A Runyon group III bacillus, MAC is a slow-growing nonchromogen that is ubiquitous in nature.15 It has been isolated from soil, water, house dust, vegetables, eggs, and milk. According to Reed et al,3 occupational exposure to soil is an independent risk factor for MAC infection, with individuals reporting more than 6 years of cumulative participation in lawn and landscaping services, farming, or other occupations involving substantial exposure to dirt or dust most likely to be MAC-positive. Cutaneous MAC infection may be associated with water exposure, as Sugita et al2 described one familial outbreak of cutaneous MAC infection linked to use of a circulating, constantly heated bathwater system. With respect to US geography, individuals living in rural areas of the South seem most prone to MAC infection.3
Primary cutaneous infection with MAC occurs after a breach in the skin surface, though this fact may not be elicited by history. Modes of entry include minor abrasions after falling,1 small wounds,2 traumatic inoculation,15 and intramuscular injection.16 Clinically, cutaneous lesions of MAC are protean. In the literature, clinical presentation is described as a polymorphous appearance with scaling plaques, verrucous nodules, crusted ulcers, inflammatory nodules, dermatitis, panniculitis, draining sinuses, ecthymatous lesions, sporotrichoid growth patterns, or rosacealike papulopustules.1,15,17 Lesions may affect the arms and legs, trunk, buttocks, and face.18
The differential diagnosis of MAC infection includes lupus vulgaris, Mycobacterium marinum infection (also known as swimming pool granuloma), sporotrichosis, nocardiosis, sarcoidosis, neutrophilic dermatosis, pyoderma gangrenosum, and cutaneous blastomycosis. Given its rarity and variability, diagnosis of MAC infection requires a high index of suspicion. Cutaneous MAC infection should be considered if a nodule, plaque, or ulcer fails to respond to conventional treatment, especially in patients with a history of environmental exposure and possible injury to the skin.
We report a rare case of primary cutaneous MAC infection arising in SCC excision sites in a patient without known immune deficiency. This presentation may have occurred for several reasons. First, the surgical excision sites coupled with the substantial occupational and recreational exposure to soil experienced by our patient may have served as portals for infection. Although SCCs are common on the hands, Mohs micrographic surgery is not always performed for excision; in our patient's case, this approach allowed for maximum tissue conservation and preserved manual function given the number and location of the lesions. Second, despite an overtly intact immune system, our patient may have harbored an occult immune deficiency, predisposing him to dermatologic infection with a microorganism of low intrinsic virulence and recurrent malignant neoplasms. This presentation may have been the first clinical indication of subtle immune compromise. For example, inadequate proinflammatory cytokines may contribute to both mycobacterial and malignant disease. A potential risk of inhibition of tumor necrosis factor α is the unmasking of tuberculosis or lymphoma.19,20 Likewise, IFN-γ is vital in suppressing mycobacteria and malignancy. Yonekura et al21 found that IFN-γ induces apoptosis in oral SCC lines. It follows that a paucity of IFN-γ could allow neoplastic growth. Normal function of IFN-γ prompts microbicidal activity in macrophages and stimulates granuloma formation, both of which combat mycobacterial infection.19 A final postulation is that a simmering cutaneous MAC infection precipitated neoplastic degeneration into SCC, much the same way that the human papillomavirus has been correlated in the carcinogenesis of cervical cancer. As an intracellular microbe, MAC could cause the genetic machinery of skin cells to go awry. Kullavanijaya et al18 described a patient with cutaneous MAC in association with cervical cancer.
Conclusion
This association of primary cutaneous MAC infection and cutaneous malignancy in a reportedly immunocompetent patient is rare. Cancer patients, as noted by Feld et al,22 are 3 times more likely to develop infections with mycobacteria, with SCC, lymphoma, and leukemia being most commonly indicated. A specific immune deficit in the IFN-γ receptor is known to confer a selective predisposition to mycobacterial infection.23,24 Toyoda et al25 outlined the case of a pediatric patient with IFN-γ receptor 2 deficiency who presented with disseminated MAC infection and later succumbed to multiple SCCs of the hands and face. The authors' assertion was that inherited disorders of IFN-γ-mediated immunity may be associated with SCCs.25 Unfortunately, our patient died before more specific immunological testing could be conducted. This case highlights the remarkable singularity of primary cutaneous MAC infection in association with multiple SCCs with seemingly intact immune status and offers some intriguing hypotheses regarding its occurrence.