41st Annual Meeting of the Society for Invertebrate Pathology
and 9th International Conference on Bacillus thuringiensis

The relationship between Nosema bombi and its bumble bee hosts has long been controversial. While its route of infection within hosts is well-known, transmission routes among hosts have remained unclarified. Furthermore, the impact of the parasite on host fitness (its virulence or pathogenicity) has been reported as high, non-existent or even, counter-intuitively, beneficial. Most of this confusion is due to a lack of controlled experiments. Recent work in a number of groups has considerably advanced our knowledge. In this talk I will review our current understanding and give directions for future work on this important host-parasite relationship

Phylogenetically, Microsporidia are now considered highly specialised parasitic fungi. They are all intracellular parasites with a characteristic and unique mode of infection. Microsporidia may infect all life forms and undoubtedly, only a small fraction of the actual number of species have been characterised. In Hymenopteran pollinators, microsporidia infections have been described from four host species only: Nosema apis infecting the European honey bee, Apis mellifera; Nosema ceranae infecting the Asian honey bee, Apis cerana; Nosema bombi, infecting Bombus spp. and Antonospora scoticae infecting Andrena scoticae. N. apis and N. ceranae are cross infective between hosts. However, N. apis does not do well in A. cerana, whereas there is a worldwide process of N. ceranae replacing N. apis in A. mellifera. N. bombi has recently become of particular interest for conservationists, since this parasite may be distributed to areas assumed free from this parasite, thereby presumably endangering endemic bumble bee spp. Furthermore, within-genome rRNA variability in N. bombi suggests that to characterize intraspecific genetic variants in the Microsporidia based on RNA sequences is not straight forward. A. scoticae infects the fat body tissue of A. scotica and may occur with an extreme prevalence in its host.