How to Choose the Right Wormer for Cattle, Sheep & Horses

Why Choosing the Right Wormer Matters

Anthelmintic (wormer) resistance is one of the biggest threats to livestock health in the UK. Using the wrong product, at the wrong dose, at the wrong time doesn't just fail to work — it actively breeds resistance. This guide helps you make informed decisions for cattle, sheep, and horses.

Understanding Wormer Drug Groups

Wormers are grouped by their active ingredient and mode of action. In the UK, the main groups are:

• Group 1 — BZ (Benzimidazoles): e.g. fenbendazole, albendazole. Broad-spectrum, but resistance is widespread in sheep.
• Group 2 — LV (Levamisole): e.g. levamisole. Fast-acting, narrow spectrum. Useful in combination strategies.
• Group 3 — ML (Macrocyclic Lactones): e.g. ivermectin, doramectin. Broad-spectrum, also controls external parasites. Resistance increasing in sheep.
• Group 4 — AD (Amino-acetonitrile derivatives): e.g. monepantel. Sheep only. Effective against multi-resistant worms.
• Group 5 — SI (Spiroindoles): e.g. derquantel (usually combined with abamectin). Sheep only.

Always rotate drug groups strategically — not randomly — and ideally based on faecal egg count (FEC) results.

Wormers for Cattle

The main internal parasites in cattle are Ostertagia ostertagi (brown stomach worm) and lungworm (Dictyocaulus viviparus). Key considerations:

• Youngstock at grass in their first season are most at risk
• Treat based on FEC results or clinical signs where possible
• Macrocyclic lactones (Group 3) are commonly used and also control lice and mange mites
• Lungworm vaccine (Huskvac) is an option for first-season grazers — discuss with your vet
• Avoid treating the whole herd routinely without evidence of need

Wormers for Sheep

Sheep face the greatest anthelmintic resistance challenge in the UK. Best practice includes:

• Use FECs to guide treatment decisions — don't worm on a calendar basis
• Identify and protect refugia (untreated animals) to slow resistance
• Use combination products (two or three drug groups together) where resistance is confirmed
• Quarantine drench all incoming sheep with a combination wormer and hold off pasture for 48 hours
• Check efficacy with a FEC reduction test after treatment

Wormers for Horses

Strategic worming has replaced routine interval dosing for horses. The approach:

• Use FECs in spring and autumn to identify high shedders (>200 epg) who need treatment
• Treat all horses in autumn/winter for encysted small redworm (cyathostomins) — only moxidectin or fenbendazole (5-day course) are effective
• Treat for tapeworm with praziquantel (usually combined in products like Equest Pramox or Equimax)
• Young horses and new arrivals should be treated regardless of FEC

General Best Practice

• Always weigh animals and dose accurately — underdosing is a primary driver of resistance
• Use a drenching gun calibrated to the correct dose
• Record all treatments in your medicine records
• Work with your vet or animal health advisor to build a herd/flock health plan

Browse our full range of Animal Health Products & Wormers — including products for cattle, sheep, and horses — to find the right solution for your farm.

Need Advice?

If you're unsure which product is right for your situation, speak to your vet or SQP (Suitably Qualified Person). Many wormer products are POM-VPS or NFA-VPS category and require a prescription or SQP recommendation.

Animal Health Information: This guide provides general information only and is not a substitute for veterinary or SQP advice. Always follow product labels and seek professional advice when selecting or administering animal medicines.