Worming guidance for horses

Horse worms are internal parasites that can affect horses throughout their lives – while a low worm burden will not have a big impact on your horse, a high worm burden can cause serious health problems. If left untreated, this can result in permanent damage or even death.

In recent years we have seen some parasites developing resistance to the wormers used to control them, so we encourage our clients to develop a targeted worming programme for their horses.

Targeted treatment with faecal worm egg counts

Faecal worm egg counts (FWECs) give us an indication of the level of worms a horse may be carrying internally. A small number of worm eggs in the faeces is acceptable for most horses, but larger numbers indicate that treatment is needed.

Studies show that 20 per cent of horses in a herd can carry 80 per cent of the herd’s worms. With regular worm egg counts, we can identify and treat those horses carrying substantial amounts of worms, targeting treatment to horses which really need it. This in turn helps to prevent resistance to wormers, which is becoming an increasing global concern.

Targeted worming plan

These are our current guidelines for targeted worming in adult horses. Each horse and situation are different, and your vet will happily discuss the best course of action. Broadly speaking, this is what we recommend:

Grazing season (early March to early October)

During this period, we recommend submitting samples for FWECs every 10-12 weeks. Ideally there should be at least three samples submitted during this time. Horses with a FWEC greater than 400 eggs per gram (EPG) should be treated with either ivermectin or pyrantel wormers. 

Please note that based on current evidence, we have increased our recommended worming threshold from 250 to 400 EPG. This helps to decrease the development of resistance to wormers but should not increase risk of clinical disease.

Any horses requiring worming should be tested for resistance at least once per year to ensure that the wormer remains effective. This test is called a Faecal Worm Egg Count Reduction Test (FWECRT) and involves submitting a second faecal sample two weeks after treatment is given. A FWECRT should also be carried out for any horses with significantly elevated worm egg counts, of more than 1000 EPG. 

Winter

In most cases, we recommend treatment in December to reduce the risk of encysted redworm, and/or tapeworm, during the winter period.

Testing for tapeworm is done via a saliva test known as Equisal, which is available from reception and can be performed on the yard by the owner. A blood test is also available. 

We find that the majority of horses tested for tapeworm do not require treatment. 

Treatment options are:

• Moxidectin, for example Equest, if treating encysted redworm only.
• Moxidectin and praziquantel, for example Equest Pramox, to cover encysted redworm and tapeworm.

We no longer recommend using a five-day course of fenbendazole, for example, Panacur, as there are concerns about its efficacy due to resistance. Occasionally, it might be appropriate to carry out a redworm blood test in certain low-risk horses, and your vet would discuss this with you if appropriate. 

Other worm control measures

1. Regular poo picking (at least twice weekly)
2. Avoid overstocking
3. All horses kept together should be tested and treated, if necessary, at the same time
4. Rotate paddocks frequently to avoid overgrazing and allow eggs to die off before horses graze the area again. A 9-12 month gap is ideal
5. Co-grazing with other species such as sheep or cattle can help to reduce egg numbers as the eggs and larvae are eaten by the other livestock species, preventing them from being eaten by horses
6. Keep dung heaps away from pasture to prevent larval migration into the fields.

Worming new horses

Any horses being introduced on the yard should be isolated and tested using a Faecal Worm Egg Count (FWEC) before being wormed, followed by treatment with moxidectin and praziquantel, such as Equest Pramox. The horse should be stabled for at least three days after worming to prevent eggs being excreted onto new pasture.

Foals and youngstock

Please remember that youngsters less than four years old are more susceptible to worms, and we advise more regular worming and testing. Please speak to your vet for a tailored plan for your horse.

If you have any questions, please contact the Dick Vet General Practice on 0131 650 6253 or email eqh@ed.ac.uk