High-Dose Fenbendazole: Worth The Cost

Since the release of moxidectin (Quest) a few years ago, small-strongyle control continues to attract the attention of deworming manufacturers and, therefore, consumers. Moxidectin caused excitement when it was proven to eliminate encysted small-strongyle larvae, which the deworming mainstay ivermectin can’t do, and was incorporated into most deworming programs. Now fenbendazole wants to jump in at a double-dose level.

Ivermectin, of course, remains the king of the dewormers, eliminating more adult parasites than any other drug without building a resistance. In December 1997, we recommended regular ivermectin use with moxidectin in the early spring months to get the encysted buggers. Now, we’re being told a five-day course of double-dose fenbendazole, called the “Panacur Powerpac,” provides superior protection to moxidectin for small strongyles.

What Is It About Small Strongyles’
Small strongyles get the dubious honor of “most significant parasite of horses,” thanks in part to manufacturer hype and to the fact they are nasty critters.

The infective immature larvae are picked up when a horse grazes or eats in a contaminated area. One female can lay thousands of eggs, and complete elimination of this parasite is virtually impossible.

When ingested, larvae reach the intestine within a day or two and burrow through the wall of the cecum and large intestine. If the horse takes in large numbers of larvae, then mild fever, decreased appetite and abdominal pain may be seen. Larvae inside the intestinal wall become encircled by a cyst (stage L3) and either continue to develop to stage L4 or “nap” for a while. Resting, inactive L3 larvae are termed arrested (“hypobiotic”) and remain in this form for weeks, months, even years. About 50% of the larvae will be in the EL3 (early L3) state at any given time, although this number may be considerably higher or lower.

In horses with low-level infestations, the larvae are ingested, penetrate the gut, mature and emerge again without causing any symptoms. Significant problems occur when the parasite burden is high and/or large numbers of larvae emerge at once.

Deworming Efficacy
All major dewormers, including the daily Strongid C, are effective against adult small strongyles. Ivermectin also works against L4 larvae that have emerged from the intestinal wall. Moxidectin was the first dewormer approved for use against encysted forms of small strongyles and will kill late L3 (LL3) and L4 life stages, which are still within their cysts, as well as newly emerged L4 and adults.

Now, Hoechst-Roussel, manufacturer of Panacur, has FDA approval for fenbendazole at double dosages (10 mg/kg) given for five consecutive days against all stages of encysted small strongyles, including the hypobiotic forms that moxidectin has not been proven to destroy.

Nothing Really New
Veterinarians were using the double-dose fenbendazole five-day treatment long before it had official FDA approval, at least as far back as the late 1980s. It was (and still is) recommended for horses that do not hold their weight well, have poor appetites and possibly low grade chronic colic and poor performance. For a number of those horses, it worked.

We know now the problem was likely related to long-term, heavy infestation with small strongyles. Periodic deworming with the usual dewormers killed off adults, but larvae were regularly emerging, maturing and laying eggs between those treatments, keeping the environment constantly loaded with small strongyles. For horses that fit this profile, the double-dose-five-day fenbendazole is still a good idea.

Indications for this treatment are less clear, however, for healthy horses on regular deworming schedules. Problems only occur if large-enough numbers of the EL3s mature and damage the intestinal wall.

The key phrase is “large-enough numbers.” If your horse is on a 30- to 60-day deworming schedule (ivermectin or rotation schedule), the odds of your horse having a problem with small strongyles is low. Good pasture management measures further decrease the chances. Under these conditions, the number of larvae of all stages will be low.

The odds the intensive fenbendazole treatment will benefit your horse are directly proportional to the presence of risk factors for high small-strongyle exposure and the medical history of the individual animal. A healthy horse probably doesn’t need it.

For high-risk horses receiving seasonal moxidectin, the picture is less clear. If you give moxidectin too early, you may miss a significant number of EL3 larvae, which can emerge and cause problems — including recontamination of pastures. Therefore, under high-risk conditions, a yearly deworming with high-dose fenbendazole, well in advance of peak grazing time, may be good insurance.

Moxidectin (Quest) runs $15 to $17 for the one necessary treatment. The high-dose fenbendazole takes ten 25-gram tubes (two per day) of either Panacur 10% paste or Safe-Guard (also Hoechst-Roussel 10% fenbendazole) at $5 to $10 per tube (amounts for a 1,000-pound horse).

The Panacur Powerpac is sold through vets, with a cost $75 to $100, plus the farm call if your vet comes out to give it. However, we found the Powerpac in the American Livestock Catalog (800/356-0700) at about $60 and learned Valley Vet Supply (800/356-1005) can special order it for you. You may also be able to get it by special order through your local farm/livestock supply store.

For even more savings, look for the paste unflavored Safe-Guard, sold in multi-dose tubes for use in cattle and horses and available in most farm/livestock stores. You can get a 92-gram tube, close enough for two double-dose daily treatments, for about $15, which comes to $37.50 for the five-day treatment. This is not an off-label, unapproved use of Safe-Guard. The multidose tube is labeled for use in cattle or horses.

Bottom Line
For horses at high risk of heavy small strongyle infestations, the double-dose-five-day deworming with fenbendazole does provide an added margin of efficacy over moxidectin. However, the treatment is more expensive than our favored ivermectin-moxidectin schedule, so consult with your vet regarding your horse’s risk factors. If you determine the double-dose fenbendazole will benefit your horse, careful shopping, including using general livestock products, can cut your costs by 60% or more.

Also With This Article
Click here to view ”Candidates For Annual High-Dose Fenbendazole.”
Click here to view ”Deworming Drugs And Brand Names.”
Click here to view ”Resistance To Dewormers.”
Click here to view ”Symptoms of Small Strongyle Parasitism.”
Click here to view ”Safety Of High-Dose Fenbendazole.”

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