Almost every horse will have at least one parasitic worm in its body. It is important to realise that these worms are often in low numbers and cause the horse no ill effects. However worm burdens can reach significant levels and may cause weight loss, diarrhoea and general ‘unthriftiness’. Large numbers of some species can also cause potentially life-threatening conditions such as colic. Generally it is possible to estimate the worm burden of a horse by carrying out a Faecal Worm Egg Count (FWEC). However there are exceptions to this rule! Small Round Worms (Cyathastomes), Pinworms and Tapeworms (Cestodes) cannot be detected on a FWEC and therefore a low or negative FWEC does not necessarily mean they do not require worming.

Tips to reduce worm burden on pasture:

  • Reduce pasture stocking density
  • Collect droppings from paddocks regularly
  • Rotation of pasture – minimum rest period of 3 months is recommended
  • Mixed species grazing – cattle and sheep act as ‘biological vacuum cleaners’

Try to graze young horses away from older horses as the younger horses carry the biggest worm burden and as such are responsible for the majority of pasture contamination.
Roundworms (Ascarids) are the major risk to young horses especially foals and yearlings. These look like large white worms, despite them mainly being an intestinal parasite, infestation usually results in coughing as the parasite migrates through the lungs. Young stock should routinely be treated against ascarids.

Pregnant Mares:

A mares’ natural resistance to worms is reduced during late pregnancy; as such pregnant mares are a significant source of pasture contamination. To help reduce this pasture contamination as well as to help keep the mare healthy both during and after pregnancy, it is important to routinely worm the animal using a product that is licensed for use in pregnant and lactating mares (e.g. Equimax or Eqvalan).
In addition the mare should be wormed just before foaling, as four to five days after birth worms often pass through the mare’s milk to the foal.


Foals are particularly susceptible to worm burdens as they have not yet built up their natural resistance, and can suffer long-term damage if not wormed appropriately.
The worming of foals should start at four to six weeks of age or as soon as they start to graze on pasture. Treatment should be repeated every four weeks until six months of age. Thereafter, worming can be continued as per the individual wormer’s recommended dosing intervals. Be aware that not all wormers are suitable to be given to young foals.

Problems faced by horse owners:

  • Resistance to worming drugs
  • Different classes of drugs treat different types of worms
  • Livery yards with many owners – individual owners responsible for worming; high turnover rates – new horses with unknown worming histories or potentially carrying resistant worms
  • Cyathostomes living in the gut lining – they hibernate in the gut wall where they can’t be detected by diagnostic tests and only a few drugs can be used to treat these larvae.

Worming protocols:

  1. Interval dosing – this involves the administration of a specific drug at regular time intervals during the high-risk summer grazing period. Some people continue to worm regularly throughout the low-risk periods (Winter) or when mostly stabled which is often expensive and unnecessary.
  2. Strategic dosing – dosing at specific times to disrupt the seasonal cycle of transmission e.g. at turnout, in the middle of the grazing season and again in the autumn. Problems can arise however in years with abnormal weather patterns leading to early or late peak pasture larval burdens. The system may also break down if heavily parasitised horses are added to the population.                                                                        
  3. Targeted strategic dosing – this is employed to suppress pasture contamination at critical times of the year and all horses have FWECs performed prior to dosing. Treatment is targeted at animals with significant burdens (>200 eggs per gram). Diagnostic
    limitations mean that Cyathastomes in the gut wall can’t be detected therefore such a program must include larvicidal dosing. 
    It is important that numbers of larvae in the gut wall are not allowed to build up in large numbers.

Golden rules to prevent resistance:

  • Ensure you are using thecorrect dose of drug, it is especially important not to under-dose. A weight band is very useful.
  • Rotate the class of drug you use annually. i.e. if you are using ivermectin or moxidectin continue to use these until next year, when you should change to a different class such as the benzimidazoles (Panacur).

Treatments that should not be missed in any worming program:

Treatment for Small Red Worms (Cyathastomes) should be included into the yearly plan. Moxidectin (Equest) or 5 days of Fenbendazole (Panacur) is effective at killing the larval stages. The best time to do this is at the end of the grazing season around October time.

Monitoring or treatment for Tapeworms. An annual blood sample or treatment should be included in the worming plan. A double dose of Pyrantel embonate (Strongid P) or a dose of Praziquantel (Equitape) is effective against Tapeworms. The best time to give this would be at the end of the grazing season about October.

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