Wolbachia effects on Rift Valley fever virus infection in Culex tarsalis mosquitoes

Abstract

Innovative tools are needed to alleviate the burden of mosquito-borne diseases, and strategies that target the pathogen are being considered. A possible tactic is the use of Wolbachia, a maternally inherited, endosymbiotic bacterium that can (but does not always) suppress diverse pathogens when introduced to naive mosquito species. We investigated effects of somatic Wolbachia (strain wAlbB) infection on Rift Valley fever virus (RVFV) in Culex tarsalis mosquitoes. When compared to Wolbachia-uninfected mosquitoes, there was no significant effect of Wolbachia infection on RVFV infection, dissemination, or transmission frequencies, nor on viral body or saliva titers. Within Wolbachia-infected mosquitoes, there was a modest negative correlation between RVFV body titers and Wolbachia density, suggesting that Wolbachia may slightly suppress RVFV in a density-dependent manner in this mosquito species. These results are contrary to previous work in the same mosquito species, showing Wolbachia-induced enhancement of West Nile virus infection rates. Taken together, these results highlight the importance of exploring the breadth of pathogen modulations induced by Wolbachia.

Fig 1. Effects of Wolbachia infection on RVFV vector competence frequencies in Cx. tarsalis.

RVFV infection 7 and 14 days post-feeding (A), dissemination 7 and 14 days post-feeding (B), and transmission rates 14 days post-feeding (C) were compared between Wolbachia-infected and control Cx. tarsalis. Bars represent data pooled from three replicates. Error bars denote binomial confidence intervals. See S1 Table for replicate-specific analyses.

Fig 2. Comparison of RVFV body and saliva titers between Wolbachia-infected and control Cx. tarsalis.

At both 7 and 14 days post-blood meal, there were no significant differences in RVFV body titers (A) or saliva titers (B) of Wolbachia-infected Cx. tarsalis compared to control Cx. tarsalis. All replicates are combined in this figure; separate replicates are provided in supplementary materials (S2 Fig). Bars represent medians and bolded numbers above the data points denote sample sizes.

Fig 3. Correlation between RVFV body titer and Wolbachia levels in Cx. tarsalis.

Wolbachia levels were normalized to the host gene actin. Normalized Wolbachia levels and RVFV body titer for each mosquito were plotted and analyzed with the Spearman rank correlation test to determine relationships. There was a moderate, negative correlation between RVFV body titer and Wolbachia levels at both day 7 (n = 46) (A) and day 14 (n = 54) (B) post-blood feeding (Fig 3). Data for all replicates were combined; see S2 Table for replicate-specific raw data.