Mosquito-borne viruses like West Nile virus (WNV) and the rare eastern equine encephalitis (EEE) are infecting people across the United States. WNV cases have been confirmed in 33 states, while EEE cases have been detected in at least five.

Bennett G, Hudgins J. What’s behind the recent surge in mosquito-borne illnesses. PBS News. August 29, 2024.

In New Hampshire, one person is confirmed to have died from EEE.

 

The number of cases—and the EEE-related death—reported is concerning. Adding to the alarm is the timing of these reports. While the beginning of September typically marks the end of mosquito season on the East Coast, it appears that this year's mosquito season is lasting longer than usual.

 

Are mosquito-borne infections linked to global warming?

 

This year’s longer mosquito season and ongoing virus appear to be a result of climate change. Climate change has impacted the seasonality of many species and bugs, which, in addition to mosquitos, include the rapid growth and spread of the spotted lanternfly and the northern expansion of blacklegged (deer) ticks.

U.S. Department of Health and Human Services. Pesticide Exposure: Another Growing Hazard for Farmworkers in a Changing Climate. September 5, 2024.

 

According to the National Oceanic and Atmospheric Administration’s (NOAA) Centers for Environmental Information, September 2023 was the hottest September on record—and the hottest September in NOAA’s 174 years of climate tracking.

NOAA National Centers for Environmental Information. Global climate summary for September 2023. October 16, 2023.

July 2024 was the hottest July ever recorded in NOAA’s now 175-year record.

NOAA National Centers for Environmental Information. Selected Significant Climate Anomalies and Events: July 2024. August 2024.

NOAA has not yet released data on August 2024’s ranking, but researchers have found that this August is tied with August 2024 as the the hottest August on record.

Peck D. Summer 2024 was the warmest on record in the Northern Hemisphere, according to latest Copernicus report. ABC News. September 5, 2024.

 

These record temperatures may not just impact mosquitos’ seasonality, but also their ability to transmit disease.

Bennett G, Hudgins J. What’s behind the recent surge in mosquito-borne illnesses. PBS News. August 29, 2024.

According to the US Department of Health and Human Services (HHS), increased temperatures and altered humidity can affect mosquitos in various ways, including by:

 

• Expanding mosquitos’ geographic range of where they live and transmit disease; 

• Accelerating the rate at which mosquitoes bite;

• Accelerating mosquito development and reproduction rates, which may increase the overall size of mosquito populations; and

• Enabling mosquitoes to “become infectious more quickly and transmit diseases faster.”

In addition to rising temperatures, climate changes like altered precipitation patterns and extreme storms can increase flooding—creating standing water—and drought, thereby increasing the number of areas where mosquitoes can lay eggs, according to HHS.

 

What to know about WNV and EEE

 

WNV is the leading cause of mosquito-borne illnesses in the US, whereas EEE is very rare in the US.

Centers for Disease Control and Prevention. About West Nile. May 15, 2024.

Centers for Disease Control and Prevention. About Eastern Equine Encephalitis. May 15, 2024.

Both viruses are caused by mosquito bites from infected mosquitoes. Typically, mosquitoes are first infected with WNV or EEE if they have bitten an infected bird.

Centers for Disease Control and Prevention. West Nile: Causes and How It Spreads. May 15, 2024.

Centers for Disease Control and Prevention. Eastern Equine Encephalitis: Causes and How It Spreads.

In either case, some people who develop WNV or EEE will have little to no symptoms, whereas others may develop severe symptoms that could lead to lasting complications or death.

 

Symptoms of WNV include fever, headache, body aches, vomiting, diarrhea, or rash.

Centers for Disease Control and Prevention. About West Nile. May 15, 2024.

 

Symptoms of EEE can include fever, headache, diarrhea, vomiting, seizures, behavioral changes, drowsiness, and coma.

Centers for Disease Control and Prevention. Eastern Equine Encephalitis: Symptoms, Diagnosis, and Treatment. May 15, 2024.

 

Currently, no vaccines or targeted treatments exist for either virus, so the best form of prevention is to avoid mosquitoes and practice habits that reduce your risk of being bitten.

Some habits that you can practice or recommend to your patients include:

• Wearing long pants and long-sleeved clothing;

• Using Environmental Protection Agency (EPA)–registered insect repellent;

• Placing screens on windows and fixing holes in the home;

• Staying inside during times when mosquitoes are most active; and

• Removing areas of standing water near the home, such as those found in places like the gutters, birdbaths, pools, or elsewhere.