The clinical signs and diagnosis of feline atypical mycobacterial panniculitis


The clinical signs and diagnosis of feline atypical mycobacterial panniculitis

Cases of AMP are infrequent, but can be exasperating. Some affected cats have skin lesions lasting months to years, despite multiple antimicrobial courses. This article tells you when to suspect AMP and how to confirm a diagnosis.

THE POPULARITY of pet cats and their importance to our society are clearly demonstrated by the growing number of private practitioners who exclusively treat cats, the increasing availability of postgraduate feline medicine training programs offered at veterinary teaching institutions, and the formation of multi-disciplinary, nationally recognized committees and task forces charged with investigating important emerging feline diseases. One disease that has arisen from this increased interest in feline disease is feline atypical mycobacterial panniculitis (AMP).

Feline AMP (also called opportunistic mycobacterial granuloma) is an uncommon infectious disease of dermal tissues and subcutaneous fat. Although the incidence of AMP is low, the disease often presents diagnostic and therapeutic challenges and can cause severe morbidity in cats.

In general, feline mycobacterial infections can be divided into three forms: classic tuberculosis (caused by Mycobacterium bovis, Mycobacterium tuberculosis, Mycobacterium microti, and a recently described new variant species), feline leprosy (caused by Mycobacterium lepraemurium), and the atypical mycobacterioses, which cause AMP.1 Feline AMP is caused by infection with rapidly growing Runyon group IV mycobacteria. This group includes Mycobacterium smegmatis, Mycobacterium phlei, Mycobacterium fortuitum, and Mycobacterium chelonae, which are ubiquitous saprophytic organisms.1,2 Runyon group IV species share morphologic features with other mycobacteria in that they are gram-positive, acid-fast rods capable of intracellular propagation within phagocytic cells.1,3

In this article, we focus on the clinical and clinicopathologic signs associated with feline AMP and the recommended methods for diagnosis. We discuss the treatment of AMP in the next article.

Patient history and clinical signs

1. A large cat bite abscess.
Feline AMP typically causes chronic ulcerative or nodular draining subcutaneous and dermal infections. It primarily affects young to middle-aged female cats, usually without concurrent immunosuppressive diseases (e.g. feline immunodeficiency virus infection, feline leukemia virus infection).1,2,4 Feline AMP is considered an opportunistic infection that most often occurs in immunocompetent cats. It is not unusual for a cat to have a several-month to multiple-year history of a nonhealing skin wound that has demonstrated no response or an incomplete response to multiple courses of antimicrobial therapy. A history of skin trauma, usually resulting from bite or scratch wounds, is frequently incriminated as the inciting cause.1,2 Infection results from inoculation or contamination of the wounds with soil containing mycobacteria2 ; it can also occur as a result of penetrating missile and vehicular injuries. Although most affected cats initially described in the literature were from tropical and subtropical climates, group IV mycobacteria have been isolated from water and soil samples from multiple temperate regions, and investigators have reported infected cats with a wide global geographic distribution.5,6