The Diagnostic Value of qPCR Quantification of Paenibacillus larvae in Hive Debris and Adult Bees for Predicting the Onset of American Foulbrood

American foulbrood (AFB) is a serious infectious disease of honeybees (<i>Apis mellifera</i>) caused by <i>Paenibacillus larvae</i>. Increased <i>P. larvae</i> count in hive-related material is associated with an increased risk of AFB. Here, we quantified <i>P. larvae</i> cells in 106 adult bee and 97 hive debris samples using quantitative PCR (qPCR); 66/106 adult bee and 66/97 hive debris samples were collected simultaneously from the same bee colony (paired-sample design). The corresponding bee colonies were also examined for the presence of AFB clinical signs. A binary logistic regression model to distinguish between AFB-affected and unaffected honeybee colonies showed a strong diagnostic accuracy of both sample types for predicting the onset of AFB based on <i>P. larvae</i> counts determined by qPCR. The colonies with a <i>P. larvae</i> count greater than 4.5 log cells/adult bee or 7.3 log cells/mL hive debris had a 50% probability of being clinically affected and were categorized as high-risk. The AFB-unaffected colonies had significantly lower <i>P. larvae</i> counts than the AFB-affected colonies, but the latter did not differ significantly in <i>P. larvae</i> counts in relation to the severity of clinical signs. Both bee-related sample types had a high diagnostic value for predicting disease outcome based on <i>P. larvae</i> counts. These results improve the understanding of the relationship between <i>P. larvae</i> counts and AFB occurrence, which is essential for early detection of high-risk colonies.