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. 2010 Apr;14(4):406-12.

Changing prevalence of tuberculosis infection with increasing age in high-burden townships in South Africa

R Wood  1 , H Liang, H Wu, K Middelkoop, T Oni, M X Rangaka, R J Wilkinson, L-G Bekker, S D Lawn
Affiliations

Changing prevalence of tuberculosis infection with increasing age in high-burden townships in South Africa

R Wood et al. Int J Tuberc Lung Dis. 2010 Apr.

Abstract

Setting: Crowded townships of Cape Town, South Africa, where human immunodeficiency virus (HIV) prevalence and tuberculosis (TB) notification rates are among the highest in the world.

Objectives: To determine age-specific prevalence rates of latent tuberculosis infection (LTBI) among HIV-negative individuals, and the annual risk and force of infection during childhood and adolescence.

Design: A cross-sectional survey using a standardised tuberculin skin test (TST) in HIV-negative individuals aged 5-40 years. A TST diameter of > or =10 mm was defined as indicative of LTBI.

Results: Among 1061 individuals, only 4.7% had low-grade TST responses of 1-9 mm. However, the proportions of individuals with TST > or =10 mm increased from 28.0% in the 5-10 year age stratum to 88.0% in the 31-35 year age stratum. The mean annual risk of infection was 3.9% up to 5 years of age. The estimated force of infection (the rate of acquisition of LTBI among the residual pool of non-infected individuals) increased throughout childhood to a maximum of 7.9% per year at age 15 years.

Conclusions: Extremely high rates of infection in childhood and adolescence result in very high LTBI prevalence rates in young adults who are most at risk of acquiring HIV infection. This may be an important factor fuelling the high rates of HIV-associated TB in southern Africa.

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Figures

Figure 1
Figure 1
Frequency distribution of diameters of induration at 48-72 hours after standardised tuberculin skin testing (TST) in 1061 individuals aged 5-40 years.
Figure 2
Figure 2
Scatter plot of diameters of tuberculin skin test (TST) induration versus age at time of performing test. Linear regression demonstrates no significant relationship between size of TST response and age.
Figure 3
Figure 3
Graph showing the relationship between the prevalence of tuberculin skin test (TST) diameters of ≥10 mm and age for 1061 subjects, using a generalized non-parametric logistic model . The 95% point-wise confidence bands are also plotted. The prevalence rate reached a maximum of 82% at an age of 27.5 years.
Figure 4
Figure 4
Fitted prevalence rate versus age for male (solid) and female (dotted) subjects, respectively, using the generalized nonparametric logistic model . The interaction term of age and gender was also considered in the model. The prevalence rate reached a maximum (points indicated) of 92% at age 31.5 years for male and the peak 84% at age 26.5 years for females.
Figure 5
Figure 5
The annual change in the prevalence of tuberculin skin test (TST) diameters of ≥10 mm versus age for male (solid line) and females (dotted line) subjects are plotted using a generalized nonparametric logistic model . The change in the prevalence of TST responses ≥10 mm increased between age 5 years and age 13 years, and reached a maximum at age 13 years in both groups.
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References

    1. World Health Organization . Global tuberculosis control 2009. Epidemiology, strategy, financing. World Health Organization; Geneva: 2009. WHO/HTM/TB/2009.411.
    1. Health Systems Trust District Health Barometer 2006/2007. 2008. Accessible at http:, www.hst.org.za/publications/717.
    1. Middelkoop K, Bekker LG, Myer L, Dawson R, Wood R. Rates of tuberculosis transmission to children and adolescents in a community with a high prevalence of HIV infection among adults. Clin Infect Dis. 2008 Aug 1;47(3):349–55. - PMC - PubMed
    1. Lawn SD, Bangani N, Vogt M, Bekker LG, Badri M, Ntobongwana M, et al. Utility of interferon-gamma ELISPOT assay responses in highly tuberculosis-exposed patients with advanced HIV infection in South Africa. BMC Infect Dis. 2007;7:99. - PMC - PubMed
    1. Rangaka MX, Wilkinson KA, Seldon R, Van CG, Meintjes GA, Morroni C, et al. Effect of HIV-1 Infection on T-Cell-based and Skin Test Detection of Tuberculosis Infection. Am J Respir Crit Care Med. 2007 Mar 1;175(5):514–20. - PubMed

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