Parasites have long been a silent, sneaky threat to horses worldwide.
From roundworms to tapeworms and small strongyles, internal parasites can sap your horse’s energy, damage vital organs, and interfere with nutrient absorption. For decades, routine deworming was the straightforward answer: dose your horse regularly with broad-spectrum anthelmintics and stay ahead of infestations.
But the landscape of parasite control is changing. Overuse and misuse of dewormers have led to growing drug resistance among common equine parasites, making many traditional treatments less effective. In response, the equine industry is evolving its approach, emphasizing smarter, targeted, and sustainable parasite management.
In this article, we’ll explore the latest parasite control strategies, why resistance happens, and how you can keep your horse healthy without over-relying on medications.
Understanding the Challenge: Drug Resistance in Parasites
Just like bacteria can develop resistance to antibiotics, parasites can adapt to survive deworming drugs. When the same class of dewormers is used repeatedly, resistant worms survive, reproduce, and multiply—making the drug less effective over time.
Small strongyles (cyathostomes) are the most notorious for developing resistance, particularly to benzimidazoles (like fenbendazole) and pyrantel. Resistance has also been reported to ivermectin and moxidectin, though these remain generally effective.
This resistance means that routine blanket deworming (giving all horses the same drugs on a fixed schedule) is no longer recommended. Instead, the goal is to reduce unnecessary treatment and only deworm horses that actually need it.
The New Approach: Targeted Deworming and Fecal Egg Counts
The cornerstone of modern parasite control is the fecal egg count (FEC)—a simple test that measures how many parasite eggs a horse is shedding in its manure. This helps identify “high shedders” who contribute most to pasture contamination, allowing for selective treatment.
Here’s how it works:
- Collect a fresh manure sample from each horse.
- Send it to a lab or use an on-farm kit for egg counting.
- Based on the results, treat only horses with egg counts above a certain threshold (usually 200 eggs per gram or EPG).
This approach reduces drug use, slows resistance development, and saves money by avoiding unnecessary treatments.
Seasonal Timing and Dewormer Rotation
Understanding when to treat and with what drug is crucial. Parasite loads often peak seasonally—usually in spring and fall when weather and grazing conditions favor larvae survival.
Dewormer rotation means alternating between drug classes to avoid selecting resistant worms. The main drug classes are:
- Benzimidazoles (fenbendazole, oxibendazole)
- Pyrantel
- Macrocyclic lactones (ivermectin, moxidectin)
However, recent research suggests rotation alone may not prevent resistance. Instead, using the right drug at the right time, guided by fecal egg counts, is more effective.
Emerging Strategies & Research
Researchers and vets are exploring innovative parasite control methods, including:
- Dewormer efficacy testing to check if current drugs work on your farm.
- Pasture management techniques: rotational grazing, mixed-species grazing, and manure removal reduce parasite eggs on pasture.
- Biological control: using natural predators like parasitic fungi to reduce larval populations.
- Selective breeding: favoring horses with natural parasite resistance.
Best Practices for Parasite Control
- Perform regular fecal egg counts, ideally twice a year (spring and fall).
- Target treatments only to horses shedding significant numbers of eggs.
- Avoid routine blanket deworming without testing.
- Rotate dewormers thoughtfully based on test results and drug class efficacy.
- Practice good pasture hygiene: pick up manure frequently, avoid overcrowding, and rotate pastures when possible.
- Consult your veterinarian to develop a tailored parasite control plan for your stable.
Why It Matters: Health and Performance Benefits
Good parasite control protects your horse’s gut health, nutrient absorption, and overall energy levels. Heavy parasite loads can cause:
- Poor weight gain or weight loss
- Diarrhea or colic
- Poor coat condition
- Reduced athletic performance
- Weakened immunity
By using smarter parasite control, you also help protect the entire horse community by slowing resistance spread.
Final Thoughts
Parasite control isn’t about blindly administering dewormers anymore. It’s a science-driven balance of testing, treatment, and management tailored to each horse’s needs and environment. With rising drug resistance, taking a thoughtful approach is the best way to keep your horse healthy and ensure effective treatments remain available for years to come.