Cyclicity in incidence variations of meningococcal infections in Bulgaria is similar to that of solar activity.

This is a retrospective study on meningococcal meningitis (MM) in Bulgaria that has, for the first time, reported results on non-linear temporal patterns of incidence and its variations. Methods of descriptive statistics, linear and non-linear modelling as well as periodogram regression analysis have been applied. A non-linear decreasing trend in crude incidence rates per 100 persons over the years 1940-1990 has been described (p < 0.0001) and cyclic variations revealed (periods T = 8.00, 18.75, 24.75 and 33.50 years, p < 0.05-0.01). Above cycles have been detected after the reciprocal trend has been removed (y = 1.04 + 15.78/t). A similar cyclicity (periods T = 8.25 and 27.5 years) in the variations of solar activity (sunspot number Rz) over the same time interval of 51 years has been established after the main cycle of 10.5-13 years has been removed by a two-step procedure. The results from this study have added to our previous findings on cyclic variations in mortality and lethality from meningococcal infections in Bulgaria (1, 2). Above similarity is also in accordance with earlier conclusions on relations of solar activity cycles with epidemics of cerebrospinal meningitis in New York and USA over the years 1800-1935 (3, 4).