Cases of eastern equine encephalitis are rare in Canada. But after an Ottawa man died of the mosquito-borne virus just weeks ago, experts and public health officials say cases are likely underreported.
The virus is rare but deadly, with a 33 to 70 per cent fatality rate for humans and the potential to cause severe long-term effects, such as paralysis and brain dysfunction for survivors.
There are no vaccines or treatment for humans, but there are also only a few cases reported annually, with the Public Health Agency of Canada reporting just one known case of eastern equine encephalitis as of Sept. 12.
But Dr. David Fisman, a professor of epidemiology at the University of Toronto, said that number may not be accurate because public health agencies don’t have to actively monitor eastern equine encephalitis cases.
“If you don’t know how much of something is happening, it’s very difficult to make smart plans,” he said.
What is eastern equine encephalitis?
The virus is passed between mosquitoes that draw blood from birds and then can further pass it to horses and people.
It’s found in a specific species of mosquito mostly found in rural, swampy areas but can then spread.
The difference between how West Nile and eastern equine encephalitis are tracked is the difference between reporting both probable and confirmed cases, or only what is confirmed through lab testing.
There are also differences in how provinces and territories track and report those cases to the federal agency for more central tracking, which means that while West Nile is tracked closely, standards around eastern equine encephalitis are more patchwork.
Some mosquito-borne viruses like West Nile fall under what’s considered “nationally notifiable” criteria, in which each case of a virus or disease is reported by provinces and can include both probable and confirmed cases, and information such as asymptomatic, non-neurological and neurological symptoms.
While cases of West Nile are reported voluntarily, PHAC notes local or provincial public health authorities follow up on cases and send data to the federal agency throughout the West Nile season and provide a final annual dataset.
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Blood donations are also routinely screened and reported to provinces and territories, which then follow up with individuals and then report to PHAC.
Eastern equine encephalitis, on the other hand, is not “nationally notifiable.”
Instead, it is identified through lab testing of sick people as opposed to clinical data, like symptoms, which aren’t collected, leading to human infections likely being underreported, experts say.
Fisman said it is “highly likely” there have been many more cases of eastern equine encephalitis that went undiagnosed, and given how dangerous the virus can be if infected, making the virus “nationally notifiable” would be wise.
“Then you can really have a more nuanced conversation about risk and whether risk is even changing,” he said. “In terms of surveillance, we’re just flying blind if we don’t proactively surveil for things that we know are probably going to be threats over the next 10, 20 years.”
According to PHAC, once a disease is deemed “nationally notifiable,” health officials determine a national case definition and “in this case-based system, provinces and territories voluntarily report cases that meet national case definitions for these diseases.”
West Nile virus became nationally notifiable in 2003, one year after being first detected.
“Some mosquito-borne diseases in Canada are not nationally notifiable and do not have a national case definition. We monitor these diseases through other methods, such as lab-based only surveillance,” PHAC said on its website.
“This method relies on lab data for diseases that are usually identified through the testing of sick people.”
The agency’s data set for eastern equine encephalitis noted infections in humans “are likely underreported, as EEEV is not a nationally notifiable disease.”
“National counts may differ from provincial and territorial counts because of different case definitions or surveillance method used. EEEV infection counts by region are not available, as they may be limited or inaccurate.”
Among the things that could be done if an accurate risk is known includes what is done now for West Nile.
That includes public education campaigns and if hot spots are found through tracking data, actions can be taken to reduce mosquito breeding areas.
Dr. Mark Loeb, an infectious diseases physician, told Global News active surveillance is typically done on viruses and diseases that have a “tremendous burden” on the public, such as West Nile, while eastern equine encephalitis sees more “passive” surveillance.
“Any physician who believes they have a patient with possible eastern equine encephalitis can test,” Loeb said. “Public health has limited resources and they have to target. It’s much easier to make an argument to target West Nile virus, whereas eastern equine encephalitis is relatively rare, so it makes sense that it’s not at the forefront of surveillance.”
He said given resources, an alternative to national surveillance could be a study in which blood is taken from people in a particular region and an analysis of antibodies can give a better idea of exposure to eastern equine encephalitis.
“I think that would be very useful to see the prevalence of exposure and then public health would make its decisions based on all these factors,” Loeb said.
In Plymouth, Mass., parks, playgrounds and sports fields have been closed from dusk to dawn due to an increased risk of eastern equine encephalitis.
Given the low risk of exposure to humans in Canada, PHAC said this would not be needed.
Loeb said he also doesn’t believe such actions are needed in Canadian communities at this time, with the risk remaining low, but there are still things people can do to limit their exposure and it’s what is already done to avoid West Nile.
This means wearing long pants, a long-sleeved shirt and socks and using a mosquito repellent containing DEET.
