A new study of genetic samples from Aedes aegypti mosquitoes from around the world finds no evidence of naturally occurring infection with Wolbachia bacteria. When artificially introduced to mosquito populations, Wolbachia can be used for reducing mosquito numbers or interrupting their ability to transmit disease-causing pathogens. The lack of naturally occurring Wolbachia removes “additional variables … that would need to be taken into account” in such efforts, researchers say. (Photo credit: Tommaso G. Chiodo)

As both the United States and countries all over the world continue to grapple with vector-borne disease, one of the latest efforts for combating mosquitoes that transmit disease-causing pathogens involves infecting them with strains of bacteria known as Wolbachia.

Benign to humans, Wolbachia occurs naturally in about two-thirds of insect species. For those into which Wolbachia is introduced, however, major biological disruptions can occur—and that’s where Wolbachia can become a tool for mosquito management. In two different variations on the method, infecting mosquitoes with Wolbachia and releasing them into the wild can be used to either reduce a wild mosquito population’s numbers overall or to interrupt wild mosquitoes’ ability to transmit pathogens. (To learn more, see the Entomological Society of America fact sheet, “Fighting Fire With Fire: How Science is Turning Mosquitoes Against Themselves.”)

As these efforts are further tested and adopted, it’s important to know whether Wolbachia occurs naturally in certain mosquito species. In that regard, a new report published this month in the Journal of Medical Entomology offers some good news.

Researchers with Yale University and the Connecticut Agricultural Experiment Station (CAES) tested DNA samples from nearly 2,900 specimens of the Aedes aegypti mosquito originating from 27 countries on six continents and found no sign of naturally occurring Wolbachia. Also known as the yellowfever mosquito, Aedes aegypti, is a primary vector of human diseases such as Zika, chikungunya, and dengue.

As Andrea Gloria-Soria, Ph.D., assistant agricultural scientist at CAES and lead author on the study, explains, the lack of naturally occurring Wolbachia in Aedes aegypti could offer a clean slate for mosquito-management efforts targeting the species with Wolbachia-driven methods.

“A naturally occurring Wolbachia infection in Ae. aegypti could introduce additional variables to the system that would need to be taken into account,” she says. “First, a native Wolbachia strain, which does not produce the desired outcome in the mosquito, could prevent the introduced strain from spreading in the population. Also, since the effect that Wolbachia has on mosquitoes varies with the strain of Wolbachia, the presence of multiple Wolbachia strains in a host may affect the expressed phenotype. The desired effect may only be seen in the absence of another strain, for example.”

In 2012, the U.S. National Institutes of Health awarded a grant to the Yale Ecology & Evolutionary Biology Lab, led by co-author Jeffrey R. Powell, Ph.D., to study the genetic diversity of Ae. aegypti worldwide. Powell’s group has since built an extensive collection of Ae. aegypti genetic material from around the world, which “presented us with the perfect opportunity to conduct a wide screen for Ae. aegypti Wolbachia,” Gloria-Soria says.

The researchers conducted PCR (polymerase chain reaction) tests on all of the samples—plus 72 samples from Aedes mascarensis, a close relative of Ae. aegypti with which it can potentially interbreed—for evidence of Wolbachia DNA. With the exception of specimens from an area where Wolbachia-infected mosquitoes were known to have been released, the team found no evidence of naturally occurring Wolbachia in any of the samples.

For mosquito management efforts using Wolbachia, this represents strong backup for a the widely held assumption that Ae. aegypti is naturally Wolbachia-free. Gloria-Soria and colleagues will continue testing new samples from new locations, but, given the geographic diversity in their testing so far, they say any natural Wolbachia infection in Ae. aegypti would be “extremely rare.”

But it does leave open the question of why that’s the case. “When we started the screen, we thought we would find Wolbachia [in Ae. aegypti] in some areas of the world that had not yet been surveyed,” Gloria-Soria says. “We were surprised to see that all of them came back negative.”