As summer draws to a close, cases of Eastern equine encephalitis are increasing in the northeastern states of the United States as well as in Quebec and Ontario. The majority of cases reported so far have involved horses, but a 41-year-old man died this week in New Hampshire from this infection caused by a virus transmitted by mosquitoes. Four other people are showing symptoms in the states of Vermont, Massachusetts, New Jersey and Wisconsin. Given this situation, Public Health is on alert on both sides of the border.
Several cities in Massachusetts have closed public parks from dusk to dawn and rescheduled outdoor public events, in addition to recommending that people limit their outdoor activities. This is because mosquitoes — which are particularly active at night — transmit the Eastern Equine Encephalitis Virus (EEEV) to humans and other mammals, such as horses. Humans and horses can only become infected from infected mosquitoes; the infection cannot be transmitted from human to human, or from an animal (a mammal such as a horse) to a human.
An EEEV infection can be particularly dangerous for the elderly (over 50 years of age), young children (under 15 years of age), and people who are immunocompromised. It is estimated that about 30% of infected people who develop Eastern equine encephalitis will die. However, according to the Centers for Disease Control and Prevention, the U.S. Public Health Agency, most infected people remain asymptomatic.
Symptoms
“Among those who are infected, about 5% will develop the severe neurological form. Among these, the mortality rate can reach 50%, and among those who survive, 50% will have neurological sequelae, such as paralysis of a limb, incontinence,” says Dr. Donald Vinh, microbiologist-infectiologist at the McGill University Health Centre.
The first phase of symptomatic infections usually begins three to 10 days after being bitten by a mosquito carrying the virus. It resembles “a non-specific flu syndrome consisting of fever, muscle and joint pain, headache and possibly nausea and vomiting. Some people will recover from this non-specific phase, which can be interpreted as influenza, gastroenteritis or COVID-19,” Dr. Vinh describes. But in a certain proportion of people, the virus reaches the brain, causing encephalitis. [une inflammation du cerveau] which can lead to convulsions, localized weaknesses that can resemble those caused by a stroke, or even a coma.”
EEEV, which is present in eastern North America and the Caribbean, has been known for longer than West Nile virus (WNV). The first outbreak described was in 1831 in Massachusetts, and the virus was isolated from a human brain in 1938, says Dr. Vinh.
Mosquitoes and birds
It is maintained in nature thanks to a transmission cycle between birds and mosquitoes of the species The melanura slide, who live mainly in marshy and wooded areas. The birdlife constitutes the reservoir of the virus. When a female mosquito bites a bird in order to collect the blood necessary for laying its eggs, it contracts the virus, which it will then transmit to the birds it will subsequently bite. And this is how the presence of the virus increases in nature during the summer and becomes more significant towards the end of the summer season, thereby increasing the risks of transmission of the infection to humans and other mammals such as horses.
Given that The melanura slide prefers to feed on birds, experts believe that another species of mosquito, such as Culexwhich bites not only birds but also mammals, would contribute to the transmission of the infection to humans and horses, explains microbiologist Christian Therrien, from the Laboratoire de santé publique du Québec.
Another hypothesis proposed: the smaller bird population as the cold season approaches, given the departure of migratory birds towards the south, would push mosquitoes, which nevertheless prefer birds, to feed more on mammals such as horses and humans.
Infected horses in the country
The Ministry of Agriculture, Fisheries and Food reported the deaths of five horses that had contracted EEV in Lanaudière (more specifically in the MRC Les Moulins). “There are also suspected cases in Lanaudière, the Laurentians and Montérégie, but these cases have not been confirmed by laboratory tests,” said Mr. Therrien, who specified that the deceased animals had not been vaccinated against the virus.
Ottawa Public Health announced that an Ottawa horse recently tested positive.
In both Quebec and Ontario, no human EEEV infections were reported this summer. Three human cases have been reported in Ontario in recent years, with “the most recent reported in 2022.”
“However, if horses are infected, it means that the virus is circulating in mosquitoes and that if a human is near a horse, he or she is at risk of being bitten and developing symptoms. But this remains rare compared to WNV,” notes Mr. Therrien.
The latter insists on the importance of also being wary of VNO, which is much more abundant in Quebec than VEEE, because the mosquito that transmits it, Culex pipienslives in urban and peri-urban areas, much closer to us, such as in the backyards of our homes. “Since the introduction of WNV in Quebec in 2002, we have experienced an epidemic peak in 2012 with 134 human cases and 5 deaths. And in 2018, 201 cases and 15 deaths.”
“About 20% of people who are infected with WNV will develop flu-like symptoms, which will generally resolve on their own. And less than 1% will have neurological damage. The fatality rate for WNV infections is about 10%, while the fatality rate for EEEV infections is 30%, which is why we are more concerned about it at the moment, because EEEV is downright dangerous,” says Mr. Therrien.
“Despite the fact that EEEV has been known for almost 200 years, we do not have a vaccine for humans or a specific antiviral treatment for EEEV. Therefore, we can only offer supportive care in hospital, such as anti-seizure medication and good hydration,” emphasizes Dr. Vinh.
The best way to avoid this infection is to avoid being bitten by wearing clothing that covers the skin and applying insect repellent to exposed areas of skin.
