Borrelia miyamotoi disease (sometimes referred to at hard tick relapsing fever or HTRF) is one of those tick-borne illnesses that seems to have gone from completely unknown to widely discussed in the blink of an eye.

The pathogen that causes the illness, Borrelia miyamotoi, was first identified in Japan in 1995 and was initially thought to be confined to that country. However, it has since been found in temperate countries around the globe. Classified as an emerging pathogen, it was first documented in Canada in 2011 and national surveillance began two years later.

Those surveillance efforts have documented Borrelia miyamotoi at low rates in ticks collected from across a wide geographic range. In all provinces east of the Rocky Mountains, Borrelia miyamotoi has been identified in blacklegged ticks (Ixodes scapularis) while in BC, the pathogen has been found in western blacklegged ticks (Ixodes pacificus). It's also likely that other Ixodes spp. ticks found in Canada carry Borrelia miyamotoi, but data in support of that theory remains slight. In all regions where Borrelia miyamotoi has been identified in this country, it infects ticks at significantly lower rates than Lyme disease bacteria.

Borrelia miyamotoi is often described as feeling like a mild case of Lyme disease. As with Lyme disease, its most common symptoms tend to be non-specific and can include chills, fever, fatigue, headaches, joint and muscle aches, nausea and vomiting. It's rare for a patient to develop severe symptoms, but when they do those symptoms can sometimes include a relapsing fever or, rarely, meningoencephalitis.

One of the more interesting things about Borrelia miyamotoi is that the pathogen doesn't follow the normal rules for borrelia bacteria. Worldwide this type of bacteria is broken down into two groups. Some, like Borrelia burgdorferi and Borrelia garinii, fall into the Lyme disease group, which infect hard-bodied ticks. Others, like Borrelia hermsii or Borrelia parkeri fall into the relapsing fever group, which infect soft-bodied ticks.

Borrelia miyamotoi caused a bit of a stir when it was first discovered because falls into the relapsing fever group and yet it is transmitted by hard-bodied ticks. That's one way to rewrite history.

Despite the similarity of symptoms between Lyme disease and Borrelia miyamotoi disease, there are some notable differences. One of the most significant is that pathogens in the relapsing fever group, like Borrelia miyamotoi, can be transmitted from one tick life-stage to the next or passed on to the next generation through its eggs. Another is that relapsing tick fever borrelia tends to be present in the blood at much higher concentrations than in the skin, which makes it somewhat easier to detect under a microscope.

Because Borrelia miyamotoi is carried by Ixodes spp. ticks, it can be transmitted to humans along with other tick-borne pathogens that infect those same ticks, including anaplasma, babesia and Lyme disease, so co-infections involving any combination of these infections can be picked up from a single tick bite.

That having been said, there is a lot that remains unclear about Borrelia miyamotoi. Although white-footed mice (Peromyscus leucopus) appear to be the most important reservoir host for the pathogen, the full scope of its hosts remains unknown although birds, field mice and voles have been implicated.

It's important to note that, as of 2024, no human cases of Borrelia miyamotoi disease have been reported in Canada although there is some data to suggest they are occurring. A survey of 10,000 blood donors in Manitoba, for instance, found that three percent had been exposed to Borrelia miyamotoi. A separate study of 250 blood samples found that between 2011 and 2014, slightly less than ten percent tested positive for antibodies to Borrelia miyamotoi. The lack of reported cases is likely an indicator that those illnesses were mild, undiagnosed or possibly misdiagnosed.

Public health experts believe that it's only a matter of time before Borrelia miyamotoi infections start to be diagnosed in Canada. As blacklegged ticks expand their territory across this country, the pathogens they carry, including Borrelia miyamotoi, will play an increasing role in the health of Canadians.