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Deworming
by David W. Ramey, D.V.M. |
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Deworming is an important part of routine horse care. Below, Dr. Ramey discusses various parasites, dewormers, and deworming schedules from his book, Horsefeathers: Facts Versus Myths about your Horse's Health.
As part of preventive maintenance for horses, people also deworm them. Horses, it seems, are the host for a variety of internal parasites. The majority of these parasites ("worms") lay eggs inside the horse and then get passed out in the manure. They lie in egg stage on the ground until they are picked up by another horse, either from blades of grass that are contaminated or from just eating manure. The eggs then mature inside the nice, warm horse and ultimately the adult worms begin to lay eggs, which starts the cycle all over again. Not all horse parasites follow this pattern of growth. Some parasites haven't read the book on this type of life cycle and choose to follow another. |
| Parasites and their Effects |
Internal parasites are associated with a number of problems in the horse. First, they can cause horses to look unhealthy and unthrifty. The worms get some of the nutrition that was rightfully meant for the horse. It takes quite a load of parasites to cause weight loss in a horse, however.
Second, parasites have been associated with colics. One type of parasite, Strongyus vugaris, undergoes one of its developmental stages in one of the major arteries that supplies the large intestines. It is thought that these larvae can clog circulation to the intestine and are therefore thought to be associated with a type of colic known as thromboembolic colic. In this type of colic, the larvae present in the arteries cause blood clots to form, presumably because they interfere with the blood flow. The blood clots (thrombi) can then break off and travel down the affected artery (the traveling clots are called emboli). The emboli then plug up the local circulation and cause the intestine to become unhealthy in the area that has its blood supply reduced or cut off.
Another parasite, the tapeworm, has been associated with a different type of colic in the horse, one in which segments of bowel slip up into each other like the parts of a telescope. This is called an intussusception.
Third, one of the parasites that didn't read the book about how parasite life cycles are supposed to go, Onchocerca, has a stage that lives in the horse's skin. Roughly 10 percent of horses can develop an allergic reaction to this parasite, which causes the skin to get crusty in a few characteristic areas (like the neck, ears and midline of the belly) and makes horses itch like crazy.
Finally, another parasite, Habronema, will sometimes get confused. This one has yet another life cycle. Habronema eggs are supposed to be laid in the mucous membranes of the mouth. Problems can happen, however, when instead of laying its eggs in the mucous membranes of the mouth like it is supposed to, the parasite deposits the eggs in the mucous membranes of the eye or the penis. When this happens, the eggs can't develop normally and the result is a granuloma -- a big red mass of inflamed tissue.
Parasites certainly can cause problems for horses, but what is more certain is that they give horse owners fits. The "best" way to kill these creatures is subject for countless research papers and presumably endless hours of stall-side discussion, but any number of ways are pretty effective. (You knew that there was not going to be a straight answer, didn't you?)
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| Deworming Compounds |
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First, a word about the deworming compounds themselves. A big myth says that deworming compounds are dangerous chemicals and that the slightest overdose can send a horse into a toxic reaction from which he may never recover. This may have been true back when horses were dewormed with things like tobacco and whiskey. Whiskey and tobacco can be very dangerous when given to horses. (They are very dangerous when given to people, too, but that has not deterred their use.) Modern deworming compounds are not at all dangerous.
Most deworming compounds work because the metabolism of a worm is very different from that of a horse. All living things function using chemical reactions. The chemicals that make the worm function do not exist in the horse. Therefore, scientists have been able to design chemicals that are specifically targeted at the worm's system. These chemicals are extremely toxic to the worm but have virtually no effect on the horse because there's nothing in the horse that the chemicals affect. Most deworming products are safe at a dose of forty times more than the horse is supposed to get. This is almost impossible to do, unless you have a lot of time on your hands and unlimited funds. The point is, these products are really safe.
Except for one (of course). There are deworming products on the market that have in them a type of chemical called an organophosphate. The products that contain it generally advertise that they specifically kill parasites known as bots. Read the label on the product to see if it has dichlorvos or trichlorfon in it. These compounds are generally safe if dosed at the manufacturer's recommended level; however, adverse reactions can occur at the recommended dose. These are older types of deworming products, and there's really no reason to use them at all. Products that contain ivermectin control bots and other parasites with a much wider margin of safety. If you do use organophosphates, watch the dose.
Ivermectin has taken the parasite control world by storm since its introduction in the early 1980s. It kills all worms except tapeworms, and it even kills the baby worms (larvae) before they grow into adults. It even kills the worms that live normally in the skin (Onchocerca) and abnormally in the mucous membranes (Habronema).
Benzimidazoles (oxibendazole, fenbendazole, mebendazole and the like) have been around for a long time. They do a good job on most of the worms but don't kill tapeworms or the ones in the stomach (bots) and mucous membranes. Given at a high dose, or on successive days, some members of this class can kill the baby worms, too. Some parasites show resistance to some members of this class of dewormers, but parasite resistance is not a major problem. These drugs are very safe and generally effective. A related chemical class of deworming products is known as febantel. This product is metabolized by the horse's body to become a benzimidazole and it works the same way as these drugs, with the same pluses and minuses.
Pyrantel pamoate is a good drug, too. It is the only one that kills tapeworms, but you have to double the recommended dose to do it (give the horse two tubes of medication). It doesn't kill the worms in the skin nor does it get the bots. Parasite resistance has not been a significant problem with these drugs.
Piperazine is an older deworming product that is used by some practitioners, primarily against ascarids, parasites affecting younger horses. It is safe and works well, but other products have a much broader range of activity and thus are more commonly used.
A final class, levami sole, is not widely used because it is relatively toxic and not particularly effective against some of the more prevalent equine parasites. It's listed here for completeness; don't ask your veterinarian to deworm your horse with levamisole, or he or she will look at you funny.
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| Deworming Methods |
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The biggest problem with establishing a deworming program for most people is that they have so many choices. Wouldn't it be nice if you could just get a straight, easy answer to a question like " What's the best program?" As long as you use something that's effective and use it periodically, you're going to kill a lot of worms. Clearly, though, this will not help you talk about worms stall-side and handle all of the myths and misconceptions.
Here's one of the biggest myths about deworming: "The route of administration matters." Or to put it another way, "You have to have your horse dewormed by a nasogastric tube (tube wormed) every once in a while."
Well, that's just ridiculous.
Back before the existence of effective paste and feed dewormers, some rationale existed for shoving a tube up your horse's nose in order to administer deworming medication to him. That is no longer the case.
You can give deworming medication to your horse in three ways. Some dewormers can be mixed with your horse's feed (one brand of these you give every day and yes, it's very safe). You can give your horse a deworming paste in his mouth. Or you can pass a tube up his nose and down his esophagus and deposit a liquid dewormer in the vicinity of his stomach. (There was an injectable deworming product, but it was pulled from the market in 1984.)
This is important. All methods of administration are equally effective, assuming two things: First, the horse must get the proper dose for his weight. Deworming a twelve-hundred-pound horse with a thousand-pound dose of dewormer doesn't work efficiently, whatever the method of administration. If you have any question about your horse's weight, overdose him. The products are safe, and what are you going to do with a teeny little bit of leftover dewormer, anyway?
Second, and this is really obvious, for the deworming product to be effective the horse must get the whole dose. If you take a tube of deworming paste and squirt it at the horse and half of it ends up on your spouse or significant other, the product is not going to work. But if you get those two little things done, then there is no difference in effectiveness between any of the deworming products. Study after study has demonstrated this.
Why does this myth about deworming by nasogastric tube persist? Is it because people believe that veterinarians have "stronger" dewormers than the ones that they can buy? (They don't.) Is it because people believe it's important to deliver the medication "right to the site?" (It isn't important and tube deworming doesn't do that anyway. The tube gets down close to the stomach and the worms are as much as eighty feet or so down the gut.) Is it because people don't want to change the way that they've done things for a hundred years? (You're getting warmer.) Is it because people don't read the current research? (Hope not.) You may feel it's necessary to run a tube up your horse's nose periodically for some reason. The horse wouldn't agree, and neither do the researchers. There's no medical reason to do it.
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| The Deworming Schedule |
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| Well, if the method of deworming isn't important, how about the schedule of dewormers? Should they be rotated or should one product be given for a year, then switched? Or does it matter? It probably doesn't matter that much. There is no firm evidence that rotating deworming products either prevents or promotes resistance to these drugs. No deworming product or deworming schedule is 100 percent effective in killing and controlling all the worms of the horse, and occasional resistance to some dewormers has been found in some worms. It does make sense that prolonged use of one single product would tend to select for a strain of parasite resistant to that one product. Since no dewormer kills all the worms, it makes sense that you should use each of them occasionally. |
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| How Often? |
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The majority of the worms of adult horses have a life cycle of six to eleven months, which is the time it takes for the worms to reach maturity after they are ingested. To control them, you need to kill worms regularly and to interrupt their life cycle so that your horse won't keep scattering eggs all over the place.
In adult horses, the frequency of deworming is primarily determined by exposure. Lush, moist pastures are ideal conditions for parasite eggs to survive. Parasite eggs can cause reinfestation in the horse for up to a year. If lots of horses live together in these conditions, there is a terrific opportunity for parasites to spread among them. If your horse is exposed to lots of parasites, you need to deworm often -- even as often as every four to six weeks -- because new adults are going to be entering your horse's intestines all the time. If you really want to deworm your horse often, one of the feed dewormers is given daily.
If, however, your horse lives in a desert or is kept in a stall and can't get to other horses' manure, the potential for him to pick up internal parasites is pretty slight. Your horse can be dewormed every three or four months and will probably be just fine.
The newest approach to deworming is to try to control them seasonally. With this method, deworming is scheduled to try to kill the egg-laying adults when they are at their highest numbers inside the horse. In the northern two-thirds of the United States, there is typically a rise in numbers of strongyle-type parasites in the spring. By deworming horses in the spring and early summer, parasite transmission is reduced because the adults are killed before they can lay eggs. This technique only works where this seasonal rise in parasites is known to occur, however. Ask your veterinarian for advice on establishing a deworming schedule.
In young horses, up to two years of age, it is especially important to control ascarids. As opposed to the parasites of adult horses, the ascarid develops fairly rapidly, taking about sixty days for the life cycle to complete itself from egg to egg-laying adult. The immature forms of this parasite migrate through the liver and lungs of the young horse and damage them, especially the lungs. Ascarid larvae are associated with the development of pneumonia in these little guys. Ascarid control is fairly important in foals. It is generally recommended that young horses be dewormed every sixty days, starting at two months of age, with a product that kills ascarids (most do). Apparently, as horses get older they develop some immunity to ascarids and so, after the horse passes his second year, it's not as important to target these worms specifically.
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| Other Methods of Control |
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Deworming is not the only effective way to help control parasites. In fact, it may not even be the best way. In addition to chemical deworming, other methods of parasite control should be used when possible. Measures can be taken to help control the transmission of the parasite eggs, for example. Simple measures such as plowing or reseeding fields can be very hard on parasite populations. Making grass into hay is very destructive to parasite larvae, and spreading of manure can leave larvae unprotected against hostile environments. In hot, dry climates, the eggs that are thus exposed to the environment dry out and die rapidly. But manure spreading can be something of a double-edged sword. In lush, moist conditions manure spreading can distribute parasite eggs all over the place, so be careful.
Removal of manure from pasture is a great way to keep horses from being parasitized. Studies have shown that this actually works better at controlling parasites than does chemical deworming. Pastures can literally be vacuumed of manure, using mechanical vacuums, or the manure can be picked up with a rake. Some people have even advocated the use of sheep and cattle as "biological" vacuum cleaners. Allowed to graze on horse pasture, sheep and cattle will happily eat up horse parasite eggs along with the grass. This poses no threat to sheep and cows, since horse parasites cannot infect them, but the effectiveness of this method of parasite control is questionable. Sheep and cattle do give a wonderful down home feeling to your pasture or paddock, however.
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| Monitoring the Program |
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| To monitor the effectiveness of your deworming program, you can have your horse's manure checked for parasite eggs. This is a reasonably effective way to see how your deworming program is going, but unfortunately many internal parasites do not shed eggs regularly. A single manure sample may not be an accurate reflection of your horse's parasite level. You'll need several consecutive manure samples to obtain meaningful results from analysis of the manure. |
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| Bots |
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A brief word about bots. Bots are parasites that live in the horse's stomach. In the parts of the country where they occur (almost everywhere except a desert), the adult botfly buzzes around the horse's legs and makes the horse nuts. The fly looks something like a big bee. The botfly lays its eggs on the horse's legs. Botfly eggs are bright yellow, and if your horse is being bothered by them, you will see all sorts of little yellow specks in the hair of your horse's legs. The horse licks the eggs off in the course of normal horse activity, which thus perpetuates the life cycle of the parasite. The ingested larvae attach in the horse's stomach and are quite happy there.
Bots apparently don't cause much damage to the horse, but people hate them and want to get rid of them. Generally, it is recommended to wait until about thirty days after the first killing frost to deworm for bots, although some people recommend more frequent deworming. Thirty days after the first killing frost, horses won't get reinfested with bots because the adult flies are all dead (they have no tolerance for cold weather). So if you only deworm for bots once, do it then. If you use ivermectin a lot as part of your regular deworming program, you'll be killing lots of bots all the time, so you probably don't have to worry about it much.
Use deworming products regularly, as your horse's exposure level dictates. Change the products every once in a while. Make sure the horse gets all of his deworming medicine. Give him the proper dose for his weight and overdose him if you're not sure. In addition to chemical deworming compounds, use other methods to control parasites, where appropriate. If you do all of this, everything will undoubtedly be just fine.
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Vet Talk with Dr. David Ramey
is a monthly column for tips on horse care. Dr. Ramey, author of numerous books and articles, is Dominion Saddlery's resident online vet. Dr. David Ramey is a 1983 graduate of Colorado State University. After completing an internship in equine medicine and surgery at Iowa State University in 1984, he moved to Southern California and began general equine practice, specializing in the care and treatment of performance horses from a variety of disciplines. Dr. Ramey is the author of numerous articles in the lay and professional press, as well as several books, including Horsefeathers: Facts vs. Myths about Your Horse's Health and the Concise Guide Series on equine health care. Look for them at Dominion Saddlery. |
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