation. E6 targets and degrades the tumor suppressor protein p53, while E7 inactivates the retinoblastoma protein (pRb). This cellular dysfunction allows the infected epithelial cells to proliferate indefinitely, leading to the development of an invasive carcinoma that can potentially metastasize to other organs.