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Worming: which is the best option for your horse?

Worms can affect all horses and ponies, whether stabled or at grass, with the young and the old being particularly susceptible.

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There are three main worming strategies
There are three main worming strategies

If worms are not controlled they can cause weight loss, colic and, in severe cases, death.


Increasingly, horses are kept in smaller paddocks, many of which are overgrazed. This in itself can lead to worm problems as more and more worm eggs are passed out in the droppings on to the pasture, contaminating it, only to be eaten again.


Therefore, to be fully effective, worming programmes must be used in conjunction with good pasture management and horse owners should ensure they:

  • Do not overstock pasture
  • Rest paddocks for several months at a time if possible
  • Graze sheep and cattle on the same pasture as horses as this can help reduce contamination
  • Pick up droppings regularly

The main types of worms found in horses are large redworms, small redworms, large roundworms, pinworms/seatworms, bots, stomach hairworms and tapeworms. No one drug will kill all types of worms so an effective programme is needed to control all worms.


There are many different trade names for wormers and new ones are constantly being marketed, so ensure you are aware of the active ingredients in each – some contain more than one.


There are at least eight different trade name wormers which all contain the drug ivermectin and are, therefore, the same wormer. So when rotating wormers, make sure you rotate the drug group and not just the trade name.


There are relatively few chemicals currently being used to treat horses against worms and, with no new drugs coming onto the market, it highlights the importance of avoiding resistance.

Natural wormers

There are more and more natural horse wormers based on herbal or homeopathic ingredients coming on to the market. Not all of these products are scientifically tested and may not be effective on some worm species.


Natural products do not kill adult worms, larvae or worm eggs – they just create an environment in the equine digestive tract which makes it uncomfortable and unattractive for worms.


They work on the principle that larvae and worm eggs are not influenced by the product, but if they mature into adult worms, they will probably prefer to ‘leave’.


Before buying natural wormers find about as much as you can about them and try to find people who have used them to share their experiences. Also consult your vet for their opinion on the product.


Worming becomes complicated because no one plan suits every situation. The age of the horse, its health, management and environment can make a big difference to worming requirements. A vet is the best person to advise on your own particular circumstances and help to develop an effective plan.

There are three main worming strategies:

Interval dosing

Regular worming every four to 13 weeks, depending on the type of wormer. This may be the only option for big, multi owner livery yards if there is no co-ordinated worming programme in place. It may result in some unnecessary treatment and the overuse of wormers will encourage resistance.

Strategic dosing

This may be the best option where many horses share grazing, especially if there is a high turnover of horses. It is specific treatment given at certain times of year, based on the parasite lifecycle and the risk of disease. It helps to avoid unnecessary treatments and the risk of resistance, making worming more cost-effective.


It does, however, require all horses grazing together to be wormed at the same time, with the same wormer. All new horses to a yard should be wormed immediately and not turned out for a minimum of 24 hours.

  • Winter: Treat for encysted small redworms – a single dose of moxidectin or a five-day course of fenbendazole.
  • Spring: Treat for roundworms and tapeworms – combination products which contain ivermectin and praziquantel treat both types of parasite in a single dose. Or a double dose of pyrantel may be used.
  • Summer: Treat for roundworms with ivermectin or pyrantel. Unless faecal egg counts are being used and indicate otherwise, give at the manufacturer’s recommended dosing intervals.
  • Autumn: Treat for roundworms, tapeworms and bots. Combination products which contain ivermectin and praziquantel treat all types of parasite in a single dose. Or separate products can be used which target each type of parasite individually.

Targeted dosing

This may be more suitable on smaller yards or where there is a stable population of horses. It involves sending faecal samples for analysis, so that the number of roundworm eggs in droppings can be counted.


A blood test can be taken for tapeworms. Your vet will be able to arrange this for you.


Initially, all horses should be tested and then based on results, treatment is targeted at the horses with significant adult worm burdens. If the egg count is negative or low there may be no need to worm. The vet should always be consulted to interpret the results of these tests and advise on the necessary treatment.


Although more expensive initially, as fewer treatments are needed, it is cost effective in the long-term.

  • Winter: Treat for encysted small redworms. Worm egg counts do not detect the encysted stages of these, so a single dose of moxidectin or a five-day course of fenbendazole is needed.
  • Spring: Arrange for your vet to take a faecal sample for roundworm infestation and a blood sample for tapeworm. He will then advise on treatment.
  • Summer: Again a faecal egg count should be taken. Your vet will advise how often this should be done. In the first year it is usually every 8-12 weeks, but should reduce in subsequent years.
  • Autumn: Treat for tapeworms and bots. There is no routine test available for bots, so annual treatment with ivermectin is recommended. A combination product containing ivermectin and praziquantel will also treat tapeworms.
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