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A Practical Guide to Feline Dermatology

Other topical antimicrobial agents, such as chlorhexidine used in a 3% shampoo or a 0.5% lotion are also beneficial

Synthetic retinoids can also be used ¹³-¹⁵. Topical tretinoin (0.01-0.05%), given once daily until resolution, is sometimes irritant Orał isotretinoin (2-5 mg/kg SID) can be given but side effects (increased pruritus, dry mucous membranes, conjunctivitis, and raised liver enzyme levels) have been reported in cats given systemie synthetic retinoids¹⁵.

Atypical mycobacterial infections

Aetiopathogenesis

Atypical mycobacterial infections are rare ubiąuitous infections, caused by facultative pathogenic bacteria of the genus Mycobacterium, excluding those causing tuberculosis and feline leprosy. The main organisms involved are Mycobacterium chelonei, Mycobacterium fortuitum, Mycobacterium phlei, Mycobacterium smegmatis and Mycobacterium thermoresistible, all of which live in the soil^li(h,8. Entry into the organism often occurs via contamination of bites, scratches and other wounds by these bacteria. Retrovirus infections could predispose to the development of these atypical mycobacterial infections. Their pathogenicity seems to be increased in the presence of lipids, which could perhaps explain the very high incidence in cats of lesions on the abdomen \

Clinical features

Dermatological signs are characterised initially by alopecic, erythematous, firm, slow-growing nodules. Within several weeks to several months, these develop into subcutaneous abscesses. They are sometimes painful (pyogranulomatous panniculitis), uleerated or accompanied by subcutaneous draining tracts (Figs 6 : 5, 6). When draining tracts are present, attempts to find a foreign body by surgical exploration lead to wound breakdown and recurrence. Main areas affected are the abdomen and inguinal regions, although other parts of the body may also be involved³- ^1<M8.

Systemie signs (fever and anorexia) are rarely seen.

Diagnosis

The diagnosis is based on lesion distribution, suggestive or even diagnostic, skin biopsies and possibly identification of the mycobacteria responsible.

Cytology of nodular contents (fine needle aspirate) and of pus reveals a pyogranuloma. Acid-fast bacilli can sometimes be seen within neutrophils when preparations are stained with Ziehl-Neelsen.

Histopathological examination of skin biopsies, taken from nodules or draining tracts, is essential. Excisional biopsies are preferable to punch biopsies because the biopsy must go deep enough to include the deep dermis and panniculus. Histopathology is characterised by nodular dermatitis or diffuse and/or pyogranulomatous panniculitis (Fig. 6 : 7). Granulomas, madę up of neutrophils, surround visually empty vacuoles (lipid vacuoles) in which acid-fast bacilli may be seen with the use of special stains (e.g. modified Ziehl-Neelsen or Fite-Faraco)³ (Fig. 6 : 8).

Bacterial culture for atypical mycobacteria needs to be carried out on skin biopsies taken under aseptic conditions. Epidermis and dermis should be dissected away. The deep dermis and panniculus should be preserved in sterile gauze soaked in sterile water before sending to the laboratory. Culture is performed on special media (blood agar, Lowenstein-Jensen medium or Stonebrink medium) at 37°C. Cultures grow rapidly but species identification is often difficult³¹⁷.

Molecular biological techniques (e.g. amplification) can now be used to identify mycobacteria.

A methodical search for an underlying illness (e.g. retroviral infection) should be routine.

Prognosis - Treatment

The prognosis must be guarded, given the freąuency of recurrence, but may now be improved thanks to the use of long-term specific antibiotic therapy.

Treatment is based on surgical excision associated with prolonged antibiotic therapy. Many

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