Studies in Sierra Leone and Nigeria suggest that Lassa fever is probably seasonal, occurring more often in the dry season than in the rainy season. Epidemiological data are scarce and some data are contradictory as the pattern is not identical from one year to the next. By studying the dynamics of the reservoir rodents, we will be able to verify whether this hypothesis is valid. Longitudinal study in Guinea, including sampling in the dry season, then at the beginning and end of the rainy season for two years, has shown a seasonal fluctuation in the number of rodents. Indeed, rodents are more numerous inside houses during the dry season than during the rainy season. The high aggregation of rodents during the dry season could explain the higher frequency of Lassa cases. However, they are 2 to 3 times less infected during the dry season. So, if the seasonality hypothesis is correct, it is not due to a higher prevalence within the rodent population, but rather to a higher frequency of contact between rodents and humans.
This frequency is therefore linked to human behaviour, which must be analysed using anthropological and sociological tools.