Deworming your horse is an important practice to prevent health complications associated with internal parasites.

Internal parasites, also known as worms or helminths, are large endoparasites that live within horses and other mammals. In high numbers, they can result in inflammation, immune dysfunction, and gastrointestinal disease.

The goal of deworming is not to eradicate all parasites from a particular individual, but rather to limit infections to animals remain healthy and do not develop clinical disease.

Until recently, it was recommended to keep all horses on a routine and frequent deworming schedule. Due to growing resistance, this approach is no longer recommended. Instead, a target approach should be used to treat horses with high infection rates.

In this article we will consider some of the common internal parasites that affect horses, as well as provide step-by-step instructions for deworming your horse.

Types of Worms Seen in Horses

Over 150 species of internal parasites can affect the horse. Today, the most clinically relevant species include small strongyles, roundworms, and tapeworms.
Common Equine Internal Parasites

Small Strongyles

Small strongyles (Cyathostomin spp.) are the most common type of worm seen in horses. It is thought that 70-100% of all horses worldwide are infected with Cyathostomin spp. [1]

Once the larvae of this parasite are ingested, they burrow and continue developing in the lining of the large colon. This life stage refers to the “encysted strongyle” population. At some point, the larvae emerge from the gut endothelium as mature adults.

Encysted strongyles can be dangerous to the horse when large numbers of encysted larvae emerge all at once. This is known as larval cyathostominosis, which can cause acute inflammation and damage to the intestinal endothelium leading to diarrhea and colic. [2] [3] [4]

Large Strongyles

Historically, large strongyles (Strongylus vulgaris) were considered the most dangerous internal parasite for horses. Part of their lifecycle involves migrating through the blood vessels that supply the bowel. In severe cases, significant damage and impaired blood flow to the affected gut can result in fatal colic. [2] [3]

However, decades of frequent deworming have greatly reduced the prevalence of this parasite. [5] As a result, large strongyles are rarely found in domestic horses today.

Round Worms

Round worms or ascarids (Parascaris equorum) are most dangerous to foals and young horses. Adult horses will develop immunity as they mature. [6]

Once ingested, roundworm larvae migrate from the gut to the liver and lungs, before once again making it back to the intestine to mature into adults. Migration through the liver and lungs can cause inflammation and tissue damage.

Horses with high roundworm loads may develop a cough or nasal discharge while the larvae move through the lungs. They may also show signs of weight loss, pot belly, and diarrhea when ascarid burdens are high. In severe cases of ascarid infestation, the adult parasites can actually obstruct the small intestine, causing colic. [6] [7]

Tape Worms

Tape worms (Anoplocephala perfoliata) are a type of cestode and can grow as long as 20 cm, though are usually around 8 cm in length.

Tapeworms gather in clusters at the junction between the small and large intestine- the ileocecal junction– where they can disrupt GI motility and lead to colic. [8] [9]

Horses become infected with tape worms through eating the intermediate host. The intermediate host in the tape worm life cycle is the orbatid mite, which is commonly found on grass, hay, and straw. When the mite is ingested by a horse, the tapeworm larvae develop into adult tapeworms. Within 6-10 weeks, the females shed proglottids, or egg cases, to be passed in the manure. [10] [11]

Pin Worms

Pin worms (Oxyuris equi) migrate through the gastrointestinal tract to lay eggs around the horses’ anus.

Whilst they cause no direct pathological damage to the horse, the eggs cause intense itching and irritation to the horse around the anal area. The irritation can be so severe that horses will scratch the area until sores develop, leading to infection and severe skin irritation which can require veterinary intervention. [12]

Thread Worms

Thread worms (Strongyloides westeri) typically only affect foals. Adult horses develop natural immunity after around six months of age.

In foals, threadworms are passed from the mare’s milk to the foal, and infected foals can show poor condition, colic, anemia, diarrhea, and a general ‘failure to thrive’. [13] [14]

Neck Thread Worms

Neck thread worms (Onchocerca cervicalis) are commonly seen in warmer climates. Mature worms are up to 30 cm long and live in the nuchal ligament, which runs from the head and along the neck.

As adult neck thread worms mature and reproduce within the nuchal ligament, they release microscopic larvae, known as microfilaria, onto the horse’s skin. Microfilaria can lead to skin irritation on the face, neck, shoulders, and stomach.

Neck thread worm infection, also known as subcutaneous filariasis, is spread by blood-sucking biting insects, such as the biting midge. [15] [16]

Lung Worms

Lung worm (Dictyocaulus arnfieldi) is commonly seen in donkeys and can spread to horses who share their pasture with donkeys.

Lung worms can lead to respiratory irritation, nasal discharge, and coughing as worms migrate and develop within lung tissue.

Unlike horses who quickly show respiratory symptoms, donkeys can be infected with a large burden of lung worms and show little-to-no symptoms. [17]

Bot Flies

Bot flies (Gastrophilus intestinalis) are a seasonal parasite, leaving eggs on the horse’s hair coat- usually the legs- during the summer grazing months. Some of the eggs are later ingested as the horse grooms itself.

Once in the mouth, bot eggs hatch into larvae that migrate to the stomach to attach themselves to the lining and continue to develop. They will then detach and pass through the manure where they will pupate into flies. [18] [19]

Generally, bots are not recognized to be of clinical significance. However, it is conceivable that large numbers of botfly larvae in the stomach can lead irritation of the stomach lining or gastric impaction.

How Do Horses Get Internal Parasites?

Intestinal parasites are transmitted from horse-to-horse via the fecal-oral route. Adult parasites reproduce in the GI tract and the eggs are passed in the manure and contaminate the surrounding environment. Another horse grazing in area will ingest the eggs or larvae, and the parasite will mature into its adult life stage internally within the host.

Under natural conditions where horses are allowed to roam freely, internal parasites rarely cause clinical disease. This is because horses live a nomadic lifestyle and graze over large areas. In this environment, they don’t eat near their piles of manure, which would be contaminated with parasite eggs.

However, the confinement of horses to small pastures has led to higher parasite burdens as parasite egg populations become concentrated in small areas.

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Symptoms of Parasite Infection in Horses

If horses become infected with high numbers of parasites, they may show clinical symptoms. These can include:

  • Poor body condition
  • Slow growth or ill-thrift in young horses
  • Diarrhea
  • Colic
  • Reduced exercise performance
  • Poor reproductive performance
  • Tail rubbing (with pinworms specifically)

However, it is also possible for a horse to have a high infection rate and show no clinical symptoms.

Genetic and environmental factors influence which animals develop clinical disease from high parasite burdens. These can include:

  • Individual genetics
  • Nutrition
  • Age, particularly young and senior horses
  • Immune compromise, such as PPID

Steps to Establishing a Deworming Schedule

It’s important to note that all horses normally have some level of internal parasite infection at any given time.

The American Association of Equine Practitioners updated their Parasite Control Guidelines in 2019, the aim of which was to help veterinarians improve strategies and programs for parasite control in horses, as well as address growing drug resistance. [9]

Their recommendations were developed based on the following: [9]

  • Today, small strongyles and tapeworms are the most clinically relevant internal parasites for adult horses, while roundworms remain the most important parasite infecting foals and weanlings.
  • Drug resistance is highly prevalent in small strongyle and roundworm species. [20] [21]
  • Parasite control programs should be individualized to the horse, as adult horses vary greatly in their susceptibility to infection with small strongyles and level of strongyle egg shedding.
  • Horses less than 3 years of age are more susceptible to parasite infection and more at risk for developing disease, and therefore require special consideration.

Fecal Egg Count Test

The first step in establishing a parasite control program for your horse(s) is to have your veterinarian perform a Fecal Egg Count (FEC) on each horse. FEC’s determine how many parasite eggs per gram of manure your horse is shedding, which equates to their total parasite burden. FEC’s also identify which parasites are present.

The results of the FEC will place your horse into 1 of 3 categories: low shedder, moderate shedder, or high shedder. [20]

  • Low shedders have FECs <200 eggs per gram (epg)
  • Moderate shedders have FECs 200-500 epg
  • High shedders have FECs >500 epg

The shedding category determines how frequently your horse should be dewormed each year.

It is estimated that 50-75% of adult horses fall into the “low shedding” category and egg shedding remains fairly consistent in adult horses throughout their lifetime. [9] [20]

There are limitations to standard FECs. [9] However, these limitations are addressed with the recommended deworming practices, outlined in later sections.

  • FECs do not detect immature or larval stages of parasites, including encysted strongyles
  • Tapeworm infections are often missed or underestimated
  • Pinworm eggs are usually missed, as eggs are not shed in the manure

By using FECs, horse owners can help preserve the effectiveness of dewormers and minimize unnecessary treatment in animals shedding a low number of eggs. Additionally, targeted treatment for high shedders can help to reduce pasture contamination and parasite burdens for the whole herd. [22]

Anthelmintic Drugs (Dewormers)

The next step will be selecting an appropriate product that successfully treats the parasites of concern for your horse.

Modern anthelmintic drugs can be classified into four main categories according to their chemical makeup and how they work in the horse: [23] [24] [25]

  • Benzimidazoles target roundworms (ascarids), strongyles (large, small and encysted), pinworms, and threadworms
  • Pyrantel pamoate targets roundworms (ascarids), strongyles (large and small), pinworms
  • Macrocyclic lactones target strongyles (large, small, and encysted), pinworms, stomach bots, and some external parasites
  • Praziquantel targets tapeworms only

Double or triple combination anthelmintic products have also been developed to provide effective control against parasites that are drug resistant. [26]

Once you’ve purchased the appropriate dewormer, dose it according to the horse’s weight. You can estimate body weight with a weight tape or using a body length x height calculation.

Please note: Equine dewormers are not designed for use in non-equine species and can be toxic to livestock, cats, dogs, and humans. Keep dewormers out of reach from animals, and children; carefully dispose of used dewormers. Appropriate parasite control for your livestock and pets should be discussed with your veterinarian. [27]

Timing of Administration

The timing of administration of these products is key to maximize efficacy and reduce environmental contamination.

It is recommended to focus anthelmintic treatments for times for the year that are most optimal transmission. Generally, this is spring and fall- when temperatures are more moderate (32-77°F/0-25°C). [9]

Conversely, it is recommended to avoid or limit treatments during peak winter and peak summer, when environmental conditions are harsh and not conducive to larval survival or transmission. [9]

For tapeworms specifically, it is recommended that treatment occur annually in late fall or early winter (after the first hard freeze) when seasonal transmission ends. Horses living in dry, arid regions may have little or no exposure to tapeworms, and therefore, may not require tapeworm-specific deworming. [9]

Anthelmintic Drug Resistance

A significant concern for modern equine welfare is the emergence of drug resistance to various anthelmintic drugs. Drug resistance is accelerated by inappropriate use of dewormers, namely, over-administration.

There is documented widespread resistance to benzimidazoles in small strongyles populations, with common resistance documented to pyrimidines and early indications of resistance against macrocyclic lactones. [#] [28]

For ascarids, there is documented widespread resistance to macrocyclic lactones, with early indications of resistance against benzimidazoles and pyrimidines. [5] [21]

The Fecal Egg Count Reduction Test (FECRT) is the only method available for detecting resistance against a particular anthelmintic drug. [9]

  • To perform the FECRT, a manure sample is collected prior to deworming and 14 days following treatment
  • FECRT results can only be interpreted at a herd level and are not accurate for the individual horse

Current Guidelines for Adult Horses

Focus on control of small strongyles.

All adult horses will benefit from a basic foundation of 1 or 2 deworming treatments per year. Low shedding horses with strong natural immunity to small strongyles do not benefit from additional treatments. [9]

  • Moderate and high shedding horses will require more frequent deworming, generally 3 to 4 times per year
  • Include a treatment effective against encysted larvae towards the end of the grazing season- fall in northern climates or spring in tropical/subtropical climates

Additionally, adult horses should be treated once per year for tapeworms, usually in late fall or early winter (after the first hard freeze). This can be accomplished with combination products containing praziquantel or a cestocidal dose (double dose) of pyrantel pamoate. [9]

Pregnant mares should also be dewormed during the last 4-weeks of pregnancy to minimize infection of the foal.

Current Guidelines for Horses Under 3 Years of Age

During the first year of life, foals should receive a minimum of 4 deworming treatments. [9]

  • 1st deworming: 2-3 months of age with a benzimidazole
  • 2nd deworming: 4-6 months of age (just before weaning) after a fecal egg count to determine primary target species (strongyles vs. roundworms)
  • 3rd deworming: 9 months of age, primarily targeting strongyles and tapeworms
  • 4th deworming: 12 months of age, primarily target strongyles

Yearlings and 2 year olds should continue to be treated as “high shedders” and receive 3 yearly treatments. [9]

Accurate dosing is important for young horses, especially because their weight changes dramatically as they grow!

Management Strategies for Controlling Internal Parasites

In addition to anthelmintic medications, several management strategies are effective in reducing parasite loads in horses. They include the following:

Avoid Overstocking

When too many horses are kept in a small pasture, it increases the amount of manure and leads to overgrazing. Both of these factors can increase parasite transmission. General recommendations for stocking pastures is 1 horse per 1-2 acres.

Rotational or Strip Grazing

Rotational or strip grazing is when horses are moved from one pasture/strip to another over the course of a year. This decreases the likelihood that horses will graze the grass close to the ground where strongyle larvae tend to stay.

Co-Grazing

Co-grazing horses with other livestock helps to reduce parasites in horses since equine-specific larvae ingested by other livestock cannot continue their life cycle. One study found that young horses grazing in the same pasture as cattle had reduced FECs. [29]

Alternate Grazing and Haying Fields

Another way to break the parasitic lifecycle is allowing horses to graze a field in spring and then cutting hay on that same field during the summer. The parasite larvae die during the drying process.

Harrowing

Harrowing involves using an implement pulled behind a tractor to break up and spread manure piles in a field. It’s a technique that’s only helpful in warm, dry climates. Ascarids can survive harrowing.

Manure Management

Regular clean-up of manure in pastures is a crucial part of parasite control. Collected manure can either be hauled off-site or composted.

Summary

There are a few key points to take away about effectively deworming your horse:

  • Intestinal parasites can negatively affect the health and wellbeing of your horse.
  • Fecal egg counts (FECs) can help you determine how high your horse’s worm burden is, and how many times throughout the year he/she should be dewormed.
  • Deworming with the right product at the right time is crucial to effectively managing internal parasites.
  • Anthelminitic resistance is a serious concern. By following these guidelines, you can help minimize the spread of resistance.

If you’re unsure if internal parasites could be affecting your horse, talk to your veterinarian.

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References

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