Infection parameters for Nosema ceranae and Nosema apis in Apis mellifera [abstract]

Nosema ceranae, a recently described microsporidian pathogen of the honey bee, Apis mellifera, is recognized as a global problem, but there are few clues about the extent of its occurrence or spread prior to 2006. In addition, the nature of its interaction with the familiar congener Nosema apis is not well understood and data from laboratories in different countries are conflicting. We compared the development, virulence, infectivity (IC50), spore production, temperature effects and direct competition in the same host of the two pathogens using U.S. isolates and bees. We found no significant difference in virulence between the two species but IC50 studies demonstrated that a 5x higher dosage of N. ceranae is needed for infection. The host response (mortality) to infection at different temperatures was not significantly different between the two pathogens. At a dosage of 1 x 105 spores per bee (200 x lowest IC50 for N. ceranae), N. ceranae produced slightly more spores than N. apis over the infection period, but when individual bees were inoculated with 1 x 105 spores of each Nosema species to produce mixed infections, N. apis produced more spores. When viruses were combined with microsporidia at different temperatures, mortality trended higher but was not significantly different from mortality due to microsporidia alone. Different results among laboratories have not been explained, but may be the result of differences in host genotypes or subspecies, or pathogen strains, and the reasons for the apparent replacement of N. apis by N. ceranae remain largely unknown.;