IT’S THE LITTLE THINGS: EXPLORING THE EFFECTS OF BACTERIA ON MOSQUITO-BORNE VIRUSES
Open Access
- Author:
- Dodson, Brittany Lee
- Graduate Program:
- Entomology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 07, 2017
- Committee Members:
- Jason Rasgon, Dissertation Advisor/Co-Advisor
Jason Rasgon, Committee Chair/Co-Chair
Matthew Brian Thomas, Committee Member
Kelli Hoover, Committee Member
Marilyn J Roossinck, Outside Member - Keywords:
- mosquito
microbiome
bacteria
viruses
infectious diseases
wolbachia - Abstract:
- Novel strategies are required to control mosquitoes and the pathogens they transmit. One attractive approach involves the maternally inherited endosymbiotic bacterium, Wolbachia pipientis (=Wolbachia). After artificial infection with Wolbachia, many mosquitoes become refractory to viruses and malaria parasites. By manipulating arthropod reproduction, Wolbachia can spread through the host population and replace the natural vector population with one that cannot maintain pathogen transmission. However, Wolbachia studies have been limited to few pathogens and mosquito hosts, and some studies have shown that Wolbachia-induced pathogen modulation may differ depending on mosquito species, Wolbachia strain, and pathogen type. Thus, we studied the breadth of effects that a somatic Wolbachia infection has on three genera of viruses, including Flavivirus, Bunyavirus, and Alphavirus. First, we studied the effects of Wolbachia (wAlbB strain) on infection, dissemination, and transmission of West Nile virus (WNV; Flavivirus) in the mosquito Culex tarsalis. After injection into adult female mosquitoes, wAlbB reached high titers and disseminated widely to numerous tissues including the head, proboscis, ovarian follicles, thoracic flight muscles, and fat body. Contrary to other systems, WNV infection rate was significantly enhanced in wAlbB-infected mosquitoes compared to the control. Next, we evaluated the effects of wAlbB on Rift Valley fever virus (RVFV; Bunyavirus) in the mosquito Cx. tarsalis. wAlbB had no effect on RVFV infection, dissemination, or transmission rates, nor on body or saliva titers in this mosquito. However, high levels of wAlbB were correlated with low viral titers or no infection, suggesting that wAlbB may suppress RVFV in a density-dependent manner in this species. Then, we tested the effects of two strains of Wolbachia (wAlbB and wMel) on two alphaviruses (Sindbis virus [SINV] and o’nyong-nyong virus [ONNV]) in Cx. tarsalis and Ae. aegypti mosquitoes. Ae. aegypti was included because it is the primary vector of select alphaviruses and is the species used in Wolbachia field releases. Neither Wolbachia strain blocked SINV or ONNV in either mosquito species, and in fact, there were instances where the viruses were enhanced. In Ae. aegypti, there was a nonsignificant trend toward wAlbB-induced enhancement of ONNV dissemination and transmission rates as well as body and saliva titers compared to a cell lysate control. SINV infection rates were significantly enhanced in wAlbB-positive Ae. aegypti. Additionally, there was a significant positive correlation between wAlbB and SINV body titer and a significant negative correlation between wMel and SINV body titer. In Cx. tarsalis, wAlbB or wMel had no effects on any SINV rates or titer. Also, we found that Cx. tarsalis is not a laboratory vector of ONNV. Furthermore, we explored the mechanism of Wolbachia-induced WNV enhancement in Cx. tarsalis by characterizing the microbiomes of Wolbachia-positive and Wolbachia-negative mosquitoes, as well as mosquitoes with varying degrees of WNV susceptibility. We found that Wolbachia introduction to Cx. tarsalis decreased the native bacteria Asaia, and Asaia was also associated with high WNV susceptibility. Finally, we tested the vector competence of several Anopheles and Culex mosquitoes for Zika virus (ZIKV). Before control tools such as Wolbachia can be deployed, we need to know which mosquito species contribute to the transmission of the virus so that they can be targeted. Except for our control Ae. aegypti mosquitoes, none of the mosquitoes we tested were positive for infectious ZIKV (Anopheles gambiae, Anopheles stephensi, Anopheles quadrimaculatus, Anopheles freeborni, Culex quinquefasciatus, Cx. tarsalis). On the basis on these results, we conclude that pathogen blocking by Wolbachia is not always a guarantee, and that in some instances, the bacterium can enhance virus infection frequency and intensity. In fact, Wolbachia-induced phenotypes are dependent on a variety of factors including mosquito host, Wolbachia strain and density, and virus type. Additionally, Wolbachia-induced enhancement of WNV infection frequency in Cx. tarsalis may be explained by Wolbachia manipulation of the microbiome. Overall, these studies stress the importance of examining the range of Wolbachia-induced pathogen modulation, because the efficacy of pathogen control efforts using the Wolbachia tool could be impeded.