Introduction: Mycobacterium avium complex (MAC) is the most common cause of pulmonary infections due to nontuberculous mycobacterium in the United States. While there have been reports of geographic variations in incidence as well as disease clusters among genetically related individuals, there have been no published reports describing clusters of co-occurring pulmonary MAC disease among genetically unrelated individuals.

Clinical Findings: Patient 1 was a 56-year-old male with worsening asthma exacerbations associated with recurrent bronchitis. Patient 2 was a 55-year-old female with worsening asthma exacerbations, progressive dyspnea, and recurrent bronchitis. The 2 patients lived together, were immunocompetent, and genetically unrelated. They were frequently exposed to aerosolized soil and inadequately heated water.

Clinical Course: Patient 1 had normal imaging and his sputum grew out MAC. He responded quickly to treatment with azithromycin, rifampin, and ethambutol. Patient 2 had computed tomographic imaging that revealed tree-in-bud and ground-glass opacities. Her sputum samples did not grow out acid-fast bacillus; however, 14 months after Patient 1 was diagnosed, her bronchoscopy samples ultimately grew out MAC. Patient 2 clinically responded quickly to the same treatment as Patient 1, with resolution of her dyspnea, bronchitis, and radiographic abnormalities.

Conclusions: We describe 2 cohabitating, genetically unrelated, immunocompetent patients with pulmonary MAC infections. The likely cause of these infections was a common exposure to aerosolized soil and inadequately heated water. We report the unusual co-occurrence of pulmonary MAC among genetically unrelated individuals living together.