A rare mosquito-borne virus recently found in Eastern Ontario is potentially more dangerous than the West Nile virus but only rarely infects humans, researchers say.
Officials with the Leeds, Grenville & Lanark District Health Unit warned this week that two horses had been infected with eastern equine encephalitis virus, which is carried by birds but can be transmitted through certain types of mosquitos.
The infections are believed to be the first-ever reported in Ontario.
The virus has been much more prevalent in the state of Massachusetts, which has seen occasional clusters of human infections occurring about once every 15 years since the 1930s and as recently as 2006.
For human patients who are symptomatic, the virus can be devastating. It causes inflammation of the brain, with an estimated 30- to 50-per-cent mortality rate and permanent neurological damage occurring among survivors.
Scientists don’t know, however, exactly what percentage of people who are infected with the virus go on to develop symptoms, which can include headache, high fever, chills and vomiting, according to the U.S. Centers for Disease Control.
As the illness progresses, it may cause disorientation, seizures and coma, the CDC says. Symptoms first occur between four and 10 days after a bite from an infected mosquito. An average of five to 10 people die in the U.S. from the virus annually.
Veterinarians recommend a vaccination for horses that also protects against West Nile but there is no vaccine for humans. Preventing mosquito bites using insect repellents with DEET and covering up exposed skin are the best defences against infection.
For those infected who become symptomatic, there is no effective treatment, as antibiotics don’t work against viruses and anti-viral drugs are not ineffective against this particular virus.
“It’s a more dangerous disease to get though it’s a lot less prevalent than West Nile,” said Alan Dupuis, a New York-based researcher and member of an expert panel on the virus convened by the Massachusetts Department of Public Health in 2012.
The type of mosquito that transmits the virus only rarely bites mammals, Dupuis said, and it is typically found in swamps and woodland areas, away from large human populations.
“But any time a mammal is infected there is this risk posed to humans.”
According to a report from Standardbred Canada, which distributes information on horses and harness racing, one of the infected horses was a 12-year-old gelding that had not been vaccinated. The animal was euthanized, the report said.
The infection of horses in Eastern Ontario may require additional surveillance to see if the virus is being transmitted by other kinds of mosquitos that feed off mammals, Dupuis said. The chances of it moving out of rural areas are low, he says.
“I wouldn’t expect anything to happen in any city. Usually these mosquitos, though they can travel, are usually found in specific swamps.”
That makes the virus harder to spread to humans than West Nile, which is transmitted by a different kind of mosquito and more likely to be found in cities.
Still, the Massachusetts report warned that while the number of human infections remains small, “the impact at the individual, family and community levels is significant, and this disease warrants significant public health attention.”
To help prevent infection, the Leeds, Grenville & Lanark District Health Unit recommends:
* Apply insect repellent containing DEET to clothing and on all exposed skin. (Don’t apply it on the palms of your hands and your fingers, and keep it away from your eyes and mouth. Wash your hands after application. Be sure to follow directions and usage appropriate concentrations DO NOT USE personal insect repellents on children under two years of age.)
* Wear light-coloured clothing, long sleeves, pants and socks when outside.
* Avoid being outside at dusk and dawn, as mosquitoes are the most active at that time.
* Ensure that all containers in or around the yard such as tires, pool covers, saucers for flowerpots, wading pools and children’s toys are regularly emptied of standing water.
* Ensure that screens, windows and doors are fully sealed to prevent mosquitoes from entering the house.