JOHANNESBURG, 30 August 2012 (PlusNews) - A new international study published in The Lancet finds that extensively drug resistant tuberculosis (XDR-TB) is more widespread than previously thought, and growing, leading researchers to reiterate calls for better infection control and new drugs.
Published on 30 August, the 8-country study was conducted among 1,278 patients with multi-drug resistant TB (MDR-TB), a form of tuberculosis (TB) resistant to the most commonly used first-line TB drugs, isoniazid and rifampicin.
Conducted by the US Centers for Disease Control (CDC) and national TB-control programmes, the study assessed the prevalence of and risk factors for XDR-TB among these patients. XDR-TB is resistant to these first-line drugs as well as at least half of the mostly commonly used second-line drugs.
Researchers found that almost 7 percent of study patients had XDR-TB. This figure is higher than the previously cited XDR-TB prevalence rate of about 5.5 percent, published by the World Health Organization (WHO) in November 2011.
What is clear from both studies is that the problem of XDR-TB is likely to get worse, said Tracy Dalton, a microbiologist in CDC’s TB-elimination laboratory branch and lead author of the new study.
More drug resistance likely
The WHO recommends a minimum of 20 months for MDR-TB treatment, which uses second-line TB drugs including daily, painful injectables that can cause hearing loss.
XDR-TB develops when patients become resistant to these second-line drugs, either because they cannot complete their treatment or because these drugs have been incorrectly prescribed. Unsurprisingly, Dalton’s study found that previous exposure to second-line TB treatment quadrupled patients’ XDR-TB risk.
“As more individuals are diagnosed with, and treated for, drug-resistant TB, more resistance to second-line drugs is expected to emerge,” Dalton said. “So far, XDR-TB has been reported in 77 countries worldwide, but exact prevalence remains unclear.”
As more countries adopt new TB diagnostics, like the GeneXpert, more cases of MDR- and XDR-TB are likely to be diagnosed.
The 3-year-long CDC study also found that XDR-TB was more common among women than men, as was TB resistant to fluoroquinolones, a class of antibiotics used to treat TB. Patients living with HIV experienced significantly less fluoroquinolone-resistant TB than HIV-negative patients, even though HIV has been shown, in other studies, to be a risk factor for XDR-TB. The study also found that unemployment, alcohol abuse and smoking were associated with resistance to second-line injectables.
Need for new TB drugs
The study is one of the first to provide important country-level data on specific drug resistance dynamics. For instance, Dalton’s work found that resistance to all three second-line injectables was significantly more frequent in South Africa’s Eastern Cape Province than in the other three South African provinces under review. She told IRIN/PlusNews that this kind of data could be important in helping national TB-control programmes responded to drug resistance and improve treatment for patients.
In a related comment published in The Lancet, Sven Hoffner from the Swedish Institute for Communicable Disease Control said that more data like this was needed.
“Most international recommendations for TB control have been developed for MDR-TB prevalence of up to around 5 percent. Yet now we face prevalence up to ten times higher in some places, where almost half of the patients with infectious disease are transmitting MDR strains,” Hoffner said. “Updated information on MDR-TB and investigation of the trends are urgently needed...especially since the true scale of the burden of MDR and XDR tuberculosis might be underestimated and seem to be rapidly increasing.”
According to Dalton, the study highlights the urgent need for new TB drugs - a call echoed in a statement issued by Tom Evans, chief scientific officer for the non-profit TB vaccine developer Aeras, in response to the study.
“Without a robust pipeline of new drugs to stay one step ahead, it will be nearly impossible to treat our way out of this epidemic,” Evans said in the statement.