A YARD with 12 adult riding horses, mainly stabled but with limited turnout on five acres and access to all-weather arenas, has followed a traditional worming programme - dosing every eight weeks and rotating wormer types. The vet now advises that the horses are being over-treated. The owner is concerned that reducing worming will cause a parasite problem.
Case background
Historically, it was recommended that horses be wormed every six to eight weeks using rotating wormers. However, modern research has shown that this approach has caused widespread worm resistance. Worms that survive frequent treatments pass on their resistant genes, leading to populations that no longer respond to common wormers.
Because adult horses develop natural immunity to most worms, they often do not require frequent treatment. Over-worming therefore provides no benefit and may even be harmful - both to the horse population and to the environment.
Changing your approach
1. Preventing wormer resistance
Frequent dosing kills off susceptible worms, leaving only resistant parasites behind.Over time, this leads to ineffective wormers, even when truly needed.By worming only when necessary, you help preserve the effectiveness of the available drugs.2. Adult horses tend to have natural immunity
Most adult horses control worm burdens well on their own.Only a small number of horses (usually 10–20%) act as high egg shedders.Targeted testing identifies these individuals so only they are treated, reducing unnecessary dosing.3. Limited pasture exposure
The horses are mostly stabled, and turnout is short and on small paddocks or all-weather surfaces.This means low exposure to infective larvae, so the risk of re-infection is very low.Worming every eight weeks in this setting provides no additional protection.4. Testing is safer and more effective
Faecal egg counts (FECs) identify which horses are shedding worm eggs and actually need treatment.Tapeworm tests (blood or saliva) and encysted redworm tests (blood) can be done seasonally to ensure targeted control.This approach maintains parasite control, while reducing resistance risk.Modern worm control plan
1. Testing-based programme
Faecal egg counts every eight to 12 weeks during grazing months (spring to autumn).Treat only if counts exceed 200 eggs per gram.Tapeworm testing once or twice yearly (autumn and/or spring).Encysted redworm treatment once a year in late autumn or early winter with moxidectin (or five-day fenbendazole if needed).2. Pasture management
Remove droppings regularly (two to three times per week).Avoid overstocking the small turnout areas.Rotate the limited paddocks as much as possible.3. Maintain some “refugia”
Not treating all horses at once leaves a small number of susceptible worms in the environment (“refugia”), which helps dilute resistant genes in the population.Reassurance for the owner
Even though it feels like “doing less”, the modern targeted approach is actually safer and more sustainable.
By reducing unnecessary worming:
You protect the effectiveness of your wormers for the future.You avoid wasting money on unneeded treatments.You reduce health risks from overuse, such as gut disturbance and potential toxicity.Your horses will still be well protected, provided regular testing and monitoring are followed.Conclusion
The old routine of worming every eight weeks is now outdated and is proven to promote resistance. Adult, mostly stabled horses with limited grazing are low-risk and should be managed through testing, targeted treatment, and good pasture hygiene.
Switching to a modern testing-based programme is the best way to ensure long-term worm control, while preserving the effectiveness of wormers.