Hold on to your pocket protectors – we’re going to get a little scientific here.
When we first got Valentine, one of the most important first tasks we had to do was deworm him (Say “Ahhh?”). This truly is an important part of horse maintenance, and should be done on a regular basis (usually every eight weeks, depending on where you live and other factors). Even the most cavalier horse owners I’ve met do not fail to deworm their horses.
We use Zimecterin Gold because it was the broadest antiparasitic we found. It contains 1.55% ivermectin and 7.75% praziquantel. They advertise that it kills roundworms, tapeworms, bots and “the arterial stages of S. vulgaris.” There are lots of different brands out there. I live in a small town, and am new to the whole horse thing, but all of the dewormers I’ve seen have the same active ingredients: Ivermectin or ivermectin with praziquantel. But what are ivermectin and praziquantel? And why do we need to use them?
Ivermectin is an anthelmintic that kills a broad spectrum of nematodes by causing muscle paralysis in parasites. That’s what the Internet says. Nematodes are roundworms; I had to look up “anthelmintic.” I think just about anyone would have. According to Wikipedia, anthelmintics are “drugs that expel parasitic worms (helminths) from the body, by either killing or stunning them.” I didn’t look up helminths. I just assumed that it means “parasitic worms.” You can look it up if you want.
Praziquantel is an antiparasitic. It kills tapeworms, which ivermectin does not.
A side note on ivermectin: there has been a lot of talk recently, especially on the Web, about fatalities in horses given various brands of dewormers containing ivermectin. It appears that these are rumors started in emails and have very little foundation in truth. Occasionally a horse will die after being administered a dewormer. Some of these are overdoses – these are usually foals, because ivermectin is very well tolerated in most animals, even at higher doses. Some are animals that have a sensitivity to ivermectin. The most likely cause for a dewormer fatality, strangely enough, is when a horse really needs deworming, because he hasn’t been treated in a while. If the horse has a very large population of parasites and they are suddenly all killed off, the toxins released by the dead parasites can kill the horse. (So if you have a horse whose deworming schedule you’re not sure of, please check with your vet before administering any dewormer.) As far as the ivermectin rumors go, the best advice I’ve heard is to check with a reliable source. Ask your vet – he or she should be up-to-date on any recalls or such, if there are any. You can even call the company that manufactures the dewormer directly. But please, please, do not skip treatment to avoid potential dangers from medication. The chances of your horse getting a bad dose of medication are very slim. The chances of your horse being infected with potentially dangerous parasites is close to 100%.
The parasites which you are trying to prevent live their life in a cycle between pasture, horse, and manure. The eggs lay around in the grass in your pasture (or the pasture of the hay supplier you buy from). They are eaten by the horse with the grass or hay. They hatch into larvae, go on holiday in various parts of your horse’s anatomy, mature into nasty, sometimes very large and numerous, worms, and lay eggs in your horse which are passed through into the manure, which ends up in your pasture, starting the cycle again.
Roundworms are a very common equine parasite, and some studies suggest that all horses are infected with these worms. Like most parasites, they take up residence in the intestinal tract, and roundworm infection can cause symptoms from chronic weight loss to severe diarrhea. Like all the parasites we will be discussing, untreated roundworm infection can even cause death. An interesting fact about roundworms is that part of their life cycle is to burrow into the intestinal wall, where they can live in a dormant state for months or even years. That is another reason why a consistent deworming program is vital to your horse.
Tapeworms are, as the name suggests, a long, flat worm. When this worm infects a horse, it does so in large numbers at the junction of the small and large intestine (known as the ileocaecal junction). Here, this giant mass of worms can cause all kinds of havoc, including bowel irritation, twisting of the intestine, or even rupture of the intestine. A large percentage of colic is caused by this parasite.
Bots are not tiny little androids. They are also not worms. “Bot” is actually short for “botfly,” a large, bee-sized fly. This fly, like other flies, congregates on your poor horse, driving him crazy. They also lay their eggs on the horse. Since the horse is bothered by the flies on him, he will lick or bite at them, thereby bringing the bot eggs into his mouth. Then – and this is really gross – the eggs hatch into larvae, which burrow into your horse’s lips, gums and tongue. This apparently doesn’t bother the horse (just the humans), but then the larvae migrate into the horse’s stomach, which does cause problems, ranging from stomach upset to a perferation of the stomach, which can cause death. I knew I hated those darn flies.
S. vulgaris is the short name for Strongylus vulgaris, a scientific name for a group of parasites that also include Strongylus equinus and Strongylus edentus. The common names for these parasites are large strongyles or bloodworms. All three of these parasites enter your horse in the same way – they lurk in your pasture in the larval form and are ingested by your grazing horse. Once inside the horse, the larvae travel through the horse to their favorite spots: for S. equinus and S. edentus, that is the liver. For S. vulgaris, it’s the intestinal cavity. Of the three, S. vulgaris is the most dangerous. While the other two can cause significant damage to the liver, S. vulgaris causes far-ranging damage. Their ultimate goal, the large intestine, can of course be severely effected by the residence of these parasites – they bite off pieces of the intestine, which can cause gastrointestinal problems such as diarrhea and the bane of all horse owners, colic. It can also cause anemia from blood loss. (By the way, none of my sources mentioned this, but it probably causes pain or at least discomfort for the poor horse too.) However, the scariest and most serious effect of these parasites is the fact that they cause damage to the arteries as they are traveling to their destination. It takes about two weeks for the larvae to get from the mouth to the mesenteric artery, which is the main artery that feeds the intestinal tract. After about 4 months in that location, in which time the larvae grow into the adult worm, they travel to the large intestine, where they feed and lay their eggs. While they are traveling around in the arteries, they leave marks on the walls of the arteries, which can lead to blood clots. Blood clots are as bad in horses as they are in humans – when they break loose, they can cause serious debilitation or even death.
So be sure to deworm your horse. Which leads to the next big question: how? As you may remember, when we were first faced with the task of deworming our brand-new horse, we chickened out and went with on-feed dewormer. This may work for you, but it didn’t for us – Valentine likes to swish his feed onto the ground, and since the dewormer is a powder, he probably didn’t get as much of it as he should have. On our next try, we used the Zimecterin Gold paste. First, we asked a couple of people how to get the syringe into the horse’s throat. Boy, did they laugh! It turns out that there is no need to get it all the way back there. Have you inspected your horse’s mouth at all? (I hope you have – there’s important information in there, and not just his age.) You’ll note that there is a large gap between the front teeth and the back – that’s where the bit lies when your horse is bridled and ready to ride. Just stick the syringe in there, back as far as it will go (not out the other side, please) and squirt away. We’re told that the horse cannot spit it out. He can drip it out a bit, though, so step back. We tried to offer Valentine some apple after this, to appease him and to help get the paste into the tummy, but he wasn’t having any of that. He seemed miffed.
For the best information on deworming – what, when, how – please ask your local veterinarian.
Strongyle information taken, in part, from Animal World Network.