Outbreak of Mycobacterium marinum infection among captive snakes and bullfrogs

Since 1985 mycobacterial infection has been observed occasionally among snakes and bullfrogs housed in the Wisconsin exhibit at the Milwaukee Zoo. Prospective screening of animals was initiated in September 1990, after two cases occurred in March and June 1990. Overall, of 47 animals that were housed in the exhibit from 1981 through its closure in 1995, 15 (31.9%) were diagnosed with mycobacterial infection. That includes 10 cases (of 24 animals; 40% prevalence) that occurred during the final 5 years, when all animals were actively being screened for infection. Infection was documented by culture for seven animals, histology for four animals, and both histology and culture for four animals. Species determination of nine of the 10 isolates revealed Mycobacterium marinum. Genetic fingerprinting of the eight available isolates using pulsed field gel electrophoresis (PFGE) showed that six animals (five snakes and one bullfrog) were infected with the same strain of M. marinum (strain A) and two snakes were infected with a second strain (strain B). Deaths of animals infected with strain A spanned 1992-1995, and for strain B 1990-1992. It is postulated that possible routes of transmission were inhalation of infected aerosols or ingestion of contaminated food, water, or fomites. These data suggest that in closed systems the presence of mycobacterial infection in one animal significantly increases the risk of infection for all animals. Moreover, individual pathogenic strains may persist for prolonged periods of time.