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A study has found that the WHO's tuberculosis (TB) programme in South Africa inadvertently helped a strain of TB-causing bacteria develop additional drug resistance.

The study will be published in the December issue of Clinical Infectious Diseases.

Researchers from the University of KwaZulu-Natal's Nelson Mandela School of Medicine tracked the development of drug resistance in a strain of Mycobacterium tuberculosis over a 12-year period.

They found that by the time the WHO introduced their programme in South Africa in 2001 — using a second-line medication to combat multidrug-resistant strains of TB — at least one strain had already developed resistance to one or more of the second-line drugs.

But because the programme didn't conduct drug susceptibility tests, the new second-line medication was not only useless to TB patients infected with the resistant strain, but also led to the strain developing additional drug resistance.

This is because when an M. tuberculosis strain is resistant to a drug, it survives and can subsequently evolve resistance to additional drugs.

The strain eventually became extensively drug-resistant (XDR-TB), resistant to seven anti-TB drugs in total, including first-line and several second-line drugs.

"The spread of a highly transmissible strain of drug-resistant TB has been facilitated by applying standard treatment regimes for susceptible and multidrug-resistant TB in the absence of drug resistance surveillance," said Willem Strum, one of the authors and dean of the University's Nelson Mandela School of Medicine, in a press release.

XDR-TB poses a grave public health threat, especially to populations with high rates of HIV where TB/HIV co-infection is common as a result of lowered immunity. The greatest concern is that XDR-TB leaves some patients virtually untreatable with currently available drugs.

The same strain caused a 2006 outbreak of XDR-TB in South Africa amongst an HIV-positive population in the rural town of Tugela Ferry in KwaZulu-Natal. Fifty-two of 53 XDR-TB patients died within an average of 25 days.

The study authors call for the increased use of drug resistance surveillance programmes to help prevent the development of drug-resistant TB.

"Public health programmes for the treatment and control of infectious diseases need to be supported by drug resistance surveillance programmes," said Strum.

Link to full paper in Clinical Infectious Diseases

Reference: Clinical Infectious Diseases doi 10.1086/522987 (2007)