Abstract
BackgroundTo investigate how the risk of active tuberculosis disease is influenced by time since original infection and to determine whether the risk of reactivation of tuberculosis increases or decreases with age.MethodsCohort analysis of data for the separate ten year birth cohorts of 1876-1885 to 1959-1968 obtained from Statistics Norway and the National Tuberculosis Registry. These data were used to calculate the rates and the changes in the rates of bacillary (or active) tuberculosis. Data on bacillary tuberculosis for adult (20+) age groups were obtained from the National Tuberculosis Registry and Statistics Norway from 1946 to 1974. Most cases during this period arose due to reactivation of remote infection. Participants in this part of the analysis were all reported active tuberculosis cases in Norway from 1946 to 1974 as recorded in the National Tuberculosis Registry.ResultsTuberculosis decreased at a relatively steady rate when following individual birth cohorts, but with a tendency of slower decline as time passed since infection. A mean estimate of this rate of decline was 57% in a 10 year period.ConclusionsThe risk of reactivation of latent tuberculosis decreases with age. This decline may reflect the rate at which latent tuberculosis is eliminated from a population with minimal transmission of tubercle bacilli. A model for risk of developing active tuberculosis as a function of time since infection shows that the rate at which tuberculosis can be eliminated from a society can be quite substantial if new infections are effectively prevented. The findings clearly indicate that preventative measures against transmission of tuberculosis will be the most effective. These results also suggest that the total population harbouring live tubercle bacilli and consequently the future projection for increased incidence of tuberculosis in the world is probably overestimated.
Highlights
To investigate how the risk of active tuberculosis disease is influenced by time since original infection and to determine whether the risk of reactivation of tuberculosis increases or decreases with age
The data presented in this table were taken from Haider and Tverdal [20], and were included to show the status for the cohorts with respect to tuberculin reactivity, BCG vaccination and X-ray findings towards the end of the study period. 1Percentage of tuberculin positive persons that later turned tuberculin negative. 2Percentage of BCG vaccinated persons. 3Percentage of tuberculin negative persons that later turned tuberculin positive without the influence of BCG vaccination. 4Percentage of persons with evidence for previous tuberculosis. 5Values represent percentage of the total population of the birth cohort
It is necessary to invest more to trace and treat patients with smear positive tuberculosis with minimal diagnostic delay. This is essential to counteract the recent spread of hypervirulent strains of M. tuberculosis [30] with an increased tendency to develop multi-drug-resistant- and extensively drug-resistant tuberculosis [31]
Summary
To investigate how the risk of active tuberculosis disease is influenced by time since original infection and to determine whether the risk of reactivation of tuberculosis increases or decreases with age. The father had smear positive pulmonary tuberculosis with a one year case history in 1961, successfully treated for 12 months with streptomycin, isoniazid and para-aminosalicylic acid. His son was 7 years old, left the home in 1972 without further contact with his father, and presented with pulmonary tuberculosis in 1994. This identical RFLP pattern of the two isolates was not found in any other strain, either among 130 historical strains collected 1961-1967 or among 4008 recent strains collected 1992-1999 [8]
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