WIKIMEDIA, SCOTT O’NEILLA bacterium known to prevent the spread of dengue and other viruses has now been shown to block transmission of Zika. Aedes aegypti mosquitoes carrying Wolbachia bacteria were highly resistant to Zika virus infection, and were unable to transmit the virus via their saliva, researchers in Brazil reported in a study published today (May 4) in Cell Host & Microbe. The findings highlight a possible mechanism for fighting the current primary viral vector in the ongoing Zika outbreak.
“It’s an exciting and encouraging study,” said Stephen Dobson, an entomologist at the University of Kentucky who studies A. aegypti biology but was not involved with the work. “To my knowledge, this is first study showing interference of Wolbachia and Zika transmission,” Dobson told The Scientist.
The Wolbachia bacterium is naturally found in at least 40 percent of all insect species, and while it’s not normally present in A. aegypti mosquitoes, it can be introduced to them. The bacterium has been shown to block transmission of dengue and chikungunya viruses, as well as the malaria parasite Plasmodium. Wolbachia is not known to infect people.
“The idea is to replace the population of [non-Wolbachia–infected] mosquitoes in a determined area” with those carrying the bacterium, said study coauthor Luciano Andrade Moreira of Brazil’s Oswaldo Cruz Foundation. Moreira is a project leader for the Eliminate Dengue program, which has been conducting trials of a similar mosquito-targeting method in dengue-affected areas since 2011.
To investigate the bacterium’s effect on Zika transmission, Moreira and colleagues exposed both uninfected female A. aegypti mosquitoes that had been captured in the wild and those infected with a strain of Wolbachia called wMel to human blood containing two strains of Zika isolated from current outbreak in Brazil (BRPE and SPH). At one and two weeks after exposing the mosquitoes to the virus, the researchers used qRT-PCR to measure the amount of virus in the insects’ heads/thoraces and abdomens.
The prevalence of Zika virus was much lower in the Wolbachia-infected mosquitoes, Moreira’s team found. One week after infection, viral loads of the BRPE strain of Zika decreased by 100 percent in the insects’ heads/thoraces and by 35 percent in their abdomens (although the latter result wasn’t statistically significant). After another week, Zika levels had dropped by 90 percent and 65 percent in their heads/thoraces and abdomens, respectively.
Similar results were seen with the SPH Zika. After the first week, this strain’s viral loads dropped by 95 percent and 67 percent in the Wolbachia-infected mosquitoes’ heads/thoraces and abdomens, respectively. They dropped by 74 percent and 68 percent, respectively, after the second week.
“The blocking effect is very [large],” Moreira told The Scientist, suggesting “we can use these mosquitoes to reduce incidence of Zika virus in the wild.”
Meanwhile, viral loads in the mosquitoes that weren’t infected with Wolbachia steadily increased over the two-week study period.
Moreira’s team next collected saliva from both groups of mosquitoes two weeks after Zika virus infection, finding that Wolbachia reduced the amount of virus in the insects’ saliva by 55 percent. To see if the saliva was still infectious, the researchers injected it into the thoraces of wild female A. aegypti. None of the insects became infected with Zika virus, compared with 85 percent of mosquitoes that were injected with saliva from Wolbachia-negative insects.
The researchers found no relationship between the density of Wolbachia in the mosquitoes and the amount of Zika virus in their bodies, suggesting the bacterium may exert its apparent protective effect against Zika indirectly.
“It’s extremely timely and important work,” Jack Werren of the University of Rochester in New York, who wasn’t involved in the study, told The Scientist. “With the looming potential crisis with Zika virus, this is an important study to do.”
The results suggest Wolbachia could play an important role in Zika control efforts, in combination with other methods, Moreira said. The next step is to assess the bacterium’s ability to block Zika infection in wild mosquito populations.
“The advantage of [Moreira’s] approach is . . . if you succeed in infecting [mosquito populations] with Wolbachia, you don’t have to keep releasing mosquitoes,” Dobson said. One potential disadvantage is that it may be difficult to get Wolbachia established in the wild, he added.
While the Wolbachia used in this study were harmless to the insects, other strains can have a sterilizing effect on the animals. Another approach to combat mosquito-borne illness—which Dobson and colleagues will soon try in California—involves releasing males infected with these strains into the wild, where they will mate with and sterilize females.
“Until we have a demonstrated tool against these pathogens, we need to be exploring multiple options,” Dobson said.
H. Dutra et al., “Wolbachia blocks currently circulating Zika virus isolates in Brazilian Aedes aegypti mosquitoes,” Cell Host & Microbe, doi:10.1016/j.chom.2016.04.021, 2016.