Know your ticks: Rocky Mountain wood tick (Dermacentor andersoni)

This is a tick species that British Columbians know well.

Rocky Mountain wood ticks are, not surprisingly, predominantly found in the Rocky Mountain region of Canada. They are widespread throughout the BC interior where they’re often found in concert with Saskatoons (Amelanchier canadensis) and native roses (Rosa spp.).

At higher elevations in British Columbia and Alberta, these ticks are known to congregate near power lines, logging roads and in grassy fields adjacent to shrubby, rocky terrain. Those that make their home in Saskatchewan mostly populate dry river beds or ravines, but can sometimes be found in open rangeland. It should be noted that campgrounds are a favourite habitat throughout this tick’s range.

Rocky Mountain wood ticks are generalist feeders, meaning they like to dine on a wide variety of animal species and commonly bite dogs and humans. Adult Rocky Mountain wood ticks are the primary vector of Rocky Mountain spotted fever (RMSF), Colorado tick fever, and tularemia in Canada. Not surprisingly, most Canadian RMSF cases have been diagnosed in BC, Alberta, and Saskatchewan.

Although tularemia is spread by this tick, it's extremely rare in this country and mostly restricted to the wood tick's range in central and western Canada.

As for Colorado tick fever, it's exceedingly rare. Only two human cases have ever been recorded in Canada (in Saskatchewan and Alberta), but it's important to note that the virus has been found in Rocky Mountain wood ticks collected in Alberta and BC and researchers have been able to determine that BC residents who were never diagnosed with the disease had been exposed to the virus and either didn't develop symptoms, developed mild symptoms for which they did not seek medical care, or had their illnesses misdiagnosed as more common ailments. Any of these scenarios would explain the lack of documented cases in BC.

Tick paralysis, which is caused by a substance in this tick’s saliva and not by an infectious agent, can also spring from a Rocky Mountain wood tick bite and, for the most part, is restricted to south-central BC. The treatment for this form of paralysis is the removal of the attached tick, which fairly universally leads to an immediate reversal of the paralysis.

Also of note: although Powassan virus, anaplasma, and Q fever haven't yet been found in any Rocky Mountain wood ticks tested in Canada, all of these pathogens have been found in Rocky Mountain ticks tested in the US and will likely one day be found in this country, assuming they aren't already present in tick populations that have yet to be sampled.

Diseases transmitted: Colorado tick fever, Powassan virus, Q fever, RMSF, tick paralysis, and tularemia.

Where found: British Columbia, Alberta, and to a lesser extent Saskatchewan.

Originally published in The Lyme Report, Issue 14.
Photo: Dr. Christopher Paddock / CDC