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Parasite Control in Horses

Intestinal worms are extremely common in grazing horses.  Establishing an effective control program is probably second only to providing horses with access to water and the appropriate nutrition.

Healthy horses at pasture

Almost all grazing horses become infected with intestinal worms (the commonest of which are the strongyles); but most have low burdens, with higher burdens in some individuals.

Younger animals (especially those ~ 1-2 years) are more at risk of infection and tend to have higher burdens.

Encysted small strongyle larvae in gut wall of a horseGenerally, the higher the burden, the higher the risk of clinical disease. The effects of strongyle infections range from a dull coat and weight loss to colic, severe diarrhoea and death. Thus, it is important that high burdens are avoided. Unless animals are showing clinical signs of disease it can be difficult to discriminate which are carrying higher burdens.

Effective dewormers (anthelmintics) have been available to treat and control equine worms for many decades; but resistance in the small strongyle (cyathostomin) group to a number of these products (especially fenbendazole) is now widespread.  This means that we need to balance control of the pathogenic stages or types of parasites with ensuring that we do not over rely on anthelmintics, especially in the delivery of unnecessary treatments. Year-round, interval treatments in all horses are to be avoided.

Strongyle eggs (top), tapeworm egg (bottom) Faecal egg counts (FECs) can be performed to assess which horses are excreting the highest levels of eggs – but these do not give an accurate indication of an animal’s overall burden, which includes immature, developing worms, as well as adult worms. 

Control using anthelmintics is only part of a complete control programme.

As the worms are primarily transferred via dung, good pasture management is essential.

Best practice guidelines

  1. Horse numbers per acre should be kept as low as possible to prevent overgrazing and to reduce contamination levels of worm eggs and larvae.
  2. Pick up and dispose of dung regularly (at least once a week). Do not spread this onto fields grazed by horses; compost it away from grazing.
  3. Rotate pastures with sheep or cattle, thereby interrupting the life cycle of the worms.  Most larvae from eggs deposited in one grazing season should be substantially reduced in number by the second half of the next grazing season. Keep in mind that fluke worms can transmit from ruminants to horses via snails and pasture – ensure appropriate control of this parasite in ruminants if the grazing is likely to encompass snail habitats.
  4. Use a targeted strategic control programme to reduce anthelmintic usage and therefore reduce selection pressure for resistance in small strongyles. Faecal Egg Count (FEC) analysis should be carried out through spring and summer, leaving individuals with negative or low FECs (usually <200 eggs per gram) untreated to promote higher proportions of less resistant worms in the population.
  5. Administer all horses in late autumn/winter with an anthelmintic effective against small strongyle encysted larvae (the best product for this use currently is moxidectin). This also provides cover against pathogenic large strongyle worms.  
  6. Ensure cover for equine tapeworm once a year in the control programme.
  7. Always ensure that anthelmintic doses are administered as recommended by the manufacturer – always work from an accurate gauge of weight by using scales or a well calibrated girth tape.  
  8. If possible, avoid administering the same ‘class’ of anthelmintic year after year. Keep in mind that ivermectin and moxidectin are in the same anthelmintic class. Test anthelmintic effectiveness once a year by undertaking a faecal egg count reduction test. Veterinary surgeons can advise on these tests.
  9. Work with your veterinary surgeon or animal health advisor to develop an sustainable deworming strategy that is tailor made to your establishment.
  10. Enforce a quarantine policy. Ensure incomers that will co-graze with resident horses are administered with an appropriate anthelmintic and have no access to grazing until 72 hours after treatment.


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Printed from on 16/08/17 02:07:25 PM

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