Ivermectin is an over-the-counter medication used to control internal parasites in horses and other animals. It is macrocyclic lactone medication and is one of the most widely used dewormers or anthelmintics in the equine industry. [1]

Ivermectin is effective against a wide range of parasites, including small and large strongyles, ascarids, pinworms, botflies as well as external parasites, such as mites or lice. This medication is available in both a paste and liquid format. [2]

Incorporating ivermectin as part of a comprehensive parasite control program enables horse owners to protect their horses from parasitic infestations. However, some parasites are developing resistance to ivermectin and other deworming agents due to the improper use of these products.

Consult with your veterinarian prior to administering ivermectin paste or liquid to your horse. Your veterinarian will help you determine the proper dosage regimen and provide guidance for an effective deworming strategy. [3][4]

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Ivermectin for Horses

Ivermectin is an antiparasitic drug that is commonly prescribed to treat and prevent a wide range of internal parasites in equids. It works by interfering with the nerve and muscle function of parasites, leading to their paralysis and death.

Common brand names for ivermectin paste formulated specifically for horses include:

  • Bimectin Paste
  • Durvet Ivermectin Paste
  • Equimec
  • Duramectin Paste
  • Powermectin
  • IverCare
  • Equimax Paste
  • Zimecterin
  • Imec (Ivermectin Oral Paste P)
  • Vetrimec Paste
  • Intermectin Paste
  • Equimectrin

Ivermectin is generally well-tolerated in horses and has a wide margin of safety. However, it’s important to administer the correct dosage based on the horse’s body weight and instructions from your veterinarian.

Parasites

Ivermectin is effective against a broad spectrum of parasites in horses, including nematodes and arthropods. It is used to treat infestations with the following parasites: [3][4][5]

  • Small strongyles (Cyathostomin spp.)
  • Large strongyles ( vulgaris, S. edentatus, S. equinus)
  • Ascarids (Parascaris equorum)
  • Botflies (Gastrophilus intestinalis)
  • Threadworms (Strongyloides westeri)
  • Pinworms (Oxyuris equi)

While ivermectin is effective against many common parasites, no dewormer is effective against all parasites. For example, tapeworms (A. perfoliata) are not susceptible to ivermectin when used on its own. The typical treatment for tapeworms consists of praziquantel with or without another medication, such as ivermectin or moxidectin. [1]

Internal vs. External Parasites

Horses can be affected by both internal parasites (endoparasites) and external parasites (ectoparasites). Internal parasites inhabit the inside of the host organism, targeting organs and systems, while external parasites live on the surface of the host, often affecting the skin and outer orifices.

Ivermectin is more commonly used against internal parasites, but may also be used to address external parasites, such as mites or lice.

Mites

Mites are small, parasitic arthropods that cause mange in horses, which is a skin condition that results in irritation, discomfort, itching and hair loss. Two common mite species in horses are Chorioptes equi (leg mites) and Sarcoptes scabiei (sarcoptic mange mites). [6]

Oral ivermectin has been used to treat different types of mange in horses, including psoroptic, chorioptic and sarcoptic mange. However, ivermectin is not officially labeled for the treatment of mange, and not all mites are susceptible to its effects. [3][4][7]

Lice

Lice are another type of ectoparasite that infest the horse’s skin and coat. Sucking lice (Haematopinus asini) feed on the horse’s tissues and blood, causing significant skin irritation, itching and hair loss.

Ivermectin (or moxidectin) has been shown to produce results in controlling sucking lice. [8]

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Anthelmintic Drugs for Horses

Determining whether a horse should be treated with ivermectin or any other deworming agent requires a comprehensive assessment of the horse’s health, history, and parasite exposure.

There are three classes of anthelmintic drugs commonly used in horses: [9]

  • Benzimidazoles: fenbendazole, oxibendazole
  • Pyrantel Pamoate: Pyrantel
  • Macrocyclic Lactones: ivermectin, moxidectin

The use of a specific anthelmintic agent should be based on evidence of parasitic infestation in the horse. Your veterinarian will use diagnostic testing and assess the following clinical signs to evaluate for internal parasites: [10]

Clinical signs do not always align with a horse’s parasite burden. Some horses have a high parasite load without showing any physical signs of infestation, while others display noticeable signs with a low parasite load.

The effects of parasites on horses are multifactorial and can depend upon species and abundance of the parasites, as well as the horse’s age and immune response.

How Ivermectin Works

Ivermectin belongs to a chemical class of macrocyclic lactones called avermectins. Avermectins are fermentation by-products derived from soil-dwelling Streptomyces bacteria.

These compounds have broad-spectrum antiparasitic properties against nematodes and arthropods. Their unique mechanism of action targets parasites effectively yet has minimal impact on the cells of mammals. [2][11]

GABA Receptors

Ivermectin eliminates parasites by targeting gamma-aminobutyric acid (GABA) receptors on their nerves and muscle cells. GABA is a neurotransmitter that regulates nerve impulses. When GABA binds to receptors on nerve cells, it typically inhibits or reduces the activity of those cells.

In horses treated with ivermectin, the drug binds to GABA receptors on parasites and enhances the inhibitory effect of this neurotransmitter. As a result, the nerve cells are less likely to fire or send signals.

Enhancing the inhibition of nerve cells makes it harder for the parasites’ muscles to contract. Sustained inhibition of nerve cells causes paralysis in the parasite’s muscles and leads to their death. [12]

Other Parasites

Some parasites, such as tapeworms (Anoplocephala perfoliata) and flatworms (flukes), are not affected by ivermectin because they do not have GABA receptors. [11] This means the drug does not cause muscle paralysis or cell death in these parasites.

To control and treat tapeworm infections in horses, other anthelmintics are used. Praziquantel, an anthelmintic belonging to the isoquinoline class, is effective against tapeworms but has no activity against other common equine parasites. It is typically administered in combination with other anthelmintic agents. [12]

Ivermectin vs. Moxidectin

Ivermectin and moxidectin are both antiparasitic medications belonging to the macrocyclic lactone class of drugs. When administered at the recommended dosage (200 µg/kg for ivermectin and 400 µg/kg for moxidectin) their efficacy against adult parasites is practically the same. [3][4][9]

However, these medications work differently when treating encystment stage infestations of cyathostomins (small strongyles). In these cases, moxidectin is the preferred dewormer. [3]

Encystment refers to the stage in the life cycle of small strongyles in which larvae penetrate the intestinal wall and become dormant before developing and emerging as adults. [3][4]

Ivermectin is ineffective against encysted cyathostomin larvae because it is rapidly metabolized and excreted by the horse’s body. The elimination half-life for Ivermectin is around 4 days, compared to 23 days for Moxidectin, meaning it is effective for a much shorter period of time.

Furthermore, by the time the encysted larvae emerge from the intestinal wall, there might be too little ivermectin left in the blood to kill the parasites.

How to Use Ivermectin

Ivermectin is available in a variety of commercial formulations and combination products, most commonly as an oral paste. While this medication was initially introduced as an intramuscular injection, this form was discontinued due to adverse reactions. [13]

In North America, ivermectin is usually available in a prefilled syringe or tube at a 1.87% concentration, or as a 1% liquid drench.

The recommended dose is 200 µg/kg or 0.2 mg/kg of the horse’s body weight. [3][#] Ask your veterinarian for help with estimating your horse’s weight to ensure you provide the correct dosage of this drug.

Is Ivermectin Safe for Horses?

Ivermectin is widely used and considered safe for horses when given at the correct dose. Its selective toxicity ensures it’s effective against parasites but has little effect on mammals, when used as directed.

The type of GABA receptors that ivermectin binds to are only found in invertebrates, such as parasites, and have not been reported in mammals, such as horses. Furthermore, horses have a blood-brain barrier that prevents many medication substances, including ivermectin, from entering the central nervous system (CNS). [3]

As a result, ivermectin does not cause the same effects on GABA receptors in horses as it does in parasites. This targeted mechanism of action contributes to its safety profile when used appropriately in horses.

Overdose and Toxicity

There have been rare cases of ivermectin toxicity in various animals, including horses. However, ivermectin toxicosis is very rare and usually only happens in young horses due to an overdose. [14]

Ivermectin overdose can result in neurological symptoms in horses, including: [14]

  • Depression
  • Ataxia (lack of coordination)
  • Tremors
  • Salivation
  • Mydriasis (dilated pupils)
  • Death

Deworming is important for horse health, and its benefits generally surpass any potential side effects. However, if you’re worried about ivermectin poisoning or overdose in your horse, always seek advice from your veterinarian.

Ivermectin Resistance

The traditional method of deworming involved giving horses anthelmintic drugs at fixed intervals, regardless of the horse’s individual parasite burden or the types of parasites present. This approach was popular until concerns about parasite resistance to dewormers became more apparent.

Because of the frequent overuse and incorrect use of deworming drugs, many parasites have become resistant to some anthelmintic agents. Using these drugs when not necessary or in too small amounts can expose parasites to sublethal doses that don’t fully kill them.

These parasite populations develop mutations that make them less susceptible to the effects of dewormers over time. These changes are passed onto parasite offspring, causing antiparasitic medications to lose their effectiveness. [1]

In a 1998 study, it was found that 96.8% of horse facilities had dewormed most of their horses in the past year. Nearly all these treatments were given on a preventative basis, and almost half of the horses received four or more treatments during that year. [15][16]

Strategic Deworming

Strategic or targeted deworming approaches involve using anthelmintic treatments selectively based on geographic risk and the horse’s current parasite burden. This method aims to optimize parasite control while minimizing the risk of anthelmintic resistance. [3][4]

To reduce the risk of parasites becoming resistant to dewormers, there are various strategies and management practices you can adopt. For the best deworming plan for your horse, always consult with a veterinarian.

Refugia

Refugia is a key component of strategic deworming programs, in which anthelmintics are used selectively based on fecal egg counts and individual horse needs. Refugia refers to the population of parasites that remain unexposed or untreated during deworming.

Refugia is a strategy that lets non-resistant parasites survive and mate with resistant ones. This increases the chances that their young will be vulnerable to deworming drugs rather than exhibiting resistance, helping to control the rise of hard-to-treat parasites in horses. [1]

Some parasite populations that might benefit from refugia are:

  • Parasites in certain life-stages (i.e. encysted cyathostomins that are not susceptible to ivermectin or other non-larvicidal treatments)
  • Parasites on pasture that have not been ingested
  • Parasites in untreated animals

By using ivermectin (and other anthelmintic agents) strategically and preserving refugia, horse owners can achieve a sustainable approach to parasite control that maintains the long-term efficacy of these drugs.

Egg-reappearance period (ERP)

Compared to the other classes of anthelmintics, the macrocyclic lactones have the longest egg-reappearance period (ERP). This describes the time for a significant number of parasites to reappear in the feces of the horse following administration of a dewormer. [9]

The ERP for ivermectin is typically between 8 to 14 weeks. The time it takes for parasite eggs to reappear in feces is influenced by various factors, including parasite species, dose of medication and drug resistance. [3][4][17][18]

A shorter ERP indicates that a drug may not be providing long-lasting protection against parasites and there is a risk of reinfestation.

Fecal Egg Count (FEC)

A fecal egg count (FEC) test is a diagnostic tool used to assess the number of parasite eggs present in a horse’s feces. FEC results can be used to monitor the horse’s parasite burden and inform strategic deworming decisions. [1][19]

Fecal egg count testing may not detect all types of parasites, as some parasites do not release their eggs into the feces or have very low egg output. FEC is most effective for determining the extent of infestation with strongyles and ascarids.
There are other limitations of FEC testing that horse owners should be aware of: [1][20]

  • Life Stages: FECs cannot detect the total number of adult ascarids or strongyles in the body. Immature or larval parasites also cannot be detected, so the estimated parasite load may be inaccurate.
  • Species identification: FEC identifies the presence and number of parasite eggs but does not specify the parasite species. Different parasites require different treatment approaches, so this is important to know for targeted deworming.
  • Tapeworm Detection: Tapeworms do not release eggs into the horse’s feces and infestations cannot be detected with an FEC.
  • Pinworm Detection: Pinworm eggs stick to the area around the anus, instead of being shed in the feces.

Fecal Egg Count Reduction Test

Usually, a FEC is done before and after a deworming treatment to see how well the treatment worked. This evaluation is known as a fecal egg count reduction test (FECRT). [1]

Before giving ivermectin, each horse should have a FEC to measure the initial amount of parasite eggs in their feces. This count is usually expressed as eggs per gram (EPG) of feces.

Ten to fourteen days after treatment, a second FEC should be conducted to determine how many parasite eggs remain in the feces. If the reduction in egg count is less than 95% after using ivermectin, this raises concerns about potential resistance development. [1][21][22]

Consult with your veterinarian for help with interpreting your horse’s FECRT results and suggestions for future treatment.

Summary

  • Ivermectin is a potent anthelmintic drug commonly used in horses to control internal and external parasites.
  • Ivermectin can kill a variety of internal and external parasites, including strongyles, ascarids, threadworms, pinworms, and botflies.
  • Strategic deworming programs, tailored to individual horse needs, are essential to prevent anthelmintic resistance and maintain effective parasite control.
  • Veterinarian consultation is recommended before giving your horse ivermectin to ensure safety and optimize parasite management in horses.

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References

  1. Nielsen, M. K. et al. AAEP Parasite Control Guidelines. American Association of Equine Practitioners (AAEP). Accessed at July 24, 2023.
  2. Barragry, T. B. A Review of the Pharmacology and Clinical Uses of Ivermectin. Can Vet J. 1987. View Summary
  3. Shoop, W. & Soll, M. Chapter 1: Chemistry, Pharmacology and Safety of the Macrocyclic Lactones. Macrocyclic lactones in antiparasitic therapy. 2002.
  4. Monahan, C. M. & Klei, T. R. Chapter 8: The Use of Macrocyclic Lactones to Control Parasites of Horses. Macrocyclic lactones in antiparasitic therapy. 2002.
  5. Ivermectin. EquiMed. 2014.
  6. Ketzis, J. K. Mange in Horses. Merck Veterinary Manual. 2023.
  7. Reichard, M. V. & Thomas, J. E. Mange (Ascariasis, Mange Mites) in Horses. Merck Veterinary Manual. 2019.
  8. Lice. American Association of Equine Practitioners (AAEP). Accessed at July 28, 2023.
  9. Schumacher, J. & Taintor, J. A review of the use of moxidectin in horses. Equine Vet Educ. 2008.
  10. Proudman, C. & Matthews, J. Control of intestinal parasites in horses. In Practice. 2000.
  11. Martin, R. J. et al. Chapter 3: Mode of action of the macrocyclic lactones. Macrocyclic lactones in antiparasitic therapy. 2002.
  12. Nielsen, M. K. Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms. Int J Parasitol Drugs Drug Resist. 2023. View Summary
  13. Campbell, W. C. et al. Use of Ivermectin in Horses. Ivermectin and Abamectin. 2012.
  14. Swor, T. M. et al. Ivermectin toxicosis in three adult horses. Am Vet Med. 2009. View Summary
  15. Kaplan, R. M. Anthelmintic resistance in nematodes of horses. Vet Res. 2002. View Summary
  16. Garber, L. Equine ‘98, Part 1: Baseline Reference of 1998 Equine Health and Management. United States Department of Agriculture, National Animal Health Monitoring System. 1998.
  17. Daniels, S. P. & Proudman, C. J. Shortened egg reappearance after ivermectin or moxidectin use in horses in the UK. Vet J. 2016. View Summary
  18. Veterinary Guide to Resistance & Parasites. Merck Veterinary Manual. Accessed at July 28, 2023.
  19. Vidyashankar, A. N. et al. Statistical and biological considerations in evaluating drug efficacy in equine strongyle parasites using fecal egg count data. Vet Parasitol. 2012. View Summary
  20. Taylor, M. & Forbes, A. McMaster worm egg count. Vetlexicon. Accessed at July 28, 2023.
  21. Swiderski, C. & French, D. D. Paradigms for Parasite Control in Adult Horse Populations: A Review. American Association of Equine Practitioners (AAEP). 2008.
  22. Porr, C. A. et al. Effects of Ivermectin and Moxidectin on Fecal Egg Count and Egg Reappearance Rate in Horses. J Equine Vet Sci. 2017.