
I’ve been asked about Powassan virus (POWV) a lot in recent months, which is a bit odd since it can sometimes be difficult to get Canadians to recognize the presence of any tick-borne pathogens in this country, let alone one that’s rather obscure. I believe I can thank the media for the sudden rise in awareness of POWV. And while you’ll never catch me arguing that awareness is a bad thing, it tends to be more useful when it comes with the proper context. So let’s give it one.
POWV is classified as an emerging illness in Canada, which isn’t really saying much. Pretty much every tick-borne illness in this country is classified as emerging. But it’s not emerging quickly. As of 2018, there have been just 21 cases of POWV reported in Canada since the virus was first identified in 1958. So the risk of acquiring the virus is extremely low, much lower than it is for Lyme disease or even anaplasma.
What’s notable about POWV is the increasing frequency with which cases in Canada have been reported over the last 15 years. It’s anticipated that upward trend will continue for the foreseeable future and likely even pick up pace due largely to the situation in the US where the number of POWV cases have started to spike.
Research into POWV in Canada up to this point can best be described as modest compared to other tick-borne illnesses. There isn’t a whole lot of it, just random papers here and there that give us pieces to the puzzle, but not yet a comprehensive picture. Largely that’s due to the limited number of cases that have thus far been reported and the wide geographic range over which those cases have been spread.
Most human cases of POWV identified in Canada were contracted in the Great Lakes region (Ontario, Quebec) as well as in New Brunswick. However, that does little to tell us where the virus can be found in nature. Mostly we have to guess based on the location of human cases, studies looking for the virus or its antibodies in humans, wildlife, and domestic animals, and limited active field research. As a result, the geographical distribution of POWV in Canada is not well understood.
We also don’t have a great handle on which animals are acting as reservoir hosts for POWV in this country although the virus has been found in dogs and multiple wildlife species, including groundhogs, squirrels, chipmunks, hares and mice. Notably 23% of groundhogs tested in Ontario were found to be infected with POWV. While that rate is alarmingly high, you would have to be bitten by a groundhog tick (Ixodes cookei) to acquire the virus and these ticks very rarely bite people. Partly that’s because they have an aversion to biting us but mostly it’s because the only place you’re likely to come into contact with a groundhog tick is by stepping on (or otherwise disturbing) a groundhog’s nest and how likely is that to happen? Not terribly.
Deer, cats, wild birds, skunks, porcupines, foxes, marmots, voles, wood rats, opossums, kangaroo rats, weasels, coyotes and raccoons are also potential hosts for POWV and more research will be needed to determine if and to what extent these creatures are infected with POWV in Canada.
Regardless of the reservoir host, POWV is spread to people almost exclusively by ticks and ticks carrying the virus have been found in Nova Scotia, New Brunswick, Prince Edward Island, Quebec, Ontario, Alberta, BC and Manitoba. But not all ticks carry POWV. In Ontario, Quebec and the Maritimes, the virus has been found in squirrel ticks (Ixodes marxi), which rarely bite humans. Groundhog ticks have been identified as carriers of POWV in Ontario, Quebec, Manitoba, and the Maritimes provinces but, as I mentioned above, rarely bite humans.
What concerns us most in all regions of Canada east of the Rocky Mountains are blacklegged ticks. Although famous for transmitting Lyme disease, they also transmit several other pathogens, including POWV. As these ticks proliferate in the US, they feed on any number of animals capable of acting as hosts to POWV and, as a result, POWV cases are starting to spike in regions of the US where blacklegged ticks make their home. Tens of millions of these ticks are subsequently brought into Canada each year by migrating birds and although that has so far translated into a modest bump in POWV rates in this country, it’s anticipated the rates in humans, domestic animals and wildlife will start rising more rapidly in the coming years and decades.
In BC, it’s Rocky Mountain wood ticks (Dermacentor andersoni) that are thought to be the primary carrier of POWV. However there is ongoing debate over whether POWV is endemic to the province. No BC tick has ever tested positive for it. And there hasn’t yet been a single human case of the disease contracted in the province. But POWV antibodies have been detected in wildlife in regions of BC where Rocky Mountain wood ticks are endemic (most notably the Okanagan Valley and the Kootenays) and this tick species has come up positive for POWV in the US. Additionally, a study of outdoor workers in BC conducted in 1968-69 found that 12.4% of those workers had been exposed to POWV. So there is valid reason to believe POWV is endemic to BC.
I won’t talk too much about Ixodes spinipalpus (no common name) since there has been very little research into this possible POWV vector to date and it would only be playing a minor role this country. But this tick is also present in Canada and capable of transmitting the virus.
It’s interesting to ponder how little we really know about POWV. Many researchers have expressed the opinion that the only POWV cases we are aware of are those that have presented with the worst symptoms. Most cases of POWV are asymptomatic. As a result, the true number of people affected by POWV is not currently known and since the virus has been aligned with catastrophic illness and death, there has been little focus on what the full range of symptoms might be.
What we know is that unlike Lyme, anaplasma, and babesia, which can take hours and even days to pass from ticks to humans, POWV can be transmitted in under 15 minutes and can start to attack the nervous system almost immediately. More typically, it takes a few days and up to five weeks for symptoms to develop. Those symptoms can include headaches, fever, stiff neck, drowsiness, nausea, vomiting, paralysis, muscle weakness, confusion, altered mental states, seizures, speech difficulties, lack of co-ordination, and cranial nerve palsies. Between 5% and 10% of POWV patients die as a result of brain inflammation while roughly 50% of survivors develop long-term neurological problems including headaches, memory deficits, and muscle wasting.
There is currently no treatment or vaccine for the POWV.