One person in New Hampshire has died from eastern equine encephalitis (EEE), a rare illness transmitted by mosquitoes. Europe, meanwhile, is seeing a rise in several more common illnesses.
ADVERTISEMENTOne of the latest mosquito-borne illnesses to put officials on alert is one that is rare yet serious, with some saying it could in the future spread more widely in the United States.
One person died in the state of New Hampshire, officials said last week, after testing positive for eastern equine encephalitis virus (EEEV), a mosquito-borne illness native to North America and the Caribbean.
There are an average of just 11 cases of the infection reported in the US each year, according to the Centers for Disease Control and Prevention (CDC), and there have been five cases of EEE in the country so far in 2024 in the states of Massachusetts, New Hampshire, Vermont, New Jersey, and Wisconsin.
In 2023, there were seven reported cases.
“It seems to be a little more widely distributed this year,” said David Hamer, a professor of global health and medicine at Boston University in the US, adding that cases are typically seen in August and September. In other words, “there's still a fair amount of time during which more transmission events might occur,” Hamer said.
There are no vaccines or treatments for the illness, which can be very serious for those who develop encephalitis or brain inflammation from it, as it kills an estimated 30 per cent of them.
The virus prompted some towns in the state of Massachusetts to close parks or fields and recommend that people stay inside in the evening when the mosquitoes that can carry the virus are most active.
The state epidemiologist in New Hampshire, meanwhile, warned of a possible “elevated risk” until the first hard frost kills mosquitoes.
“Cases are increased in some parts of the country compared to previous years. There is always the question of whether there is a veritable increase in cases or just an increase in our ability or effort to recognise and report cases,” said Matthew Ippolito, an assistant professor at the Johns Hopkins University School of Medicine in the US and expert on another insect-borne virus, malaria.
“I think the consensus is that there is a true rise in cases over previous years. But outbreaks are known to occur and it is not a cause for alarm at this stage,” he added.
Rise of mosquito-borne illnesses in Europe
Mosquito-borne illnesses also have health officials on alert in Europe, with West Nile virus and dengue, in particular, spreading.
While a large majority of West Nile infections are asymptomatic, fewer than one per cent of people infected can develop neurological complications such as meningitis, which is inflammation of the spinal cord and membranes around the brain, or encephalitis.
While European countries have had cases of West Nile since the 1960s, the number of infections has increased over the last 15 years, according to the French Pasteur Institute.
Since the beginning of the year, 13 European countries have reported human cases of West Nile. Late last year, the EU Food Safety Authority (EFSA) said cases of West Nile had a more than sevenfold increase in 2022 due in part to outbreaks in Italy and Greece.
The European Centres for Disease Prevention and Control (ECDC) added in mid-August that the number of cases in 2024 was within the expected range, even though cases were higher in Greece and Spain than in previous years.
ADVERTISEMENTLike eastern equine encephalitis, mosquitoes become infected with West Nile virus by feeding on birds. However, humans do not have high enough concentrations of the virus to transmit these viruses to mosquitoes when they bite a human.
This is different from other mosquito-borne illnesses, where humans can amplify the impact of the virus.
“If you look at something like dengue or chikungunya, humans end up being very linked with high concentrations of virus, and we can infect mosquitoes and that’s when you end up with these big urban outbreaks of dengue fever,” said Hamer.
You also “need to have the right vector present” for the virus to spread,” he said.
ADVERTISEMENTCulex mosquitoes, which spread the West Nile virus, are present in Europe, and Aedes, the tiger mosquito that can spread dengue, chikungunya, and zika, is becoming increasingly prevalent.
According to the ECDC, the Asian tiger mosquito or Aedes albopictus is “firmly established in 13 European countries” which marks a “significant increase from 10 years ago” when it was limited to eight countries in Europe.
“In recent years, we have been seeing increases in the number of locally acquired cases of dengue and West Nile virus infections, particularly in southern European countries. We also see more regions affected each year and for longer periods,” the ECDC said in a statement to Euronews Health.
Impact of climate change
There have been more than 12 million dengue cases globally since the beginning of 2024 and thousands of deaths, mostly in the region of the Americas. In Europe this year, there have been locally acquired dengue cases in both France and Italy.
ADVERTISEMENT“The introduction and at least focal outbreaks of dengue and chikungunya, I think, are going to become increasingly frequent in many, parts of Western Europe, the United States, and even into Canada,” said Hamer.
Experts say climate change is likely to exacerbate mosquito-borne illnesses by affecting the migration of birds, for instance, but also by shortening the winter season so mosquito populations are active for longer.
“Climate change is expected to extend the mosquito-borne disease transmission season by creating more suitable conditions for mosquitos to thrive,” the ECDC said in a statement to Euronews Health.
“This means that the season starts earlier and finishes later in various parts of Europe, hence increasing the period when we can observe cases and potentially the number of cases”.
ADVERTISEMENTThe risk that eastern equine encephalitis is introduced to the EU, however, is low, the ECDC said, in part because humans do not contribute to its transmission.
Ippolito at Johns Hopkins University pointed out that “different mosquitoes thrive under different conditions”.
“So, while it is true that warmer temperatures generally mean a bigger geographic range for mosquitoes and other vectors of disease - for example, we see incursions of mosquitoes at higher altitudes in certain parts of the world that were previously free of malaria - there are many other factors that come into play,” he said.
While he thinks there’s less of a risk of eastern equine encephalitis spreading more globally since it is so rare and fatal, “the hope is always that we carry forward lessons of the past and maintain a strong monitoring and response system”.
ADVERTISEMENTExperts say people can protect themselves by wearing protective clothing and mosquito repellent as well as eliminating standing water where they lay eggs.
