Some viruses prefer mosquitoes to humans, but people get sick anyway − a virologist and entomologist explain why

Some viruses prefer mosquitoes to humans, but people get sick ...

Humans have an exceptional ability to deal with viruses. In most cases, your immune system is able to fight an infection. On the other hand, your body provides a spa-like environment that is temperate and stable, optimal for viruses to replicate. Human behavior, including close contact with animals and frequent travel, also increases the likelihood of becoming infected.

From the perspective of viruses spread by insects, or arboviruses, making the evolutionary leap from insects to humans is a tough battle. Viruses cannot replicate very well in humans, which means transmission from mosquitoes is often very difficult.

One might think arboviruses continually evolve in ways that enable them to infect more species. But do they?

We are a virologist and an entomologist who study insect-borne and viral diseases and how human and insect immune systems respond to invading pathogens. Our work provides insights on the complex journey of an arbovirus as it cycles between insect and vertebrate hosts.

As an example, let’s use a Togavirus, the mosquito-transmitted arbovirus that causes eastern equine encephalitis, or EEE. This rare but serious disease can cause a potentially fatal neurological condition in humans and horses. Although EEE is primarily endemic to the eastern United States, its incidence in recent years has increased in regions farther north, with several reported cases in states such as Michigan, Massachusetts and New York.

While rare, a EEE infection in people can lead to severe complications or death.

From animals to mosquitoes

A female mosquito’s inner workings – particularly its guts and salivary glands – create the perfect environment for a virus to flourish.

When a mosquito bites an infected nonhuman host, such as a sick bird, the virus is transported with freshly ingested blood into the mosquito’s midgut – the equivalent to the human stomach and intestines where food is stored and digested. The virus quickly infects midgut cells to avoid a hostile digestive environment and quietly replicates without activating the mosquito’s immune pathways.

Within days, the virus will be released by damaged midgut cells to migrate to the mosquito’s salivary glands, where it will be positioned for transmission. Now, each time the mosquito feeds, it will pump virus-saturated saliva into its new animal host and continue the disease transmission cycle.

Microscopy image of oblong blobs filled with small red circles, surrounded by blue cicular blobs

This image shows a tissue section of the salivary gland of a mosquito infected with EEE. The virus particles are colored red.
Fred Murphy and Sylvia Whitfield/CDC

It is easy for the virus to avoid detection by the mosquito’s relatively primitive immune system. Compared with humans, the immune system of mosquitoes can launch only a generalized and overall less effective attack on pathogens. This means an arbovirus can usually establish a persistent, lifelong, almost…

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