A nosocomial outbreak of multidrug-resistant Mycobacterium bovis among HIV-infected patients. A case-control study.

To identify risk factors in a nosocomial outbreak of multidrug-resistant Mycobacterium bovis (MDRMB) tuberculosis (TB) among HIV-infected patients. We evaluated the study period (from the first to the last MDRMB smear-positive patients hospitalized in the unit) using a case-control study with three control groups. Since MDRMB is extremely rare, we assumed that a single strain was responsible for all six cases. A 19-bed infectious diseases unit in Paris, France. The index case was an AIDS patient who was hospitalized in September 1989 because of MDRMB TB. The cases were five HIV-infected patients who developed MDRMB TB between January 1990 and October 1991. Controls were randomly selected from HIV-infected patients in our unit during the study period (case-control study 1, 15 patients), during the contact period (at least one MDRMB smear-positive patient hospitalized in the unit; case-control study 2,20 patients), and patients matched according to the length of contact (case-control study 3, 24 patients). After detecting the nosocomial outbreak, we took respiratory isolation precautions for all patients suspected of having active TB. Risk factors for MDRMB nosocomial transmission, and the occurrence of new cases of MDRMB infection in HIV-infected patients and health-care workers after the introduction of isolation precautions. The most important predictor of nosocomial transmission of MDRMB to HIV-infected patients was the (mean +/- s.d.) length of contact in days [cases, 22 +/- 15.8; study 1 controls, 11.2 +/- 18.9 (P = 0.07); study 2 controls, 14.6 +/- 8.5 (P = 0.043)]. Only one case of MDRMB TB resulted from exposure to MDRMB-smear-positive patient after the introduction of respiratory isolation measures. The incubation period in the single health-care worker who developed MDRMB TB was longer than in the cases. In a nosocomial outbreak of MDRMB TB, the contact time was the main risk factor of transmission to HIV-infected patients. Respiratory isolation measures appear to be effective.