A researcher studying Aedes mosquitoes infected with Wolbachia. [Peter Iliccieve, Fiocruz]
As warm weather approaches the Northern Hemisphere, public health agencies are taking measures to control mosquito populations as well as preparing for the influx of new Zika cases. While knowledge of the Zika virus has existed for decades, recent cases in South America that have been linked to increased virulence and birth defects have the public, understandably, on edge. Scientists are working to address the threat from the Zika virus by attempting to uncover the underlying biology that allows mosquitoes to transmit the virus to humans.
Now, researchers at Brazil's Oswaldo Cruz Foundation (FIOCRUZ) have discovered that the bacterium Wolbachia—found inside the cells of 60% of all insect species—can prevent the transmission of Zika virus in Aedes aegypti mosquitos.
While the study of Wolbachia in Aedes mosquitoes is not new, as the microbes were originally inserted into Aedes eggs as part of the Eliminate Dengue Program, this is the first report on the effect of Wolbachia bacteria on Zika virus. Because the bacterium is passed on from mother mosquitoes to offspring, it is a sustainable control agent that could subdue Zika transmission.
Wolbachia bacteria were first identified in 2005 as a way to combat mosquito-borne infections. After 4 years, researchers were successful in their attempts to isolate the bacterium from fruit flies and get it inside Aedes mosquitoes' eggs without using any genetic alteration.
The researchers expected Wolbachia to shorten mosquitoes' lifespans, but the bacterium provided an added bonus, in that it heavily reduced dengue virus replication in the mosquito—it also has a similar effect on Zika transmission. Moreover, the same results were previously observed for chikungunya virus.
“The idea has been to release Aedes mosquitoes with Wolbachia in the field over a period of a few months, so they mate with Aedes mosquitoes without Wolbachia living in the place and, over time, replace the mosquito population,” explained senior study author Luciano Moreira, Ph.D., team leader at FIOCRUZ. “Zika and dengue belong in the same family of viruses, so with the outbreak in Brazil, the logical idea was to test the mosquitoes carrying Wolbachia by challenging them with Zika virus and see what would happen.”
Dr. Moreira and his colleagues gave Brazilian field mosquitoes and Wolbachia-infected mosquitoes Zika virus by feeding them human blood infected by two recent strains of the virus that are circulating in Brazil. After 2 weeks, the researchers saw that mosquitoes carrying Wolbachia had fewer viral particles in their bodies and saliva.
“Aedes aegypti harboring Wolbachia are highly resistant to infection with two currently circulating Zika virus isolates from the recent Brazilian epidemic,” the authors wrote. “Wolbachia-harboring mosquitoes displayed lower viral prevalence and intensity and decreased disseminated infection and, critically, did not carry the infectious virus in the saliva, suggesting that viral transmission was blocked.”
The findings from this study were published recently in Cell Host & Microbe in an article entitled “Wolbachia Blocks Currently Circulating Zika Virus Isolates in Brazilian Aedes aegypti Mosquitoes.”
Currently, the molecular mechanism underlying the drop in viral reproduction is unknown, but one theory is that because Wolbachia lives inside of the mosquito's cells, if the virus enters such a cell to replicate, then there is an internal competition for resources. Surprisingly, this drop held true no matter how many Wolbachia the mosquito carried.
“Wolbachia showed to be as effective on Zika as the most important dengue experiments we did,” Dr. Moreira noted. However, he cautioned that the strategy is not 100% effective, nor will it eliminate the virus. “We know that there will not be only one solution for Zika—we have to do this alongside different approaches, like vaccines or insecticides, besides the public measures to control Aedes breeding sites.”