Searching for an American foulbrood early detection threshold by the determination of paenibacillus larvae spore load in worker honey bees

In the present study, the relationship between Paenibacillus larvae (White) spore load per adult bee and the appearance of American Foulbrood (AFB) symptoms in the colony was investigated. A total of 44 beehives from 8 apiaries located in Sardinia island (Italy) were involved in this research. Adult bee samples were collected from colonies either showing or lacking the disease symptoms, and the average number of P. larvae spores per adult bee was determined through microbiological culture, followed by bacterial identification through a combination of morphological, biochemical and genetic assays. Correspondence between the detection of disease symptoms in the hives and the number of spores per adult bee samples was obtained. In line with previous studies, the outcome of the present investigation supports the employment of adult bee spore load determination as a tool to get information on the actual health status of the colony. This allows an early detection of AFB disease, especially when clinical symptoms in colonies are still not observable. In addition, the results suggest that a level around 3,000 P. larvae spores per adult bee, obtained with our experimental conditions, might represent a threshold for the appearance of clinical symptoms in the colony.