Infectivity and virulence of Nosema ceranae (Microsporidia) isolates obtained from various Apis mellifera morphotypes

Abstract

The infection of honey bees, Apis mellifera L. (Hymenoptera: Apidae), by the microsporidian Nosema ceranae is one of the factors related to the increase in colony losses and the decrease in honey production observed in recent years. However, these effects seem to differ depending on the climate zone. The range and prevalence of N. ceranae have increased significantly in the last decades, with different consequences in northern and southern temperate areas. The existence of various isolates of N. ceranae from distant geographical areas, which probably exhibit different degrees of virulence, could explain the different responses of the bee to the infection. The aim of this work was to compare the effects of two N. ceranae isolates from different host populations from Argentina on honey bee survival at two ages post-eclosion. Using cage experiments, we compared the development of infection of worker bees through the estimation of daily bee mortality and spore counts. Host subspecies identity analysis showed a strong similarity with Apis mellifera scutellata morphotype for the northern region, with a greater hybridization between subspecies with European origin toward the central and southern regions. Genetic characterization of isolates from the three regions indicated only the presence of N. ceranae. Infected bees survived longer than control bees, and bees infected at 5 days had a lower survival than those infected at 72 h with isolates from the three regions. These differences in survival matched the development of the N. ceranae infection, with differences in spore loads for infected bees at 5 days. Our studies showed that Nosema infection and survival varied among the different ages post emergence of workers, and both increased as the honey bee aged. These differences in susceptibility to infection could be related to the immune response of bees of different ages or to changes in the composition and succession of the intestinal microbiota throughout its ontogeny.