Parasite Dewormer (Anthelmintic) Resistance in Horses: Causes & Prevention Strategies

Anthelmintic resistance, more commonly known as Parasite Dewormer Resistance, is one of the most pressing concerns in the world of equine health. Resistance occurs when internal parasites affecting horses becoming less susceptible to parasite control medications (dewormers), rendering the medications ineffective over time.

Without effective deworming medications, the parasitic load in horses can increase unchecked, potentially resulting in severe, life-threatening conditions such as colic. Horse owners, veterinarians, and researchers must work together to develop effective parasite management strategies without compromising the efficacy of dewormers.

It’s important for horse owners and caretakers to familiarize themselves with the causes of anthelmintic resistance, how conventional deworming strategies contribute to the problem, and current best practices to combat this growing concern.

By understanding the mechanisms of resistance, the equine community can better safeguard the health and longevity of their horses now and in the future.

What Causes Anthelmintic Resistance in Horses?

Anthelmintic resistance refers to anthelmintic drugs (dewormers) becoming less effective against their target parasites over time. This means some parasites are considered “resistant” to a specific type or class of medication, preventing those drugs from having the desired effect.

Horse owners have a vested interest in maintaining the efficacy of dewormers, as these medications are the only method for combating the potentially severe health effects of a high parasite burden.

Research shows that anthelmintic resistance is developing in all of the common internal parasites affecting horses. ^[1]

Exposure to Anthelmintics

One of the main triggers of anthelmintic resistance is exposure to anthelmintic medications. Each individual parasite has small genetic variations that may make it more or less susceptible to anthelmintic drugs. ^[2]

When owners administer anthelmintic medications to their horses, only the parasites susceptible to that drug die. This leaves behind a population of non-susceptible parasites, which continue to reproduce and pass their resistance genes onto the next generation of parasites. ^[2]

Over time, more and more parasites in the population have resistance genes against traditional anthelmintic medications.

This is effect is referred to as selection pressure, meaning each subsequent generation of parasites is more likely to pass on the resistant genes. This is one of the main drivers of anthelmintic resistance. ^[2]

Low Doses of Anthelmintics

Another concern regarding anthelmintic resistance is the use of low doses of anthelmintic medications. Many owners do not weigh their horses prior to drug administration, frequently resulting in inaccurate drug dosing.

Low doses of anthelmintics allow the parasites to modify their body metabolism to increase excretion of the drug from their systems, or prevent binding of the drug entirely. ^[2] These parasites survive the anthelmintic treatment, resulting in more resistant parasites.

Elimination of Refugia

Refugia is an important concept in parasitology. This term refers to existing populations of parasites not targeted by anthelmintic treatments. These untreated parasites survive and continue to reproduce, which helps maintain a population of parasites that remain susceptible to the treatment.

By maintaining a refugia, the overall population of parasites contains both resistant and susceptible individuals. When susceptible parasites from the refugia mate with any resistant parasites that survive the treatment, the resulting offspring are less likely to be fully resistant.

This slows down the spread of resistance genes in the parasite population, extending the efficacy of the deworming medication even as new generations of parasites are produced under the selection pressure of medical parasite control.

Common refugia for equine parasites include: ^[3]

• Parasitic life stages in the environment
• Mature parasites within horses not treated with anthelmintics
• Parasitic life stages in treated horses that anthelmintic medications do not target

Many internal equine parasites use infected horses as their main refugia. ^[3] For example, cyathostomins (small strongyles) can encyst themselves in the intestinal lining and remain encysted for years. ^[3] These cysts are not susceptible to most traditional anthelmintics, making these cysts a valuable refugia for this parasite species. ^[3]

Conventional deworming programs that aggressively treat for parasites eliminate these “horse-associated” refugia, decreasing the variety of genes available in the parasite population. Over time, decreased gene variety, combined with selection pressure from anthelmintic use, further increases the number of parasites with resistance genes. ^[3]

Therefore, maintaining parasites in refugia is important to ensure that susceptible genes continue to pass along to new parasite generations. ^[3]

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Effects of Conventional Deworming Programs

Conventional deworming programs largely rely on two main strategies: interval dosing and rotating dewormer products.

However, new research into anthelmintic resistance suggests these strategies increase parasitic resistance to dewormers.

Interval Dosing History

Conventional deworming programs developed when parasites such as Strongylus vulgaris were very common. ^[4] In fact, estimates suggest that in the 1960s and 1970s, up to 90% of colic cases were due to Strongylus vulgaris. ^[4]

During this time, veterinarians recommended an interval dosing strategy of benzimidazole anthelmintics, such as fenbendazole, to prevent S. vulgaris-related colic. ^[4]

With the interval dosing strategy, owners gave anthelmintic drugs every 8 weeks, corresponding to the maturation time for parasites to grow from larvae to adults. ^[4]

This protocol virtually eliminated S. vulgaris parasites from the horse population, as the protocol killed all adult parasites before they were able to form larvae. ^[1][4] S. vulgaris is now a rare finding in horses. ^[1]

Now, the most common equine parasite are cyathostomins. ^[4] These parasites are less likely to cause significant disease than S. vulgaris, however they can cause colic and inflammation of the cecum and colon. ^[1]

Modern Implications of Interval Dosing

Cyathostomins have a different life cycle than S. vulgaris as they form cysts within the intestinal wall that anthelmintic medications cannot target. ^[4]

These cysts rupture and release adult parasites at regular intervals, with individual cysts potentially lasting for years before rupturing. ^[4] Once a cyst ruptures, it takes five weeks or more for the adults to start laying eggs. ^[5]

The regular release of adults from cysts means that interval dosing, which many owners continue to use as a deworming strategy, continuously exposes the parasites to anthelmintic medications. ^[4]

Although many adults die initially from an anthelmintic treatment, any adults that emerge several days after administering the treatment only receive a low dose of medication. This exposure to a low dose of medication, combined with survival of completely resistant parasites during the initial treatment, promotes anthelmintic resistance in these parasite species. ^[4]

Current research shows widespread anthelmintic resistance in cyathostomin parasites, likely due to the use of interval dosing strategies. Studies report resistance in cyathostomin parasites against benzimidazoles and pyrimidines (e.g., pyrantel) on multiple continents, with affected farms having over 80% of horses harboring resistant parasites. ^[1]

Rotating Dewormers

The rotation of dewormers as a treatment strategy originated in the 1960s, when only a few deworming products were available. ^[4] The products available at the time targeted one or two parasite species, meaning that horses required multiple products to cover the wide range of internal parasites affecting their health. ^[4]

Modern dewormers, such as ivermectin, cover a broad range of target species. The development of these broad-spectrum dewormers meant it was no longer necessary to cover multiple parasites, eliminating the need for rotation. ^[4]

Many veterinarians continue to recommend dewormer rotation under the assumption that parasites that are resistant to one drug are not resistant to another. ^[4] With this theory, veterinarians aim to slow the development of anthelmintic resistance. ^[4] However, there is no scientific evidence that shows dewormer rotation slows anthelmintic resistance. ^[4]

In fact, dewormer rotation may hide the development of anthelmintic resistance on a property. ^[4] Although using a second drug may kill the parasites resistant to the first drug, the drug cannot target the life stages of the resistant parasites in refugia. ^[4]

This means although the horses show low parasite burdens on fecal testing, the parasites in refugia will reinfect them with parasites resistant to the first drug. Over time, those resistant parasites may also develop resistance to the second drug. ^[4]

Monitoring Current Anthelmintic Resistance Status

The most common equine parasites developing anthelmintic resistance globally are: ^[1]

• Cyathostomins (small strongyles)
• Pinworms
• Parascaris, a common roundworm in young horses

There are only three main classes of anthelmintics currently available to target roundworms. ^[1] Although rare, parasites on farms can become resistant to multiple classes of anthelmintics.

For example, there is a recent report of triple anthelmintic resistant parasites in France. ^[6] This means none of the currently available anthelmintics would be effective in treating this population of parasites.

Table 1. Current levels of resistance against the main classes of anthelmintics ^[1]

Identification of Anthelmintic Resistance

Identifying anthelmintic resistance on a farm is critical, as these farms require a custom deworming protocol to ensure the efficacy of the medications.

The gold standard test for identifying anthelmintic resistance is a fecal egg count reduction test (FECRT).

Fecal Egg Count Reduction Test

In this test, veterinarians perform two or more fecal egg counts (FEC) tests to determine changes in egg shedding before and after anthelmintic treatment. ^[1] In a fecal egg count, veterinarians count the number of eggs in the horse’s feces to get an estimate of the eggs per gram. ^[1]

Since these tests measure eggs in the feces, they are only suitable for diagnosing parasites such as small strongyles and Parascaris. ^[7]

There are multiple steps to a FECRT: ^[1]

• Collection of feces for a fecal egg count prior to deworming
• Administration of a dewormer product
• Collection of feces for a second fecal egg count 14 days after treatment
• Statistical analysis to identify the efficacy of the dewormer product used

At least five horses from the same herd should be examined for an effective FECRT, however testing even one horse can still be an adequate indicator of anthelmintic resistance on a farm. ^[1]

As part of the statistical analysis, the veterinarian compares the number of eggs found in the FEC before and after deworming. At least 90% reduction in fecal egg counts is required to consider deworming protocols effective. ^[1]

Other Tests

Other tests are available for identifying the presence of specific parasites. ^[8] However, the efficacy of these tests in identifying anthelmintic resistance is largely unknown.

Tests may have potential applications for testing parasite resistance include: ^[8]

• Antibody testing for tapeworms
• “Tape test” for pinworms
• Enzyme-based testing for Strongylus vulgaris
• Enzyme-based testing for liver flukes

Preventing Anthelmintic Resistance

Due to increasing anthelmintic resistance, prevention strategies are critical for maintaining the efficacy of existing deworming medications. The main recommended strategies are selective deworming programs and pasture management.

Selective Anthelmintic Programs

Selective anthelmintic programs are primarily based on the results of fecal egg counts. ^[5] In this approach, veterinarians recommend only deworming horses showing egg counts of more than 200 eggs per gram of feces. ^[5]

The goal behind this treatment protocol is to reduce contamination of pastures. ^[5] Fecal egg counts are not accurate representations of the horse’s parasite load, however the amount of eggs they shed in their feces does indicate the degree of environmental contamination that horse will produce. ^[5]

By treating horses who are “high shedders“, environmental contamination goes down, and overall parasite burden in the herd decreases. ^[5]

Implementation of a selective anthelmintic program involves: ^[5]

• Annual fecal egg counts for all horses on the property
• Deworming horses with >200 eggs per gram in their feces using a product effective against the target parasite
• Deworming horses showing symptoms of parasitism, regardless of their fecal egg count result
• Regular FECRT to identify anthelmintic resistance early

Only treating “high shedders” allows small numbers of parasites in untreated horses to continue producing eggs. ^[5] These horses are unlikely to develop symptoms from their low parasite burden, but the continued production of eggs within the tract maintains susceptibility genes in the parasite population. ^[5]

This promotes continued anthelmintic efficacy, while ensuring horses do not reach high parasite burdens that may cause symptoms. ^[5]

Work with your veterinarian to design an appropriate selective anthelmintic program for your herd based on fecal egg count results.

Pasture Management

Pastures are the main source of parasite infection for horses, so proper pasture management is an important part of reducing parasite burden. ^[5] Strategies to reduce parasite contamination of pastures include: ^[5][9]

• Frequent removal of feces from the pasture (2-3 times weekly)
• “Cross-grazing” with sheep or cattle
• Resting pastures for 6 months or more
• Ensuring a stocking density of no less than 1.5 acres per horse
• Positioning muck heaps more than 10 m away from water sources or grazing areas
• Composting manure for at least six months before spreading on fields
• Keeping new horses off pastures until they have undergone a deworming protocol

Summary

Anthelmintic resistance refers to the growing problem of reduced dewormer efficacy against equine internal parasites.

• Causes of anthelmintic resistance include repeated exposure to drugs, exposure to low doses of drugs, and removal of susceptible parasites from refugia
• Conventional deworming practices only further contribute to existing anthelmintic resistance
• Many major parasites of horses are showing resistance to commonly used dewormers
• A fecal egg count reduction test is the best method for identifying anthelmintic resistance
• Selective deworming and pasture management practices are critical to preventing anthelmintic resistance from developing