Clustering of American foulbrood in managed honey bees (Apis mellifera) in Ontario, 2015-2020.

American foulbrood (AFB) is a highly virulent bacterial disease of managed honey bee colonies. The objectives of this study were to analyze retrospective honey bee inspection records to summarize the descriptive epidemiology of AFB inspection data in Ontario, examine temporal trends of AFB prevalence, and assess the geographic distribution of AFB cases at the colony, apiary, and operation level. Analysis was performed on 2,789 records collected by the Ontario Ministry of Agriculture, Food and Rural Affairs apiary program on registered honey bee colony inspections during the 2015 through 2020 beekeeping seasons. Observed prevalence was calculated at the colony, apiary, and operation level; within hobbyist and commercial beekeeping operations; and for each year. Smoothed prevalence data using Bayesian estimation was used for choropleth mapping and Moran’s I analysis. The data was examined for high and low disease prevalence clusters using the flexible spatial scan test. Prevalence of AFB was 1.6%, 4.2%, and 5.8% at the colony, apiary, and operation levels, respectively. There were no apparent temporal or spatial trends in disease occurrence. Moran’s I correlation coefficient was 0.09, 0.13 and -0.01 at the colony, apiary, and operation level, respectively. The flexible spatial scan test identified two disease clusters at the colony level with standardized mortality ratios (SMRs) of 4.54 (95% CI=3.62-5.57) and 2.42 (95% CI=2.03-2.85). One disease cluster was detected at the apiary level with a SMR of 3.14 (95% CI=2.08-4.41). The findings were consistent with low levels of endemic AFB within the Ontario honey bee population. Spatial analysis was supportive of the main mode of AFB transmission occurring within beekeeping operations as a result of management practices. These results will be useful for veterinarians working on AFB management protocols within operations, and to inform research on
AFB mitigation and surveillance strategies.