D3234: Comparative analysis of Nosema infection level in individual bees vs. pooled honey bee samples

The fungal pathogens Nosema apis and Nosema ceranae cause Nosemosis, a severe intestinal disorder in honey bees, associated with overwinter colony losses in Western Canada. The widely accepted method for determining Nosema colony infection level for risk assessment and antifungal treatment (fumagillin treatment threshold ≥ 10⁶ spores/bee) is based on microscopic spore counts of 60 pooled worker bees. However, a colony can consist of up to 50,000 individuals and a single worker bee can carry over 50 million Nosema spores. Hence, pooled samples containing low numbers of heavily infected bees among high numbers of healthy bees may misrepresent the overall colony infection level, resulting in decisions for antifungal treatment of predominantly uninfected bees. Nosema surveillance in our research colonies during summer 2021 revealed that two sub-samples collected in one day from the same colony showed a tremendous difference in spore counts and Nosema spp. composition. Therefore, we collected samples from artificially inoculated colonies to correlate the frequency and severity of Nosema infection in individual bees to average spore counts in pooled samples containing incremental numbers of bees. This data allows us to discuss an optimal sampling strategy that can be easily performed by beekeepers in the field to distinguish between high and low infected colonies. Furthermore, the resulting reliable prediction of risk of clinical Nosemosis can ultimately help for strategic decision of evidence-based fumagillin therapy to prevent from potential risk for antimicrobial resistance.