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Drug-resistant TB can be eliminated

Model shows that drug-resistant strains may be on the decline

False-colour SEM of Mycobacterium tuberculosis

Source: CDC/Janice Haney Carr

Multidrug-resistant tuberculosis (MDR-TB) can be eliminated, according to a modelling study published today in Science Translational Medicine. The authors, led by Christopher Dye from the World Health Organization (WHO), used data on the incidence of drug-susceptible and drug-resistant tuberculosis spanning six years to model the ‘reproductive number’ — the number of secondary cases generated by each case of the disease.

They found that in all countries and regions studied except for Russia, the reproductive number for MDR-TB was less than 1, indicating that rates of the disease are in a slow decline. But the data indicate that at the current rate, tuberculosis will not be eliminated before 2050, which is the WHO’s current target.

“If we make best use of the drugs that we have, we really can turn this epidemic around,” says Dye. “But TB control has to be for the long term.”

Earlier attempts to estimate the reproductive number through direct measurements had suggested it was much higher. “These so-called direct measurements incorporate a lot of assumptions,” Dye points out. “They tend to measure only part of the [transmission] process.” Because this model is based on data collected over time from country-wide surveillance systems, it takes the whole process into account.

The authors built the model using data from 25 European countries, Hong Kong, the USA, and Russia, but say that the results are transferable to other countries — even those with a high prevalence of MDR-TB. The only question mark hangs over countries with a high prevalence of HIV, which could affect the way MDR-TB behaves, he says.

Data showing incidence trends are not available for the two countries with the highest TB burden — India and China — but Dye is optimistic about prospects for controlling the disease in these countries too. Limited data from Shenzhen, a city on the Chinese mainland, show that MDR-TB may be on the decline, according to Dye. Both countries have started collecting systematic data and so trends should be measurable within a few years.

Although the model estimates a reproductive number greater than 1 for MDR-TB in Russia, Dye says the latest data show that recent control efforts have paid off. The country has seen a downturn in MDR-TB cases. “If you can do it in Russia, you can probably do it anywhere,” he says.

But not all experts share Dye’s optimism. “What this study doesn’t do is take into account that evolution is ongoing,” says Megan Murray, Associate Professor of Epidemiology at Harvard University, USA. Resistant strains may evolve to become more transmissible, she points out. “That to me is what’s frightening.”

Murray also doubts that the declines in TB incidence seen in the study are down to the success of treatment programmes. “TB is declining for reasons that have not too much to do with interventions,” she says. “We’ve known for decades that if socioeconomic circumstances are good, TB is going to decline anyway.”

In countries like India and China or in countries with a high incidence of HIV, it’s a different story. “The data I’ve seen suggest China has an emerging problem,” Murray says. There is particular concern over the Beijing Strain — a highly transmissible strain identified in the 1990s that is relatively common in China and may be particularly adapted to developing resistance.

Reference and links

1.	Dye C, Williams BG. Slow Elimination of Multidrug-Resistant Tuberculosis. Science Translational Medicine 2009. 1:3ra8