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Drug-resistant TB strains 'could become widespread'

drug restsistant tb could become widespread Virtually untreatable strains could spread globally

TB incidence is falling in many regions of the world, but the emergence of new strains resistant to antibiotics could see an upsurge of untreatable cases, according to a new study by Australian scientists.

Although previous studies have suggested that resistant strains of TB cause longer lasting infections, they were also thought to have lower transmission rates. This is why scientists have questioned whether drug-resistant TB strains are more likely than drug-sensitive strains to persist and spread – an important question for predicting the future impact of the disease.

This is a vital question because 1 in 3 people are estimated to carry the TB bacterium, though it remains latent in most cases.

The World Health Organization estimates that there over 9m new cases of TB in 2007, with 1.6m deaths recorded in 2005.

In the new study strains of the bacterium were isolated from various parts of the world to estimate the rate of evolution of drug resistance and to compare the relative "reproductive fitness" of resistant and drug-sensitive strains.

"We found that the overall fitness of drug-resistant strains is comparable to drug-sensitive strains," says Dr Tanaka of the Evolution and Ecology Research Centre. "This was especially so in Cuba and Estonia, where the there is a high prevalence of drug-resistant cases."

This could reflect an inconsistency in drug treatment programs in these countries. Indeed, Estonia now has one of the highest rates of multi-drug resistance in the world.

The study also shows that the spread of drug resistant strains was mainly due to the transmission of these strains to other patients, rather than the bacterium acquiring resistance in each patient due to incomplete treatment.

"Our results imply that drug resistant strains of TB are likely to become highly prevalent in the next few decades," says Dr Fabio Luciani, the study's lead author. "They also suggest that limiting further transmission of TB might be an effective approach to reducing the impact of drug resistance."

The study was published in the Proceedings of the National Academy of Sciences.

This article was published on Fri 14 August 2009

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