The 3D Approach to Worming

virbac logoBy Callum Blair BVMS MRCVS, Virbac Animal Health


There are a number of factors that will influence the effectiveness of a worm control programme, which can be considered under the 3D's of worming:
  1. Direction
  2. Dose
  3. Delivery



In the past the selection of horse wormers was largely left to horse owners with purchases regularly based on price or whim. Often there was little or no guidance on worming, no rationale behind the use of drugs and little consideration given to the best plan of action.

New legislation has changed that and horse wormers are now classified as POM-VPS (Prescription Only Medicines - Veterinarian Pharmacist SQP). This offers an ideal opportunity for horse owners to tap in to the reservoir of knowledge that prescribing professionals possess.

The prescription consultation will involve guidance on the correct use of wormer drugs and can help identify the most appropriate strategy for effective parasite control taking into account a horse's circumstances.
A good starting point for establishing a coherent worming plan is to identify and understand which parasites are significant to us and when, and then which of these can be detected easily. Infestations that can be easily detected are amenable to a targeted use of drugs, reserving treatment only for situations where test results indicate it is necessary.

Parasite Time of Year Detection

Bots Autumn/early Winter Endoscopic examination.
The presence of eggs on a horse's legs during the Summer is indicative of a probable infection.

Encysted small redworms Winter Not possible to diagnose by routine methods. Diagnosis is based mainly on clinical signs and history

Roundworms Spring and Summer Easily done during grazing season using Faecal Egg Counts (FECs)

Tapeworms Spring and Autumn Possible to detect using specialised FEC technique but test is not readily available, is not very sensitive and does not provide any indication of the level of infection.
A blood test is available from Diagnosteq that measures the level of antibodies produced by the horse against the main tapeworm Anoplocephala perfoliata

It has been established that in a population of horses 20% will carry 80% of the parasites. It follows then that 80% of horses carry very few parasites. By using regular faecal egg counts “wormy” horses can be identified and treated.
As Roundworm infections are the most significant in terms of the risk of resistance and the most easily detected they are the most appropriate parasite for targeted treatment, allowing a less drug dependent approach.
Not all horses will be suitable for a reduced drug regime, however. For instance, young horses, those on restricted grazing or where pasture management (e.g. poo picking) is lacking, are more likely to require more conventional, interval-based worming plans.
Where detection of parasites is either not possible or fairly expensive, it is probably better to err on the side of caution and assume that they are present. This will then involve specific treatments at certain times of the year to control encysted small redworm, tapeworms and bots.
Your yearly plan can therefore be built around a framework of strategic treatments for the more evasive parasites and, over the summer, either routine wormer treatments or the use of FECs (Faecal egg counts) to determine if and when treatment is required.
It is important that in our desire to find alternatives to conventional wormer drugs that we only use methods for parasite control with robust scientific evidence to support their effectiveness. If we do not we risk putting horses' health at risk.
Having established the most appropriate approach to worm control for your horse, it is essential that if or when drugs are used they are used properly.


The dose of wormers is determined through extensive research to establish the optimal amount of drug to achieve the desired effect (kill parasites) whilst avoiding undesirable side effects. Accordingly, if the full recommended does is administered the level of efficacy can be predicted under normal circumstances.
One stumbling block is that dose rates are based on a horse's bodyweight and guesses of weight are notoriously inaccurate, routinely 20% less than the actual weight. It is advisable that before any horse is treated it should either be weighed or a good estimation of its weight obtained by using a weigh tape.
Access to a weighbridge may be easier than you think and if you are given the opportunity to use one you should take it, as it will ensure that you use the right amount of drug for your horse which could ultimately save you money and protect them from the risk of over- or under-dosing.
Do not be tempted to under-dose your horse for any reason, especially to make your horse fit in with the weight of one tube/sachet. This is false economy and not only puts your horse at risk but could also create problems for the horse population as a whole. Even if you do establish the correct weight of your horse and have sufficient wormer to treat him, the drug is only effective if it is actually fully administered - which leads to the third D for consideration.


For many horse owners administering wormers from a syringe does not present any trouble, but problems are more common than you may think. This is largely due to the fact that owners are often embarrassed to admit that they encounter difficulties when worming their horse.
Many syringes are designed for large hands and can be awkward to handle; add to this the common discrepancy between the height of a horse and its owner and a horse's natural aversion to potential threats and it shouldn't be surprising that it can be a struggle.
This can lead to owners being tempted to empty syringes into feed buckets, but even a thin film of product round the bucket or apparently small spillages can represent a significant proportion of the administered dose.
If your horse falls into the “syringe averse” category it may be worth considering some form of desensitisation programme for this perceived threat. There are also an increasing number of devices intended to facilitate accurate delivery of the whole dose.
Alternatively there are now different formulations of all drug groups that are designed with such horses in mind. Granules have been available for a long time but can still be associated with remnants in feed buckets and are only available for fenbendazole and Pyrantel. Recently palatable tablet forms of Ivermectin and Ivermectin in combination with Praziquantel have been launched which provide flexibility of drug delivery and will take the stress and guesswork out of worming.