More than half of tuberculosis infections among men, women, and children were seen to derive from contacts with adult men, according to findings from research teams working in the United Kingdom, Zambia, and South Africa.
Understanding TB infection incidence by age and sex of source cases, and where and how often contacts occur, is critical to controlling transmission and directing prevention and treatment efforts. For this study, published online in the American Journal of Epidemiology (Am J Epidemiol. 2015 Dec 8. doi: 10.1093/aje/kwv160), researchers compared TB prevalence data in adults from a recent study in South Africa and Zambia and from an earlier, related study in children with data on daily contact patterns gathered through interviews with more than 3,500 adults 18 and older (53% female).
Close contacts were defined as face-to-face conversations, while casual contacts were defined as sharing indoor space, a potentially important source of TB transmission. In the interviews, adults reported an average of 4.9 close contacts and 10.4 casual contacts daily. They also reported the age groups and sexes with which their contacts occurred.
The authors of the study, led by Dr. Peter J. Dodd of the University of Sheffield (U.K.), said the finding on adult males as the likely drivers of TB transmission in both genders was surprising, because the study also revealed that daily close contacts occurred preferentially among age groups and among members of the same sex.
More than 63% of women’s close contacts and 61% of men’s were among members of the same sex. Adults averaged 0.8 close contacts per day with children under 12, though women had a slightly higher rate of contact than men. Among adults, estimated infections from contact with adult men was 57.3% in South Africa and 65.7% in Zambia. Estimated transmission from contact with men was 50% or higher in all age groups, in both sexes, and in both countries, except in girls 12 and younger and boys 4 and younger in South Africa.
“We noticed in the study that there was preferential mixing by gender,” Dr. Dodd said in an interview. “If there was the same amount of TB in men and women, you’d have expected women to be responsible for the majority of transmission to women and vice versa. But in fact there’s so much more TB in men, that they’re still responsible for the majority of transmission in women.”
Young children’s high proportion of TB infection attributable to contact with men was surprising, they noted, because their contact rates with women were higher. This suggests “that even in this age group, the higher prevalence in males tended to outweigh the higher contact rates between young children and women,” the researchers wrote in their analysis.
Prevalence of TB was markedly higher in men than women (0.9% vs. 0.4% in Zambia, and 3% vs. 2% in South Africa). While the reasons for more TB in men are not well understood, these likely include both biological and social factors, including time before seeking care, and access to care. If men are slower to report or get diagnosed, because of work or other considerations, they will have higher prevalence of infectious TB disease, Dr. Dodd said. Preferential mixing by gender means that men are also more likely to be exposed to infectious TB by other men.
The study also found that incidence of infection was between 1.5 and 6 times higher in adults than was measured in children. “Adults are likely to have higher rates of exposure to TB, because adults on average have more contact with other adults than they do with children – and it’s the adults who tend to have infectious TB disease,” Dr. Dodd said.
Tuberculosis infection incidence based on surveys in children might underestimate true incidence in adults in TB-endemic settings, the researchers cautioned in their analysis.
“For hyperendemic communities in South Africa, which may have an incidence of M. tuberculosis infection in children as high as 4% per year, our findings imply that incidence rates in never-infected adults may be as high as 10% per year, rates which have rarely been seen outside institutional settings.”
Care and control of tuberculosis in males, the researchers concluded, “is critical to protecting men, women, and children from tuberculosis.”
Individual researchers were funded by the Bill and Melinda Gates Foundation, the Medical Research Council (U.K.), and the Wellcome Trust. None of the researchers reported conflicts of interest.