What has Karonga taught us? Tuberculosis studied over three decades

Abstract

This paper summarises tuberculosis (TB) research over almost 30 years in Karonga District, northern Malawi, an area typical of much of rural Africa. The dominant factor has been the human immunodeficiency virus (HIV), which arrived in the district about 1980, leading to an increase in TB incidence to a peak of approximately 65 smear-positive pulmonary cases per 100000 population in 2000. Tuberculin surveys indicate annual risks of Mycobacterium tuberculosis infection of approximately 1%; thus, most of the population is uninfected and at risk of primary infection and disease. Molecular epidemiological studies demonstrate that about two thirds of TB arises from recent infection, but recognisable recent contact is responsible for only about 10% of disease. By 2001, 57% of TB was directly attributable to HIV, implying that it would have declined were it not for HIV. HIV infection increases the risk of TB most among young adults, and greatly increases the risk of recurrence from new infection after treatment. Mortality rates in the HIV-infected are high, but there is no association of HIV with drug resistance. Other risk factors with relatively smaller effects include age and sex, contact, several genetic polymorphisms and area. Neither one nor two doses of the bacille Calmette-Guérin (BCG) vaccine provides protection against adult pulmonary TB, despite protecting against leprosy. Skin test surveys, cohort studies and comparative immunological studies with the UK suggest that exposure to environmental mycobacteria provides some protection against TB and that BCG's failure is attributable partly to this widespread heterologous exposure masking effects of the vaccine. Drug resistance has remained constant (<10%) over more than 20 years. Immunotherapy with M. vaccae provided no benefits, but treatment of HIV-positive patients with cotrimoxazole reduced mortality. The Karonga programme illustrates the value of long-term population-based studies to investigate the natural history of TB and to influence TB control policy. Current studies focus on immunological markers of infection, disease and protection, and on elucidating the impact of antiretroviral treatment on TB incidence at population level.

Figure 1

Map of Karonga

Figure 2

Frequency distributions of induration to RT23 in females with no BCG scar by age, Karonga district 1980-1984

Figure 3

Prevalence of induration greater than 10 mm, by age, sex and BCG scar status, Karonga 1980-1984.

Figure 4

DTH response to tuberculin and time since vaccination, among individuals aged <15 who were BCG scar negative at vaccination.

Figure 5

Rate of smear-positive pulmonary tuberculosis in adults aged 15-49, Karonga, 1998-2005.

Figure 6

Relative risk of TB in HIV-positive vs HIV-negative adults in northern Malawi, by age group

Figure 7

Ratio of male to female cases of smear-positive pulmonary tuberculosis by age group over time.