Susceptibility of Apis mellifera Larvae of Different Ages to Infection from Melissococcus plutonius, an European Foulbrood Disease-causing Pathogen

Abstract

European foulbrood (EFB) is a bacterial disease caused by Melissococcus plutonius. The pathogen is lethal to the infected larvae and the decomposition of dead larvae results in the development of a foul smell. This study aimed to evaluate the susceptibility of different Apis mellifera larva stages to M. plutonius in the apiary and the response of the honeybee colony to pathogen exposure. In total, 1069 larvae of different ages were artificially infected with M. plutonius. The larvae were allowed to be naturally fed by the nursing bees in the hive. The eggs, as well as the first instar and second instar larvae, were completely removed from the comb on day 7 post-infection. The survival rate of third instar larvae was 1.53% upon exposure to the pathogen (pathogen number: 2.67×10<SUP>6</SUP> to 2.67×10¹). The survival rate of the larvae increased to 37.39 and 59.09% of day 4 and day 6 larvae, respectively. These survival larvae developed without exhibiting any symptoms of EFB. However, polymerase chain reaction (PCR) detection showed that these larvae carried a large amount of M. plutonius, and could be a potential source of EFB disease in the colony. The result of this study could be helpful for understanding the progress of disease development when honeybee colony is exposed to the pathogen in natural condition, by which a strategy for efficient diagnosis and treatment of EFB could be established.