Cats are susceptible to a number of viral infections, some of which may cause cutaneous and/or mucocutaneous lesions as part of their inherent cytopathic effects, or may render the animal susceptible to other dermatological diseases due their effects on the immune status of the patient.

Feline Herpesvirus

Infection with feline herpesvirus 1 classically causes rhinotracheitis, but cats may also develop oral and/or cutaneous lesions. An ulcerative and necrotising facial and nasal dermatitis or stomatitis may occur with or without active respiratory or ocular signs or history of previous upper respiratory tract disease. Stress or steroid administration can precipitate cutaneous lesions, as well as the more classical upper respiratory signs, in carrier animals.

Biopsy specimens reveal ulceration and necrosis with mixed inflammatory cell infiltrate, including eosinophils, and basophilic intranuclear inclusions may be seen in keratinocytes. Multinucleated keratinocytic giant cells can be seen in the superficial and follicular epithelium and necrosis of epitrichial sweat glands. Herpesvirus may be cultured from the affected skin or demonstrated by immuno histochemistry or electron microscopy.

Supportive care and antibiotic cover may allow resolution of lesions. Agents that have been used include lysine, alpha-interferon and aciclovir. A recent report of an open and uncontrolled study documented the use of famciclovir in naturally occurring cases of herpesvirus infection with skin and ocular lesions. Improvement was seen in 4 weeks in all cats, with complete remission in seven of nine cats with no adverse effects noted.

Exfoliative erythema multiforme has been documented in cats following acute upper respiratory tract infections, which resolved after several weeks.

Feline Calicivirus

Oral ulceration is much more common in calici-virus infection than rhinotracheitis infection. There are reports of occasional cases of skin lesions affecting the feet or perineum in association with calicivirus infection, characterised by swelling, tenderness and ulceration.

Orthopox (Feline Cowpox) Virus

Lesions caused by orthopoxvirus are seen sporadically in hunting cats. The natural reservoir of the virus is small wild mammals, and cats become infected via wounds and also by the oronasal route. Many cases occur in the late summer and autumn, coinciding with peak numbers of small mammals. Cat-to-cat transmission and transmission to other species can occur.

The primary lesion is typically at the site of a bite, on the head, neck or forelimb. This is followed by a viraemic phase when pyrexia, anorexia and depression may be seen. Secondary lesions appear 10-14 days later and may be extensive and generalised; macules progress to ulcerated papular or nodular lesions with central crust. Oral lesions are seen in up to 20% of cases. If animals are immunocompromised or receive glucocorticoids a fatal necrotising pneumonia may ensue. In otherwise healthy cats lesions heal slowly over 3-4 weeks.

The disease must be differentiated from bacterial and fungal infections, eosinophilic granuloma complex and tumours. The diagnosis confirmed by typical histopathology: hyperplasia of epidermis; ballooning degeneration of epidermal cells; intracytoplasmic eosinophilic inclusion bodies; necrosis; virus isolation; and serology at appropriate laboratories.

There is no specific treatment. Supportive therapy including antibiotics is indicated as appropriate; euthanasia may be necessary in severely affected animals. Glucocorticoids are contraindicated. Infection of humans is uncommon, but can be serious in immunocompromised patients and owners should always be warned about zoonotic potential.

Feline Infectious Peritonitis

Cutaneous lesions have been documented rarely in cats with coronavirus infection, consisting of ulcerative lesions around the head and neck due to a superficial vasculitis, with viral antigen demonstrable in the vessel walls. Early lesions show a hydropic interface reaction and vasculitis, progressing to necrosis and ulceration. The prognosis is grave.

Feline Leukaemia Virus

Infection with this retrovirus can give rise to chronic oral and cutaneous infections associated with immunosuppressive effects of infection, poor wound healing, seborrhoea, exfoliative dermatitis, generalised pruritus, cutaneous horns (positive gp70 staining) and neoplasia.

Series of cats have been described with pruritic crusting dermatosis involving the face; other junctional areas were also commonly involved. Definitive diagnosis consists of demonstrating positive feline leukaemia virus (FeLV) serology and histological demonstration of viral association in skin biopsy specimens: irregular hyperplasia; syncytial-type giant cell formation in epidermis and outer root sheath of hair follicles; positive gp70 staining.

Whilst most cases of lymphosarcoma are FeLV-associated, most cases of feline cutaneous lymphoma are FeLV negative, although FeLV antigen may be demonstrable by immunohistochemical or polymerase chain reaction (PCR) techniques. Lesions in non-epitheliotropic lymphoma are usually multiple and generalised, with variable clinical appearance and usually in association with systemic involvement. Epitheliotropic lymphoma is uncommon in cats, usually of T-cell origin, and whilst affected animals are serologically negative for FeLV there are reports of FeLV-provirus amplification from tissue by PCR, suggesting it may be involved in the aetiology.

Multiple cutaneous horns on the footpads can be seen in FeLV-infected cats, occasionally on the face. Histologically there is extensive, compact, laminated hyperkeratosis, but the base of the lesions is characterised by parakeratosis, apoptosis and multinucleate keratinocytic giant cells. Lesions arising under the claws may be seen on FeLV-negative cats and have different histological features.

There are occasional reports of association with development of liposarcoma in experimental FeLV infection.

Demodectic mange is uncommon in cats and generalised demodicosis is seen in immuno-compromised animals, associated with both FeLV and feline immunodeficiency virus (FIV) infection, often complicated by staphylococcal infection. Good control of clinical lesions can be achieved, with treatment of secondary infection and use of miticidal topical therapy such as lime sulphur or dilute amitraz. Systemic endectocides have also been used, but toxicity is a risk due to slow metabolism and long half-life of drugs.

Sarcoptic mange is rarely reported in the cat, but has been seen in immunosuppressed animals in association with FeLV infection. Lesions are variably pruritic pinnal and facial papules, sometimes severe pododermatitis, self-inflicted damage, crusting and scaling. Topical endectocide therapy is the treatment of choice in such cases.

Feline Immunodeficiency Virus

Infection with FIV and resultant immunosuppression can give rise to chronic recurrent oral and cutaneous infections, and increased susceptibility to infection with Cryptococcus neoformans, Candida albicans, Microsporum canis and demodicosis.

A generalised skin disorder has been reported in a number of cats characterised by generalised papulocrustous eruptions with alopecia and scaling, most severe on the head and limbs. Histological features are hydropic interface dermatitis with occasional giant keratinocytes and peculiar pallor of basal epidermal cells. The diagnosis is supported by positive serology

Co-infection may occur with FeLV may occur.

Feline Papillomavirus Infection

Papillomata are occasionally seen in cats, which may be virally induced. At least two types of feline papillomavirus have been identified. In young cats lesions can be seen on the tongue, usually the ventral aspect. Solitary cutaneous papillomata on adult cats are rare and not known to be virus-associated. Multiple viral papillomata are recognised in middle-aged to older cats. Lesions begin as pigmented macules anywhere on the body, mainly on the trunk, progressing to variably sized, multiple hyperkeratotic plaques. Surface and follicular epithelial hyperplasia and dysplasia are seen with intracytoplasmic inclusions and papillomavirus-like particles demonstrated by electron microscopy (EM). Papillomavirus antigen may be demonstrated by immunohistochemistry.

Cases of multicentric squamous cell carcinoma in situ (Bowen's disease) also show epithelial hyperplasia and dysplasia, with more cellular atypia; in many cases intranuclear papillomavirus antigen can be demonstrated. It is suggested that feline papillomavirus may induce long-lasting dysplastic lesions that eventually become neoplastic. Demodex cati may be found in the lesional areas, but not unaffected tissue, often in association with concurrent FIV infection.

Surgical removal is not an option if there are multiple lesions, but there are reports of effective treatment of early lesions with beta-radiation therapy in lesions <4 mm thick and also anecdotal reports of the efficacy of alpha-interferon. Other treatment options include topical 5-fluorouracil and oral acitretin (3 mg/kg/day)

Feline Sarcoma Virus

In young cats feline sarcoma virus (FeSV) causes multiple cutaneous fibrosarcomas. Cats with FeSV-induced fibrosarcomas are FeLV positive. C-type virus particles have also been found in a case of subcutaneous haemangioma.