How viruses travel from mosquitoes to humans: the journey of EEE

Humans have a strong defense mechanism against viral infections, with the immune system typically fighting off threats. However, viruses thrive in the human body, which provides a stable, ideal environment for replication. Despite human defenses, behaviors like close animal contact and frequent travel increase the risk of infections, especially from insect-borne viruses like arboviruses.

Arboviruses, such as the Togavirus that causes Eastern Equine Encephalitis (EEE), face tough challenges transitioning from mosquitoes to humans. Though these viruses struggle to replicate in humans, they find mosquitoes to be the perfect hosts. When an infected mosquito bites a bird, the virus enters the insect’s digestive system. There, it avoids immune responses and moves to the salivary glands. When the mosquito feeds again, it transmits the virus through saliva to new hosts.

Mosquitoes have evolved to harbor these viruses without suffering severe harm. However, the virus manipulates mosquito behavior to increase its chances of transmission. It can make mosquitoes feed more often and become more attracted to infected hosts, though this comes at the risk of being killed by agitated animals. The virus can even affect mosquito reproduction, sometimes sterilizing them.

For the virus to infect a human, it must adapt to the higher body temperature and evade the human immune system. Once it enters the bloodstream and possibly the central nervous system, the immune system can inadvertently cause more harm through inflammation. Fortunately, the EEE virus cannot be transmitted between humans, as it relies on mosquitoes for further transmission.

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