Objective To obtain a better understanding of a disease affecting dogs in which nodular mycobacterial granulomas are present in the subcutis or skin.
Design Retrospective survey
Procedure A diagnosis of an unidentified mycobacterial infection was made in 45 dogs following detection of acid-fast bacilli surrounded by granulomatous or pyogranulomatous inflammation in tissue sections. Eight cases were identified from the records of our diagnostic laboratory. In all cases culture for mycobacteria was unsuccessful. Another 37 cases were identified by practitioners and veterinary pathologists in response to a survey mailed to veterinary clinics throughout Australia. Data from these 45 cases were entered into a data base. The data set was incomplete, as some respondents did not answer all questions.
Results Over 90% of affected dogs had short coats and about half were Boxers or Boxer-cross dogs. A weak seasonal trend for the development of primary lesions in autumn and winter was identified. Cases were recorded from New South Wales (35 dogs), Western Australia (4), Queensland (4), Tasmania (1) and New Zealand (1). The subcutis and skin of the ears and head (including the ears) were involved in 64% and 85% of cases, respectively, for which lesion site was recorded. Culture was attempted but was unsuccessful in 19 cases, including 11 cases where material was submitted to our laboratory and/or a Mycobacteria Reference Laboratory. Lymph node enlargement, internal organ involvement or constitutional signs were not a feature of the syndrome, and most lesions did not worry the dogs. Although many practitioners recorded a favourable response to therapy with doxycycline (response rate 57%) or amoxycillin-clavulanate (63%), spontaneous resolution of infection was thought to have occurred in six of seven dogs (86%) not given systemic antimicrobials, and three dogs where antimicrobials had failed previously. A minority of cases failed to respond to antimicrobial therapy and continued to have chronic lesions. There was no discernible trend for dogs of a particular age or sex to be affected.
Conclusion This syndrome is caused by saprophytic mycobacteria of limited pathogenicity that give rise to lesions restricted principally but not exclusively to the subcutis and skin of body extremities. Fastidious growth requirements have prevented their isolation on synthetic media used for culture of mycobacteria. Organisms presumably enter the subcutis following a breach in integrity of the epidermal barrier and produce self-limiting disease in immunocompetent dogs. Lesions tend to resolve spontaneously. The possibility of a public health threat from affected dogs is highly unlikely.