Objective: To identify risk factors in a nosocomial outbreak of multidrug-resistant Mycobacterium bovis (MDRMB) tuberculosis (TB) among HIV-infected patients.
Design: 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.
Setting: A 19-bed infectious diseases unit in Paris, France.
Patients: 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).
Interventions: After detecting the nosocomial outbreak, we took respiratory isolation precautions for all patients suspected of having active TB.
Main outcome measures: 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.
Results: 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.
Conclusion: 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.