Cross-Municipality Migration and Spread of Tuberculosis in South Africa

Tuberculosis (TB) prevalence varies geographically in South Africa, but the drivers of this spatial heterogeneity are poorly understood. Like many infectious diseases, internal migration of infected people may contribute to the spread of TB and emergence of new hotspots via TB introduction and increased encounters between infectious and susceptible individuals. 

Cross-border migration has been shown to impact international variation in TB prevalence. Particularly in low-TB-burden countries, migrants from high-burden countries represent a large share of new infections and a focus population for treatment and preventive interventions. As such, understanding internal migration patterns in people with TB in high-burden settings is critical to preventing TB and drug-resistant TB spread within countries.

In a new journal article published in Nature’s Scientific Reports, Jacob Bor and coauthors analyze longitudinal data on all tuberculosis test results recorded by South Africa’s National Health Laboratory Service (NHLS) from January 2011–July 2017, alongside municipality-level migration flows estimated from the 2016 South African Community Survey. They first assess migration patterns in people with laboratory-diagnosed tuberculosis and analyze demographic predictors. They then quantify the impact of cross-municipality migration on tuberculosis incidence in municipality-level regression models.

Main findings:

• The NHLS database included 921,888 patients with multiple clinic visits with TB tests. Of these, 16 percent submitted TB test samples to clinics located in more than one municipality, which indicates that they migrated between municipalities. 
• The median distance between origin and destination municipalities was 304 kilometers.
• Migration was most common at ages 20–39 years and rates were similar for men and women. 
• In municipality-level regression models, cross-municipality migration among people with laboratory-diagnosed TB was positively associated with future drug-susceptible TB and drug-resistant TB incidence within municipalities. 

High migration of people with TB could have important implications for TB care and management. As more people with laboratory-diagnosed TB migrate between municipalities, it might be more difficult to ensure continuity of care, successful treatment and post-treatment and prevent recurrence. Consequently, spatial targeting of interventions using these types of data could be critical to improve the care of individual patients with tuberculosis, as well as reduce population-level transmission and prevalence. The findings can also help local and national TB programs and policymakers predict future geographic patterns of TB to mitigate the spread of the disease.