Some strains of multidrug resistant tuberculosis (MDRTB) may have a lower fitness (be less capable of spreading) than drug-susceptible tuberculosis bacteria, according to a study published this week in PLOS Medicine. The study, conducted by Louis Grandjean of Imperial College London, and colleagues, compared new tuberculosis cases among household contacts of tuberculosis patients in South Lima and Callao, Peru to determine the relative fitness of MDRTB versus drug-susceptible tuberculosis.
The study followed 1,055 household contacts of 213 individuals with MDRTB infection (defined by resistance to the drugs rifampicin and isoniazid), and 2,362 household contacts of 487 individuals with drug-susceptible tuberculosis for up to three years. Thirty-five (3.3%) of MDRTB contacts and 114 (4.8%) of drug-susceptible tuberculosis contacts developed tuberculosis. When the authors adjusted for risk factors such as HIV status, socio-economic status, and sputum smear grade (a measure associated with higher risk of transmission) of the index case, household contacts of MDRTB cases were 44% less likely to contract tuberculosis than were contacts of drug-susceptible tuberculosis cases.
Previous laboratory findings as well as estimates of fitness based on genetic clustering of strains in the population suggested a lower relative fitness for MDRTB compared to drug-susceptible tuberculosis, but few studies have directly measured the incidence of second cases of tuberculosis among contacts of both MDRTB and drug-susceptible tuberculosis. The researchers note that they did not have genoptyping data for infections in household contacts, so some of these secondary cases may have been transmitted from someone outside the household. Additionally, transmission dynamics may be different in the community setting outside households or in different countries, and more fit MDRTB strains may emerge in the future.
Despite these limitations, the authors say that their findings are "welcome and encouraging news for tuberculosis control programs and health services attempting to contain the spread of MDRTB."
Funding: This work was supported by the Wellcome Trust (Grant Number WT088559MA, http://www.wellcome.ac.uk). The funding body had no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Citation: 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
Wellcome Centre for Clinical Tropical Medicine, Imperial College London, London, United Kingdom
Universidad Peruana Cayetano Heredia, Lima, Peru
TB Centre, London School of Hygiene & Tropical Medicine, London, United Kingdom
Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, United States of America
Laboratorio de Mycobacteriologia, Dirección Regional de Salud-Región Callao, Lima, Peru
Unidad Técnica de TB-MDR, Ministerio de Salud, Lima, Peru
Estrategia Sanitaria Nacional de Prevención y Control de la Tuberculosis and Laboratorio de Mycobacteriologia, Dirección de Salud II-Lima Sur, Lima, Peru
Instituto Nacional de Salud, Lima, Peru
Office for HIV/AIDS, Malaria, Tuberculosis and Neglected Tropical Diseases, World Health Organization, Geneva, Switzerland
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Dr Louis Grandjean
Imperial College London, UNITED KINGDOM