Long-Term Temporal Trends of spp. Infection Prevalence in Northeast Germany: Continuous Spread of , an Emerging Pathogen of Honey Bees (), but No General Replacement of .

The Western honey bee () is widely used as commercial pollinator in worldwide agriculture and, therefore, plays an important role in global food security. Among the parasites and pathogens threatening health and survival of honey bees are two species of microsporidia, and is considered an emerging pathogen of the Western honey bee. Reports on the spread of suggested that this presumably highly virulent species is replacing its more benign congener in the global population. We here present a 12 year longitudinal cohort study on the prevalence of and in Northeast Germany. Between 2005 and 2016, a cohort of about 230 honey bee colonies originating from 23 apiaries was sampled twice a year (spring and autumn) resulting in a total of 5,600 bee samples which were subjected to microscopic and molecular analysis for determining the presence of infections with or/and . Throughout the entire study period, both - and -infections could be diagnosed within the cohort. Logistic regression analysis of the prevalence data demonstrated a significant increase of -infections over the last 12 years, both in autumn (reflecting the development during the summer) and in spring (reflecting the development over winter) samples. Cell culture experiments confirmed that has a higher proliferative potential than at 27° and 33°C potentially explaining the increase in prevalence during summer. In autumn, characterized by generally low infection prevalence, this increase was accompanied by a significant decrease in -infection prevalence. In contrast, in spring, the season with a higher prevalence of infection, no significant decrease of infections despite a significant increase in infections could be observed. Therefore, our data do not support a general advantage of over and an overall replacement of by in the studied honey bee population.