A Quick Guide to Horse Worming

Author: Danielle Cousins
Published: Monday 23rd September 2013
Updated: Tuesday 23rd April 2024

As with any animal, worms can seriously affect the health of your horse or pony. With so many worming products available and reports of growing resistance within parasite groups, getting your head around a successful worming programme can be difficult.

Please note: since Equitape (a praziquantel only wormer) was discontinued, there is no longer a product that targets only tapeworm. Only 20% of horses carry worm burdens that need treating, so performing a simple saliva test to see if your horse requires treatment is important for minimising giving your horse medication unnecessarily, and also reducing the risk of worm resistance developing (making treatments less effective in the future).

Here is a general worming plan to target key parasites at certain times of the year in adult horses:

  • Autumn (Sept-Oct): carry out a saliva test for tapeworm and FEC for roundworm. If they get a positive result, target tapeworm with a wormer containing Praziquantel or an elevated dose of Pyrantel.

  • Winter (Nov-Feb): target encysted larval stages of small redworm with a Small Redworm Blood Test carried out by your vet and treat with a Moxidectin product if required. This will also kill botfly larvae. Alternatively, use an elevated dose of Fenbendazole but this will not target any botfly larvae.

  • Spring (Mar-Apr): test again for tapeworm and FEC for roundworm. Only treat horses with a positive result. Target tapeworm with a product containing Praziquantel or use an elevated dose of Pyrantel. If your horse needs worming for roundworm too, then a combination wormer is ideal.

There are general rules that apply to all horse owners to reduce the number of worm eggs in the animal's environment

  • Regularly remove faeces from the grazing pasture- at least once weekly to reduce the number of eggs and larvae that may be ingested during grazing.

  • Do not over-stock pastures- one and a half acres per horse is recommend although individual requirements may vary owing to size and weight.

  • Sharing pastures with sheep and cows will reduce the number of eggs in the environment- the ruminants are a dead-end for parasites that are specific to equine animals so when they ingest the eggs during grazing, the worms do not develop and so cannot produce more eggs.

  • Rotate pastures and rest them- ideally for for at least three months. Strong sunlight and hard frost help to reduce eggs and larvae surviving in the paddocks.

It is also important to remember that each horse will carry different levels of worm burdens and will require a personalised worming programme

  • A full guide on worm counts and tapeworm tests can be found here on our blog "Why is worm counting important?".
  • Faecal Egg Counts (FECs) - these indicate the level of adult egg-producing roundworms. FECs should be carried out every 3-4 months to identify horses with high burdens that need treatment- a good kit by Westgate Labs can be found here. However, they do not detect encysted small redworms (a common type of roundworm) or tapeworms. FECs can also be useful in identifying horses showing resistance to treatment when the sample is taken a short while after worming.

  • EquiSal Tapeworm Test - the latest way to test for tapeworm. A simple swab of saliva is taken from the horse and sent off to Austin Davis Biologics Ltd to analyse. They carry out ELISA tests on the saliva taken but as blood sample is not needed, these test are far more affordable and easy to fit into a worming plan. The kits are available here.

  • Blood sample ELISA - this test will identify if your horse has, or has had a tapeworm infection. It involves taking a blood sample from the horse which is then exposed to tapeworm antigens (unique molecules that cause a reaction from the immune system). A high positive result means that a horse requires worming, while a low positive result can mean that your horse has a low level infection that may not need treatment, or that your horse has once been infected but the horse's immune system remembers the tapeworms and has antibodies to protect the gut from infection. These tests are generally expensive and so tapeworms can be wormed for twice a year to prevent significant infestations from occurring. Tapeworms can cause colic and are easily treated with a Praziquantel product.

  • You can also new test for Pinworm with Westgate's Pinworm Test Kit, available from VioVet.

  • Only worm when necessary- horses have evolved with their parasites and a small worm burden is not detrimental to their health. Exposing small populations to a wormer on a regular basis encourages resistance to develop and this has become a real problem. It is important to maintain worm populations that have not been exposed to anthelmintics (worming drugs). While some parasites can be targeted at specific times of year, it is not always necessary to treat for roundworms, so egg counts should be carried out in Spring, summer and Autumn.

  • Give the correct dose- underdosing leads to resistance building in surviving worms and overdosing is not beneficial. Make sure you know the weight of your horse by using a weighbridge or weighing tape and dose accordingly. Always round up! Dosing for slightly higher weight than your horse is perfectly safe (eg dosing a 525kg horse for 550kg) - never round down as this will not be an effective dose. You can estimate your horse's weight here with our handy weight calculation tool.

  • Spitting out just 5 grams of paste can lead to an underdose of 200-250kg, so ensure they swallow the full amount required/eat all their food with the wormer mixed in in one go.

The exception to this rule is with new arrivals because it is unlikely that you will know their worming history. Worm all new arrivals for:

  1. Encysted small redworm - larval stages that when the burden is high, can cause severe colic when they emerge from the gut lining in spring.

  2. Tapeworm. Use the EquiSal Tapeworm Test to determine whether there is actually a burden to treat. The EquiSal Test requires a simple saliva swab from your horse, which then needs sending off for analysis.

There are three main groups of chemicals used in horse wormers:

Benzimidazoles (1-BZ): Fenbendazole, Mebendazole, Oxibendazole

Tetrahydropyrimidines (2-LM): Pyrantel/ Pyrantel Pamoate

Macrocylic Lactones (3-ML, formerly 3-AV): Moxidectin and Ivermectin

Praziquantel does not fit into these three groups and is effective against tapeworms only. However, there is no longer a praziquantel only wormer for horses, so it is often combined with ivermectin or moxidextin (3-ML) which together provide a broader spectrum of anthelmintic activity i.e. more species of worms are killed by the product.

It is a good idea to rotate the class of active ingredients to reduce chances of resistance developing. For example, if you use Moxidectin (3-ML) to worm one year, use Fenbendazole (1-BZ) the next. Take care to alternate the chemical class, not just the product brand. When using combination products such as Equest Pramox which contain two active ingredients - Praziquantel will only target tapeworms while the other ingredient will be effective against other types of parasite so try and take note of which ingredients and chemical class they are.

There is so much that can be discussed with horse worming and you will find that there are many different opinions and approaches. This article aims to summarise our opinion of a general targeted worming plan and give some advice on pasture maintenance. Don't be discouraged if you can't remember the ingredients - just make a note of the horse wormers you have used and seek some advice. Feel free to contact us for more information and of course, discuss a worming plan with your vet.