Seasonality in pulmonary tuberculosis among migrant workers entering Kuwait

Background: There is paucity of data on seasonal variation in pulmonary tuberculosis ( TB) in developing countries contrary to recognized seasonality in the TB notification in western societies. This study examined the seasonal pattern in TB diagnosis among migrant workers from developing countries entering Kuwait. Methods: Monthly aggregates of TB diagnosis results for consecutive migrants tested between January 1, 1997 and December 31, 2006 were analyzed. We assessed the amplitude (alpha) of the sinusoidal oscillation and the time at which maximum (theta degrees) TB cases were detected using Edwards' test. The adequacy of the hypothesized sinusoidal curve was assessed by chi(2) goodness-of-fit test. Results: During the 10 year study period, the proportion (per 100,000) of pulmonary TB cases among the migrants was 198 (4608/2328582), (95% confidence interval: 192 - 204). The adjusted mean monthly number of pulmonary TB cases was 384. Based on the observed seasonal pattern in the data, the maximum number of TB cases was expected during the last week of April (theta degrees = 112 degrees; P < 0.001). The amplitude (+/- se) (alpha = 0.204 +/- 0.04) of simple harmonic curve showed 20.4% difference from the mean to maximum TB cases. The peak to low ratio of adjusted number of TB cases was 1.51 ( 95% CI: 1.39 - 1.65). The chi(2) goodness-of-test revealed that there was no significant (P > 0.1) departure of observed frequencies from the fitted simple harmonic curve. Seasonal component explained 55% of the total variation in the proportions of TB cases (100,000) among the migrants. Conclusion: This regularity of peak seasonality in TB case detection may prove useful to institute measures that warrant a better attendance of migrants. Public health authorities may consider reallocation of resources in the period of peak seasonality to minimize the risk of Mycobacterium tuberculosis infection to close contacts in this and comparable settings in the region having similar influx of immigrants from high TB burden countries. Epidemiological surveillance for the TB risk in the migrants in subsequent years and required chemotherapy of detected cases may contribute in global efforts to control this public health menace.