Mosquito spit and skin welts boost viral infection

How your body reacts to a virus-laden mosquito bite may lead to illness more than the virus itself. Anthea Batsakis reports.

If a mosquito is infected with a virus, that pathogen can use aspects of the unfortunate host to its advantage.

Mosquito spit and inflamed welts following a bite give viruses such as dengue fever a boost by helping them hijack our immune system, two recent studies in mice showed.

American researchers led by the University of California, Berkeley’s Michael Schmid found mosquito saliva, which is injected into your skin when the insect draws blood, amplifies dengue fever's symptoms. They published their work in PLOS Pathogens last week.

And a European team led by Marieke Pingen at the University of Leeds, UK discovered the swelling that arises after a mosquito’s blood feast gives viruses a leg-up around the host’s body. Their research was published in the journal Immunity today.

“It’s the body’s reaction to the mosquito bite as much as the potential introduction of the virus which may lead to illness,” says Cameron Webb, an entomologist at Australia’s University of Sydney.

The work may help develop vaccines or treatments that calm a host’s vigorous immune response to infection.

Mosquito saliva is a complex fluid. It’s a lubricant, an anaesthetic and an anticoagulant to help with the blood-feeding process. It also transports viruses.

When a mosquito ingests infected blood, the virus starts out in their gut, then makes its way to the insect’s salivary glands after a few days where it hitches a ride on the salivary injection to the next host.

“Mosquitoes aren’t like a dirty syringe that transfers droplets of blood from person to person,” Webb says.

“The mosquitoes themselves need to be infected with a virus before they can pass it on.”

The US team found mosquito saliva has a role in the host's body, too – it can increase the severity of dengue infection.

There are at least four types of dengue virus, known as serotypes. After a person is infected with any of the serotypes, their immune system manufactures antibodies that match it to identify it and clear it away, should reinfection occur.

But when a patient is infected down the track by one of the other serotypes, antibodies from the first don’t destroy the new virus. Rather, they can force immune cells in the host to become replication stations for the new virus.

As a result, dengue virus becomes potentially fatal for second-round infection.

Without an influx of white blood cells, the viruses couldn’t replicate and didn’t spread.

The researchers tested two groups of mice – one with serotype antibodies, the other without – and their reaction to dengue with and without mosquito saliva.

They found adding mosquito saliva to the dengue infection made the mice with antibodies much sicker than their antibody-free counterparts. They think it's partly because a compound in the saliva makes blood vessels "leaky" and allows the virus to spread further.

The researchers from Europe took a similar approach but were more concerned with the lumps that emerge on your skin after being bitten. They examined the Semliki Forest virus and Bunyamwera virus in mice.

When a mosquito pierces your skin and the saliva starts flowing, your body sends white blood cells to rush the area and dispatch the invasion.

But instead of clearing the pathogens away, the white blood cells become infected with the virus, which replicates within them and spreads rapidly around the body.

This immune cell infection seems to be a necessary step for the viruses, the researchers found – without an influx of white blood cells, the viruses couldn’t replicate and didn’t spread.

Mice with inflamed welts suffered more severe reactions to the viruses compared to those with no inflammation. Their findings could mean topical anti-inflammatory creams that help treat swelling could stop the virus carrying out its full potential.

Webb says prevention, though, is key – wearing insect repellent during times and places mosquitos are active is still the most effective way to avoid contracting a mosquito-borne virus.

“Until we have vaccines available to all mosquito-borne pathogens, avoiding mosquito bites are going to remain the most important way to minimise public health risks.”

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