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.

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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.

How common and dangerous is the virus?

EEE is rare in humans. From 2003 to 2023, there have been 196 cases across the US, according to data collected by the Centers for Disease Control and Prevention (CDC).

The country reports an average of 11 cases annually. The largest outbreak occurred in 2019 with 38 cases and 12 deaths nationwide.

Public health authorities consider it a serious disease because of its impact on the brain and 30 percent mortality rate.

Even in the event of survival, many of those who have been infected go on to experience long-term neurological problems.

How can the virus be controlled or treated?

Public health authorities have said prevention is crucial because there is no specific treatment or vaccine for humans. Control measures include:

Applying government-approved insect repellents
Carrying out mosquito control programmes, including aerial and truck-mounted spraying of pesticides
Wearing long sleeves and pants during peak mosquito hours (dusk to dawn)
Installing anti-mosquito screens on windows and doors
Draining standing water around homes
Avoiding outdoor activities during peak mosquito times

Treatment focuses on managing symptoms and providing supportive care, such as prescribing painkillers.

What action are authorities taking?

States are taking several measures to curb the spread of the virus, which includes varying levels of lockdowns.

The town of Plymouth in Massachusetts is closing public outdoor facilities from dusk to dawn. The northeastern state has also urged residents to avoid outdoor activities during those peak mosquito hours.

Aerial and truck-mounted spraying of insecticides is also being conducted in several areas.

What other viruses or diseases are spread by mosquitoes in North America?

The mosquito-borne but less deadly West Nile virus has infected 289 people in the US this year.

The country has also recorded 3,861 dengue fever cases this year, already exceeding the 3,352 cases reported across all of 2023.

Dengue fever causes high fevers and muscle and joint pain and has been reaching record levels across the world, according to the CDC. Mortality rates can be up to 13 percent in untreated patients.

Is climate change causing the spread of this virus?

Experts have been warning that climate change is lengthening the duration of mosquito season, which is characterised by a certain level of humidity and hot temperatures.

The prevalence of the virus within mosquitoes may also be an indicator of risk to humans.

“Years with large populations of infected mosquitoes tended to have more human/horse cases,” Hill said, adding that climate change may foster larger populations of mosquitoes that can carry EEEV but would not introduce more of the virus.

This year’s cases are likely part of the normal dynamics of the virus, she said.