Dynamics of tuberculosis infection in Sweden

Sweden provides a special setting for epidemiological and demographic studies of tuberculosis (TB) infection over time for principally two reasons; first, the Swedish TB epidemic has undergone a tremendous transition since the end of the 19th century, when TB was highly endemic, to the current situation with practically interrupted indigenous transmission since several decades. Second, an increasing proportion of persons who grew up before TB transmission virtually disappeared in the 1960s are reaching advanced age, and thus creating conditions that predispose to reactivation of latent TB infection (LTBI). Since it can be assumed that most subjects in these cohorts have LTBI, increasing rates of reactivation TB would be expected if they actually harbored viable TB bacilli. However, case rates in the indigenous Swedish population continue to decline suggesting that spontaneous clearance of LTBI occurs over time. Hence, data from Sweden can provide new insights into the natural course of LTBI. This thesis shows that about 10% of infected individuals ever develop active TB but the majority does so within the first years from infection. When transmission is halted in the society only about 2% reactivate LTBI. In our setting clearance of M. tuberculosis was shown to occur at an annual rate of 4.5% when following subjects with known year of infection.
The demographic pattern of TB in Sweden has turned to a disease principally affecting the immigrant population. Recent immigration from countries with high TB incidence constitutes the highest risk as shown by a fourfold higher risk for active TB among immigrants from the Horn of Africa in East Denmark compared to immigrants from the same area with residence in Scania, Sweden.
The use of an interferon-gamma release assay for identifying active TB was unsatisfactory, especially in cases of pulmonary disease, and thus the usefulness of the assay for this purpose is questionable. The test performed better in patients with suspected lymph node TB, and hence it might be considered for inclusion in the diagnostic protocol in that context.