One person has died in New Hampshire from Eastern Equine Encephalitis (EEE), a rare disease transmitted by mosquitoes. Europe, meanwhile, is seeing a surge in several more common diseases.
One of the latest mosquito-borne diseases to alarm authorities is both rare and serious, and some say it could spread more widely in the United States in the future.
A person has died in New Hampshire, officials said last week, after testing positive for Eastern equine encephalitis virus (EEEV), a mosquito-borne disease native to North America and the Caribbean.
According to the Centers for Disease Control and Prevention (CDC), on average, only 11 cases of infection are reported in the United States each year, and five cases of EEE have been reported in the country so far in 2024 in the states of Massachusetts, New Hampshire, Vermont, New Jersey, and Wisconsin.
In 2023, seven cases were reported.
“It looks like the spread of the virus is a little bit broader this year,” said David Hamer, a professor of global health and medicine at Boston University in the United States, adding that cases are typically seen in August and September. In other words, “there is still some time in which additional transmission events could occur,” Hamer said.
There is no vaccine or treatment for this disease, which can be very serious for people who develop encephalitis or brain inflammation, killing about 30% of them.
The virus has prompted some cities in Massachusetts to close parks or fields and recommend people stay indoors at night, when mosquitoes that can carry the virus are most active.
New Hampshire’s state epidemiologist warned of a possible “high risk” until the first hard frosts kill the mosquitoes.
“Cases are up in some parts of the country compared to previous years. The question remains whether this is a true increase in cases or just an increase in our ability or efforts to recognize and report cases,” said Matthew Ippolito, an assistant professor at Johns Hopkins University School of Medicine in the United States and an expert on another insect-borne virus, malaria.
“I think the consensus is that there is a real increase in cases compared to previous years. But outbreaks are known and it is not a cause for concern at this point,” he added.
Mosquito-borne diseases on the rise in Europe
Mosquito-borne diseases are also putting health authorities on alert in Europe, including the West Nile virus and dengue fever, which are spreading.
Although the vast majority of West Nile virus infections are asymptomatic, less than one percent of infected people may develop neurological complications such as meningitis, which is inflammation of the spinal cord and membranes surrounding the brain, or encephalitis.
While European countries have been experiencing cases of West Nile virus since the 1960s, the number of infections has increased over the past 15 years, according to France’s Pasteur Institute.
Since the beginning of the year, 13 European countries have reported human cases of West Nile virus. Late last year, the European Food Safety Authority (EFSA) said West Nile virus cases had increased more than sevenfold in 2022, partly due 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, although cases were higher in Greece and Spain than in previous years.
As with Eastern equine encephalitis, mosquitoes become infected with West Nile virus by feeding on birds. However, virus concentrations in humans are not high enough to transmit these viruses to mosquitoes when they bite them.
This differs from other mosquito-borne diseases, where humans can amplify the impact of the virus.
“If you look at cases like dengue or chikungunya, humans end up being very linked to high concentrations of virus, and we can infect mosquitoes and that’s when you end up with these big urban outbreaks of dengue,” Hamer said.
“The right vector also needs to be present for the virus to spread,” he said.
Culex mosquitoes, which spread West Nile virus, are present in Europe, and Aedes, the tiger mosquito that can spread dengue, chikungunya and Zika, is becoming increasingly common.
According to the ECDC, the Asian tiger mosquito or Aedes albopictus is “firmly established in 13 European countries”, marking a “significant increase from 10 years ago”, when it was limited to eight countries in Europe.
“In recent years, we have seen an increase in the number of locally acquired dengue and West Nile virus infections, particularly in southern European countries. We also see that more regions are affected each year and for longer periods,” ECDC said in a statement to the Europe Health Observatory.
Impact of climate change
Since the beginning of 2024, more than 12 million cases of dengue fever have been reported worldwide and thousands of deaths have been recorded, mainly in the Americas. In Europe, this year, locally acquired cases of dengue fever have been reported in France and Italy.
“The introduction and at least focal epidemics of dengue and chikungunya will, I think, become increasingly common in many parts of Western Europe, the United States and even Canada,” Hamer said.
Experts say climate change is likely to exacerbate mosquito-borne diseases by affecting bird migration, for example, but also by shortening the winter season, so mosquito populations are active for longer.
“Climate change is expected to extend the transmission season for mosquito-borne diseases by creating more favourable conditions for mosquitoes to thrive,” ECDC said in a statement to the Europe Health Observatory.
“This means that the season starts earlier and ends later in different parts of Europe, increasing the period during which we can see cases and potentially the number of cases.”
The risk of Eastern equine encephalitis being introduced into the EU is, however, low, according to the ECDC, partly because humans do not contribute to its transmission.
Ippolito, of Johns Hopkins University, pointed out that “different mosquitoes thrive in different conditions.”
“While it is true that warmer temperatures generally mean a greater geographic range for mosquitoes and other disease vectors – for example, we are seeing mosquito incursions at higher altitudes in parts of the world that were previously malaria-free – many other factors are at play,” he said.
Although he believes there is less risk of global spread of eastern equine encephalitis because it is so rare and deadly, “the hope is still that we learn from the past and maintain a robust surveillance and response system.”
Experts say people can protect themselves by wearing protective clothing and mosquito repellent and eliminating standing water where mosquitoes lay their eggs.
