The distribution of Paenibacillus larvae spores, the causative agent of American foulbrood, was studied on three different levels in the honey bee system; the apiary level, the colony level and the individual honey bee level. The increased understanding of spore distribution has been used to give recommendations regarding sampling of adult honey bees. The vertical transmission of P. larvae spores through natural swarms has been described for the first time and artificial swarming as a method for control of American foulbrood have been evaluated. The results demonstrated that there is no practical difference in spore load between supers and brood chambers, and that the spore load in samples of adult honey bees on the different levels correspond to the clinical disease status of the colony. The study on individual bees showed that spores are unequally distributed among the bees and that as more bees get contaminated each positive bee also contains more spores. This may present a problem when sampling from colonies with low levels of clinical disease, although the study on colony and apiary level showed no false negatives. A model for calculating the number of bees that needs to be sampled to detect P. larvae in a composite sample of adult bees, given certain detection levels and proportions of positive honey bees in the sample, was developed The swarm study demonstrated vertical transmission of P. larvae spores. Furthermore, the artificial swarm study showed that single and double shaking are equally effective treatment methods, and that the original disease status is of little importance for the spore load decrease
