Bovine Papillomavirus (BPV) is a highly prevalent double-stranded DNA virus that infects cattle worldwide, causing a condition known as bovine papillomatosis. This virus primarily stimulates the proliferation of epithelial and connective tissues, resulting in the formation of benign tumors commonly called warts or fibropapillomas. While often considered a mild disease, BPV infection can lead to significant economic losses for the livestock industry. These losses stem from the devaluation of hides due to permanent scarring and production issues, such as difficulty in milking and secondary infections.
Classification of Viral Types
The BPV group is highly diverse, encompassing at least 14 distinct genotypes, classified into genera like Deltapapillomavirus, Epsilonpapillomavirus, and Xipapillomavirus. These types exhibit a distinct preference, or tropism, for specific tissues within the bovine host. BPV-1 and BPV-2, belonging to the Deltapapillomavirus genus, primarily cause fibropapillomas, involving both the epidermal and dermal layers of the skin.
BPV-3 typically causes true epithelial papillomas that are less invasive and appear as white, smooth warts. Conversely, BPV-4 has a strong tropism for mucosal surfaces, targeting the upper gastrointestinal tract and the urinary bladder. The differences between these types dictate the clinical outcome, ranging from simple skin lesions to severe internal disease. The viral particle itself is small and non-enveloped, housing its circular double-stranded DNA genome within a protein shell.
Clinical Manifestations in Cattle
The most recognizable sign of BPV infection is the appearance of cutaneous lesions, or warts. These manifest in various forms, including small, smooth raised nodules that can progress into rough, cauliflower-like, or pedunculated growths. External fibropapillomas are most frequently observed on the head, neck, shoulders, teats, and genital areas of younger cattle. Warts located on the teats can interfere with milking, cause pain, and increase the risk of mastitis. Those on the penis can impede breeding.
The disease progression becomes more serious with internal manifestations, particularly those caused by BPV-4 in the alimentary tract, leading to papillomas in the mouth, esophagus, and rumen. These growths can cause digestive disturbances, such as chronic bloat or difficulty swallowing. A more concerning outcome involves the synergistic relationship between BPV (primarily BPV-1, BPV-2, and BPV-4) and the ingestion of bracken fern, which contains carcinogenic compounds. This combination can lead to the malignant transformation of internal papillomas into squamous cell carcinomas of the upper gastrointestinal tract and transitional cell carcinomas of the urinary bladder, a condition known as enzootic hematuria.
Routes of Transmission and Prevention
BPV is highly contagious and spreads primarily through two mechanisms: direct contact and indirect transmission via contaminated objects, known as fomites. Direct spread occurs when an uninfected animal rubs against the lesions of an infected one, allowing the virus to enter through microscopic abrasions or cuts in the skin. Indirect transmission is a significant route in farming operations, often involving shared equipment that has come into contact with viral particles.
To mitigate this spread, strict biosecurity protocols must be implemented, focusing on the thorough disinfection of all instruments used on multiple animals. Contaminated items like halters, ropes, nose leads, milking clusters, or tattooing instruments can harbor the virus for extended periods. Instruments should be cleaned to remove organic material and then immersed in a potent solution, such as 2–4% formaldehyde, to inactivate the virus. Segregating visibly infected animals from the rest of the herd, especially younger, more susceptible calves, is also important for limiting the viral load in the environment.
Treatment and Human Health Relevance
In many cases, BPV lesions are self-limiting, undergoing spontaneous regression as the animal’s immune system mounts an effective response, often within a year. Intervention is necessary for lesions that cause functional impairment, such as those that impede movement or interfere with milking.
Treatment Options
Treatment options include surgical excision, cryotherapy using liquid nitrogen, or electrocautery to remove the growths. Another veterinary approach involves the use of autogenous vaccines. These are custom-made preparations created from the homogenized tissue of the animal’s own warts, intended to stimulate a stronger immune reaction.
Human Health Relevance
BPV is considered strictly species-specific, meaning it does not pose a significant risk of infection to humans. While the virus shares structural similarities with Human Papillomavirus (HPV), BPV is confined to its bovine host. However, BPV-1 and BPV-2 can occasionally infect horses, causing equine sarcoids.