Worms and worming

Worming horses is an important part of every horse owner’s basic horse care routine. There are different worms that need to be removed from your horse’s guts to keep him healthy. A major worm burden can be life threatening, with potential damage to the gut leading to colic, diarrhoea and other significant problems for the horse, even after the worms have been treated.

Worms found in horses

The internal parasites which are cause for concern include:

• Small strongyles (small redworms)
• Large strongyles (large redworm)
• Tapeworms
• Roundworms
• Pinworm

Encysted small redworm is particular worrying. The larval stage of the small redworm bury into the lining of the gut, where they lie dormant. They can then develop and emerge en masse from the gut wall in the early spring, causing diarrhoea and colic, with a mortality rate of up to 50%.

Treating worms in horses

Due to parasites becoming increasingly resistant to the active ingredients used in equine deworming products (anthelmintics), it is recommended that you test for the presence of worms using a faecal worm egg count before deworming horses. This will help you to decide whether your horse needs deworming or not. This will show the presence of most adult worms, but not tapeworm, nor encysted small redworm.

Tapeworm can be tested for via either a blood test or a saliva test. A new small redworm blood test is due to become commercially available from September 2019 which will identify burdens of small redworm, included those at the encysted stage in their life cycle. Speak to your vet if you like more information. Otherwise it is recommended that all horses are treated to remove encysted small redworm in late autumn or early winter. All other worming should be undertaken only when tests show a horse is carrying a specific worm burden.

Before worming your horse use a weight tape or weigh bridge to find out your horse’s actual weight. Do not guess. It is important that you give the correct amount of a relevant dewormer to treat the worms you are looking to target. If you guess your horse’s weight you risk under dosing which could lead to the worms currently affecting your horse developing a resistance to the active ingredients, making it more difficult to remove them in future.

The regular (at least weekly) removal of droppings from turnout paddocks and grazing land is a highly effective way of preventing the infective stages of worms passing from a fresh pile of droppings on to the surrounding grass and thereby on to other horses. Avoiding overstocking paddocks with large numbers of horses; rotating and resting paddocks to allow grass to recover; and alternating horses with grazing cattle and sheep will all help reduce worm infestions on pasture.

Horse wormers: active ingredients

Not all dewormers (anthelmintics) are effective against all types of worms so if your horse does need worming make sure the brand of wormer you choose contains the correct active ingredients for the worms you are looking to treat.

Moxidectin: this ingredient is effective against encysted developing small redworm larvae as well as large redworm, pinworms, intestinal threadworms, ascarids (adult and larval stages) and will also treat bots.

Ivermectin: lvermectin-based wormers are effective against both larval and adult stages of small redworms as well as pinworms, intestinal threadworms, stomach worms, lungworms, neck threadworms, ascarids and bots.

Pyrantel: this ingredient is effective against most adult roundworms and needs to be administered at twice the standard dose when being used for the treatment of tapeworm. 

Praziquantel: this ingredient is effective against all three known species of equine tapeworm in a single dose. It is not effective against other worm types.

Fenbendazole/Mebendazole: these are less commonly used for worming as there is confirmed resistance to them. A five-day course of a fenbendazole-based wormer can be given between October and March to remove small redworm encysted larvae.

Equine deworming products can be purchases from vets, via online pharmacies and over the counter at equestrian retailers which have an AMTRA SQP [Animal Medicines Training Regulatory Authority suitably qualified person] on the staff. You may be asked to provide your horse’s passport when purchasing deworming products.