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Larvicidal Dose Deworming For Horses

Dr. David Laird of Texas is nationally recognized for his expertise in internal parasite control and, more recently, for his work on encysted larvicide treatments for cyathostomiasis, a condition that leads to poor performance and colic in calves. horses:

According to Dr. Laird, Quest (moxidectin) will actually only kill about 15% of encysted strongyles, while Panacur (fenbendazole) will kill much more than that and is also much safer than Quest/moxidectin (note: Panacur and Safeguard are identical products with different trade names; the chemical name for Panacur and Safeguard is “fenbendazole”).

Dr. Laird devoted most of his discussion to the life cycle of the small strongyle worm. When your horse goes out to graze or rummage in his stable or paddock, he is ingesting thousands and thousands of larvae that can remain alive in the soil for up to a year.

The tiny larvae move through your horse’s digestive tract until they reach their cecum and colon. It then penetrates the wall of the cecum or colon and remains there for a minimum of 45 to 60 days and up to 2 to 3 years. Impossible, you think. You religiously deworm yourself every 8 weeks, right? There is no way those grubs can live there if you deworm every 8 weeks. Also, there are a lot of worms that kill the little strongyles. Heck, they ALL kill little strongyles. Right?

Correct…however, they only kill ADULT worms, or larvae that have not yet burrowed into the lining of your horse’s cecum and colon.

These larvae are known as encysted larvae and Dr. Laird compared them to a hibernating bear. He explained that they have a very, very slow metabolism. When you deworm your horse, that dewormer stays in your horse’s gut for about 18 hours. Because encysted larvae have a very slow metabolism, the dewormer simply doesn’t do the job over an 18-hour period. It does not affect the little ones. So the encysted larvae sit there making waste in the lining of your horse’s gut, and when they finally decide to emerge in your horse’s stomach, they leave all this cellular debris behind, and that’s when your horse can get sick. This condition is known as cyathostomiasis (small stronyl infection).

Symptoms may include:

*Cow manure as diarrhea
*Mild recurrent colic (2-3 days)
* Listless, weak
*Fast and dramatic weight loss
*Peripheral edema (swollen legs)
*May or may not be eating

So you think, I am a smart and experienced horse owner. I know when to do a fecal egg count. Well here is the interesting part of trying to diagnose this condition. If you deworm your horse every 8 weeks, your fecal egg count will most likely drop to zero, but your horse may still have hundreds of thousands of encysted larvae. Okay, you say, you use a daily dewormer, like Strongid C. I couldn’t have “encysted larvae”. Right? Wrong! Okay, you used the ivermectin, do your daily dewormer every day, except when you were on the show last month, you forgot to bring it with you, but that was just 2 horrible days. Guess what? your horse ingested thousands upon thousands of maggots those two horrible days, and since Strongid C only kills the maggots on the way to the cecum and colon, once the little “bugs” have encysted, your daily dewormer has no effect on them. Also, what about all the encysted grubs that were already there before I started using Strongid C? Remember, they can live on your horse for 2-3 years.

Treatment: Two times the normal dose of Panacur dewormer for five consecutive days (note: Panacur and Safeguard are identical products with different trade names, the chemical name for Panacur and Safeguard is “fenbendazole”).

He explained what LD-50 means. “LD” stands for lethal dose. “50” means 50%. LD-50 means the dose of drug that will kill 50% of the animals that take it. Ivermectin has an LD-50 of 15. This means that if you give 10 horses 15 tubes of ivermectin dewormer all at once, 5 of those 10 horses are likely to die. Quest, has an LD-50 of only 3. So if you give 10 horses 3 Quest wormers, 5 will probably die.

Well, interestingly, Panacur (fenbendazole) just doesn’t kill a horse, no matter how much you give it (although, some recent studies have noted that the chemical fenbendazole can have an LD-50 of around 200, so it’s still very sure). So that hibernating baby worm that has gotten into the lining of your horse’s gut can be killed by a double dose of Panacur (fenbendazole) for 5 days and it won’t harm your horse. But it will kill all those encysted larvae, and long story short, if you use Panacur (fenbendazole) twice a year in conjunction with a regular 8-week deworming schedule (or gear your deworming schedule toward proper handling depending on where you live). (extreme heat actually kills worm larvae, while cold or subzero temperatures do not), by rotating the type of dewormers, you will have an optimal deworming schedule.

He emphasized that a high performance horse will benefit greatly from the treatment.

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