Ever since I got interested in horse medicine, I heard nothing but how horrible internal parasites were for horses. Once I got into veterinary school, I saw post-mortem pictures of horses with purpley dead intestinal parts due to parasite problems, or horses that were really skinny and nearly dead due to parasite problems, or horses that were anemic due to parasites: terrible stuff. I came away with the impression that the worst thing that could ever happen to a horse would be if it had an internal parasite.
This idea was quite happily promoted by veterinary experts on parasitology, and especially happily promoted by companies and feed stores and catalogues that sold deworming products. So for years horse people were taught – some of them by me – that you had to deworm horses…. every day, or month, or every two months, or quarterly, using the same deworming product every day, or the same deworming product once a month for a year and then switching, or “rotating” dewormers all of the time, or give a paste or put it in the feed or run a tube down the nose or give a shot (until that didn’t work out)…
I had learned all about parasite life cycles, too. There are quite a number of them, with different life cycles, but the ones that led to all of the ugly pictures that I had seen were of particular concern. This family of parasites was/is known as “Stronglyes” (large and small). CLICK HERE is you want to see a more detailed explanation. But here’s how it goes.
- The adult parasite lives in the horse’s intestinal tract
- The adult lays eggs
- The eggs get pooped out onto the grass or into the stall
- The eggs mature into larvae over a few weeks
- The horse eats the larvae off the pasture
- The larvae grow into egg-laying adults
I’ve lived and practiced in southern California for essentially my whole career (except for one glorious year interning at Iowa State University). SoCal is just great for people (there are a lot of them) for a lot of reasons, not including traffic. Over the years, I’ve
Anyway, as I went along in my career, blithely deworming horse-after-horse that was generally living in a clean stall in a desert, it occurred to me that while I loved living in SoCal, it must be some sort of a parasite hell. Steps 3 and 4 in the life cycle are a particular impediment to any parasite that needs to develop in any situation, and on grass in particular. If the horse poops in the stall, the poop gets scooped up and thrown away. If it gets pooped out outside of the stall, it’s usually onto dry, sometimes very hot, sand. SoCal is not any sort of parasite Club Med, for sure.
Over time, all of the non-stop deworming had a completely predictable effect. Internal parasites, it seems, like to live, too, and over time, they develop mechanisms to get around the effects of the deworming agents. So, over time, the antiparasitic agents stopped
Recognizing this, in the early 2010’s, parasitology experts started recognizing that internal parasite resistance to common deworming agents was starting to become a significant problem. Something had to change. So, the experts came up with guidelines that suggested that a low-level of parasite burden was probably not a big deal, that people should check for parasites before they deworm, and many other things. You can take a look at the internal parasite control guidelines of the American Association of Equine Practitioners if you CLICK here.
Realizing that I lived in a parasite wasteland, and what with the new guidelines and all, in 2012, I started collecting fecal samples on many of my clients’ horses and deworming only if they were positive (this is slightly different than AAEP recommendations – mine
If you don’t want to read the whole study, the bottom line is that my suspicions were right. If your horse lives in a dry climate and/or has his manure picked up regularly, it’s going to be pretty hard for him to get Strongyle parasites. Otherwise stated, there’s no reason to give your horse a dewormer if he doesn’t need a dewormer. That fits in well with my general philosophy – don’t give your horse medicine unless he needs medicine (I’m like that with my own health, too).
I’ve got a couple of other research papers in the pipeline that I hope to get submitted for publication in the next few months. But as far as internal parasites go, you shouldn’t just go running to the feed store to stuff some dewormer down your horse without thinking about it first. And I’ve done research to prove it!