Deworming your horse: The inside story

Dr Ciaran Gobl


Internal parasites can have a negative effect on your horse, particularly in the young, growing animal and the older horse or pregnant mare. While horses can tolerate low levels of internal parasitism, an increased worm burden can cause ill thrift, weight loss, diarrhoea, colic, peritonitis or even pneumonia (secondary to migrating larvae). Deworming is an integral part of horse ownership however, in recent times, the overuse or the inappropriate use of commercially available anthelmintics (dewormers) has led to populations of parasites that are resistant to these chemical dewormers.


Common Internal Parasites

There are numerous species of worms that can infect horses and several of these are of significance in Australia. These include roundworms (or ascarids, Parascaris equorum), large (Strongylus spp) and small (cyathostomes) strongyles, and tapeworms (Anoplocephala spp).
Threadworm (Strongyloides westeri) infections are typically temporary and occur only in foals.
Other common parasites of horses are pinworms (Oxyuris equi), and bots (the immature or larval form of adult botflies, including Gasterophilus intestinalis).


Parasite Control

Chemical dewormers are readily available from veterinarians, pet stores and online. Despite a wide array of commercially available products with a variety of trade names, there are actually only a few licensed ingredients that can be used in horse wormers. This can contribute to resistant worm burdens as horse owners can be confused, believing they are using a different dewormer when in fact it is the same wormer only with a different trade name. Not all dewormers are effective against all types of worms. The main anthelminitcs available are as follows.


  • Macrocyclic lactones
    • Ivermectin – targets all parasites except tapeworms and is effective against some migrating strongyle larvae
    • Moxidectin – targets all parasites except tapeworms and is partially effective against encysted small stronglyes
  • Benzimidazoles
    • Fenbendazole – targets large and small strongyles, pinworms, and roundworms and it is effective against migrating strongyle larvae and encysted small strongyles when used at double the normal dose for five consecutive days;
    • Oxibendazole – targets large and small strongyles, pinworms, roundworms, and threadworms;
  • Tetrahydropyrimidines
    • Pyrantel pamote – targets large and small strongyles, pinworms, and roundworms and it is effective against migrating strongyle larvae and encysted small strongyles when used at double the normal dose for five consecutive days;
  • Prazino-isoquinolines
    • Praziquantel – targets tapeworms only


Herbal medicines

Currently, there are no studies to prove the effectiveness of any herbal medicine commercially available for treating equine internal parasites.

Pasture Management

Collecting manure from pasture on a regular basis (every day/alternate days) can reduce the chance of your horse picking up worm eggs from the faeces. Rotating paddocks, particularly after deworming your horse, can reduce the risk of reinfection with internal parasites. Feeding horses in proper feeders off the ground will reduce the intake of parasite eggs off the grass. Adequate time and an appropriate mixture of greens and browns is important if you are composting your horse’s manure. It is critical to make sure the compost is “cooked” properly so that parasite eggs are not reintroduced back onto the pasture.


Anthelmintic resistance is defined as the development of populations of internal parasites that are not killed after recommended doses of anthelminitc drugs. Over dosing, under dosing and inappropriate scheduling of anthelmintic treatments have been cited as causes of anthelmintic resisitance in horses. To find out which anthelminitic is appropriate for your horses, a faecal egg count (FEC) should be carried out by your veterinarian. Your veterinarian can then determine which anthelmintic, how often you use it and what other strategies you can use to help decrease the worm burden in your horses. Resistance is a growing concern and steps we take now could
make life in the future a lot less difficult.


Deworming is an essential part of horse ownership. To carry out this task effectively it is important to discuss your situation with your veterinarian and to carry out some basic tests on your horses (FEC). Using a dewormer inappropriately could be more detrimental that not using a dewormer at
all. Different populations of horses have different deworming requirements (foals, yearlings, mares, aged horses) and with the help of your veterinarian you can find out which strategy best suits your needs.