Feline leukaemia virus (FeLV) belongs to the family Retroviridae, genus Gammaretrovirus, and can be transmitted both vertically and horizontally. In this latter case, the virus can be transmitted by a bite or through close contact between cats living together (which share, for example, food bowls and litters and practice reciprocal grooming) given that the virus is excreted predominantly in the saliva and secondarily in other body secretions.
FeLV causes generalised immunodeficiency and, for this reason, infected subjects can develop the same opportunistic bacterial infections (bacterial, fungal and parasitic) as those that can be found during infection by feline immunodeficiency virus (FIV). FeLV infection is also associated with two cutaneous diseases attributed to the cytopathic effect of the virus: giant cell dermatosis and cutaneous horns.
GIANT CELL DERMATOSIS
FeLV-associated giant cell dermatosis was described for the first time at the beginning of the 1990s. This dermatological disorder seems to be associated with particular viral strains and is characterized by alopecia, adherent scales, erosions and crusts first involving the head (muzzle, peri-oral region, pre-auricular region and pinna) and secondarily the trunk, limbs, foot pads and mucocutaneous junctions. Pruritus is almost always reported. In most cases the clinical picture is dominated by an exfoliative dermatitis, localised to the head and neck or generalised, and less frequently by an ulcerative dermatitis with crusting. The affected animals may have systemic signs including anorexia, lethargy, loss of weight and fever already from the first examination or show progressive worsening of their clinical condition over a period of a few weeks.
The differential diagnoses vary depending whether the clinical picture is predominantly systemic or dermatological, but include allergic dermatitides, parasitic diseases (in particular cheyletiellosis, otoacariasis and demodicosis), dermatophytoses, bacterial or viral infections, such as infections by herpesvirus or poxvirus, thymoma-associated exfoliative dermatitis, pemphigus foliaceus and other autoimmune diseases and adverse drug reactions.
The diagnosis is based on histological examination of the lesions, aided by immunohistochemistry and polymerase chain reaction (PCR) analysis, two collateral investigations that can confirm the viral aetiology of the cutaneous lesions. The main dermatopathological finding is the presence of giant, multinucleated cells in the epidermis and follicular epithelium. These cells are syncytial cells formed from the fusion of several keratinocytes induced by the virus and cellular dyskeratosis. The collateral investigations of immunohistochemistry and PCR can be carried out on the deparaffinised biopsy samples of suspicious lesions. In positive samples, immunohistochemical studies can show viral antigen (the gp70 envelope protein and the p27 group-specific determinant), whereas PCR analysis can detect sequences of proviral DNA.
Measuring the FeLV antibody titre must be included in the diagnostic protocol since the infected animals with this form of dermatitis were all FeLV-positive and some had also been vaccinated against FeLV in the past. No effective treatment has yet been reported for this form of dermatitis and the prognosis is currently poor.
CUTANEOUS HORNS
The second cutaneous disorder induced by FeLV is the formation of cutaneous horns. This keratin tumour can also be induced by papillomavirus, actinic keratosis, bowenoid carcinoma in situ and squamous cell carcinoma. The cutaneous horns induced by FeLV develop almost exclusively on the digital, central or metacarpal/metatarsal foot pads and only very rarely also on the muzzle. These tumours may be single or multiple, conical or cylindrical and of different sizes, ranging from a few millimetres to up to 2 cm. Histologically these lesions are characterized mainly by marked compact orthokeratotic hyperkeratosis and the presence of giant multinucleated cells. The principal differential diagnosis is tumours induced by papillomavirus, which can have a similar clinical and histological picture, even if true viral papillomata have never yet been described on the foot pads of cats. Confirming the true viral aetiology of the lesions is important above all for prognostic purposes and for this reason it is recommended that serological studies for FeLV should be carried out and also immunohistochemical investigations for both FeLV and papillomavirus in the lesions.
Surgical excision or laser therapy can be used to remove FeLV-induced cutaneous horns. The prognosis in these cases does, however, remain uncertain given that the possibility of recurrence of these tumours increases considerably in cases with a viral aetiology. One therapeutic option which could help to control the viral infection and, therefore, also the cutaneous symptoms, seems to be feline interferon. This has been shown to be effective at reducing clinical signs and mortality in cats infected by FeLV, or co-infected with FIV and FeLV, when given at the dose of 1 MU/kg/die in 5-day cycles repeated on days 0, 14 and 60.
There are also case reports in the literature of FeLV-negative cats with cutaneous horns for which a viral aetiology was not demonstrated. In these cases the cutaneous horns involved the digital foot pads and were located immediately beneath the nail.
Finally, the presence of FeLV was also demonstrated in a cat with cutaneous nodular lesions due to a non-epitheliotropic, T-cell cutaneous lymphoma not responding to treatment with lomustine, and serologically negative for FeLV. It was suggested that the FeLV was replicating exclusively in the skin in this case as a possible explanation for the lack of circulating antibodies.
Suggested readings
- Center SA, Scott DW, Scott FW.Multiple cutaneous horns on the footpad of a cat. Feline Practice 1982; 12:26-30.
- Favrot C, Wilhelm S, Grest P et al. Two cases of FeLV-associated dermatoses. Veterinary dermatology 2005; 16:407-12.
- Gross TL, Clark EG, Hargis AM et al. Giant cell dermatosis in FeLV-positive cats. Veterinary Dermatology 1993; 4:117-22.
- 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; 562.