Previous research has shown that Wolbachia – a genus of bacteria that live in insects – renders mosquitoes resistant to pathogen infection, preventing the mosquitoes from infecting humans with the pathogens. As a result, researchers are currently releasing Wolbachia-infected mosquitoes into the wild as part of a strategy to control Dengue virus. They also are investigating Wolbachia as a possible control strategy for malaria.
Expecting to find that Wolbachia would block infection by West Nile virus in the same way that it blocks Dengue virus, injected the Wolbachia into adult female Culex tarsalis mosquitoes, allowed them to grow, then fed the mosquitoes a meal of blood infected with West Nile virus. Wolbachia infection did not block West Nile virus in the mosquito, instead these mosquitoes had significantly higher West Nile virus infection rates seven days after they were fed the infected blood. Wolbachia infection allowed the mosquitoes to become infected with West Nile virus faster than the controls.
These results point to a previously unforeseen complication – the possibility that mosquitoes rendered resistant to one pathogen by Wolbachia infection might become better vectors of an alternative pathogen. The team also found that West Nile virus enhancement in the Wolbachia-infected mosquitoes occurred in conjunction with the suppression of genes associated with the mosquitoes’ anti-viral immune response.
This is the first study to demonstrate that Wolbachia can enhance a human pathogen in a mosquito. The results suggest that caution should be used when releasing Wolbachia-infected mosquitoes into nature to control vector-borne diseases of humans.
Wolbachia Enhances West Nile Virus (WNV) Infection in the Mosquito Culex tarsalis. (2014) PLoS Negl Trop Dis 8(7): e2965. doi:10.1371/journal.pntd.0002965
Novel strategies are required to control mosquitoes and the pathogens they transmit. One attractive approach involves maternally inherited endosymbiotic Wolbachia bacteria. After artificial infection with Wolbachia, many mosquitoes become refractory to infection and transmission of diverse pathogens. We evaluated the effects of Wolbachia (wAlbB strain) on infection, dissemination and transmission of West Nile virus (WNV) in the naturally uninfected mosquito Culex tarsalis, which is an important WNV vector in North America. After inoculation into adult female mosquitoes, Wolbachia reached high titers and disseminated widely to numerous tissues including the head, thoracic flight muscles, fat body and ovarian follicles. Contrary to other systems, Wolbachia did not inhibit WNV in this mosquito. Rather, WNV infection rate was significantly higher in Wolbachia-infected mosquitoes compared to controls. Quantitative PCR of selected innate immune genes indicated that REL1 (the activator of the antiviral Toll immune pathway) was down regulated in Wolbachia-infected relative to control mosquitoes. This is the first observation of Wolbachia-induced enhancement of a human pathogen in mosquitoes, suggesting that caution should be applied before releasing Wolbachia-infected insects as part of a vector- borne disease control program.