Equine Care

WORMS - general principles to control internal parasites in your horse

The most important worms to be aware of are:

Small Redworms (Cyathostomins)
Large Stongyles
Tapeworms

Of lesser importance:

Bots
Pinworms

The main wormers available:

Benzimidazoles (BZs) eg. Fenbendazole (Panacur)
Macrocylic lactones include: Ivermectin (Equalan); Moxidectin (Equest)
Tetrahydropyrimidines such as Pyrantel (Strongid)
Pyrazino-isoquinolines e.g Praziquantel (Equimax = combination product)

Resistance to wormers (anthelmintics) has been a problem since the 1970s and it is vital that horses are wormed appropriately. Small redworms are the most important as they are the most common and can ‘hide’ in the intestinal wall thus making treatment difficult. No resistance in bots or tapeworms have yet been reported. Large strongyles are relatively rare in well-maintained animals. Pinworms are common but easily managed when treating strongyles. Here are some guidelines to slow the development of resistance:

Test for Resistance:

Initial microscopic examination should be carried out. Horses should only be treated when faecal egg counts exceed 200 eggs per gram. Our in-house faecal egg count costs £8.50 plus VAT for the first horse, then £7.20 plus VAT for additional animals and for future tests in the same season.
Yards should try to establish whether they have resistance on their premises by doing an egg count immediately before treatment and then again 14 days later. We would recomemnd testing a group of 5 or 6 horses on the same date.

Benefits of Testing for Resistance:

Fewer wormers are used hopefully delaying the development of resistance.
Initial set up costs may be higher than the traditional system but there should eventually be a cost saving on the purchase of wormers.
Fewer drugs are administered making this a greener system of managing your horse.
Monitoring faecal egg counts confirms the worm control strategy is working well.

Rotation of Wormers:

Change the product you use annually but first your should ensure you do not have a resistance problem using faecal egg counts. For example it is known there is wide resistance to BZs.
To treat for both adult and inhibited small redworms use moxidectin (Equest) or five-day course of fenbendazole (Panacur). Although ivermectin (Eqvalan) does NOT kill inhibited small redworms, it kills the adults thus making resistance less likely to occur by maintaining a number of worms unexposed to the wormer. (This is known as refugia.) However only treating the adult stages can also cause mass reactivation of the inhibited larval stages.
Tapeworm treatment should be carried out at least annually (and at 6 monthly intervals in high-risk animals.) As tapeworm infection is not seasonal, the timing of the treatment does not matter.

Anthelmintic dosing:

Dose for the correct weight, one can use girth tapes to ensure therapeutic levels are reached as under-dosing will encourage resistance.
Avoid over worming by checking the worm status with faecal egg counts.

Pasture Management:

Avoid high stocking density and over grazing.
Pick up equine faeces from the field twice weekly in the summer and once weekly in the winter.
Rotate the pastures grazed, particularly by the foals, to reduce Parascaris equorum. (Adult horses play no part in the spread of the P equorum.)
Mixed grazing with sheep and cattle is beneficial as they can ‘hoover up’ the eggs without harming themselves. T axei is the only species that can be shared: however it is does NOT cause disease in horses.
Ideally, avoid mixed age groups of horses in the same field where possible.

Foals:

The most important parasites in foals are small redworms, P equorum and Strongyloides westeri. Most studs tend to administer anthelmintics monthly to foals from 6 weeks of age until 6 months of age and then 6 weekly until the foal is a yearling. This is because foals have no immunity to gastro-intestinal parasites. So check the worm status closely until 3 years of age. In practice the need for routine treatment of foals will depend upon management, the treatment of the mare and any previous parasite problems on the yard. It is a good idea to treat the mare just before foaling to lower her faecal worm egg count, reducing spread to the foal and shedding of Strongyloides larvae in the mare’s milk. Additionally, try to avoid breeding from wormy horses.

There are no new wormers currently being developed for use in horses and therefore we need to use the drugs responsibly.

If you would like further information please do not hesitate to contact us on 01291 672637.