FELINE NONHEALING nodular or ulcerative dermatoses can present veterinary practitioners with diagnostic and therapeutic challenges. These lesions can initially be mistaken for cat bite abscesses, but empirical abscess management consisting of drainage, cleansing, and short-term systemic antibiotic treatment routinely fails. Feline atypical mycobacterial panniculitis (AMP) (also called opportunistic mycobacterial granuloma) is one of many causes of chronic, draining, nodular skin and subcutaneous diseases in cats. Feline AMP results from infection with rapidly growing Runyon group IV mycobacteria. In the previous article, we reviewed the clinical and diagnostic features of AMP. This article focuses on the treatment options available in cats with AMP.
To initiate the best therapeutic plan, confirm an etiologic diagnosis of AMP by morphologically identifying the infecting mycobacteria in cytologic or histologic samples and by obtaining a positive, noncontaminated tissue culture. Performing antimicrobial susceptibility profiles on atypical mycobacterial isolates also facilitates successful treatment planning and implementation. Once a definitive diagnosis of AMP is established, practitioners and clients are faced with several treatment choices, including medical therapy only or a combination of medical and surgical therapies.
The Runyon group IV mycobacteria that cause AMP include Mycobacterium smegmatis, Mycobacterium phlei, Mycobacterium fortuitum, and Mycobacterium chelonae. These agents are rapid-growing, nontuberculous mycobacteria that do not respond to antitubercular drugs. Ideally, selecting an antimicrobial drug is based on individual susceptibility profiles. Antibiotic susceptibilities vary among strains of mycobacteria. They also occasionally differ within strains in different geographic regions and can be influenced by previous and concurrent antibiotic treatment.1-3 For instance, multiple studies have demonstrated that strains of M. smegmatis are usually susceptible to more antibiotics than strains of M. fortuitum are, although both of these mycobacteria are often susceptible to fluoroquinolones, gentamicin, and doxycycline.1,4 Previous publications also report that M. fortuitum is more commonly isolated from North American cats with AMP, while M. smegmatis was the most prevalent strain seen in a survey of 49 Australian cats.1,5,6 Certain pathogenic mycobacteria, most notably Mycobacterium tuberculosis, have been known to rapidly develop antibiotic resistance during treatment, and increasingly resistant strains may be an emerging feature of atypical mycobacterial infections as well.1,7
No well-established guidelines allow a clinician to predict whether a cat with AMP will have an adequate clinical response to medical therapy or will eventually require medical and surgical therapy (Figure 1). Since many authors have reported cures with single-agent antimicrobial therapy1,7,8 and appropriate antimicrobial therapy is recommended before performing most surgical procedures in cats with AMP,1,7,8 we recommend that antibiotic administration be considered the first step in the comprehensive treatment of all cats with AMP.
1. This cat was presented with a chronic lesion on the caudal ventral abdomen. The arrowheads outline the extent of the wound at presentation. The diagnosis was AMP, and the cat was treated for six weeks with appropriate antibiotics, followed by aggressive surgical excision.