Infection rate based on quantitative real-time PCR of Melissococcus plutonius, the causal agent of European foulbrood, in honeybee colonies before and after apiary sanitation

Abstract - Since 1999, the incidence of European foulbrood (EFB) in Switzerland has been rising constantly. To understand the reason for this increase, an epidemiological study of the efficacy of sanitation measures was carried out. Workers from brood nests and flight entrances at infected apiaries were collected from colonies with and without clinical symptoms in 2005-2006. In order to quantify bacterial loads, a novel real-time PCR assay was developed for Melissococcus plutonius. Our data show that workers from brood nests have about 20-times higher bacterial loads than those from flight entrances, suggesting that the former are more suitable for EFB-monitoring. Moreover, current sanitation measures appear to be insufficient because only three out of eight apiaries were free of M. plutonius one year after sanitation. While no clinical symptoms are observed below 50 000 CFU of M. plutonius per bee, workers can nevertheless be carriers and likely responsible for bacterial propagation.