Chinese Sacbrood virus infection in Asian honey bees (Apis cerana cerana) and host immune responses to the virus infection

Chinese Sacbrood virus (CSBV) is a positive-stranded RNAvirus that infects both the European honey bee (Apis mellifera) and the Asian honey bee (A. cerana). However, CSBV has much more devastating effects on Asian honey bees than on European honey bees, posing a serious threat to the agricultural and natural ecosystems that rely on A. cerana for pollination service. Using quantitative RT-PCR method, we conducted studies to examine the CSBV infection in Asian honey bee colonies and immune responses of individual bees in response to CSBV infection. Our study showed that CSBV could cause infection in different developmental stages of workers including eggs, larvae, pupae, newly emerged workers, and foraging workers. In addition, evaluating the tissue tropism and transmission of CSBV in infected bees showed that CSBV was detected in the ovaries, spermatheca, and feces of queens as well as semen of drones of the same colonies, suggesting an existence of vertical transmission of CSBV in Asian honey bees. Further, the detection of CSBV in colony food suggests that healthy bees could pick the infection by the virus-contaminated food, and therefore, a possible existence of a food-borne transmission pathway of CSBV in Asian bee colonies. The expression analysis of transcripts (defensin, abaecin, apidaecin, and hymenoptaecin) involving innate antiviral immune pathways showed that CSBV infection could induce significant immune responses in infected bees. However, the immune responses to CSBV infection varied among different development stages with eggs exhibiting the lowest level of immune expression and forager workers exhibiting the highest level of immune gene expression. The results obtained in the study yield important insights into the mechanisms underlying disease pathogenesis of CSBV infections in Asian honey bees and provide valuable information for a rational design of disease control measures.