Fig 1.
Visual schematic of sampling collection from individual hives.
A total of 2 apiaries were sampled for each beekeeping operation (A, B, and C; letters denote operation ID): an apiary with an index case of clinically and laboratory confirmed AFB disease (index apiary) and a second apiary with no detected clinical evidence of AFB disease (unaffected apiary). Twenty randomly selected hives were sampled from the index apiary in each operation (represented by grey-shaded boxes), and ten randomly selected hives were sampled from each unaffected apiary. Samples of honey super honey (HS honey), brood chamber honey (BC honey), and brood chamber bees (BC bees) were collected from each sampled hive, when available.
Fig 2.
Concentrations of Paenibacillus larvae spores for individual hives from index and unaffected apiaries.
A single index apiary and single unaffected apiary were sampled from each operation (six apiaries total). Apiaries are grouped vertically by sample type (BC honey, HS honey, and BC bees) and horizontally by beekeeping operation (A, B, and C; letters denote operation ID). Dotted lines represent median values and bars represent interquartile ranges. ** denotes statistical significance where p < 0.01 (Wilcoxon rank sum test).
Table 1.
Summary statistics of concentrations of Paenibacillus larvae spores from individual hives.
Fig 3.
Comparison of concentrations of Paenibacillus larvae spores between HS honey and BC honey and BC bees within individual hives.
Data include all apiaries of all beekeeping operations. a) HS honey is positively correlated with BC honey (rs = 0.76, p < 0.0001); b) HS honey is positively correlated with BC bees, albeit less strongly (rs = 0.50, p < 0.0001). Hives with only one of the two samples under comparison are excluded from each plot. Circles = operation A, squares = operation B, triangles = operation C; black = hive from apiary unaffected by American foulbrood (AFB), red = hive from index apiary with a recent, confirmed case of AFB in a single hive.
Fig 4.
Spores of Paenibacillus larvae per gram of honey from pooled honey samples collected during routine extraction at the end of the honey-producing season for each beekeeping operation.
Each operation submitted 18 samples representing six randomly selected apiaries or lots; three unique samples were collected from each apiary or lot. Dotted lines represent median values and bars represent interquartile ranges. The solid grey line represents an arbitrary threshold value of one spore per gram of honey. Capital letters denote operation ID. ** denotes statistical significance where p < 0.01; **** denotes statistical significance where p < 0.0001 (Kruskal-Wallis test).
Fig 5.
Conceptual representation of the detection of a spore “signal” in pooled, apiary-level honey samples.
The corresponding apiaries the pooled samples are derived from are indicated by letters for operations A, B, and C. Small boxes within an apiary represent individual bee hives, and red boxes represent those hives heavily contaminated with spores of P. larvae. A lighter “signal” (light pink/red) in the pooled samples reflects operations with very little contamination in individual hives, whereas higher concentrations of spores (stronger “signal”–red) in pooled samples correspond to more widespread and/or chronic contamination. Index and unaffected apiaries are used in this theoretical example.