Larvicidal Dose Deworming For Horses

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

According to Dr. Laird, Quest (moxidectin) in reality will only kill about 15% of the encysted strongyles, whereas Panacur (fenbendazole) will kill a much higher amount than that and is also a lot safer then Quest/moxidectin (note: Panacur and Safeguard are identical products with different trade names the chemical name for both 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 in the pasture or picks around in his stall or paddock, he is ingesting thousands and thousands of larvae that can stay alive on the ground for up to a year.

The little larvae move along your horses digestive tract until it comes to his cecum and colon. Then it penetrates the wall of the cecum or colon and stays there for a minimum of 45-60 days and as long as 2 to 3 years. Impossible, you think. You worm religiously every 8 weeks, right? There is no way those larvae can live in there if you worm every 8 weeks. Besides, there are plenty of wormers that kill small strongyles. Heck, they ALL kill small strongyles. Right?

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

These larvae are known as encysted larvae, and Dr. Laird likened them to a hibernating bear. He explained that they have a very, very slow metabolism. When you worm your horse, that wormer is in your horse’s gut for about 18 hours. Because the encysted larvae have a very slow metabolism, the wormer simply doesn’t do the job over an 18 hour period. It doesn’t effect the little guys. So, the encysted larvae sit there making waste in the lining of your horse’s gut, and when they finally decide to emerge into your horses stomach, they leave behind all this cellular debris, and this is whenyour horse can get sick. This condition is known as Cyathostomiasis (small stronyle infection).

Symptoms can include:

*Cow manure like diarrhea

*Mild reoccuring colic (2 3 days)

*Listless, weak

*Rapid and dramatic weight loss

*Peripheral edema (swollen legs)

*May or may not be eating

So you think, I’m a smart and experienced horse owner. I know when to do a fecal egg count. Well, here’s the interesting part of trying to diagnose this condition. If you worm your horse every 8 weeks, your fecal egg count will very likely come up a big zero, but your horse can still have hundreds of thousands of encysted larvae. Okay you say, you use a daily wormer, such as Strongid C. Couldn’t have “encysted larvae”. Right? Wrong! Okay you used the ivermectin, do the daily wormer every day, except when you were at the show last month, you forgot to bring it with you, but that was only 2 crummy days. Guess what? your horse ingested thousands and thousands of larvae those two crummy days, and since Strongid C only kills the larvae on the way to the cecum and colon, once the little “buggers” have encysted, your daily wormer has no effect on them. Besides, what about all the encysted larvae that were already there before you started using Strongid C? Remember, they can live in your horse for 2 to 3 years.

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

He explained what LD-50 means. “LD” stands for Lethal Dose. “50” stands for 50%. LD-50 means the dosage of medication that will kill 50% of the animals taking it. Ivermectin has an LD-50 of 15. This means that if you gave 10 horses 15 tubes of ivermectin dewormer all at one time, it would be likely that 5 of those 10 horses would die. Quest, has an LD-50 of only 3. So, if you gave 10 horses 3 Quest dewormers, 5 would probably die.

Well, interestingly, Panacur (fenbendazole) just doesn’t kill a horse, no matter how much of the stuff you give it (although, some recent studies have noted that the chemical fenbendazole may have an LD-50 of somewhere around 200, so still very safe). Therefore, that hibernating baby worm that has burrowed into the lining of your horse’s gut gets to have the livin’ daylights kicked out of it with a double dose of Panacur (fenbendazole) for 5 days and it won’t hurt your horse. But it will kill all of those encysted larvae and in a nutshell, if you use Panacur (fenbendazole) twice a year along with a regular deworming program every 8 weeks, (or gear your deworming program toward proper management depending on where you live. FYI: extreme heat actually kills worm larvae, whereas, cold or sub-freezing temperatures does not), rotating the type of wormers, you will have yourself an Optimal deworming program.

He stressed that a high performance horse will benefit tremendously from the treatment.

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