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Deworming Horses

Resistance of intestinal parasites in horses to commonly used dewormers is becoming increasingly recognised. Therefore deworming recommendations for adult horses have changed in recent years to reduce resistance of parasites to dewormers in our horses.

The main intestinal parasites of concern in horses are the small strongyles or cyathostomins. These recommendations are for cyathostomin control in ADULThorses; i.e. they DO NOT APPLY TO FOALS. Small strongyles cause disease because larval stages burrow into the wall of the colon where they form cysts around themselves and can remain dormant for extended periods of time. This can interfere with normal function of the colon, or if mass emergence of larvae from the cysts occurs severe colitis (diarrhoea) can result.

The main goals of a deworming strategy are to prevent parasitic disease by minimising pasture contamination with parasite eggs, and minimise the development of parasite resistance to dewormers.We can do this by reducing the numbers of eggs laid by adult worms, and also where possible removal of faeces from paddocks before eggs develop to infective larvae. The preservation of refugia (population of worms that have not developed resistance to dewormers) can be achieved by NOT deworming all horses. Selective deworming acknowledges that horses don’t need to be parasite-free in order to prevent clinical disease, and in fact, that achieving a parasite-free horse is neither possible nor desirable. It also considers egg reappearance periods (the length of time from deworming until eggs can be found in faeces again) in determining when and how often to use different anthelmintics.

This method relies on the identification of horses that are shedding large numbers of parasite eggs onto the pasture, as these horses are the main culprits contaminating the pasture for everyone. Only these individuals are dewormed, thus minimising pasture contamination. A faecal egg count (FEC) is performed on each horse at intervals according to the egg reappearance period of the dewormer being used, and horses are categorised as high shedders (those with a FEC of greater than 500 eggs per gram [EPG]), moderate shedders (200-500 EPG) and low shedders (FEC of less than 200 EPG). All the high shedders are selected for treatment. In some instances, some moderate shedders may also require treatment, for example if they are showing clinical signs of parasitic disease. Dewormers are selected based on an initial faecal egg count reduction test, which makes sure there isn’t already resistance to the dewormer selected, so we know it is effective.

What do you need to do?
  • Talk to your vet about starting a selective deworming program on your farm.
  • Perform faecal egg counts. Start by doing a faecal egg count reduction test (take a FEC, deworm your horse with the product you wish to use, then repeat the FEC in 14 days’ time). This will tell you if you have resistance to that particular dewormer. Faecal egg counts should then be repeated at regular intervals – talk to your vet about the egg reappearance period of your dewormer, most are around 8 weeks. 
  • When you get your FEC results, select the horses with high FECs (>500 EPG) for deworming. Talk to your vet about any horses that have a moderate (200-500 EPG) FEC about whether they should be dewormed as well.
  • Avoid using dewormers that target the encysted larvae, such as moxidectin, on a regular basis. These dewormers should be reserved for horses that need treatment for parasitic disease.
  • Keep pastures as clean as possible. It takes at least 2 days (longer in cold weather) for eggs to hatch and develop to infective larvae that horses ingest. This is an ideal window to remove faeces from paddocks if possible, which will go a long way to reducing the number of worm eggs and larvae on the pasture.
  • Once a year, preferably in autumn, all horses should be dewormed with a product containing praziquantel for possible tapeworm infections.
  • Tell your friends! This works best if everyone does it, so spread the word!