Humans possess a remarkable ability to combat viruses. Our immune systems are usually capable of fighting off infections. However, our bodies also provide an ideal environment for viral replication. Furthermore, human behaviors like close contact with animals and frequent travel increase our risk of infection.

For arboviruses (insect-borne viruses), the jump from insect to human is difficult. Viruses often struggle to replicate effectively in humans, hindering transmission from mosquitoes. While it might seem like arboviruses would constantly evolve to infect more species, this isn't always the case.

We, a virologist and an entomologist, study insect-borne viral diseases and how human and insect immune systems respond to pathogens. Our research sheds light on the complex journey of an arbovirus as it cycles between insect and vertebrate hosts.

Take Eastern Equine Encephalitis (EEE), caused by a mosquito-transmitted Togavirus, as an example. This rare but serious disease can cause a potentially fatal neurological condition in humans and horses. Although primarily found in the eastern U.S., EEE has recently increased in northern regions.

From Animals to Mosquitoes:

A mosquito's gut and salivary glands provide the perfect environment for a virus. When a mosquito bites an infected animal (like a bird), the virus enters its midgut. The virus quickly infects midgut cells, replicates, and then migrates to the salivary glands. Now, every bite transmits the virus to a new host.

Mosquitoes have relatively simple immune systems, allowing arboviruses to establish persistent infections without harming the mosquito. Mosquitoes have evolved to tolerate these infections, maintaining viral populations without significant immune responses. However, viruses can manipulate mosquito behavior, like increasing feeding frequency or attraction to infected hosts, and can even affect mosquito reproduction.

From Mosquitoes to Humans:

For a virus to infect a human, it must overcome several barriers. The first step, transmission, occurs when an infected mosquito bites a human. The virus then faces the challenge of adapting to the higher human body temperature and evading the body's defenses, including physical barriers and immune cells. If the virus reaches the central nervous system, the immune response can cause inflammation and damage, potentially leading to long-term effects.

Viruses use various strategies to survive in humans, such as mutating to avoid immune detection and hijacking human cells for replication. However, the complexity of the human immune system makes it difficult for viruses to survive and spread between humans.

From Human to Human?

Like many arboviruses, EEE cannot be transmitted from person to person, limiting its spread. Humans are a "dead end" for the virus. The EEE virus prefers its transmission cycle between mosquitoes and birds. Human infections occur when a mosquito bites a human instead of a bird.

The EEE virus gains little, if anything, by infecting humans. Human bodies are not its preferred environment.