Internal Parasites in Horses – Types, Symptoms, Treatment & Prevention

Internal parasites– or worms- are a common concern for many horse owners. They can cause inflammation, immune dysfunction, and gastrointestinal disease.

In a parasitic relationship, the parasite lives on or inside another organism (host), relying on it for nourishment, shelter, and reproduction, while causing harm or damage to the host. The host-parasite relationship has developed over millennia, with the parasite and host species co-evolving together.

All horses are susceptible to parasites and infections can lead to a number of negative health consequences. Signs of equine internal parasites include weight loss, colic, ill thrift, poor coat quality, and lethargy.

The level of parasitic infection for individual horses will depend on a variety of factors. These include feeding and grazing conditions, pasture management, and deworming practices. ^[1]

If you suspect your horse has a high parasite load, consult with your veterinarian to receive a diagnosis and treatment options. Making changes to your horse’s feeding program and management can help to reduce the risk of internal parasites.

How Horses Get Internal Parasites

All grazing animals have an intestinal parasite load. Genetic and environmental factors influence which animals develop clinical disease from high parasite burdens.

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 an 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.

Unless something is done to interrupt this cycle, parasites will continue to reproduce, infecting any horse that grazes in the pasture.

High parasite loads are more common in young horses because their GI immunity is still developing. Adult horses do gain some immunity to certain types of parasites, however some adult horses remain susceptible to high parasite burdens throughout life.

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Types of Internal Parasites Affecting Horses

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

Large Strongyles (Strongylus vulgaris)

Historically, large strongyles were considered the most dangerous internal parasite for horses. Part of their lifecycle involved migrating through the mesenteric artery, the main blood supply to the bowel. This resulted in significant damage and impaired blood flow to the affected gut- termed “verminous arteritis”. In severe cases, horses died of complications related to colic.

The good news is that since the 1970s, frequent deworming practices have greatly reduced the prevalence of this parasite. ^[2] As a result, large strongyles are rarely found in domestic horses today.

Small Strongyles (Cyathostomin spp.)

Small strongyles are the most common internal parasite in horses today. ^[3] Once the larvae of this parasite are ingested, they burrow into the gut lining of the large colon where they continue to develop. This life stage refers to the “encysted strongyle” population. At some point, the larvae emerge 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. Most horses do not show clinical signs of infection, however larval cyathostominosis has been reported to be fatal in 50% of all cases. ^[4]

Young horses between the age of 1-4 are the most sensitive to strongyles since they are still developing immunity to this parasite. ^[5]

Ascarids (Parascaris equorum)

Roundworms, also known as ascarids, can grow to be 50 cm in length and are especially dangerous to foals and young horses.

Once ingested, roundworm larvae move through the gut wall to the liver and then the lungs where they are coughed up and swallowed. The larvae mature into adults in the intestine.

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]}

Tapeworms (Anoplocephala perfoliata)

Tapeworms are another large and potentially dangerous species of internal parasite, growing up to 20 cm in length.

They are transmitted through an intermediate host, the oribatid mite. This mite is found in grass, harvested forage, and even grain.

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.

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]}

Young horses and those over 15 years of age appear to be most at risk of tapeworm infection.

Pinworms (Oxyuris equi)

Pinworms lay eggs around the outside of the horse’s anus, causing itching and irritation. Horses will rub against trees, stall walls, or anything they can to relieve the itching.

This frequent rubbing can lead to hair loss and create sores around the anus which may become infected.

Threadworms (Strongyloides westeri)

High threadworm infections in foals can lead to delayed growth, lethargy, diarrhea, and anemia. However, most horses develop natural immunity to threadworms by 6 months of age.

Transmission can occur via milk. Therefore, deworming the mare during pregnancy can also help to reduce threadworm transferral to the foal.

Bots (Gastrophilus intestinalis)

Botflies are a seasonal parasite, leaving eggs on the horse’s hair coat 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.

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.

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.

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 goal of deworming is not to eradicate all parasites from a particular individual, but rather to limit infections so 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 drug resistance, this approach is no longer recommended. Instead, a targeted approach should be used to treat horses with high infection rates.

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]

1. 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.
2. Drug resistance is highly prevalent in small strongyle and roundworm species. ^{[10] [11]}
3. 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.
4. 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. ^[10]

• 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] [10]}

Shedding categories are used for adult horses, or those older than 4 years old. Foals and young horses will follow special guidelines, outlined in later sections.

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. ^[12]

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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:

• 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. ^[13]

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. ^[14]

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. ^{[4] [10]}

For ascarids, there is documented widespread resistance to macrocyclic lactones, with early indications of resistance against benzimidazoles and pyrimidines. ^{[2] [11]}

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]

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!

Alternative Deworming Treatments

Increased anthelmintic drug resistance and some horse owners’ desire to avoid chemical dewormers has resulted in a growing market for herbal and alternative deworming products. It should be noted that there is limited research into the efficacy of these products. ^[9]

Another alternative deworming treatment that holistically-minded horse owners may inquire about is Diatomaceous Earth (DE). Long-term use of DE is not recommended due to the risk of secondary nutrient deficiencies.

Additionally, there is little research to back the efficacy of DE. To our knowledge no studies have been done in horses. Research in cattle and sheep has shown no effect on FEC or growth and performance measures. ^[15][16] One study in hens showed that those fed DE had lower FECs than the control group. ^[17]

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

First and foremost, it’s important not to overstock pastures. 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.

The recommendations for stocking pastures will vary depending on the type of terrain and grass species present, but is generally 1 horse per 1-2 acres of pasture.

Rotational or Strip Grazing

Another strategy for managing internal parasites is rotational or strip grazing, where 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. This strategy helps reduce parasite transmission.

Co-Grazing

Co-grazing horses with other livestock can also reduce the parasite burden in horses. One study found that young horses grazing in the same pasture as cattle had reduced FECs. ^[5]

Co-grazing helps to reduce parasites in horses since some equine-specific larvae are ingested by other livestock. These parasites do not harm other species as they are unable to continue their life cycles in other animals.

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. You can allow horses to graze on the pasture again in late summer or fall.

Harrowing

Depending on the climate you live in, harrowing can either be helpful or harmful as a parasite management strategy. Harrowing involves using an implement pulled behind a tractor to break up and spread manure piles in a field.

• In warm, dry climates, harrowing a pasture (and keeping horses off of it for several weeks) can cause existing larvae to die off and may be helpful
• In moderate, moist climates, this tactic likely won’t kill parasite larvae and will increase transmission

However, harrowing is mainly effective for strongyles. Ascarids can survive harrowing.

Manure Management

The average horse produces between 60 to 70 pounds of waste per day. Regular clean-up of manure in pastures is a crucial part of parasite control. Collected manure can either be hauled off-site or composted.

The concept behind manure management is simple: less manure in the pasture means fewer parasite larvae available for your horse to ingest.

Nutritional Support

A high parasite burden in the gut can negatively affect nutrient absorption and your overall horse’s health. This is apparent when symptoms such as weight loss and diarrhea are seen.

Certain types of parasites, such as roundworms, can affect organs beyond the gut including the liver and lungs. If your horse tests positive for roundworms, you may want to provide additional support for liver and respiratory function.

To support liver function, consider providing milk thistle supplementation during and after deworming. You can support respiratory health by ensuring you are feeding low-dust hay, soaking the hay or providing it in elevated hay nets.

Following deworming, it is important to re-establish healthy hindgut function to recover from the negative impacts of parasites. Using highly digestible fibre sources such as beet pulp or soybean hulls can nourish the beneficial microbes of the hindgut and support hindgut fermentation.

Probiotic supplements, which are live beneficial microbes, can be given after deworming to help support fibre digestion in the hindgut. Look for supplements that have a high CFU count which ensures that enough microbes reach the hindgut to have an appreciable effect on digestion.

Internal parasites are here to stay, but understanding how they affect our horses and implementing appropriate treatment and management strategies can help keep parasite numbers in check and decrease negative effects on your horse’s health.

Frequently Asked Questions

Here are some frequently asked questions about internal parasites in horses:

Summary

Internal parasites are common in horses and can cause weight loss, colic, poor coat, lethargy, and broader immune and gut problems.

• Parasites spread by the fecal-oral route on pasture, and the risk is higher with crowded or poorly managed grazing.
• The most prevalent parasites today are small strongyles (including encysted stages), ascarids in foals, and tapeworms, though pinworms, threadworms, and bots also occur in horses.
• Diagnosis and control should be targeted: use fecal egg counts to classify shedders, deworm based on results, and time treatments mainly to spring and fall while addressing tapeworms once yearly.
• Drug resistance is widespread, so rotate products thoughtfully (including praziquantel for tapeworms) and verify efficacy with fecal egg count reduction tests at the herd level.
• Reduce exposure with management by avoiding overstocking, picking manure, rotating or co-grazing pastures, consider haying fields, and using harrowing only in hot/dry conditions.
• Support recovery post-deworming with good forage, salt/water, and probiotics as needed.