Nosema ceranae, a newly identified pathogen of Apis mellifera in the USA and Asia

Globalization has provided opportunities for parasites/pathogens to cross geographic boundaries and expand to new hosts. Nosema disease is one of the most serious adult honey bee diseases and has high prevalence in honey bee colonies. For years, Nosema apis was thought to be the only microsporidian infecting domestic bee colonies. However, recently it was discovered that N. ceranae could cross the species barrier from Asian honey bees (Apis cerana) to European honey bees (Apis mellifera) that are widely used for crop pollination and honey production. Over the last few years, considerable progress has been made in our understanding of Nosema infections in honey bees. This review summarizes previous findings and recent progress in the understanding of Nosema infection of A. mellifera in the USA and Asia, with particular emphasis on the comparative epidemiological, morphological, pathological, and genomic organization of two Nosema species. The prospects of future research and remaining unresolved questions associated with the study of honey bee Nosema diseases are also discussed.