Planning a safe and effective parasite control program for your horse should include a consultation with a veterinarian. This article is not a substitute for a consultation with a veterinarian.
The fenbendazole purge, also called the Panacur purge, Panacur PowerPac, and Safe-Guard Power-Dose, is a multi-day deworming treatment that targets specific life stages of the small strongyle, an internal parasite that infects horses. Fenbendazole is a member of the benzimidazole class of deworming drugs, and is the generic name of the active ingredient in Panacur and SafeGuard dewormers.
Why target small strongyles as part of your deworming program?
For most of the United States, from about October until the following May or June, your horse can become infected by strongyles by ingesting L3 stage large and small strongyle larvae from soil, water, or blades of grass. Strongyles in the microscopic L3 larval stage often do not survive long enough to infect your horse in hot, dry weather but tolerate freezing temperatures quite well. Large strongyles are less of a serious threat to your horse’s health than small strongyles, but share some life cycle characteristics and are affected by the same deworming chemicals.
Almost immediately after ingestion, small strongyle L3 larvae embed themselves in the mucosa (lining) of the large intestine, where they are protected from the horse’s immune system, and most deworming medications. They may continue to mature after encapsulation, or they may remain suspended in this stage for up to two years. Large strongyle larvae do not encyst, but continue to develop into adults as they migrate through various tissues in the body.
The life cycle of the small strongyle is affected by environmental conditions, with encysted small strongyles tending to emerge together in groups from the lining of the intestines in winter or spring. A mass emergence of large numbers of encysted strongyles can cause a syndrome called larval cyathostomosis, which poses a serious health risk for your horse.
Cyathostomosis may also occur several weeks after a deworming with a chemical that kills the adult stage of small strongyles. To prevent cyathostomosis, keep the population of encysted small strongyles low with a well-planned parasite control program, and use fecal egg counts on all horses in a herd to monitor your program’s effectiveness.
Which dewormers target strongyles?
The following deworming chemicals target various life stages of large and small strongyles:
Ivermectin at a dose of 0.2 mg/kg/once (EquiMax, Eqvalan, Ivercare, Rotectin 1, Zimecterin, Zimecterin Gold, generic ivermectin) kills adult large and small strongyles, and migrating large strongyle larvae.
Moxidectin at a dose of 0.4 mg/kg/once (Quest, Quest Plus, Combocare) kills adult large and small strongyles and migrating large strongyle larvae, plus the encysted LL3/L4 larval stages of small strongyles.
Fenbendazole at a dose of 10 mg/kg/day; 5 days (Panacur PowerPac, Safe-Guard Power-Dose) is effective against all of those stages and types of strongyles, plus the encysted EL3 stage of small strongyles.
(Note: updated information on moxidectin added 08-15-10)
In the past, moxidectin was thought to be effective against only some of the life stages of small strongyles, an internal parasite that infects horses. However, newer research indicates that moxidectin is very effective against all stages of small strongyles, including encysted small strongyles.
Read more in the article “Are Moxidectin Horse Dewormers Effective against Encysted Small Strongyles?”
The study discussed in the article found that a single dose of moxidectin horse dewormer is as effective against encysted small strongyles as the five-day double-dose fenbendazole deworming regimen known as the Panacur Powerpac or Safe-guard Power-dose.
That’s good news for penny-pinchers, since a single dose of moxidectin costs around $10 compared to $30-$60 to administer the five-day double-dose fenbendazole regimen. It’s also good news for horses on farms where strongyles have developed resistance to fenbendazole.
The fenbendazole purge is a double or purge dose of fenbendazole, administered each day for five days in a row. The Panacur PowerPac or Safe-Guard Power-Dose is a conveniently bundled package of five syringes, each containing a double dose of fenbendazole paste dewormer for an average size horse. Safe-Guard Cattle Drench, which contains 10% fenbendazole in a liquid suspension, can also be used to administer the fenbendazole purge to horses, and is roughly half the cost per horse compared to the Panacur PowerPac product. To use this product to deworm horses, read the article “Using Safe-Guard Cattle Dewormer to Deworm your Horse”.
Isn’t a double dose of dewormer toxic?
Older deworming chemicals, such as trichlorphon and phenothiazine, were actually somewhat toxic to the horse at the same dose needed to effectively control parasites. Newer generations of deworming chemicals are effective against parasites at small fractions of the dose that would be harmful to the horse. Fenbendazole has a particularly high safety margin, with the dose needed to cause toxicosis (toxicity) in horses at 100 times the normal dose for deworming. So, even a double dose is only 1/50th the amount that would cause a toxic reaction in most horses.
However, administering any dewormer may cause an inflammatory reaction from a die-off of parasites. Endotoxins are released from the dead parasites inside your horse’s body following a deworming, and can lead to colic, laminitis, or other disorders triggered by bodywide inflammation.
The risk for an inflammatory reaction is higher for horses with a high parasite load or unknown worming history, or aged, underweight, or otherwise debilitated horses. Talk to your veterinarian about keeping Banamine on hand in case of a severe inflammatory response to deworming, and schedule dewormings when you will be able to keep a close eye on your horse for several days after treatment.
In the case of a horse that you suspect has a heavy parasite load, or who has an unknown deworming history, you may choose to deworm with a single dose of ivermectin one or two weeks before administering the fenbendazole purge or moxidectin. Horses can have an inflammatory reaction to large numbers of internal parasites dying off during a multi-day purge, so it may be wise to reduce the parasite population as much as possible before administering the purge.
When and where is the fenbendazole purge most effective?
Although you can administer a fenbendazole purge at any time of year, March or November may be the optimal times of year to schedule it as part of a comprehensive deworming program designed for the climate of the United States. In most cases, the fenbendazole purge should only be administered once per year, and may not be of any additional benefit if your horse is on a solid parasite control program that includes moxidectin. There is parasite resistance to fenbendazole at the single-dose rate, and evidence exists that the double-dose regimen does not provide a high level of control on some farms.
Check the references and further reading list at the end of this article to learn more about the life cycle of large and small strongyles and how they can affect your horse. This website also features a companion article that describes how to use Safe-Guard 10% fenbendazole cattle drench to safely and economically administer the fenbendazole purge to horses:
References and further reading:
The Horse, April 2004, Strongyles: The Worst of the Worms, pp15-18
The Horse, Aug 2004, Drugs for the Dewormer War, pp35-38
The Horse, Dec 2004, Control Programs for Mature Horses, pp52-58
Selected articles from back issues of The Horse are available free to registered users at:
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