🐴 Quest (Moxidectin) Dewormer in Horses: 2025 Vet Guide to Safer Parasite Control

By Dr Duncan Houston BVSc

Parasite control is a cornerstone of equine health, and in 2025, moxidectin—the active ingredient in Quest®—remains one of the most effective treatments, especially for targeting encysted small strongyles (cyathostomes). 🐛

🔬 Why Encysted Small Strongyles Matter

Small strongyles are the most common and potentially damaging parasites in horses. These worms follow a unique life cycle:

• 🪱 Horses ingest larvae from pasture
• 🔒 Larvae encyst (hide) in the intestinal lining
• 🩸 Encysted larvae evade most dewormers and can later emerge en masse, causing colic, diarrhea, and severe inflammation

Regular dewormers—including ivermectin—can’t kill these hidden stages. That’s where Quest comes in. 💥

💊 What Makes Quest® Different?

Quest contains moxidectin, a macrocyclic lactone like ivermectin, but with added potency. It’s currently one of only two medications proven to kill encysted larvae. 🧬

💡 Comparison with Other Dewormers:

• Ivermectin: Kills adult strongyles but not encysted larvae
• Fenbendazole (Panacur®): Can kill encysted larvae but requires a 5-day double-dose protocol and is less effective against other parasites
• Quest®: Effective against encysted larvae in a single dose and also targets a broad range of other parasites

⚠️ Dosing and Safety Considerations

Quest is potent—and that’s exactly why correct dosing is so important. Overdosing, especially in young or underweight horses, can lead to serious side effects. 🧪

• 📏 Use a weight tape or scale to estimate your horse’s weight accurately
• 🐎 Do not estimate by eye—this is where most errors happen
• ⛔ Do not use in debilitated or miniature horses unless under veterinary supervision

Pro Tip: If you’re unsure, your vet can help calculate the correct dose based on weight and health status. 💉

🧬 How Moxidectin Works

Moxidectin interferes with the nervous system of parasites, paralyzing and eliminating them from the body. Its ability to penetrate tissues makes it effective against encysted stages. It also provides a longer period of protection compared to some other dewormers. ⏳

🧾 Is Quest® Right for Your Deworming Program?

Not every horse needs moxidectin every time. Here's when Quest may be recommended:

• 🐴 In horses with high small strongyle counts on fecal testing
• 🔁 As a once- or twice-yearly deep-clean in strategic deworming programs
• 🏥 In horses at risk of larval cyathostominosis (severe reaction to emerging larvae)

But remember—it’s not a cure-all. Fecal egg counts, seasonal planning, and pasture management all matter. 🌾💩

📅 When to Use It in 2025

Veterinarians often recommend Quest in the late fall or early spring, when encysted larvae are most likely to emerge. However, it should only be used as part of a larger plan tailored to your herd and environment. 🧑‍⚕️

🤒 What to Watch For Post-Dosing

Side effects are rare if Quest is dosed properly. However, watch for:

• 💤 Lethargy
• 😰 Excessive sweating or tremors
• 🚫 Inappetence or colic signs

If any of these occur, contact your vet immediately. Most issues arise from overdose or incorrect use. ✅

🏁 Final Thoughts

Quest® (moxidectin) is a powerful tool in equine parasite control, especially when targeting encysted small strongyles. When used wisely—with accurate dosing and veterinary guidance—it can dramatically reduce parasite loads and help keep your horse healthy from the inside out. 💪🐎

For seasonal guidance and personalized deworming protocols, consult the Ask A Vet app or visit AskAVet.com. 🐴📱

— Dr Duncan Houston BVSc