
Our emerging concerns about Rocky Mountain Spotted Fever (RMSF) in Long Point, Ontario, have led to a variety of questions from veterinarians and dog owners. (It’s also led to a lot of rumours, and sorting out what’s real illness vs internet chatter and hype is tough – but that’s another story.)
We’re still trying to figure out what’s really going on, but in the meantime we also need to increase awareness about the disease itself and our tracking efforts. Better awareness can lead to earlier testing and treatment, which can save lives (of both dogs and people). So here’s a quick rundown of what you need to know about RMSF.
What causes RMSF, and what signs does it cause?
RMSF is caused by the bacterium Rickettsia rickettsii, which is transmitted by a few different types of ticks. Here in Ontario, the most common tick vector for this bacterium is the American dog tick, Dermacentor variabilis. In other areas, the brown dog tick (Rhipicephalus sanguineus) and the Rocky Mountain wood tick (Dermacentor andersoni) are the main vectors. Historically, the American dog tick has largely been considered a nuisance but not a disease threat in Ontario, with most of the attention focused on the black legged tick (Ixodes scapularis) which transmits the causative agents of Lyme disease and anaplasmosis. That thinking needs to change a bit now, at least around Long Point.
The signs of RMSF are quite variable and often vague. Mild cases are probably missed regularly as they’re easily overlooked or dismissed, with vague signs such as lethargy, decreased appetite or non-specific pain or stiffness. Fever is probably pretty common at some point in disease but is not often detected by owners. More severe cases can involve a wide range of signs, including neurological disease, ocular disease, rash or other skin issues, enlarged spleen or liver, enlarged lymph nodes and increased bleeding tendencies. The latter is related to one of the more consistent findings on bloodwork in affected dogs: thrombocytopenia (decreased platelet count). Severe thrombocytopenia can lead to bleeding in any tissue, but may first be noticed due to petechia or ecchymoses (small dots of patches of bruising) on the gums or on the belly.
How long does it take an infected dog to get sick with RMSF?
The incubation period is the time from exposure to the development of the first signs of disease. That’s particularly relevant in dogs that may have transient exposure to high risk areas (like Long Point), but may not get sick or be tested until they’ve returned to a lower risk area. For RMSF, the incubation period is about 2-14 days. The cases we’ve seen so far in dogs that visited Long Point all developed disease within that window.
How is RMSF diagnosed in dogs?
Most often, PCR is used to detect the DNA of the R. rickettsii in the blood of infected dogs. This test is commonly included in some of the “vector borne” or “fever of unknown origin” PCR panels offered by commercial veterinary diagnostic labs.
We can also test blood samples for antibodies against the bacterium, but detecting antibodies alone is not enough to confirm infection. To do that, we need to test two blood samples, one at the start of disease and one a few weeks later, and show at least a 4-fold increase in antibodies (which is what we’d expect soon after and active infection). This is less less commonly used compared to PCR, and there are fewer labs that offer this kind of serology, because it can only provide a retrospective diagnosis several weeks after the dog gets sick.
It’s important to consider running a PCR for RMSF in any dog that has recently visited Long Point or adjacent areas (or anywhere else RMSF might be present) that also has compatible clinical signs such as fever, decreased platelet count and/or pretty much any non-specific disease of unknown cause. Since signs of RMSF can be so variable, querying exposure history and knowing where the disease is present are important to help know when to test.
Is it easy to miss RMSF in a dog?
Yes, and that’s the big concern because RMSF is very treatable IF it is caught early, but it can be very serious if it’s not.
One of the big diagnostic challenges right now is making sure it’s on the radar for veterinarians and dog owner. In areas where RMSF is endemic, it’s pretty high on the list for sick dogs, so dogs get tested and will even start empirical treatment before test results are available in highly suspect cases. In areas where it’s not a currently known issue (like most of Ontario), it’s easy to miss, which is why it’s so important to spread the word about the current situation. Dogs (and people) may visit areas that are high risk but live in areas that are low risk, where their local veterinarian (or physician) may not have RMSF on the radar at all.
Another major challenge is the often vague clinical signs in dogs, especially early in disease. Most often, RMSF is but one of many potential causes, and the signs of RMSF often overlap with those of immune-mediated diseases, particularly immune-mediated thrombocytopenia (IMTP), a disease caused by the immune system attacking the body’s own platelets. The main treatment for IMTP is immunosuppressive drugs, which are the last thing we want to give a dog with a serious infectious disease. So it’s critical to figure out which disease may be the cause of thrombocytopenia is a dog has potentially been exposed to RMSF before starting treatment.
Does being on a tick preventive rule out RMSF in a dog?
Wouldn’t that be nice… but no it doesn’t, for a few reasons:
- Compliance. Sometimes people forget to give their dog its preventative, or they get off schedule. Even when they think the dog is up-to-date, a close review sometimes finds lapses.
- Efficacy. Tick preventives are very good, but they are not 100%. We definitely want dogs on them, but they are not a guarantee that tickborne disease cannot occur, especially diseases where it takes less attachment time for the tick to infect the dog (see below).
- Speed of kill. How long it takes for a tick to transmit the bacterium and how quickly tick preventives work to kill a tick once it bites has to be considered. The transmission time is very well described for dogs; the Companion Animal Parasite Council says a tick must attach for 5-20 hours to transmit R. rickettsii, although some data from other species suggests the transmission time could be even shorter. Some tick preventives for dogs work very quickly and should kill ticks within minutes to a couple hours, but others can take 12 hours. That’s fine for preventing Lyme disease, since it takes 24-48 hours for Borrelia burgdorferi to be transmitted after a tick attaches, but it may not be quick enough to prevent transmission of RMSF.
- Even shorter acting tick preventives should still only be be considered a very important aid in the prevention on RMSF. Regardless of the product, it’s still important to avoid ticks, do regular tick checks, and consider RMSF if the dog is sick and has visited a potentially high risk area.
Can RMSF be treated?
RMSF can be effectively treated, but catching it early is very important. The sicker the dog is when treatment is started, the poorer the prognosis. Doxycycline is the treatment of choice, but other things might be needed depending on the type and severity of disease. Usually, the response to doxycycline is quick and obvious, and the prognosis is good if treatment is given early enough.
Spread the word!
We’re continuing to investigate this situation, in both dogs and ticks in the Long Point area, and we have a great team of colleagues working together nationally, provincially and locally, in human and animal health, to maximize the response and communication.
As ever, we’re trying to walk the line between increasing awareness and causing paranoia. We want people (dog owners, veterinarians, general public, physicians) to have RMSF on the radar, and take tick avoidance seriously as always. At the same time, we don’t want to cause panic, feed internet rumour mills, or make people afraid of going outside. It’s always a tricky balance with emerging infectious diseases.