American researchers, working with local scientists in the Northern Cape, have suggested that while HIV increases the likelihood of contracting the globe’s leading infectious killer – tuberculosis – it isn’t the only risk factor for TB.
Every year, about 1.5m people die because of TB, with about a quarter of the world’s population infected with the disease’s causative agent – Mycobacterium tuberculosis.
Additionally, HIV is the largest known risk factor for progression to active TB due to its immense immunosuppressive effect, says Oshiomah Oyageshio, a doctoral student in the University of California-Davis Department of Anthropology and corresponding author of a recent study published in PLOS Global Public Health.
“It increases TB risk by about 20-fold,” he told MedicalXPress.
But, said the researchers, despite this, HIV isn’t the only risk factor associated with TB: in fact, gender, and whether people lived in towns or rural areas, also increased or decreased the odds of getting the disease.
“Recent national health surveys in South Africa have found that 82% of people with TB do not have HIV,” Oyageshio said. “This motivates the understanding of what drives high TB incidence in situations without HIV ‘masking’ other susceptibility signals.”
In the study, Oyageshio and colleagues investigated the epidemiological risk factors, outside HIV, associated with TB in the Northern Cape. In this understudied area, most people are likely to be exposed to TB by the time they are young adults. It is estimated that 80% to 90% of the population is infected with latent TB, suggest the researchers.
The study was conducted in collaboration with Marlo Möller, a professor at Stellenbosch University, and Möller's lab.
The researchers found that male gender and, separately, residency in large towns, were the strongest risk factors associated with active TB progression, expected findings based on previous TB studies.
They also found that socioeconomic status, age and whether or not people moved in their lifetime had a significant active TB risk. These findings surprised the researchers.
“We did not expect TB risk to increase at older ages for people with higher socio-economic status, neither did we expect individuals living in urban areas who had migrated to rural areas to be just as protected from active TB as lifetime small-town dwellers,” Oyageshio said. “Both trends warrant further study.”
The researchers said these findings may reflect how pre- and post-apartheid environments affected social factors and thus lifetime TB risk.
Socio-economic status and residency
For their study, the team generated a cohort of 774 individuals with suspected TB from 12 community health clinics in the Northern Cape. They then assessed important risk factors by analysing the cohort with three statistical models.
“The province has a complex socio-demographic history, so we wanted to design a set of models that represented that to the best of our ability,” Oyageshio said.
“Our first model was informed by common risk factors from previous TB epidemiology studies, serving as a baseline comparison. The residence and socio-economic models were formulated to include variables relevant to South African populations, involving the change in socio-economic status over time, and migration between rural and urban areas.”
The researchers found that higher socio-economic status is a protective factor among 18- to 39-year-olds. Among older aged cohorts, the trend reversed, with those of high socio-economic status being more at risk for active TB. The finding runs counter to similar research conducted in the United States and Mexico.
“We expect a positive relationship where the more socio-economic status you have, the more education you have, the better your well-being and access to health care tends to be,” said Justin Myrick, a study co-author and field manager in the Henn Lab for Population Genetics.
Brenna Henn is an associate professor in the Department of Anthropology and senior author of the study.
The researchers suggested that these trends in the Northern Cape may be echoes of the societal and economic shifts that occurred pre- and post-apartheid.
“You only really see the benefits of higher socio-economic status in those individuals who were born – or were young – right towards the end of apartheid,” Myrick said.
Researchers also found that residency affected active TB risk. Town-living individuals had higher odds of active TB compared with lifetime rural dwellers and those who moved to rural areas from towns. Those who moved to rural areas from towns had the overall lowest risk of active TB.
The researchers hypothesised that this may be due to vaccinations being more common in urban areas. Those born in towns may have a higher likelihood of being vaccinated at a young age. As they get older and move to more rural areas, their chances of being exposed to the TB bacteria are lower.
Study details
Strong effect of demographic changes on Tuberculosis susceptibility in South Africa
Oshiomah Oyageshio, Justin Myrick, Jamie Saayman, Lena van der Westhuizen et al.
Published in PLOS Global Public on 23 July 2024
Abstract
South Africa is among the world’s top eight tuberculosis (TB) burden countries, and despite a focus on HIV-TB co-infection, most of the population living with TB are not HIV co-infected. The disease is endemic across the country, with 80%–90% exposure by adulthood. We investigated epidemiological risk factors for (TB) in the Northern Cape Province, South Africa: an understudied TB endemic region with extreme TB incidence (926/100,000). We leveraged the population’s high TB incidence and community transmission to design a case-control study with similar mechanisms of exposure between the groups. We recruited 1,126 participants with suspected TB from 12 community health clinics and generated a cohort of 774 individuals (cases = 374, controls = 400) after implementing our enrolment criteria. All participants were GeneXpert Ultra tested for active TB by a local clinic. We assessed important risk factors for active TB using logistic regression and random forest modelling. We find that factors commonly identified in other global populations tend to replicate in our study, e.g. male gender and residence in a town had significant effects on TB risk (OR: 3.02 [95% CI: 2.30–4.71]; OR: 3.20 [95% CI: 2.26–4.55]). We also tested for demographic factors that may uniquely reflect historical changes in health conditions in South Africa. We find that socioeconomic status (SES) significantly interacts with an individual’s age (p = 0.0005) indicating that protective effect of higher SES changed across age cohorts. We further find that being born in a rural area and moving to a town strongly increases TB risk, while town birthplace and current rural residence is protective. These interaction effects reflect rapid demographic changes, specifically SES over recent generations and mobility, in South Africa. Our models show that such risk factors combined explain 19–21% of the variance (r2) in TB case/control status.
See more from MedicalBrief archives:
More than half of South Africans do not seek TB treatment – HSRC survey
TB clusters show where HIV treatment is missing in South Africa