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Multidrug-resistant TB spreads less easily than ‘normal’ TB

Close contacts of MDRTB cases 44% less likely to get TB than contacts of drug-susceptible TB

Louise Prime

Wednesday, 24 June 2015

Multidrug-resistant tuberculosis (MDRTB) seems to spread less easily within households than drug-susceptible TB, according to research* funded by the Wellcome Trust. This good news means that, at least for now, we have improved chances of controlling the disease, said the authors of the study – published online in PLOS Medicine.

Researchers led from the Wellcome Centre for Clinical Tropical Medicine at Imperial College London wanted to investigate the relative ‘fitness’ (or ability to spread) of MDRTB and drug-susceptible TB bacteria. They identified 1,055 household contacts of 213 people with TB resistant to rifampicin and isoniazid, and 2,362 household contacts of 487 people with drug-susceptible TB, living in South Lima and Callao, Peru.

During up to three years’ follow-up, 35 (3.3%) of MDRTB contacts and 114 (4.8%) of drug-susceptible tuberculosis contacts developed tuberculosis. The researchers adjusted for risk factors including human immunodeficiency infection, socioeconomic status and sputum smear grade (a measure associated with higher risk of transmission) in the index case, and then calculated that household contacts of people with MDRTB were 44% less likely to contract tuberculosis than contacts of people with drug-susceptible tuberculosis.

The study authors said their results – which they called “welcome and encouraging news” in the attempt to contain the spread of MDRTB – tallied with those from earlier laboratory and population-based studies that had suggested a lower relative fitness for MDRTB compared with drug-susceptible TB. They acknowledged that they couldn’t be sure whether incident cases of TB resulted directly from household contact with the index cases or were caught from elsewhere – because they didn’t genotype the second cases – and also that MDRTB could develop more ‘fit’ strains in the future.

Nevertheless, they concluded that their study “suggests that MDRTB is less fit (less transmissible and/or less able to cause disease) than drug-susceptible tuberculosis, at least in households. The fitness of MDRTB relative to drug-susceptible tuberculosis is one of the most important determinants of future MDRTB spread. A low relative fitness of MDRTB improves the chances of containing and diminishing the spread of drug-resistant tuberculosis.”

* Grandjean L, Gilman RH, Martin L, Soto E, Castro B, Lopez S, et al. (2015). Transmission of Multidrug-Resistant and Drug-Susceptible Tuberculosis within Households: A Prospective Cohort Study. PLOS Med 12(6): e1001843. doi:10.1371/journal.pmed.1001843

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