BACKGROUND: Effective treatment for drug-suscepti-ble tuberculosis (TB) rapidly renders patients noninfectious,
long before conversion of sputum acid-fast smear or culture to negative. Multidrug-resistant TB
(MDR-TB) patients on treatment are currently assumed to remain infectious for months. While the resources
required for prolonged hospitalization are a barrier to the scale-up of MDR-TB treatment, the safety of
community treatment is clear.
OBJECTIVES : To estimate the impact of treatment on infectiousness among MDR-TB patients.
METHODS: A series of five human-to-guinea pig TB transmission studies was conducted to test various
interventions for infection control. Guinea pigs in adjacent chambers were exposed to exhaust air from a
hospital ward occupied by mostly sputum smear- and culture-positive MDR-TB patients. The guinea pigs then
underwent tuberculin skin testing for infection. Only the control groups of guinea pigs from each study (no
interventions used) provide the data for this analysis.
The number of guinea pigs infected in each study is reported and correlated with Mycobacterium tuberculosis
drug susceptibility relative to treatment.
RESULTS : Despite exposure to presumably infectious MDR-TB patients, infection percentages among guinea
pigs ranged from 1% to 77% in the five experiments conducted. In one experiment in which guinea pigs were
exposed to 27 MDR-TB patients newly started on effective treatment for 3 months, there was minimal
transmission. In four other experiments with greater transmission, guinea pigs had been exposed to patients
with unsuspected extensively drug-resistant tuberculosis who were not on effective treatment.
CONCLUSIONS : In this model, effective treatment appears to render MDR-TB patients rapidly noninfectious.
Further prospective studies on this subject are needed.