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

The US has reported five cases of eastern equine encephalitis this year.

The United States has recorded this year’s first death from a rare mosquito-borne virus.

Officials in New Hampshire announced the patient’s death on Tuesday, marking the state’s first human case in a decade and the fifth US case of the virus this year.

Mosquitoes in several areas within the state are believed to be infected with the virus while surrounding areas are on high alert, particularly in the neighbouring state of Massachusetts

What is the mosquito-borne virus, and how far could it spread?

What is the virus?

The virus is officially called eastern equine encephalitis virus (EEEV), also known as “Triple E”. Rare but severe, it was first identified in horses in Massachusetts in 1938.

Since then, there have been 118 human cases and 64 deaths from the virus in the state, based on data from the Massachusetts Department of Public Health

In humans, the virus attacks the central nervous system and can cause inflammation or swelling of the brain.

Where is the virus found?

The virus is found in North America and the Caribbean while human cases primarily occur in eastern and Gulf Coast states of the US.

This can be attributed to a “complex ecology of several different bird species and mosquitoes which are reliant on arboreal swamps for breeding”, said Verity Hill, associate research scientist at Yale University’s School of Public Health.

Moreover, the black-tailed mosquito – the main carrier of the virus – is found primarily in the eastern US, Mexico and the Caribbean.

How does the virus spread?

The virus typically circulates in birds located in hardwood swamps. Mosquito species that feed on both humans and mammals spread the virus when they bite an infected bird and then a mammal and inject the virus into its bloodstream.

Unlike birds, infected humans and horses are “dead-end hosts”, meaning they do not have enough virus in their blood to transmit EEEV to a mosquito that may bite them, Hill told Al Jazeera. This means they cannot pass on the virus to other animals or humans

Infections tend to be asymptomatic in birds but deadly in horses.

The most common carrier of EEE is the black-tailed mosquito (Culiseta melanura) while others include Aedes, which transmits the dengue virus, and Coquillettidia.

Summer to autumn is mosquito season in the US, making it a particularly risky time for such viruses.

What are the symptoms?

Symptoms in humans typically appear four to 10 days after infection. These include:

Sudden onset of fever and chills

Headache

Vomiting and diarrhoea

Seizures and behavioural changes

Drowsiness and disorientation

In severe cases, brain swelling (encephalitis)

EEE is diagnosed by observing symptoms and testing spinal fluid or blood, which can show if the virus or viral antibodies are present.

How many cases are there of the encephalitis virus this year?

There have been five confirmed human cases of EEEV in the United States in 2024 – one each in Massachusetts, New Jersey, Vermont, Wisconsin and New Hampshire.

In Oxford, Massachusetts, an 80-year-old man was infected in mid-August, becoming the first human case in the state since 2020.

The only death reported so far has been the one in New Hampshire.