Sometimes in medicine, where there is so much that we don’t know, it’s pretty easy to get trapped in the familiar--stick with the drugs and dosing we know, run the tests we’ve run before, assume that if a disease hasn’t been reported in our area before we aren’t going to see it. The familiar might be a comfortable place, but it is also a deceptively dangerous place. Clinging to what we think we know or the way we’ve always done something limits us and, in the case of medical practitioners, might damage (or at least not optimally benefit) our patients.
I spent this afternoon learning about diseases I thought I knew and diseases I’ve never met. Equine piroplasmosis, a tick-borne disease, is considered an exotic disease in the U.S. I’ve never seen a case and don’t know anyone who has, so I might have been tempted to skip a talk on treatments for the condition. I’m glad, though, that I didn’t. I had no idea (well, I suppose I might have learned it for a mid-term once) that imidocarb, the standard treatment, causes severe colic signs in horses. So, of course, I didn’t know that pre-medication with another drug such as atropine or glycopyrrolate can mitigate those signs. Well, if piroplasmosis isn’t endemic to my state, will I ever need to know? Here’s the answer--once upon a time, West Nile virus was considered an exotic disease.
Never having lived even east of the Sierra Nevadas, let alone the Mississippi, I’ve never seen a case of Potomac horse fever (PHF), either. In fact, I didn’t even realize that the disease had wandered out of the mid-Atlantic. The cases presented this afternoon in a discussion of factors affecting survival of the disease hailed from Indiana and Kentucky. Just the knowledge that it isn’t confined to the Eastern Seaboard suddenly makes PHF seem a lot closer. I paid attention to the talk. Oxytetracycline. If you’re a veterinarian who's even thinking PHF, give oxytetracycline. Got it.
I’m a California girl, so there are a few bugs with whom I’m well-acquainted. Talk to veterinarians in parts of the country that are actually green year-round about Corynebacterium pseudotuberculosis (which causes pigeon fever), and you might get the same look I would give someone going on about Potomac horse fever--the look that says, “I think I learned about that--once …” I, on the other hand, used to treat three or four cases a week during the peak season. We had pigeon fever horses coming out of our ears; I would go to bed at night dreaming of plastic bags filled with pus. Having treated horses with all three reported forms of C. pseudotb infection (and some that looked like the reported forms’ alien cousins), I’ve gotten pretty cocky about my Coryne knowledge over the years. But, assuming I know it all would have been another mistake. It turns out that the test I’ve used for years to determine whether a horse has internal abscesses doesn’t work if the horse also has an external abscess. Huh. I did not know that.
Equine protozoal myeloencephalitis (EPM) is another old friend. It was the bright, shiny, new kid in town when I was in veterinary school, the diagnostic darling of every dressage barn. Now, granted, EPM hasn’t been recognized as long as pigeon fever, and the information has evolved pretty rapidly over the years, but having treated patients and written an article or two on the subject, I thought I had a reasonable handle on it. But, I didn’t know that one of the causative agents, Neospora hughesi, could be transmitted across the placenta from dam to foal, and even into a second generation.
For those of you on the other side of the country who may never have heard of pigeon fever, and those for whom EPM is a rare sighting, pay attention. As Dr. Jennifer Jeske said during her presentation, Coryne pseudotb is expanding its range. "If you're not dealing with it now, you probably will be in the future." Dr. Nicola Pusterla also showed a map demonstrating that both Sarcocystis neurona (the other causative organism of EPM) and N. hughesi EPM are becoming widespread across the United States.
With all of this in mind, I paid close attention to a presentation on an emerging disease of horses in Scandinavia called acquired equine polyneuropathy. The video and slides shown by Dr. Gittan Grondahl depicted a devastating disease characterized by an inflammatory demyelination of the long nerves of the hind limb. The horses start by losing some hind-end function and rapidly progress to knuckling over. If they go down, they usually can’t get back up and are euthanized. Will I ever see this disease? Maybe not. But I do know that it isn’t safe to assume that I know what I think I know.