What is the deadly ‘Triple E’ mosquito virus spreading in northeastern US?

The United States has reported its first death of the year from a rare mosquito-borne virus. New Hampshire officials announced the fatality on Tuesday, marking the state's first human case of the virus in a decade and the fifth reported case in the U.S. this year.

Mosquitoes in various parts of New Hampshire are believed to be infected, prompting heightened alert, especially in neighboring Massachusetts.

What is the Virus?

The virus, known as Eastern Equine Encephalitis Virus (EEEV), or "Triple E," is rare but severe. It was first identified in horses in Massachusetts in 1938. Since then, Massachusetts has recorded 118 human cases and 64 deaths due to the virus, according to the Massachusetts Department of Public Health.

In humans, EEEV attacks the central nervous system, potentially causing brain inflammation or swelling.

Where is the Virus Found?

The virus is prevalent in North America and the Caribbean, with most human cases occurring in the eastern and Gulf Coast regions of the U.S. This distribution is linked to a "complex ecology of several bird species and mosquitoes that rely on arboreal swamps for breeding," explained Verity Hill, an associate research scientist at Yale University's School of Public Health. The primary carrier of the virus, the black-tailed mosquito, is found mainly in the eastern U.S., Mexico, and the Caribbean.

How Does the Virus Spread?

The virus circulates primarily in birds residing in hardwood swamps. Mosquitoes that feed on both birds and mammals transmit the virus when they bite an infected bird and then a mammal, injecting the virus into its bloodstream. However, unlike birds, humans and horses are "dead-end hosts," meaning they cannot spread EEEV to other mosquitoes, animals, or humans, Hill told Al Jazeera. While the infection is generally asymptomatic in birds, it can be deadly in horses.

The most common EEEV carrier is the black-tailed mosquito (Culiseta melanura), but other carriers include Aedes mosquitoes, known for transmitting dengue virus, and Coquillettidia. The risk of infection is highest from summer to autumn, the peak mosquito season in the U.S.

What Are the Symptoms?

Symptoms in humans typically appear four to 10 days after infection and may include:

• Sudden onset of fever and chills
• Headache
• Vomiting and diarrhea
• Seizures and behavioral changes
• Drowsiness and disorientation
• Brain swelling (encephalitis) in severe cases

Diagnosis is based on symptoms and testing spinal fluid or blood for the presence of the virus or viral antibodies.

How Many Cases Have Been Reported This Year?

In 2024, there have been five confirmed human cases of EEEV in the U.S., one each in Massachusetts, New Jersey, Vermont, Wisconsin, and New Hampshire. The first case in Massachusetts occurred in mid-August when an 80-year-old man was infected, marking the state's first case since 2020. The only death so far this year was in New Hampshire.

How Common and Dangerous is the Virus?

EEE is rare in humans, with 196 cases reported across the U.S. from 2003 to 2023, according to the Centers for Disease Control and Prevention (CDC). The U.S. averages 11 cases per year, with the largest outbreak in 2019, resulting in 38 cases and 12 deaths. Due to its severe impact on the brain, EEE is considered a serious disease with a 30 percent mortality rate. Survivors often suffer from long-term neurological issues.

How Can the Virus Be Controlled or Treated?

There is no specific treatment or vaccine for humans, making prevention crucial. Control measures include:

• Using government-approved insect repellents
• Implementing mosquito control programs, such as aerial and truck-mounted pesticide spraying
• Wearing long sleeves and pants during peak mosquito activity (dusk to dawn)
• Installing screens on windows and doors
• Draining standing water around homes
• Avoiding outdoor activities during peak mosquito hours

Treatment focuses on managing symptoms and providing supportive care, such as pain relief.

What Actions Are Authorities Taking?

States are taking various steps to curb the virus's spread, including local lockdowns. Plymouth, Massachusetts, has closed public outdoor facilities from dusk to dawn, and residents have been urged to avoid outdoor activities during peak mosquito hours. Aerial and truck-mounted spraying of insecticides is also underway in several areas.

What Other Mosquito-Borne Viruses Are Present in North America?

The less deadly West Nile virus has infected 289 people in the U.S. this year. Additionally, there have been 3,861 dengue fever cases, surpassing the 3,352 cases reported in 2023. Dengue fever, which causes high fevers and muscle and joint pain, has reached record levels globally, with mortality rates up to 13 percent in untreated patients, according to the CDC.

Is Climate Change Contributing to the Virus's Spread?

Experts warn that climate change is extending mosquito season, characterized by certain humidity levels and warm temperatures. The prevalence of the virus among mosquitoes may indicate human risk. "Years with large populations of infected mosquitoes tended to have more human and horse cases," Hill said, adding that climate change might foster larger populations of mosquitoes capable of carrying EEEV, although it would not increase the virus itself. This year's cases likely reflect the normal dynamics of the virus, she added.