Aflatoxin M1 is a carcinogenic and genotoxic metabolite of Aflatoxins present in food contaminated by fungi for
lactating cattle, it is excreted through milk and when used to make cheese, the toxin will also be transferred to
the dairy. The contamination of unripened hard white cheese with AFM1 seems to vary according to the season of
the year, possibly due to the change of foodstuff, from fresh pasture in the rainy season to dried foods in the dry
season and vice versa. This research determined both the prevalence and contents of AFM1 in cheeses of local and
Nicaraguan origin marketed in El Salvador, as well as the changes occurred according to the season and the
association between levels of AFM1 with meteorological parameters. The significantly higher prevalence of AFM1
contamination in both local cheeses and Nicaraguans, was found in the dry season and the lowest in the rainy
season (41 % vs. 20 %; 31 % vs. 0%, respectively), the same trend was observed in AFM1 contents (0.076 vs.
0.036 μg/kg; 0.050 vs. 0.021 μg/kg, respectively). A significant association was demonstrated between levels of
AFM1 with the averages of accumulated rainfall and relative humidity according to the sampled season. The
prevalence of AFM1 in cheeses indicate that El Salvador and Nicaragua are endemic to dairy contamination by
that mycotoxin. Seasonal variation may be due to a lack of rainfall, that promotes the growth of aflatoxigenic
fungi in the crops of raw materials, which will be used for feedstuff intended for dairy cattle, thus, the consumption
of contaminated food will cause the temporary increase of AFM1 in milk and their derivatives.