Background Host-related and environment-related factors have been shown to play a role in the development of tuberculosis (TB), but few studies were carried out to identify their respective roles in resource-poor countries.

Few studies have investigated the risk factors for tuberculosis (TB) infection in highly endemic countries. We conducted a household study in The Gambia, in which a tuberculin skin test (TST) was performed in members of the households of 315 smear-positive pulmonary TB cases and 305 community control subjects. The risk of being TST positive (10 mm or more) was higher in contacts of cases than in contacts of control subjects. It increased with age, male sex, and duration of stay in the household but was not associated with the presence of a bacille de Calmette-Guérin scar. Within the households of the TB cases, the risk of TST positivity was higher in males and was increased with age, social proximity to the case, and the radiologic extent of the disease in the case's chest X-ray. Adjusting on these, the risk of TST positivity was higher in first-degree relatives compared with more distant relatives and nongenetically related household members, but the effect was not statistically significant. In highly endemic areas, the risk of TB infection in contacts of TB infectious cases is associated with age, sex, intensity of exposure to the case, and severity of disease in the case, but it is possible that genetic factors contribute to the susceptibility to Mycobacterium tuberculosis infection.

Activation of Th1 lymphocytes, IFN‐γ production and macrophage activation are crucial in defense against Mycobacteria. In developing countries, Th2 activation and IL‐4 production have been associated in vitro with tuberculosis and with poor clinical outcome after treatment. Serological markers of Th1 [soluble lymphocyte activation gene (LAG)‐3] and Th2 (IgE, solubleCD30, and CCL22/macrophage‐derived chemokine) activity were measured in 414 HIV‐negative tuberculosis patients from The Gambia and Guinée and in 414 healthy household and community controls. Measurements were repeated during treatment to assess the effect of therapy on Th1/Th2 ratio. At diagnosis, sLAG‐3 levels were lower in patients than in community controls (p<0.0001), but were higher in household controls exposed to contact with patients than in community controls (p<0.0001). In comparison with community controls, patients had consistently higher levels of IgE, sCD30, and CCL22 (p<0.0001), whereas household controls had lower levels of indicators of Th2 activity (p<0.0001). After treatment, cured patients had higher levels of Th1 (p<0.0001) and lower levels of Th2 (p<0.0001) activity than patients who were not successfully treated or interrupted therapy. In Africa, tuberculosis is associated with low Th1 and high Th2 activity in vivo, whereas close exposure to tuberculosis is associated with a high Th1/Th2 ratio. Patients with favorable outcome after treatment exhibit a higher Th1/Th2 ratio compared to patients with poor clinical outcome.