Decoding Dewormers: Types, Resistance Concerns, and Use for Horses


Horses are magnificent creatures that bring joy and companionship to many. However, like animals, they are susceptible to various health issues, including internal parasites. Deworming is a crucial aspect of horse care that helps maintain their well-being. This article will explore different types of dewormers, concerns regarding resistance, and the proper use of dewormers for horses.

Understanding Internal Parasites in Horses

Internal parasites, also known as worms, can compromise the health and performance of horses. These parasites can reside in various parts of a horse’s body, including the gastrointestinal tract, lungs, and blood vessels. Common equine internal parasites include roundworms, tapeworms, strongyles, and bots.

When infested with worms, horses may exhibit symptoms such as weight loss, poor coat condition, diarrhea, colic, reduced energy, and even respiratory issues. Deworming is necessary to prevent these symptoms and the potential long-term damage caused by internal parasites.

The Different Types of Dewormers for Horses

Several types of dewormers are available for horses, each with its unique mode of action. It is essential to understand the different categories of dewormers and their effectiveness against specific types of internal parasites.

1. Benzimidazoles

Benzimidazoles are a group of dewormers targeting most equine worms, including roundworms, strongyles, and tapeworms. They work by inhibiting the parasites’ energy production, ultimately leading to their death. Common benzimidazole dewormers include fenbendazole and oxibendazole.

2. Macrocyclic Lactones

Macrocyclic lactones, also known as avermectins and milbemycins, are highly effective against a broad range of internal parasites. They target roundworms, strongyles, and bots, among others. These dewormers work by paralyzing the parasites’ nervous system, leading to their demise. Common macrocyclic lactones include ivermectin and moxidectin.

3. Pyrantel Pamoate

Pyrantel pamoate is a dewormer primarily effective against roundworms and some species of strongyles. It works by interfering with the neurotransmission in the parasites, causing paralysis and eventual elimination. This dewormer is often combined with other active ingredients to broaden its spectrum of action.

4. Praziquantel

Praziquantel is targeted explicitly against tapeworms, an internal parasite commonly found in horses. It disrupts the tapeworm’s metabolism and leads to its disintegration. This dewormer is often combined with other deworming drugs to provide comprehensive protection against multiple parasites.

It is essential to consult a veterinarian to find the most suitable dewormer for your horse since different environments and infestation levels may require specific protocols.

Resistance Concerns in Deworming

Over time, internal parasites can develop resistance to certain deworming drugs, reducing their effectiveness. Resistance primarily arises due to the misuse or overuse of dewormers. When parasites are repeatedly exposed to a particular dewormer, the susceptible ones die, leaving behind the resistant strains to multiply and thrive. To combat resistance, it is crucial to implement strategic deworming practices.

Veterinarians often recommend performing fecal egg counts (FECs) to assess the worm burden in horses. This helps determine whether deworming is necessary and which dewormers will be most effective. By utilizing targeted treatments based on FEC results, horse owners can reduce the chances of overexposing parasites to deworming drugs, thus minimizing the development of resistance.

Proper Use of Dewormers for Horses

It is essential to follow proper administration and dosage guidelines to maximize the efficacy of dewormers and minimize the risk of resistance. Here are some important tips:

  1. Consult a veterinarian to establish a deworming schedule tailored to your horse’s needs. This schedule may vary depending on factors such as age, health, and geographical location.
  2. Administer dewormers at the appropriate dosage based on the horse’s weight. Underdosing can lead to ineffective treatment, while overdosing can increase the risk of adverse effects.
  3. Rotate the classes of dewormers used to prevent the development of resistance. Strategic rotation involves using dewormers from different courses at specific intervals, ensuring parasites are not repeatedly exposed to the same drug.
  4. Practice good hygiene and pasture management. Regularly removing manure from paddocks and pastures can help reduce the number of parasite larvae in the environment.
  5. Quarantine new horses before introducing them to the herd to prevent the introduction of resistant parasites.


Deworming plays a vital role in maintaining the health and well-being of horses. By understanding different types of dewormers, being aware of resistance concerns, and implementing proper deworming practices, horse owners can effectively combat internal parasites and provide their equine companions with a healthy and parasite-free life.


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