Mycobacterial diseases affecting the skin or subcutis of cats and dogs

Abstract

Mycobacteria are gram‐positive bacterial rods that have as their distinguishing feature a cell wall rich in mycolic acids and esters. This waxy layer imparts two important features of the genus: an acid‐fast staining characteristic, and the ability to survive and replicate within phagocytic cells. In the history of microbiology, infections caused by mycobacteria feature prominently as two important diseases of antiquity, tuberculosis and leprosy. In veterinary medicine, most attention has been paid to tuberculosis and Johne's diseases (paratuberculosis). The best way to understand mycobacterial syndromes is to develop a conceptual framework based on the host‐pathogen relationship. Thus, the mycobacterioses can be divided into: (1) diseases caused by obligate mammalian parasites; (2) localized infections of immunocompetent hosts; and (3) disseminated infections in hosts with defective cell‐mediated immunity. Tuberculosis is the prototypic disease caused by mycobacteria that are so adapted that they cannot survive without a mammalian host, i.e. they represent true bacterial parasites. The Mycobacterium tuberculosis complex (M. tuberculosis, M. bovis, M. africanum and M. microti) are primary pathogens capable of producing contagious disease in immunocompetent hosts, although they can also produce devastating disease in immunodeficient hosts. It is important to remember that although the tubercle bacillus can produce florid clinical signs, localized inapparent infections that are constrained and eventually eliminated by a competent immunological response are much more common than cases with symptomatic disease. An atypical form of tuberculosis attributable to M. microti is seen with some frequency in cats in the UK as a result of cats being bitten by a vole (field mouse), an environmental reservoir. Feline leprosy attributable to M. lepraemurium is probably best considered a mycobacterial disease caused by an obligate mammalian parasite, the murine leprosy bacillus, inoculated subcutaneously following bite injuries from infected rats. Most mycobacterial diseases in cats and dogs fall into the second category of localized infections of immunocompetent hosts. The majority of these infections result from inoculation of environmental mycobacterial species through a breach in the epidermis. Typically, this occurs following a penetrating injury in which there is subsequent contamination of the wound with soil or dirt, although the insult may be as subtle as an insect bite. Rapidly growing mycobacteria like M. fortuitum or M. smegmatis have a predilection for producing infections in fatty tissues, such as the subcutis of cats and dogs or the mammary glands of cattle. These infections can, with time, become deep‐seated and extensive. Treatment typically involves long courses of antimicrobials chosen on the basis of culture and susceptibility data, although some cases require en bloc resection of infected tissues and reconstruction of the surgical wound using advancement flaps. Occasionally, M. avium and other slow‐growing saprophytic species can also give rise to localized cutaneous disease, typically following a cat‐scratch injury. Leproid granulomas are localised infections of short‐coated dog breeds that probably result from inoculation of a fastidious saprophytic mycobacterial species into the subcutis following fly bites. The majority of these infections resolve spontaneously as a result of an effective host response, although rare cases require systemic or topical therapy. The least common form of mycobacterial disease occurs in patients with immunologic compromise. In these patients, involvement of the skin is the most obvious manifestation of a widespread disseminated process. Surprisingly, these cases may respond favourably if treated in a timely manner with appropriate combinations of antimicrobials, e.g. clarithromycin and rifampicin, and possibly an additional agent.