The intestinal honey bee parasite Nosema ceranae (Microsporidia) is at the root of colony losses in some regions while in others its presence causes no direct mortality. This is the case for Spain and France, respectively. It is hypothesized that differences in honey bee responses to N. ceranae infection could be due to the degree of virulence of N. ceranae strains from different geographic origins. To test this hypothesis, we first performed a study to compare the genetic variability of an rDNA fragment that could reveal differences between two N. ceranae isolates, one from Spain and one from France. Then we compared the infection capacity of both isolates in Apis mellifera iberiensis, based on the anatomopathological lesions due to N. ceranae development in the honey bee midgut, N. ceranae spore-load in the midgut and the honey bee survival rate. Our results suggest that there is no specific genetic background of the two N. ceranae isolates, from Spain or France, used in this study. These results agree with the infection development, honey bee survival and spore-loads that were similar between honey bees infected with both N. ceranae isolates. Probably, differences in honey bee response to infection are more related to the degree of tolerance of honey bee subspecies or local hybrids to N. ceranae, or experimental conditions in the case of laboratory trials, than to differences between N. ceranae isolates. Further studies should be done to estimate the contribution of each of these factors on the response of the honey bees to infection.
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