Tuberculosis (TB) has been known to mankind since ancient times, and is likely to have caused more deaths in human history than any other infectious disease. Unlike HIV and malaria, globally rates of disease are falling very slowly and last year it once more became the leading cause of death due to a single infectious disease, causing 1.5 million deaths every year. Despite its importance, there is great uncertainty about the transmission of M.tb (the infection causing TB disease) – where it happens and who infects whom.
Rates of M.tb infection are usually estimated by surveys of latent infection in children – a population less likely to be infected, which makes it easier to detect new infections in that age group. However, recent work by researchers at the London School of Hygiene &Tropical Medicine, The Zambia AIDS Related Tuberculosis Project and the Desmond Tutu TB Centre, suggests that these rates may substantially underestimate rates of exposure in adults in South African and Zambia, and that men may be the main source of infection in men, women and children.
To explore potential drivers of M.tb transmission, we conducted a survey of social contact patterns in 16 communities in Zambia, and eight communities in the Western Cape of South Africa. This involved interviewing individuals to find out who they had contacted and where they had spent time the previous day.
Like other social contact surveys, we found strong patterns of mixing within similar age groups. Interestingly, we also found that in these settings, men and women mixed more with their own genders.
Surveys of latent M.tb infection that had been conducted in those same communities provided us with estimates of rates of infection in children, and a prevalence survey quantified the levels of undiagnosed TB disease in adults.
Using our observed contact patterns, and comparing the infection rates in children against the prevalence of disease in other age groups, we were able to estimate rates of infection in all age groups, with interesting results.
Annual rates of M.tb infection in adults might be 1.5–6 times higher than was empirically measured in children. This is due to adults having higher rates of contact with active TB cases (usually other adults) than children do.
The much higher prevalence of TB in men suggested they were the main source of infection in both genders. This, and the within-gender mixing patterns, indicates that TB exposure in men is particularly intense. Separately, a recent systematic review implies that men not only have more TB than women, but also a lower chance of being diagnosed.
What does all this mean? Since most infections are likely due to contact with adult men, more attention must be given to ensuing that TB diagnosis is accessible to men. This could be critical to protecting men, women, and children from tuberculosis.
Peter J. Dodd, Clare Looker, Ian D. Plumb, Virginia Bond, Ab Schaap, Kwame Shanaube, Monde Muyoyeta, Emilia Vynnycky, Peter Godfrey-Faussett, Elizabeth L. Corbett, Nulda Beyers, Helen Ayles, and Richard G. White* Age- and Sex-Specific Social Contact Patterns and Incidence of Mycobacterium tuberculosis Infection. American Journal of Epidemiology doi: 10.1093/aje/kwv160
Image: Scanning electron micrograph of Mycobacterium tuberculosis bacteria, which cause TB. Credit: NIAID