European isolates of the Microsporidia Nosema apis and Nosema ceranae have similar virulence in laboratory tests on European worker honey bees

Nosema apis and Nosema ceranae are gut parasites that infect western honey bees (Apis mellifera) worldwide. N. ceranae is an exotic infectious disease agent of A. mellifera, having been originally described in the Asian honey bee (Apis cerana), while N. apis is native to the western honey bee. To better understand the dynamics and epidemiology of the two pathogens, we examined the impact of European isolates of both Microsporidia on the longevity of European A. mellifera in a controlled laboratory experiment. N. ceranae caused slightly higher host mortality compared to N. apis, but differences in virulence were subtle and non-significant. Variation across published studies may reflect geographic differences in the coadaptation of hosts and parasites and seasonal differences in host susceptibility.