Effects of varroa mites on the immune system and pathology of honey bees

Varroa mites (Varroa destructor) are serious ecto-parasites of honey bees (Apis mellifera), causing severe bee colony loss worldwide. This thesis research focuses on the interactions among the parasite, host, and pathogen, using the mite-bee-virus system as a model to reveal the biochemical principles behind the immunological interactions among them, as well as to provide knowledge for the control of the harm from varroa mites. I examined the effects of varroa mites on the bee's physiological aspects, survivorship, hemocytes profiles, cellular immune responses, expression of genes encoding antimicrobial peptides (abaecin, defensin, and hymenoptaecin) and immunity-related enzymes (phenol oxidase (PO), glucose dehydrogenase (GILD), glucose oxidase (GOX), and lysozyme), amplification of deformed wing virus (DWV), and activities of the antimicrobial peptides and enzymes. Varroa mites reduced the body weight, total soluble protein concentration, and survivorship of honey bees, and adversely changed bee hemocyte profiles and cellular immune responses. Varroa mites significantly suppressed the expression of genes encoding these antimicrobial peptides and immunity-related enzymes at both transcription and protein levels. Varroa mites drastically promoted the replication of DWV in bees. Deformed wing bees were heavily infected by DWV. DWV titers were significantly positively related to mite density but negatively related to the immunity-related enzyme mRNA levels. It could be concluded that the mechanism of how varroa mites kill honey bees is by significantly suppressing bee immunity in both cellular and humoral immune responses and by boosting the replication of DWV. The immune system of varroa-infested, especially the deformed wing bees, was heavily impaired. This serves the first evidence that an ecto-parasite immunosuppresses its invertebrate host. The correlations among the gene expressions of antimicrobial peptides and immunity-related enzymes were also illustrated. This research also interestingly demonstrated that the replication of DWV needs both varroa infestation and a bacterial challenge, which can lead to a practical application for controlling the harm of varroa mites to honey bees by using antibiotics in beehives. To further reveal the mechanism of this immunosuppression, using proteomics approaches, I studied on the collection, purification, and identification of varroa salivary proteins. Mass spectrometry analyses demonstrated that these salivary proteins may be unique.