An infected mosquito spit can help spread Zika virus, two new studies have found. In mice tests, mosquito saliva was proven a vital player in the spread of a virus that in certain cases can turn a benign infection into a life-threatening one.

When an infected mosquito bites a human host, its saliva attracts the body's immune cells to the bitten area. The mosquito saliva then tricks the immune cells into spreading any kind of virus throughout the body.

Scientists are already aware that bites from mosquitoes can cause inflammation. The mosquito saliva promotes an immune response. The injected saliva, which is lower than a microliter, contains a mix of powerful molecules that numb the pain from the insect bite and prevent blood clotting.

The two research teams wanted to see how this immune response can seriously and easily affect the mosquito-borne viruses' ability to infect human hosts.

First Study: Semliki Forest Virus

A research team from the University of Leeds injected mice with Semliki Forest virus, which is a close relative of dengue, chikungunya and Zika viruses, with and without the saliva of Aedes aegypti mosquitoes.

Among the mice injected with the virus without the mosquito saliva, the virus didn't spread too much. On the other hand, among the ones injected with the virus and the mosquito saliva, the virus spread faster.

The team also found that the mosquito saliva was able to transform a typically harmless Semliki Forest virus strain into a deadly infection that killed many of the test mice. These findings were published in the journal Immunity on June 21.

"Mosquito bites are not just annoying — they are key for the virus to cause disease," said virus expert Clive McKimmie from the University of Leeds, who led the Semliki Forest virus study.

McKimmie added that the findings can help in "repurposing" common anti-inflammatory drugs in preventing the virus from spreading and causing infections if administered quickly.

Second Study: Dengue Virus

A separate team from the University of California Berkeley, which was led by Eva Harris, conducted a similar experiment in mice using the dengue virus.

This mosquito-borne virus comes in four strains. When people get infected with one strain of the dengue virus, their condition worsens when another strain infects them.

Using the same technique, the researchers found that mosquito saliva allows the dengue virus to spread and infect people. Moreover, the saliva enables the virus to contaminate the body's immune cells that were sent to fight off the infection.

"Mosquito saliva and enhancing antibodies thus need to be considered when developing vaccines and drugs against dengue," wrote the researchers.

The second study was published in the PLoS Pathogens journal on June 16.

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