Histopathological Impact in the Larval Gut of the Honeybee, Apis mellifera jemenitica, Upon Infection with the American Foulbrood Bacterium, Paenibacillus larvae

Background: American foulbrood (AFB), caused by Paenibacillus larvae (P.larvae) bacterium, is one of the major threatening disease to the global apiculture industry. This Apiary-threatening bacterium has recently been detected in some of the Saudi Apiaries. The current study herein was conducted to investigate the impact of the locally isolated P. larvae infection on the histological integrity of the alimentary tract of the indigenous Saudi honeybee brood, Apis mellifera jemenitica (A.mellifera jemenitica). Methods: The 1st instar larvae were orally infected with the P. larvae. Non-infected (control) or infected (at 48 and 72h post-treatment) alimentary canals were dissected for histological investigations. Alimentary canals were separated into three parts; foregut (proventriculus), midgut (ventriculus) and hind gut (intestine) prior to histological investigation. Gut sections were subject to light and electron microscopy investigations at 48 and 72h post infection. Results: Data showed histopathological alterations in the infected alimentary canal as early as 48h post-P. larvae infection. Infected proventriculus showed many vacuoles, separation of the epithelial layer and damage of circular muscles. Lacerations were observed in the basement membrane surrounding the midgut in the infected larvae. Vacuoles of different sizes were also observed in the hindgut of infected larvae. Moreover, aggregations of P. larvae bacterial cells were detected in the gut epithelium of infected larvae. At 72h post-P. larvae infection, infected broods showed separation and elongation of the midgut epithelial cells. These histological alterations may have led to the severe laceration observed in the layers of infected stomach. Finally, deterioration of muscular and epithelial layers of infected stomach was observed at 48h post-infection. In addition, increased vacuoles were also observed in the epithelial cells of the hindgut of infected larvae. These histological alterations have been confirmed on the subcellular level via transmission electron microscopy as it showed subcellular degradation in terms of degraded mitochondria and nuclei. Conclusion: These histopathological deteriorations are similar to that globally recorded in the AFB, and hence, provides warning alarm to the local authorities for taking immediate actions towards restricting the epidemics of this disease within the Saudi Apiaries. In parallel, utilizing the larval immune system against P. larvae infection is currently under investigation for producing natural pharmaceutical treatment of the AFB.