I’ve moved around North America a fair bit and as a result have first-hand experience with the regional differences in veterinary medicine (both as a doctor and pet owner). Many owners don’t fully appreciate how important local variations in disease prevalence can be to designing an effective preventative care strategy (e.g., which vaccinations to give when) and diagnosing an animal’s illness. Let me use the example of Potomac horse fever to illustrate.
The first cases of Potomac horse fever (PHF)were recognized around the Potomac River in Maryland and Virginia. Since then, cases have been confirmed through most of the United States and Canada, but PHF is still most prevalent in the eastern U.S. Horses living in close proximity to creeks and rivers are at an increased risk, and most cases are diagnosed in the spring, summer, or early fall.
Let’s look at two scenarios involving a horse with the following symptoms: fever, depression, loss of appetite, diarrhea, and abdominal discomfort. In one case, I’m still practicing in Virginia, it’s July, and the horse in question lives on a beautiful little farm with a stream meandering through the middle of it. Fast forward 10 years to case two. It’s February and the frigid wind is kicking up epic amounts of dust around my patient’s home, an arid ranchette on the eastern plains of Colorado.
In scenario one, Potomac horse fever is at or near the top of my rule out list; in case two, I’m not sure it would even appear at the bottom. Obviously, the way I would approach the diagnosis and treatment of these two patients would therefore be very different. PHF testing in case one, and if positive potentially a recommendation for vaccination of all the horses on the farm. (Even though the vaccine for PHF isn’t that great, once the disease is diagnosed at a particular location I want to do everything possible to reduce the severity of future cases). For case two, I’m probably thinking salmonellosis until proven otherwise and would order tests, empiric treatment, and isolation of the affected individual with that in mind.
The local/regional variation and seasonality seen with PHF occurs because it is a vector-borne disease. Research has shown the bacteria that causes PHF, Neorickettsia risticii, is carried by freshwater snails and flying, aquatic insects (e.g., caddisflies, mayflies, damselflies, dragonflies, and stoneflies ) that are infected with a certain type of parasite. The exact mechanism of transmission is still not known, but clearly, freshwater snails and aquatic insects are most prevalent close to water sources and in the warmer months of the year.
Potomac horse fever is not the only disease that demonstrates such quirky, situational attributes. This is why it’s so important to talk to your veterinarian about where and when your pets have spent or are going to be spending time. If your vet fails to ask about your pet’s travel history and plans, make sure to bring it up yourself.
Dr. Jennifer Coates