Understanding the ecology and risks of Powassan virus in Canada

Powassan virus has been generating headlines in Canada in recent months following news reports that an Ottawa woman developed encephalitis in the wake of a tick bite more than a year ago and has been hooked up to a ventilator ever since.

Powassan virus is a tick-borne pathogen that has been found in many parts of Canada, but is of greatest concern in Ontario and New Brunswick. First identified in 1958 when a five-year old boy from Powassan, Ontario fell ill, the ecology of Powassan virus is complicated and known to involve several species of ticks, hosts, and reservoirs.

The virus is primarily spread by the blacklegged tick -- the same tick that spreads Lyme disease -- which has been increasing its geographic range in Canada since the first specimens were identified at Long Point, Ontario in the early 1970s. Blacklegged ticks are generalist feeders that target a wide variety of hosts, including deer, rodents, birds and humans. When an infected tick feeds on someone, it can transmit the virus to them through its saliva.

While the blacklegged tick is the primary vector for Powassan virus, other tick species may also play a role in its transmission, including groundhog ticks and squirrel ticks. Somewhat disconcertingly, the virus can be present in tick populations in the absence of vertebrate hosts and infected ticks can survive for extended periods between feedings.

Because of the complex ecology of Powassan virus, limited research efforts in Canada, and the rareness of identified infections in this country, it can be difficult to determine exactly where risk is the greatest.

As with most tick-borne illnesses, climate change is thought to play a role in the geographic spread of Powassan virus as temperatures warm to the point where blacklegged and domestic ticks can survive in an increasing number of areas. It can also cause ticks to be active for longer periods of time each year, increasing the risk to people.

There are no specific treatments for Powassan virus and while cases of severe illness and death are associated with the pathogen, researchers believe most cases are likely mild or asymptomatic. The only cases they really know about are the ones with the most severe outcomes. Because the illness is unfamiliar to most people, many who contract the virus likely attribute their flu-like symptoms to better known illnesses.