A statistical analysis of the seasonality in pulmonary tuberculosis

The present study examines whether pulmonary tuberculosis (PTB) has an annual seasonal pattern. A mathematical model is also obtained to forecast the pattern of incidence. The data for the study are the cases of PTB reported throughout Spain, published in the Epidemiology Bulletin by the Carlos III Health Center of the Spanish Ministry of Health in a 26-year period, 1971-1996. The analytical results show that the low rates in tuberculosis notifications over the period 1971-1981 have changed, halting in 1982 and reversing with high incidence from 1983 onwards. An annual seasonal pattern was also shown with higher incidence during summer and autumn. With the mathematical model we predicted the disease behaviour in 1997 and the results were compared to the reported cases. In Spain, as in several industrialised countries, the reason for this recent increase in the number of reported cases is, mainly, the human immunodeficiency virus (HIV) infection. The seasonal trend, with higher incidence in winter, can be attributed to the increase in indoor activities, much more common than in a warm climate. The tubercle bacilli expelled from infected persons in a room with closed windows may remain infectious for a long time, increasing the risk of exposure of healthy persons to the bacilli. As the preclinical period, from exposure to clinical onset, may be of several weeks, the high incidence in spring would be explained. Moreover, in winter and spring the infections of viral aetiology, like flu, are more frequent and cause immunological deficiency which is another reason for the seasonal trend observed. An incidence greater than that foreseen by the mathematical model would express a failure in epidemiologic surveillance, and thus the results of this study may be used to assess a quality of the preventive measures.