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A study in Peru has demonstrated the importance of early drug susceptibility tests to diagnose multidrug-resistant tuberculosis (TB) and the necessity of environmental control measures to prevent airborne TB transmission.

The investigation, carried out in a hospital in Lima, was published in PLoS Medicine last week (16 September).

The researchers adapted a detection system developed in the 1950s, which passes all the air from a ward of patients co-infected the HIV and TB over guinea pigs housed in an animal facility on the Dos de Mayo Hospital's roof. A set of control guinea pigs was separated from the test group and breathed fresh air.

Marcos Ñavincopa, a medical doctor at the hospital and author of the research, told SciDev.Net that Mycobacterium tuberculosis strains were isolated from the patients and animals.

The researchers then used DNA and drug susceptibility tests to identify which patients had infected which guinea pigs, and ensure the guinea pigs had not been infected by other means.

They found that just ten HIV/TB patients out of 97 were responsible for virtually all the cases of TB in the guinea pigs. Seven of them had drug-resistant TB that had been inadequately treated.

"This research shows it is important to diagnose multidrug-resistant TB quickly, rather than to wait for these patients to fail normal treatment, because while such patients are inadequately treated, they may be highly infectious, and spread the disease," says Roderick Escombe, lead author of the study and a medical doctor in the department of infectious diseases and immunity at Imperial College London, United Kingdom.

"This shows clearly that a contaminated environment without proper ventilation is a powerful transmitter of the disease and that the smaller and less ventilated it is, the higher the chance of transmission," adds Ñavincopa.

"Effective TB infection control measures, fresh air and ventilated areas are essential in hospitals, but also in other health settings such as antiretroviral therapy rooms and rural medical centers, to prevent airborne dissemination of the bacteria," he adds.

The average patient infectiousness over the 505 days of the study was six times greater than that recorded in the 1950's study.

"We do not know if the infective dose of TB for guinea pigs is the same as the dose for humans. But compared to other published data on human infectiousness, the patients in our study were highly infectious," Escombe adds.


PLoS Medicine doi 10.1371/journal.pmed.0050188 (2008)