In recent years, one hot topic among equine veterinarians is parasite resistance; that is, the development of parasites which resist being killed by the deworming medication given to a horse. We think we know what’s causing it. But first, let’s back up…
cause severe colic, diarrhea, weight loss, anemia (loss of red blood cells), and edema (fluid accumulation within the subcutaneous space, due to loss of blood protein). Additionally, the larval stage of large strongyles can migrate through the arteries of the large colon, causing damage and blood clots which can result in a sudden, deadly colic.
When effective deworming products were developed, they were generally given via a nasogastric tube – that is, a tube placed by a veterinarian, from the nostril to the stomach. Because these medications could only be given by a veterinarian, and had a moderate cost associated (farm call fee, possible sedation, placing a nasogastric tube, and the medication itself), they were
only given as necessary to prevent parasite overload.
Once paste dewormers were developed, it was possible for horse owners to treat their own horses with deworming medication. This was good for a variety of reasons. First, these medications were safer for the horse, meaning less risk of accidental overdose. Second, no nasogastric tube was required, meaning the horses generally tolerated the procedure better. Finally, it was much cheaper. A tube of deworming medication now costs between $5-20, depending on the drug, instead of a tube deworming plus a farm call.
But, of course, there are downsides to this method. First, it is easy to underdose – if your horse has a mouthful of hay hidden, and spits it out after the medication is given, who knows how much he actually received. Second, since a veterinarian is not required, it is easy to give at inappropriate times – too frequently (many people are giving these drugs every 6-8 weeks!), or when your horse does not need it. All in all, the advantages outweigh the negatives, and by no means am I recommending a switch back to tube deworming (my rotator cuffs are very thankful for pastes!).
We can’t forget to mention the daily dewormers, which were developed as feed additives in order to provide a low-level, constant dose of deworming medication. No offense intended to those that developed or use these products (I used to use them, too!), this is about as good an idea as treating a bacterial infection with a daily, ½ dose of antibiotic. You’re not killing a whole lot of bugs, and the majority of those that survive have been exposed to just enough drug to make them resistant to a higher dose. The infection remains, and you have created super-bugs. The same applies to our dewormers.
To sum up, we have a situation where many horses are receiving deworming products entirely too frequently, and many times at too low a dose. The result has been that many of the products available to us are not effective, and resistance to the remaining products is rising. And the sad fact is, there are no new equine deworming products in development. What we have, we’re stuck with. So we need to act fast to diminish the resistance. So, how do we do this?
Tune in next week for part 2, where we'll go over methods to deal with parasite burdens while decreasing resistance. Until then, stay warm!