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Fairfield T. Bain, DVM, MBA, Dipl. ACVIM, Dipl. ACVP, Dipl. ACVECC, Merck Animal Health

We don’t like to think about them, but we know they’re there. Those pesky parasites living large on our horses’ pastures and paddocks and – eventually – making themselves at home inside our horse, thriving to ensure the next generation of parasitic threat.

For years, deworming programs were geared to eradicating parasites from horses. This was not only counterproductive but also unrealistic. Today, the goals of deworming are three-fold:

1) Eliminate enough of the parasite burden to help prevent clinical disease

2) Control parasite egg shedding and environmental contamination

3) Preserve the effectiveness of the dewormers we have for the future

Time moves on and we have evolved in our thinking, learning and understanding. Sometimes, what is old is new again and we revisit the very compounds that have dramatically changed the way intestinal parasites are managed in horses. Including exactly how they work in the horse to remove the target parasites. One such compound, fenbendazole, has been around for more than six decades, but has a story that has stood the test of time.

Going to battle at the cellular level

How a dewormer works – known as mode of action – plays an important role in determining the efficacy and safety of the treatment. Fenbendazole goes to battle at the cellular level, safely destroying the threat before it can cause problems for your horse.

Inside the cells of animals and parasites are structures called microtubules, which are important in a number of cellular processes. When fenbendazole is absorbed by the worm, it works by binding to the beta-tubulin within the microtubules and inhibiting their formation, resulting in disruption of cell division. This potent disruption blocks the parasite’s ability to produce energy, starving the parasite until it dies.

What makes fenbendazole unique is that it is more attracted to parasite beta-tubulin than that of the animal being treated. This means the animal’s cells are not destroyed along with the parasite. This is also the reason the parasite is slowly killed, and why fenbendazole is a safe choice for deworming in so many different situations and for every horse, regardless of age, size or body condition. As well as safe for the environment and many other species.

Tackling two of the toughest parasite threats

Fenbendazole’s mode of action is uniquely suited to treating ascarids, the primary parasite concern in foals and weanlings, and small strongyles, the primary parasite risk in adult horses.

Ascarids (roundworms)

Ascarids are developing worldwide resistance to ivermectin, moxidectin and pyrantel pamoate.[1]Fenbendazole is the preferred choice for treating ascarids, including ivermectin-resistant ascarids.

Ascarids present a two-pronged threat. When eggs are ingested, the larvae hatch and migrate, eventually invading the lungs, creating inflammation and ill thrift. Larvae in the lungs get coughed up, swallowed and passed into the intestinal tract where they become adults in the small intestine. Severe ascarid infestations can create intestinal obstruction that could lead to impaction colic or intestinal rupture, which can be fatal.

Fenbendazole targets adult ascarids in the small intestine. Fenbendazole’s potent, but safe mode of action, starves the ascarid until it dies. This results in a slow kill, which decreases the likelihood of potential side effects (endotoxic shock, impactions), which may occur after deworming foals with heavy ascarid infections using other classes of dewormers.[5]

Encysted small strongyles

Small strongyles pose a significant threat because of their ability to encyst. When small strongyle larvae are ingested, they burrow in the wall of the intestine. At this time the larvae may stop maturing and simply hibernate – or encyst – for prolonged periods of time. As many as 90% of the larvae may become encysted in this manner and remain in this stage of development for 4 months or up to 3 years.[7] Over time, encysted larvae continue to accumulate in the intestinal wall. When they start developing again at once – due to seasonal influences – and they all emerge at once (typically spring), this may cause severe problems for the horse, and even death.

The larvicidal dose of fenbendazole, Panacur® POWERPAC, is the only dewormer proven to treat all stages of encysted small strongyles, including the early third-stage that encyst. It can uniquely penetrate the mucosal wall and kill the encysted larvae in hiding before the threat of mass emergence. Because fenbendazole is less soluble it persists longer in the host. Efficacy can be expanded by extending the duration of exposure to the drug, which is the basis of the five-dose regimen.

Standing the test of time

Times have changed along with our approach to deworming horses. Now more than ever, it is important to work with your veterinarian to implement deworming programs informed by science to ensure the anthelmintics we have may continue to stand the test of time. This means there is not a one-size-fits-all deworming prescription.


[1] Craig TM, Diamond PL, Ferwerda NS, et ak. Evidence of ivermectin resistance by Parascaris equorum on a Texas horse farm. J Eq Vet Sco 2007;27:67-71

[2] Hearn FP, Peregrine AS. Identification of foals infected with Parascaris equorum apparently resistant to ivermectin. J Am Vet Med Assoc. 2003; 223(4):482-485.

[3] Boarsema JH, Eysker M, Nas JW. Apparent resistance of Parascaris equorum to macrocyclic-lactones. Vet Rec 2002; 150(9) 279-281

[4] Schougaard H, Nielson MK. Apparent ivermectin resistance of Parascaris equorum in foals of Denmark. Vet Rec 2007;160: 439-440

[5] AAEP Parasite Control Guidelines. Revised February 2016.

[6] Cribb, NC, Cote, NM, Bouré, LP, Peregrine, AS. Acute small intestinal obstruction associated with Parascaris equorum infection in young horses: 25 cases (1985-2004). N Z Vet J. 2006 Dec;54(6):338-43.

[7] PANACUR (fenbendazole) Paste 10% equine dewormer product label. 

Take a closer look at Panacur® and discuss its benefits with your veterinarian. Visit for more information.

Consult your veterinarian for assistance in the diagnosis, treatment, and control of parasitism.

Do not use in horses intended for human consumption. When using Panacur® (fenbendazole) Paste 10% concomitantly with trichlorfon, refer to the manufacturer’s labels for use and cautions for trichlorfon.

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