I think it's safe to say that Colorado tick fever (CTF) is one of those tick-borne illnesses that few Canadians have ever heard of -- and there's a good reason for that.

The illness is understudied. Period. The virus that causes CTF has been documented in this country and the US, but a lack of targeted, ongoing research makes it difficult to find historical or current information about CTF, especially in Canada. Very few cases have ever been diagnosed here, and the authors of a 2018 report describing a rare case of CTF diagnosed in Saskatchewan note that we don’t even have a test for Colorado tick fever virus (CTFV) in Canada. If a doctor is suspicious that a patient has contracted the illness and wants laboratory confirmation, a sample must be sent to the US Centers for Disease Control’s Arbovirus Diagnostic Laboratory in Fort Collins, Colorado for testing.

CTFV is spread by Rocky Mountain wood ticks and, for the most part, its symptoms make it seem very much like a mild case of Rocky Mountain spotted fever (RMSF) for which CTF is often confused. Those symptoms include fevers, chills, headaches, lethargy, muscle pain, and generalized weakness. Neurological symptoms, though rare, affect children more often than adults. Roughly half of all patients develop relapsing fevers and about a quarter experience gastrointestinal symptoms. Unlike RMSF, only five percent of CTF patients develop a rash and very few ever die.

Canadian investigators first isolated CTFV from Rocky Mountain wood ticks that were collected in Banff, Alberta between 1952 and 1954. Over a decade later, in 1965 and 1966, a small number of Rocky Mountain wood ticks collected in the Selkirk mountains of southeastern BC came up positive for CTFV and limited research conducted in the BC Interior in the 1960s revealed that a small number of the region's residents carried antibodies that suggest they’d been exposed to CTFV at least once in their lives. Although no small mammal hosts were identified in either province, research out of the US suggests that golden-mantled ground squirrels -- which are ubiquitous in southern BC and Alberta -- are likely carriers of the virus.

Research out of the US also suggests that the CTFV is still circulating in nature at roughly the same levels that it was 50+ years ago. Although CTF may have faded from our collective memory, that doesn't mean it has gone away. What it means is that due to a lack of research and awareness, most of the information we have about this disease is more than 50 years out of date. That goes doubly for Canada where there are no recent studies to draw on, leaving us to rely largely on information collected in the 1950s and 1960s.

More recent efforts involving computer modelling suggest that the range of Rocky Mountains wood ticks is likely to expand in the coming decades, which means that CTFV's range could also expand. But since we don't have a great idea where the virus flourishes in nature right now, we can't really say how or where it's going to expand in the future.

The good news, however, is that if you manage to pick up an infection, it will likely manifest mild symptoms or remain asymptomatic.

Photo of golden-mantled ground squirrel (Spermophilus lateralis) taken near Lake Isabelle in the Indian Peaks Wilderness Area, Colorado, USA by Oz Tangles.