Etiology

• Canine papillomavirus type 1 Papillomavirus.

Predisposing factors

• Exposure to dogs with oral papillomas may predispose to disease, although this is usually only observed in larger dog colonies.
• Dogs that have recovered from oral papillomas are protected from further papilloma development and this disease is seen in young dogs that have not been previously exposed to the papillomavirus.
• Immunosuppression will not usually predispose to disease development but may allow the development of a greater number and size of the lesions and may delay spontaneous regression.

Pathophysiology

• Microtrauma in the oral cavity allows canine papillomavirus type 1 access to the basal cells of the epithelium. Papillomaviruses have a tropism for squamous epithelial tissues.
• The papillomavirus infects basal cells and, as part of a productive infection, stimulates massive hyperplasia of keratinocytes.
• This hyperplasia causes marked thickening of the epidermis.
• The thickened epidermis folds resulting in an exophytic lesion within the mouth.
• The characteristic 'cauliflower' appearance of the papillomas is caused by the irregular folding of the hyperplastic epithelium.
• Early in the infection, the dog produces antibodies against canine papillomavirus type 1. Although these antibodies prevent additional papilloma development, they do not cause papilloma regression.
• Papilloma regression is dependent on a cell-mediated immune response which develops after a variable amount of time.
• Once the cell-mediated immune response has been initiated, papilloma regression is rapid.
• The dog is subsequently protected from papilloma development, although some evidence suggests a high proportion of dogs continue to have sub-clinical infection by canine papillomavirus type 1.

Timecourse

• Experimental infection reveals the incubation period is 4-8 weeks.
• The typical time to regression of the papillomas is also 4-8 weeks.
• However, regression in some cases can take as long as 6 months.
• In humans, around a quarter of cutaneous viral papillomas take over a year before spontaneous regression and even when persistence of the papillomas is prolonged, eventual regression should still be expected.

Epidemiology

• The epidemiology of infection is poorly understood.
• Currently it is believed that both dogs with oral papillomas and those without visible papillomas can shed infectious virus particles.
• The rate of shedding of papillomavirus from a dog with visible papillomas is much higher than a dog without oral papillomas.