Background
The current understanding of airborne tuberculosis (TB) transmission is based on classic
1950s studies in which guinea pigs were exposed to air from a tuberculosis ward. Recently we
recreated this model in Lima, Peru´, and in this paper we report the use of molecular
fingerprinting to investigate patient infectiousness in the current era of HIV infection and
multidrug-resistant (MDR) TB.
Methods and Findings
All air from a mechanically ventilated negative-pressure HIV-TB ward was exhausted over
guinea pigs housed in an airborne transmission study facility on the roof. Animals had monthly
tuberculin skin tests, and positive reactors were removed for autopsy and organ culture for M.
tuberculosis. Temporal exposure patterns, drug susceptibility testing, and DNA fingerprinting of
patient and animal TB strains defined infectious TB patients. Relative patient infectiousness was
calculated using the Wells-Riley model of airborne infection. Over 505 study days there were
118 ward admissions of 97 HIV-positive pulmonary TB patients. Of 292 exposed guinea pigs,
144 had evidence of TB disease; a further 30 were tuberculin skin test positive only. There was
marked variability in patient infectiousness; only 8.5% of 118 ward admissions by TB patients
were shown by DNA fingerprinting to have caused 98% of the 125 characterised cases of
secondary animal TB. 90% of TB transmission occurred from inadequately treated MDR TB
patients. Three highly infectious MDR TB patients produced 226, 52, and 40 airborne infectious
units (quanta) per hour.
Conclusions
A small number of inadequately treated MDR TB patients coinfected with HIV were
responsible for almost all TB transmission, and some patients were highly infectious. This result
highlights the importance of rapid TB drug-susceptibility testing to allow prompt initiation of
effective treatment, and environmental control measures to reduce ongoing TB transmission in
crowded health care settings. TB infection control must be prioritized in order to prevent health
care facilities from disseminating the drug-resistant TB that they are attempting to treat.