Feline leprosy due to Mycobacterium lepraemurium.

Objectives: This paper, the second in a series of three on 'feline leprosy', provides a detailed description of disease referable to Mycobacterium lepraemurium, the most common cause of feline leprosy worldwide.
Methods: Cases were sourced retrospectively and prospectively for this observational study, describing clinical, geographical and molecular microbiological data for cats definitively diagnosed with M lepraemurium infection.
Results: A total of 145 cases of feline leprosy were scrutinised; 114 'new' cases were sourced from the Victorian Infectious Diseases Reference Laboratory records, veterinary pathology laboratories or veterinarians, and 31 cases were derived from six published studies. Sixty-five cats were definitively diagnosed with M lepraemurium infection. Typically, cats were 1-3 years of age when first infected, with a male gender predilection. Affected cats were generally systemically well. All had outdoor access. Lesions tended to consist of one or more cutaneous/subcutaneous nodules, typically located on the head and/or forelimbs, possibly reflecting the most likely locations for a rodent bite as the site of inoculation for organisms. Nodules had the propensity to ulcerate at some stage in the clinical course. The cytological and histological picture varied from tuberculoid, with relatively low bacterial numbers, to lepromatous with moderate to high bacterial numbers. Treatment was varied, although most cats underwent surgical resection of lesions with adjunctive medical therapy, most often using a combination of oral clarithromycin and rifampicin. Prognosis for recovery was generally good, and in two cases there was spontaneous remission without the requirement for medical intervention. Untreated cats continued to enjoy an acceptable quality of life despite persistence of the disease, which extended locally but had no apparent tendency to disseminate to internal organs.
Conclusions and Relevance: M lepraemurium causes high bacterial index (lepromatous) or low bacterial index (tuberculoid) feline leprosy. The infection typically causes nodules of the skin and/or subcutis (which tend towards ulceration) on the head and/or forelimbs. The disease usually has an indolent clinical course and infected cats have a generally favourable response to therapeutic interventions, with rare cases undergoing spontaneous resolution. Genomic analysis may yield clues as to the environmental niche and culture requirements of this elusive organism. Prospective treatment trials and/or additional drug susceptibility testing in specialised systems would further inform treatment recommendations.