| Summary: | (1) Background: Microsporidiosis (nosemosis) is an intestinal disorder of adult honey bees caused by the microsporidian pathogens <i>Vairimorpha apis</i> and <i>Vairimorpha ceranae.</i> In Canada, fumagillin is an approved antibiotic used to treat this disease. However, the recommended dosage is based on efficacy studies for <i>V. apis</i>, the native pathogen in European honey bees. Since the detection of <i>V. ceranae</i> in <i>Apis mellifera</i>, <i>V. ceranae</i> became more prevalent in managed European honey bees and seems to have replaced <i>V. apis</i> due to yet unknown reasons. (2) Methods: This colony study investigated the efficacy of fumagillin administered in the fall to colonies infected with both <i>V. apis</i> and <i>V. ceranae</i> and its effects on the <i>Vairimorpha</i> species’ prevalence overwinter. Spore loads in control and fumagillin-treated colonies were analysed by microscopy; <i>Vairimorpha</i> species prevalence was determined molecularly and infection and treatment effects on colony productivity were assessed. (3) Results: Fall fumagillin treatment was associated with a temporary reduction in spore load, but there was no difference in spore loads between treated and control colonies the following spring. Interestingly, fumagillin-treated colonies had a significantly greater prevalence of <i>V. ceranae</i> relative to <i>V. apis</i> the following spring, suggesting fumagillin is less effective in controlling <i>V. ceranae</i>.
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