Papillomaviruses are highly contagious, usually species-specific, DNA viruses that tend to develop in the stratum corneum of the epidermis causing its proliferation. Papillomavirus infections have been reported both in numerous feral felidae and in domestic cats. Papillomavirus in the skin does not necessarily lead to the development of cutaneous lesions; on the basis of the few cases reported in the literature, skin lesions seem to be more common in immunodepressed animals and in the Persian breed of cats. However, a recent study that investigated the presence of Papillomaviruses in the normal skin of cats positive or negative for Feline immunodeficiency virus, based on the hypothesis that the state of immunodepression induced by a retroviral infection would favour the development of Papillomaviruses and enhance their pathogenicity, did not find significant differences between the two groups.
In the cat there are reports of infections by both feline Papillomaviruses and bovine Papillomaviruses. The lesions induced by the feline Papillomaviruses are viral plaques, cutaneous papilloma and oral papilloma, while feline fibropapilloma, also called feline sarcoid, seems to be due to infection by bovine Papillomaviruses.
Viral plaques are usually flat, slightly raised lesions with an irregular surface and desquamation; they are distributed mainly on the head, neck, dorsal regions of the thorax or ventral regions of the abdomen and on the proximal extremities of the limbs. These lesions often present as pigmented macules that grow but that do not, usually, exceed 8 mm.
The diagnosis is made from histological studies and identification of Papillomavirus antigens or DNA in the lesions, depending on whether an immunohistochemical technique or polymerase chain reaction (PCR) analysis is used, respectively. These lesions can be difficult to differentiate from actinic keratosis or other lesions undergoing neoplastic transformation. It has been hypothesised that they are an initial form of Bowen’s disease or squamous cell carcinoma in situ and it has been demonstrated that 48% of cats with bowenoid carcinoma have Papillomavirus antigens even though the exact pathogenic role of the virus in these cases is unknown.
Oral papillomasare very rare and present as sessile, often multifocal neoformations which involve the ventral surface of the tongue. There is one case report of an exophytic palpebral papilloma.
Cutaneous papillomas present as horny, pedunculated structures of about 0.5 cm; they generally appear in adult cats with no particular preferential site. Cutaneous horns on the plantar foot pads can also develop in animals with papillomatosis and for this reason, in the presence of foot pad lesions, Papillomavirus infection must be differentiated from the more probable Feline leukemia virus infection. In all cases, the diagnosis is confirmed by histological examination of the lesions and identification of the virus by immunohistochemistry or PCR.
There are numerous therapeutic options available for controlling the skin lesions induced by Papillomaviruses. Although it should not be forgotten that many of these lesions undergo spontaneous regression, the potential for some to undergo neoplastic transformation in cats certainly argues in favour of active therapy.
Among the reported pharmacological treatments, imiquimod would seem to have better future prospects than retinoids or interferon. The therapeutic protocols used involve topical application on the lesions every 48-72 hours or every 24 hours for 2-3 consecutive days a week, with clinical remissions possible in many cases.
Cryosurgery or laser therapy may be the treatment of choice, particularly for multiple or very large lesions; it should, however, be appreciated that several treatment sessions are usually necessary. Surgical excision is another possible treatment, particularly for single lesions, although the possibility of recurrences following surgery cannot be excluded.
In the cat there are reports of a proliferative, fibroblastic formation, called fibropapilloma or feline sarcoid, which is induced by bovine Papillomavirus. Infected cats have one or more nodular growths, generally distributed on the muzzle (lips and nose), ears, paws and tip of the tail, and only rarely also in the oral cavity. These lesions have been observed particularly in stray cats and cats living in the countryside. Their differentiation from other spindle cell tumours can be very difficult both histologically and using immunohistochemical techniques. Surgery is one therapeutic option, although the rapid recurrence usually seen after surgical excision has led to other techniques such as cryotherapy, radiotherapy or amputation, being recommended as the treatment of choice.
A recent study showed that the topical use of imiquimod led to an improvement of 75% or more in 80% of equine sarcoid lesions. Although studies on feline sarcoid tumours still need to be carried out, this drug seems to be a reasonable option as initial therapy in cats before proceeding to more aggressive interventions if necessary.
Suggested readings
- Favrot C. Le infezioni cutanee da papillomavirus nel cane e nel gatto. Quaderni di dermatologia 2007, 2: 13- 15.
- Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Epidermal tumors. In: Skin diseases of the dog and cat. Clinical and histopathologic diagnosis, 2nd ed. Blackwell Science, 2005; 574-5.
- Medleau L, Hnilica KA. Viral, Rickettsial, and protozoal skin diseases. In: Small Animal Dermatology. A color atlas and therapeutic guide. 2nd ed. Saunders, 2006; 141-3.
- Muday JS, Willis KA, Kiupel M et al. Amplification of three different papillomaviral DNA sequences in a cat with viral plaques. Veterinary Dermatology 2008, 19: 400-404.
- Munday JS, Witham AI. Frequent detection of papillomavirus DNA in clinically normal skin of cats infected and noninfected with feline immunodeficiency virus. Veterinary Dermatology 2010, 21:307-10.