West Nile and EEE: What to Know About Mosquito-Borne Illness in the U.S.

Two mosquito-borne viruses are making headlines this week: West Nile virus for hospitalizing Anthony Fauci, MD, and eastern equine encephalitis (EEE) virus for prompting curfews in some Massachusetts towns.

While both are transmitted through the bite of an infected mosquito, and both can lead to severe brain swelling, that’s largely where the similarities end.

Here’s what you need to know about each virus — and about other mosquito-borne illnesses that sometimes cause a buzz in the U.S.

West Nile Virus

Ian Lipkin, MD, of Columbia University in New York City, first identified West Nile virus as the cause of an outbreak of encephalitis in New York in 1999.

By using a broader sequencing method than what was generally available at the time, Lipkin matched sequences from human brain tissue to those in a database and was able to narrow it down to a “West Nile-like” virus. The virus had been isolated in Uganda in 1937, and had caused outbreaks in Europe, Africa, the Middle East, and parts of Asia before being brought into the U.S.

“We now know it truly was West Nile virus,” Lipkin told MedPage Today. “No one really knows for sure how it got here, but there have been suggestions that there might have been a transfer by mosquitoes from the Middle East.”

There’s particular suspicion that geese imported for foie gras that came through LaGuardia Airport in Queens, New York, may be to blame, Lipkin said, as the first cases started popping up in the summer of 1999 around that airport.

Since that time, West Nile has become endemic in the U.S., peaking at around 10,000 cases in 2003, then averaging about 2,000 cases per year, according to CDC data. So far in 2024, there have been 289 cases of the disease identified in 33 states.

In about 80% of cases, West Nile is completely asymptomatic. For the 20% who become symptomatic, a smaller subset have severe illness including encephalitis, and overall only 1% of people die from the virus, according to Amesh Adalja, MD, of Johns Hopkins University in Baltimore.

“For the vast majority of people who get it, they may not even know that they were sick,” Adalja told MedPage Today.

Severe illness due to infection with the virus is most likely to strike older or immunocompromised people, and symptoms can include fever, chills, and headache. Fauci told STAT of his bout with West Nile, stating that he’s “never been as sick in my life,” as he experienced extreme fatigue, “profound” weakness, hours of “shaking chills” over several nights, and a fever that spiked at 103°F.

West Nile virus is part of the genus Flavivirus, which also includes other viruses that typically infect humans through the bite of an infected mosquito including dengue, yellow fever, and Zika, Adalja said.

Birds are the main viral reservoir, particularly corvids (ravens, magpies, crows, and ot