Causes

The route of inoculation is unknown, but biting flies, midges, mosquitoes, or other arthropods may introduce mycobacteria into the host based on the following evidence: the presence of potential vectors around affected dogs, lesions occurring at sites favored by such vectors (e.g., the dorsal fold of the ears), and the prevalence of CLG in short-coated breeds and dogs housed outdoors (Malik et al, 1998).

Signalment and Clinical Findings

CLG occurs almost exclusively in short-coated breeds, with boxers and boxer crosses remarkably overrepresented. Staffordshire bull terriers and Doberman pinschers also are commonly affected (Malik et al, 2001). The finding of multiple characteristic lesions in typical locations in a short-coated breed suggests the diagnosis of CLG.

CLG usually is manifest as single or multiple firm, granulomatous to pyogranulomatous, well-circumscribed nodules in the skin or subcutis (Figure 104-1). Nodules typically are located on the head, particularly the dorsal fold of the ears. Some nodules ulcerate and are usually painless, but may be pruritic when secondary infection with Staphylococcus pseudintermedius occurs. Affected dogs are otherwise healthy.

Diagnosis

The differential diagnoses include infectious, inflammatory, and neoplastic diseases of the subcutis and skin. When samples are collected, the skin surface should be disinfected first because environmental mycobacteria may be present on the epidermis and can cause erroneous results in both culture and polymerase chain reaction (PCR) studies.

On cytologic examination, mycobacteria appear as bacilli that do not stain with Romanowsky-type stains (e.g., Diff-Quik) but do stain with a modified acid-fast procedure such as the Ziehl-Neelsen method, and thus are so-called acid-fast bacilli (AFB). Organisms appear in variable numbers either within the macrophages or giant cells, or extracellularly. Absence of visible AFB organisms on cytologic examination does not rule out CLG.

Histopathologic analysis reveals pyogranulomatous dermatitis with AFB. The bacteria are highly variable in appearance, ranging from long, slender filaments to short, variably beaded bacilli to coccoid forms. Sections may require a lengthy search, even by experienced pathologists, to locate foci in which AFB organisms are evident. A histologic diagnosis of “sterile” pyogranuloma syndrome should prompt a search for an infectious agent.

The CLG organism is not possible to culture because its growth requirements have not been determined. Other organisms that can be cultured, particularly S. pseudintermedius, can secondarily infect leproid granulomas.

PCR testing provides a definitive diagnosis by amplifying regions of the bacterial 16S ribosomal RNA gene using mycobacterium-specific primers. The test is more sensitive when performed on DNA extracted from fresh tissue; however, published PCR protocols generally are successful even when used on paraffin-embedded, formalin-fixed tissue, although false-negative results may occur when contact time with formalin is over 48 hours. Retrospective studies using PCR have demonstrated the presence of microbial DNA (e.g., from mycobacteria and Leishmania spp.) in specimens previously diagnosed as sterile. Recently, one of the authors (JF) has developed a real-time PCR test for the CLG organism that is sensitive and specific and improves the availability and accuracy of molecular diagnostics (Smits et al, 2012).