I’ve heard of a few possible cases of Rocky Mountain Spotted Fever (RMSF) in dogs in Ontario in the past week. It’s pretty preliminary information, but it’s relevant to both dog and human health so it’s worth getting some information out about it now. At least one case was a pretty solid diagnosis: Rickettsia rickettsii was identified on PCR, and the dog had clinical signs that fit with RMSF. I’ve also heard rumours of a couple of others from the same general area; in these situations it’s always tough to know how much stock to put into online comments about other cases, but I’m still trying to chase down more information to confirm.

RMSF isn’t considered an endemic disease in Ontario.  We do have one of the main tick vectors (Dermacentor variabilis, the American dog tick, see image) in the province, and locally acquired cases have occasionally been found in dogs and people, but it’s not a disease that’s generally considered much of a risk locally. That’s probably because, despite having a competent tick vector, the causative bacterium (R. rickettsii) wasn’t really prevalent in our wildlife reservoirs (e.g. rodents, rabbits). However, when ticks move they can bring pathogens with them, so it’s possible for new pathogens to gradually work their way into an area and start building up in the reservoir hosts before we recognize the problem.

It’s possible this current situation is:

• A single unusual but precedented case, plus online chatter
• A cluster of cases from a point source of exposure that will die out
• One or a cluster of cases from exposure to infected adventitial ticks (i.e. ticks from high risk areas that dropped off traveling birds)
• An indication that RMSF is emerging in some areas in Ontario

It’s hard to say which of those is most likely. Strange disease events occur all the time, and most often aren’t the harbinger of a new ongoing problem, but tickborne diseases are changing and we have to be concerned about the last possibility.

Sorting out dog origin and travel history in these cases can be challenging too. At least one of the affected dogs most likely acquired the infection in the Long Point area, a location in Ontario that was one of our first high risk regions for ticks and tickborne disease, and one that would be a logical place to find something new or evolving. More time is needed to sort out how many cases there actually are, and where the dogs might have been exposed.

RMSF is a concern because it can cause severe disease. At least one of these recently affected dogs died. RMSF can also kill people. The prognosis is worse when diagnosis and treatment are delayed, so it’s important for veterinarians and physicians to be aware of any changing regional potential for this disease to occur.

In dogs, RMSF infection can result in a wide range of clinical signs, from no apparent disease to rapid death. Classical signs include fever, weakness, petechia/ecchymoses (dots or blotches from bleeding under the skin), other signs of bleeding (e.g. in the eyes), a rash, joint pain/swelling, swelling of the spleen, and a range of other, often non-specific signs.

Decreased platelet counts are the most common abnormality on a complete blood cell count (CBC), but there may be changes in white blood cell numbers too. Sometimes, other tickborne diseases (e.g. Lyme disease, anaplasmosis) are considered first. Sometimes, it looks like an immune mediated disease, which is more common around here, and which is treated with immunosuppressant drugs – something we don’t want to give a patient with RMSF.

Diagnosis of RMSF is usually based on detection of bacterial DNA in blood by PCR. Serology (antibody testing) is nice to have too, but isn’t as readily available. Antibiotic therapy can be effective, and the prognosis is very good if antibiotics are started before disease is very severe, so early recognition is key.

People don’t get infected with RMSF from dogs, but a diagnosis of RMSF in dogs is relevant to human health since both people and dogs get infected the same way – from an infected tick. If a dog gets infected, it means there are infected ticks in the area that could also infect people. A case report from 2003 about presumptive cases of RMSF in two dogs and their owner highlights this.

• The report is from the southern US, where a person’s two dogs died. The first had signs that fit with RMSF and was taken to a veterinarian, but because the dog died overnight, samples weren’t submitted for testing. Another dog in the family died 8 days later. A couple of weeks after that, the owner went to the ER with vague signs, then went to her doctor a few days later with progressive disease (that included a fever and rash, which are signs of RMSF). She then was admitted to hospital and shortly thereafter was started on antibiotics, including doxycycline (the drug of choice for treating RMSF). Unfortunately, she deteriorated quickly and died. She was ultimately diagnosed with RMSF. She had two other dogs that also got sick, and one was confirmed to have RMSF.
• So, this person and their dogs had abundant exposure to infected ticks, and developed RMSF. The dogs got sick in advance of the owner, but that early warning sign was missed. If the person had known to think about RMSF (and mention it to her physicians), it’s possible she would have received earlier treatment and survived. But, the owner didn’t think about it, the veterinarian didn’t raise the issue when the dog was seen, and the physicians presumably didn’t query anything about pets. Multiple opportunities were missed to raise the question of “could this be RMSF?”

That’s why we need to pay attention to animal and human diseases in parallel. There’s lots of talk about One Health, but the talk:action ratio is really high. That’s my ongoing frustration with it: while the One Health concept is great, there’s very little actually being done to improve animal/human/environmental health as a whole. This situation is an area where we can act to spread awareness to maybe help both human and animal health.

American dog tick image from: https://wcvm.usask.ca/learnaboutparasites/parasites/dermacentor-variabilis-american-dog-tick.php