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Corro Stories

Horse Wormer Guide: The Ultimate Guide To Equine Internal Parasites and Deworming Your Horse

By Caroline Cochran

Internal parasites are tiny worms that can cause big problems in horses. Following the emergence of effective deworming agents, most vets advocated treatment at regular intervals to limit health risks. However, decades of this aggressive approach created a new problem for horse owners as populations of certain parasites developed resistance to common dewormers.

Today, working with your veterinarian to develop a strategic deworming plan is vital for your horse's health and the health of future generations of equines. Keep reading to learn more about common types of worms in horses, methods for assessing your horse's parasite load, and how to choose the best horse wormer for your horse.

 

Equine Internal Parasites

While most horse owners understand the importance of deworming, many don't appreciate the differences between common internal parasite species and how to treat them effectively. Each type of worm has a different impact on the horse, and understanding your horse's risk will help you prioritize a parasite management strategy to keep him healthy.

Health Risks of Internal Parasites For Horses

If your horse has a heavy worm burden, he may show external signs like weight loss, a dull coat, excessive tail scratching, or a potbelly. In addition, young horses may experience frequent diarrhea, and certain parasites may even increase the risk of colic.

These external symptoms are usually the result of more serious internal damage that can result from parasitic infection, like gastrointestinal lesions, impactions, or liver damage. However, it is normal for all horses to have some level of parasite burden. An effective parasite control strategy protects horse health and limits resistance by keeping parasite loads manageable, not eliminating them.

How Worms Infect Your Horse

Internal parasites are tiny organisms that live in the horse's body and follow a life cycle that includes stages outside and inside their host. Horses accidentally consume the eggs or larvae of parasites while grazing, and the worms mature inside their body. Many parasites cause horses to shed eggs in manure, restarting the cycle.

Worms can cause internal damage to organs while migrating throughout the body, and some may stay inside your horse for years. While most parasites stay within the gastrointestinal tract, some migrate to other tissues like the liver, lungs, or heart.

Types Of Equine Internal Parasites

Traditional deworming approaches of the past were based on large strongyle bloodworms. Today, small strongyles and tapeworms are the most troublesome internal parasites in adult horses.

Large Strongyles

Horses ingest the larvae of large strongyles while grazing on pasture. Strongylus vulgaris is the most dangerous type of large strongyle. The species is commonly known as bloodworms as they often burrow through arterial walls, disrupting blood flow and increasing the risk of colic.

Other species of large strongyles can migrate to the liver, but they do not have the same damaging effect on the bloodstream. Thankfully, this type of parasite is no longer a significant concern for domestic horses.

Small Strongyles

Small strongyles, also known as Cyathostomes, have a similar lifecycle to large strongyles, but they do not migrate to other tissues after reaching the large intestines. In recent years, the prevalence of dewormer resistance in small strongyles has led to significant changes in worming recommendations.

These worms can also be challenging to manage as they often become encysted by burrowing into intestinal walls. Most horses with access to grazing have small strongyles, but it's rare for this infection to cause severe clinical signs.

Ascarids

Ascarids, commonly known as roundworms, primarily affect young foals. Most horses develop some level of immunity by the time they are adults. While a small infestation is negligible, a heavy load of roundworms can stunt a young horse's growth.

Ascarid eggs develop into larvae in the small intestine before migrating through the liver to the lungs. Horses then cough up the larvae and reingest the roundworms before they mature in the small intestine. Like small strongyles, ascarids are also developing resistance to some wormers.

Tapeworms

Tapeworms have two hosts in their life cycle, the horse and the oribatid mite. Mites eat tapeworm eggs, and horses ingest the mites while grazing. Adult tapeworms attach to the ileocecal junction, where the cecum meets the ileum of the small intestine. This attachment can cause inflammation and contribute to ulceration or obstruction.

Pinworms

Pinworms mature inside the horse's intestines after being ingested as eggs. When the adults emerge from the anus, they'll lay eggs on the horse's skin. While pinworms are relatively harmless internally, the eggs can be itchy and lead to excessive tail rubbing and hair loss.

Bots

Bots are both internal and external parasites. Adult bot flies are flying insects that lay distinct yellow eggs on your horse's coat. Most people find bot eggs on the lower forelimbs, where the eggs remain until contact with the horse's saliva stimulates hatching. Although bots don't generally cause any severe health problems before passing in manure, they can damage the stomach lining and cause minor oral lesions.

Threadworms

Like ascarids, threadworms primarily infect foals as most adult horses develop immunity. Foals can ingest threadworm larvae through their mother's milk or from the environment, but the larvae can also penetrate the foal's skin. These larvae then migrate to the lungs and small intestine. Severe infestations can cause diarrhea and dehydration, but threadworms are rare today, thanks to dewormers.

Effective Parasite Control With Horse Deworming

Traditional deworming programs consisted of constant treatment every two months with a rotation of different classes of wormer paste. However, modern approaches to parasite control have shifted away from indiscriminate treatment.

The objective of deworming is not to kill all of the adult worms living in your horse. Instead, the goals of effective equine internal parasite control are:

  • Limit the parasite load in individuals so that horses remain healthy and do not develop clinical symptoms.
  • Control environmental contamination with the infective stages of parasites during seasons suitable for larval development.
  • Schedule the use of dewormers to prevent the spread of infective eggs between horses.

Deworming is not as simple as picking out the most popular wormer paste and popping it in your horse's mouth. Instead, an effective worm control strategy involves a careful assessment of the parasite burden in your herd and the appropriately timed use of specific classes of dewormers.

Anthelmintic Resistance

Parasite populations can become resistant to worming treatments over time. With no new drugs on the horizon, anthelmintic resistance poses a significant threat to equine health. This problem emerged from the overuse of anthelmintic drugs due to misguided attempts to eliminate parasites in horses.

Frequent exposure to the same type of wormer allowed worm populations to adapt as resistant worms survived and continued to reproduce. Today, worming recommendations have shifted to maintain refugia, allowing susceptible worms to remain in the environment and compete with resistant worms.

Assessing Parasite Burden In Horses

All horses will have a different profile of internal parasites and require a tailored deworming strategy, even herdmates residing on the same farm. Therefore, assessing your horse's overall parasite burden is a vital first step when choosing what dewormer to use and when.

Fecal Egg Count

A fecal egg count is a valuable tool that helps horse owners understand what's going on inside their horse. This diagnostic test allows your veterinarian to measure the number of strongyle eggs that your horse passes in each gram of manure. Horses with a high fecal egg count are high shedders, and horses with a low fecal egg count are low shedders.

The results of a fecal egg count help owners identify horses that may need more frequent deworming. Ideally, you should perform a fecal egg count at least once a year before deworming in the spring. A second fecal egg count 10 to 14 days after deworming can reveal if your dewormer worked and help detect dewormer-resistant parasites.

Seasonal Considerations

Considering the timing of treatments based on seasonal transmission patterns can help enhance effectiveness and limit the risk of resistance by maintaining refugia.


Dewormers that target tapeworms and bots should be used in late fall, while the treatment for encysted small strongyles is most effective at the beginning and end of the grazing season. The typical deworming schedule in the spring and fall avoids administering dewormers when temperature extremes prevent larval transmission.

Considerations For Worming Young Horses and Foals

While fecal egg counts are valuable for determining the efficacy of deworming programs for young horses, current guidelines advocate standardized deworming intervals for all foals and yearlings, regardless of shedding status. Unlike most adult horses, young horses also usually require treatment for threadworms and roundworms.

Types Of Horse Dewormer

Available paste wormer brands contain different anthelmintic drugs with varying effectiveness in treating specific types of worms. Below are the four classes of drugs commonly found in FDA-approved dewormers.

Macrocyclic Lactones: Ivermectin and Moxidectin

Ivermectin and moxidectin are the foundation for strongyle control in horses. Although there is some resistance to these drugs in ascarid populations, these potent wormers are effective against nearly all types of worms, including bots, lungworms, and even some external parasites.

Many veterinarians recommend ivermectin paste as the basis of most deworming programs. This drug will kill parasite larvae, and use every six months will eliminate large strongyles. Moxidectin is the only product licensed against encysted small strongyles without documented resistance, but it is not suitable for young horses.

Dewormers with Ivermectin or Moxidectin:

Benzimidazoles: Fenbendazole and Oxibendazole

Benzimidazoles have played a key role in horse deworming programs for over 40 years. These drugs are often used in young horses as an effective treatment for ascarids and are available in a safe-guard single dose or as a larvicidal dose in a Panacur powerpac. Resistance to fenbendazole and oxibendazole is prevalent in small strongyles, but benzimidazoles are relatively safe and unlikely to cause side effects.

Dewormers with Fenbendazole or Oxibendazole:

Tetrahydropyrimidines: Pyrantel Pamoate and Pyrantel Tartrate

Pyrantel works rapidly by paralyzing worms, but these drugs cannot penetrate the intestinal wall to kill encysted small strongyles. There is significant resistance to pyrantel among strongyles, but this drug class is more effective against pinworms than ivermectin.


However, this class of drugs is very safe for all ages of horses and is available in several forms. Pellets containing low levels of pyrantel can provide daily worming for young horses or high shedders, but you should consult your veterinarian before starting a continuous wormer.

Dewormers with Pyrantel Pamoate or Pyrantel Tartrate:

Isoquinoline-Pyrazines: Praziquantel

Praziquantel is more than 95% effective at killing tapeworms in horses. It is generally used in combination wormer pastes that also include Ivermectin or Moxidectin.

Veterinarians often recommend treating tapeworm in the fall when transmission ends due to cold weather. The life cycle of tapeworms takes six months to complete, so worming is not necessary more than twice per year.

Dewormers with Praziquantel:

Sample Worming Schedule For Adult Horses

Every horse is different. Always consult your veterinarian when creating a deworming schedule for your horse and ensure that you use the appropriate dose of wormer for your horse's weight.

Low Shedders (<200 EPG)

Fecal Egg Count: At least once per year before deworming in spring

  • March: Ivermectin, Moxidectin
  • October: Ivermectin with Praziquantel or Moxidectin with Praziquantel

Moderate Shedders (200-500 EPG)

Fecal Egg Count: Twice per year before deworming in spring and fall

  • March: Ivermectin, Moxidectin
  • July: Pyrantel Pamoate, Fenbendazole
  • October: Ivermectin with Praziquantel or Moxidectin with Praziquantel

High Shedders (>500 EPG)

Fecal Egg Count: Performed before and after worming to monitor for resistance

  • March: Ivermectin, Moxidectin or Fenbendazole
  • June: Pyrantel Pamoate, Fenbendazole or Oxibendazole
  • September: Ivermectin with Praziquantel or Moxidectin with Praziquantel
  • November: Pyrantel Pamoate, Fenbendazole or Oxibendazole

Environmental Parasite Management

Effective parasite management should not rely solely on the use of anthelmintic drugs. But even a well-executed deworming protocol has a limited impact if horse owners fail to incorporate environmental control strategies. These include:

  • Frequent manure removal from dry lots and pastures to limit chances of horses ingesting eggs passed in feces while grazing.
  • Rotating and resting pastures for the minimum duration needed to reduce infection risk based on local temperature and moisture.
  • Properly composting manure to kill parasites before spreading on pastures.

Horses kept in larger areas are less likely to graze near manure, so avoiding overgrazing and providing supplemental hay when needed can also help parasite control.

Frequently Asked Questions

Here are some answers to commonly asked questions about worming horses.

Does my horse have worms?

It's normal for every horse to have some internal parasites, but regular deworming helps keep horses healthy by keeping parasites at manageable levels. Horses with heavy parasite burdens may not show external signs, so it's essential to perform regular fecal egg counts to test if your horse has worms.

What is the best worming schedule for horses?

There is no single best deworming schedule for all horses. Effective deworming protocols are tailored to the individual horse's needs. Check out the AAEP Parasite Control Guidelines for more information on formulating the best strategy for your horse.

What horse wormer should I use?

Paste dewormer options for horses have different effects on different parasites. Therefore, you should select horse wormers based on the time of year, your horse's fecal egg count, and the specific parasites you want to target.

How often do horses need to be wormed?

Low shedders benefit from worming once to twice per year, while high shedders need to be wormed three to four times per year, with consistent fecal egg counts to confirm effectiveness.

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References

Kaplan, R.M., Nielsen, M.K., 2010. An evidence-based approach to equine parasite control: It ain’t the 60s anymore. Equine Vet. Educ. 22, 306-316.

Peregrine, A.S., Molento, M.B., Kaplan, R.M., Nielsen, M.K., 2014. Anthelmintic resistance in important parasites of horses: does it really matter? Vet. Parasitol. 201, 1- 8.

Armstrong, S.K., Woodgate, R.G., Gough, S., Heller, J., Sangster, N.C. Hughes, K.J., 2014. The efficacy of ivermectin, pyrantel and fenbendazole against Parascaris equorum infection in foals on farms in Australia. Vet. Parasitol. 205, 575-580.

Boersema, J.H., Eysker, M., Maas, J., van der Aar, W.M., 1996. Comparison of the reappearance of strongyle eggs in foals, yearlings, and adult horses after treatment with ivermectin or pyrantel. Vet. Quart. 18, 7 – 9.

Demeulenaere, D., Vercruysse, J., Dorny, P., Claerebout, E., 1997. Comparative studies of ivermectin and moxidectin in the control of naturally acquired cyathostome infections in horses. Vet. Rec.15, 383–386.

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