Horse deworming has been an ongoing debate in online forums and social media groups alike. The protocols and recommendations have become more specific to where your horse lives, even to their specific farm. Many horse owners for years have routinely rotated dewormers throughout the year, but as antibiotic resistance has reared its ugly head, so have anthelmintic-resistant parasites.

Rotational Worming is Out

Rotational worming involves using different dewormers (anthelmintics) every 6 to 8 weeks. However, there’s been a surge in drug-resistant parasites that’s attributed to rotational worming or excessive deworming with a single dewormer. Intestinal parasites are susceptible to different anthelmintics, and no single one is effective against all parasites.

Why the Concern?

Encysted small strongyles are a more concerning parasite that has shown resistance to two of the three classes of dewormer ingredients on the market. The concern about this parasite becoming too resistant lies in the condition known as parasitic colitis (larval cyathostominosis). 

Encysted small strongyles are a larval-stage parasite. They burrow into a horse’s intestinal wall, can remain dormant and unaffected by most dewormers, and suddenly erupt from the intestinal wall, causing potentially fatal damage. 

Currently, ivermectin and moxidectin (Quest®) are still considered effective dewormers against these strongyles.

Vets Recommend a Fecal Egg Count Before Deworming

Most veterinarians recommend a fecal egg count at least once a year. I request a fecal egg count at least twice a year, more frequently (every 2-3 months) for horses under 2 years old or if anyone is itching their tail off.

What is a Fecal Egg Count?

When conducting a fecal egg count, fresh manure is examined under a microscope for parasitic eggs. Parasite eggs look different depending on the species, and your vet can recommend the most effective dewormer ingredient for your horse based on these findings. This helps reduce over-deworming or overusing a single kind of dewormer.

What Parasites Show Up in a Fecal Egg Count?

Parasites that can be identified in a fecal egg count include:

• Large strongyles (Strongylus spp.)
• Small strongyles (cyathostomes)
• Ascarids (parascaris equorum, a type of round worm)
• Stomach worms (Habronema spp.)

Parasites that are not identifiable in a fecal egg count include:

• Pinworms (can be detected by anal swab)
• Bots
• Tapeworms
• Encysted red worms (large strongyles) 

Current Deworming Recommendations

Each vet will have an intimate understanding of each property they service and will make recommendations for each horse based on symptoms, property history, or fecal egg count results. Typical deworming schedules are in the Spring and Fall (March and October). 

For individual parasites, the current deworming guidelines include:

• Strongyles – Ivermectin or Moxidectin
  • Resistant to pyrantel and benzimidazoles
• Tapeworms – Praziquantel (Quest Plus®, Zimectrin Gold®, Equimax®) or a double dose of pyrantel pamoate
• Ascarids – Fenbendazole (Panacur®) and oxibendazole (Anthelcide®EQ) or pyrantel salts, remain effective against these roundworms.
  • Resistant to ivermectin, moxidectin, pyrantel, benzimidazole in certain stages
• Pinworms – Benzimidazoles
  • Resistant to ivermectin 

Your vet may recommend a specific brand of dewormer based on the ingredients; some combine anthelmintics to cover a broader spectrum of parasites.

Low, Moderate, and High Egg Shedding in Horses

Each horse has a shedding rate, meaning the number of eggs it passes into the environment through its manure. Fecal egg counts measure eggs per gram (EPG) of manure, establishing the shedding rate. Horses that are high egg shedders may be recommended to be dewormed more frequently (my mare is one of those), while low shedders get dewormed less frequently, and moderate shedders land somewhere in the middle.

Other Considerations for Weanlings and Yearlings

Most deworming recommendations online are for horses 3 years old and older. Young horses, especially those under 2 years old, are often more susceptible to parasites. One such parasite is Ascarids. At about 18 months old, horses start to acquire immunity to ascarids, with adult horses having very few cases of ascarid infestation (those that are immunocompromised are at risk).

Don’t Sweat the Creepy Crawlies

While drug-resistance parasites might be the stuff of nightmares, deworming for horses doesn’t have to be one. Talk to your vet, get a fecal egg count, and start your deworming protocol with your vet’s recommendations. Over-deworming is the big boogie man here.