Mosquito-to-mosquito infections keep dengue circulating

While mosquitoes acquire dengue viruses from people when they feed on blood, the insects can also infect each other, a recent study finds. 

Under normal conditions, when mosquito and host populations are robust, dengue is transmitted in a cycle from mosquitoes to human hosts and back to new mosquitoes, which keeps the virus in circulation.

But the study – published Aug. 31 in the journal Public Library of Science Neglected Tropical Diseases – reveals mother Aedes aegypti mosquitoes transmit dengue viruses to their offspring and, for the first time, finds evidence of male mosquitoes infecting females when they mate.

The research answers a big question among disease ecologists: how the virus is maintained during periods when mosquitoes become less active or when populations drop – such as in dry and cold spells – and when hosts are less susceptible.

“The study highlights how much we still need to know about the biology of these viruses and their interactions with the mosquitoes,” said Laura Harrington, professor of entomology at Cornell. Harrington is a co-lead author of the paper along with Irma Sanchez-Vargas, a research scientist at Colorado State University.

Now that researchers have proven these modes of transmission in the lab, next steps will be to test if they similarly occur in the field.

The research opens the door for potential new virus control methods that focus more on male mosquitoes, which tend to be ignored because they don’t take blood meals. “It indicates to me that males could actually be directly involved in transmission in the virus cycle,” Harrington said. “If we could understand in more detail what’s happening in the field, we might be able to target the populations when they are very low and minimize carry-overs of the virus from one epidemic to the next.”

The results have implications for other disease-causing viruses where mosquitoes are vectors, including yellow fever, Zika and chikungunya. These viruses tend to infect all tissues in the mosquito’s body before they reach the salivary glands, Harrington said. More research is needed to identify the exact mechanisms that allow transmission from one mosquito to another, but possibilities include eggs being infected when females fertilize them, and through seminal fluids during mating when males infect females, Harrington said.

Many other researchers have tested mosquitoes for transmissions between mothers and offspring and from males to females after mating, but those studies tested infections after a single blood meal. The meal helps the female develop a clutch of eggs. 

“When they take one blood meal there is often not enough time for the virus to actually escape and make it into the ovarian tissue,” Harrington said. The current study re-created natural conditions with mosquitoes taking multiple blood meals, which led to higher infection rates between mosquitoes. Males acquire the virus in the egg and pass it on to females when they mate as adults.

The researchers began with a large number of wild-caught mosquitoes and conducted blind experiments, waiting to determine whether mosquitoes were infected. They separated females and gave them an infectious blood meal and a second noninfectious meal. They collected the second batch of eggs from females, surface sterilized the eggs, hatched and reared offspring to adulthood, and tested them for virus infections. From a subset of infected progeny, they reared a second set of progeny, then mated infected males from that second generation to uninfected females. They then went back and tested the male and the females he mated with for infection.

The paper’s senior co-author is virologist Ken Olson at Colorado State. Other co-authors include Jeffrey Doty at the Centers for Disease Control and Prevention and William Black at Colorado State.

The study was funded by the National Institutes of Health, Cornell Affinito Stewart Sabbatical Grant, and the Regents of the University of California through the Foundation for the National Institutes of Health and Grand Challenges in Global Health Initiative.

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Lindsey Knewstub