Seasonality of tuberculosis in an Eastern-Asian country with an extreme continental climate.

Aggregate monthly notifications of incident sputum smear-positive tuberculosis (TB) in Mongolia, stratified by sex and age groups, were analysed separately for Ulaanbaatar, Mongolia, and the rest of the country for the 9-yr period from 1998 to 2006. TB notifications were compared with ambient surface temperature. More than twice as many TB cases were notified in the peak month (April) compared with the trough months (October-December), paralleling the temperature curve. The fluctuations recurred consistently over the entire observation period, were identical in the capital compared with the rest of the country, and were independent of age and sex. TB notifications parallel the temperature amplitudes and have a magnitude not reported elsewhere. We hypothesise that the influence of temperature on life either indoors or outdoors is consistent with the transmission probability of Mycobacterium tuberculosis and the subsequent delay to disease recognition and notification with the incubation period, possibly co-determined by other factors, rather than accessibility to services.