Rapid Gastrointestinal Passage May Protect Bombus terrestris from Becoming a True Host for Nosema ceranae.

Pollination provided by managed honey bees as well as by all the wild bee species is a crucial ecosystem service contributing to the conservation of biodiversity and human food security. Therefore, it is not only the health status of honey bees but also the health status of wild bees that concerns us all. In this context, recent field studies suggesting interspecies transmission of the microsporidium parasite from honey bees () to bumblebees ( spp.) were alarming. On the basis of these studies, was identified as an emerging infectious agent (EIA) of bumblebees, although knowledge of its impact on its new host was still elusive. In order to investigate the infectivity, virulence, and pathogenesis of infections in bumblebees, we performed controlled laboratory exposure bioassays with by orally inoculating the bees with infectious spores. We comprehensively analyzed the infection status of the bees via microscopic analysis of squash preparations, PCR-based detection of DNA, histology of Giemsa-stained tissue sections, and species-specific fluorescence hybridization. We did not find any evidence for a true infection of bumblebees by Through a series of experiments, we ruled out the possibility that spore infectivity, spore dosage, incubation time, or age and source of the bumblebees caused these negative results. Instead, our results clearly demonstrate that no infection and production of new spores took place in bumblebees after they ingested spores in our experiments. Thus, our results question the classification of as an emerging infectious agent for bumblebees. Emerging infectious diseases (EIDs) pose a major health threat to both humans and animals. EIDs include, for instance, those that have spread into hitherto naive populations. Recently, the honey bee-specific microsporidium has been detected by molecular methods in field samples of bumblebees. This detection of DNA in bumblebees led to the assumption that infections represent an EID of bumblebees and resulted in speculations on the role of this pathogen in driving bumblebee declines. In order to address the issue of whether is an emerging infectious agent for bumblebees, we experimentally analyzed host susceptibility and pathogen reproduction in this new host-pathogen interaction. Surprisingly, we did not find any evidence for a true infection of by , questioning the classification of infections as EIDs of bumblebees and demonstrating that detection of microsporidian DNA does not equal detection of microsporidian infection.