High detection rate of the virulence plasmid pMP19 from <i>Melissococcus plutonius</i>-positive honey and larvae samples

European foulbrood caused by <i>Melissococcus plutonius</i> is a serious infectious disease of honey bee brood. pMP19 is known as an important virulence plasmid of <i>M. plutonius</i>. Nevertheless, pMP19-negative <i>M. plutonius</i> has often been isolated from EFB cases. Since pMP19 is unstable in <i>M. plutonius</i> during propagation in laboratories, such pMP19-negative isolates may have lost this plasmid during the cultivation steps for isolation. Therefore, the retention rate of pMP19 in <i>M. plutonius</i> in the field may be higher than that of isolates. To gain insight into the distribution of pMP19-positive <i>M. plutonius</i> in the field, we directly detected pMP19 by PCR from 103 Japanese honey and 41 diseased larvae samples that were known to be contaminated by <i>M. plutonius</i> and revealed that all the <i>M. plutonius</i>-positive samples were also positive for pMP19. These results suggest high pMP19 prevalence in <i>M. plutonius</i> in the field and provide deeper insight into this disease.