Impact of an ectoparasite on the immunity and pathology of an invertebrate: evidence for host immunosuppression and viral amplification

Abstract

Varroa mites (Varroa destructor) are ectoparasites of honey bees (Apis mellifera) and cause serious damage to bee colonies. The mechanism of how varroa mites kill honey bees remains unclear. We have addressed the effects of the mites on bee immunity and the replication of a picorna-like virus, the deformed wing virus (DWV). The expression of genes encoding three antimicrobial peptides (abaecin, defensin, and hymenoptaecin) and four immunity-related enzymes (phenol oxidase, glucose dehydrogenase, glucose oxidase, and lysozyme) were used as markers to measure the difference in the immune response. We have demonstrated an example of an ectoparasite immunosuppressing its invertebrate host with the evidence that parasitization significantly suppressed expression of these immunity-related genes. Given that ticks immunosuppress their vertebrate hosts, our finding indicates that immunosuppression of hosts may be a common phenomenon in the interaction and coevolution between ectoparasites and their vertebrate and invertebrate hosts. DWV viral titers were significantly negatively correlated with the expression levels of the immunity-related enzymes. All bees had detectable DWV. Mite-infested pupae developed into adults with either normal or deformed wings. All of the deformed-wing bees were greatly infected by DWV (approximately 10(6) times higher than varroa-infested but normal-winged bees). Injection with heat-killed bacteria dramatically promoted DWV titers (10(5) times in 10 h) in the mite-infested, normal-winged bees to levels similar to those found in mite-infested, deformed-wing bees. Varroa mites may cause the serious demise of honey bees by suppressing bee immunity and by boosting the amplification of DWV in bees exposed to microbes.

Fig. 1.

The relationship between bee wing deformity and the density of varroa mites. (A) A scanning electron microscope photograph of a female varroa mite. (B) A newly emerged worker bee with a typical symptom of deformed wings with two mites on her thorax as the white arrows indicate. (C) Linear regression analysis for the relationship between the degree of wing deformity and the number of mites per cell (mean ± SEM). Three groups of newly emerged worker bees were distinguished: DW, NW, and MF. (A is used with the permission of Scott Camazine; B is used with the permission of Maryann Frazier.)

Fig. 2.

The effect of varroa infestation on the expression of genes encoding the antimicrobial peptides in newly emerged worker bees: DW, NW, and MF. The bees were either injected with bee saline or heat-killed E. coli suspended in the bee saline. NTC refers to the nontreatment control (i.e., the MF bees without injection). All of the values shown are mean ± SEM. (A) Varroa infestation suppressed the expression of hymenoptaecin gene. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's protected least significant difference (PLSD), P ≤ 0.0003). (B) Varroa infestation suppressed the expression of defensin gene in the DW bees injected with bacteria. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.049). (C) Varroa infestation suppressed the expression of abaecin gene in the DW bees injected with the bacteria. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.0029).

Fig. 3.

The effects of varroa infestation on the expression of genes encoding the immunity-related enzymes in newly emerged worker bees: DW, NW, and MF. The bees were either injected with bee saline or heat-killed E. coli suspended in the saline. NTC refers to the nontreatment control (i.e., the MF bees without injection). All of the values shown are mean ± SEM. (A) The expression of GLD gene was suppressed in the mite-infested bees. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.0159). (B) The expression of GOX gene was suppressed in the varroa-infested bees. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.0386). (C) Varroa infestation suppressed the expression of PO gene. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.0495). (D) Varroa infestation suppressed the expression of lysozyme gene. The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.0069).

Fig. 4.

The effect of varroa infestation and microbial exposure on DWV genomic RNA levels (mean ± SEM) in newly emerged worker bees: DW, NW, and MF. Deformed-wing bees are heavily infected by DWV, and varroa infestation and microbial exposure boost the replication of DWV (note that the y axis is a log scale). The bees were either injected with bee saline or heat-killed E. coli suspended in the saline. NTC refers to the nontreatment control (i.e., the MF bees without injection). The bars with different letters are significantly different (ANOVA, P < 0.0001; pairwise comparison with Fisher's PLSD, P ≤ 0.042).