Worm Egg Counts

Importance of a targeted approach to worming

A targeted approach is the right approach

Dr Emma Batson of Merial Animal Health explains why it is increasingly important to monitor the worm burden of your horse and how a targeted approach to worming is vital for all horse owners.

We now know that the level of worm infestation can vary considerably from one horse to another – even those kept together on the same yard. As a general rule, it is estimated that 80% of the equine worm population is carried by only 20% of horses, so clearly a blanket approach to worming may be unnecessary for the majority of horses.

Targeted programmes incorporating faecal worm egg counts (WECs) play an important role in managing equine parasites, reducing unnecessary treatments so that each individual horse is only treated as needed.

WECs are easy to undertake, simply take a fresh sample from each horse, send them away to be analysed by your vet or laboratory and the results will be returned, along with a recommendation of whether worming is necessary at that time. WECs are a vital part of EQVALAN®’s SMART worming campaign, which encourages owners and yard managers to Simply Monitor, Assess Risk and, if necessary, Treat with the most appropriate wormer. A simple to use online planner at www.smartworming.co.uk takes the guesswork out of worming.

Target tapeworms too

WECs are a great indicator of most species of adult roundworms; however, immature and encysted small redworm cannot be detected on a WEC test. Tapeworms can only be properly detected with a blood test. Research has shown that tapeworms are associated with certain types of colic and so in the absence of a blood test, it is important to treat these parasites with the right product at the right time.

Whilst traditionally a double dose of pyrantel-based wormer was the only treatment available for tapeworms, more recent developments mean that a single dose of combination wormer such as EQVALAN® Duo which contains both ivermectin and praziquantel – a known tapeworm treatment – is all that is needed.

Twice yearly treatments for tapeworm in the spring and autumn will act as the cornerstone of your programme. So just one dose, twice a year, will help keep the tapeworm burden at bay. An annual treatment for immature and encysted small redworms is also usually recommended during the winter months, and in between times regular WECs recommended every 8-10 weeks can be performed.

The combination of twice yearly tapeworm treatments, an encysted treatment and regular WECs will provide optimal results for all horses, reducing the amount of wormer required as well reducing the threat of worm resistance.

Providing a Faecal Sample 


  1. Collect dung from each animal to be tested – do not combine dung from several animals. Fresh dung is of paramount importance as worm eggs can begin hatching shortly after collecting and this can make your WEC (worm egg count) result inaccurate.
  2. Collect 3 - 4 boli from various places in the dung pile. Mix them together in a bag and then take a smaller sample from this for the WEC – a small handful is plenty.
  3. Place this small sample into a small bag, and then the small bag into a larger bag. Ensure the larger bag is labelled with the horse’s name, your surname and the date the sample was collected (you might want to do this before you place the sample in the bag!).
  4. Keep the sample out of direct sunlight and in a cool place. If you cannot deliver or post the sample immediately, keep it in the fridge, but aim to send or deliver the sample on the day of collection.
  5. Your WEC result will usually be ready within 2 days

Wormer Resistance

Anthelmintic resistance occurs when a proportion of the parasites picked up from a particular pasture are no longer affected by the chosen worming treatment. A number of factors can contribute to the development of resistance, including underdosing and the frequent, perhaps unnecessary, administration of wormers over time.

All worms in any given population (i.e. inside a horse) are each as genetically different from each other as humans are genetically different from one another. A very few of those worms will have a natural genetic variation that can enable them to survive a dose of anthelmintic, or wormer. This sort of variation exists within all populations of all organisms. This occurs partly because random mutations occur in the genome of an individual organism, and these mutations can be passed to offspring. Throughout the individuals’ lives, their genomes interact with their environments to cause variations in traits. The process of natural selection, as detailed by Charles Darwin’s theory of evolution, states that those living things that are the best adapted to survive in their environments will survive and be the most successful at reproduction, therefore having more offspring, which also have the genetic mutation. Eventually, through natural selection such mutations become the norm.

How does wormer resistance develop?

Using too low a dose of wormer is one of the key ways that helps those slightly, naturally resistant worms to survive. These then breed with each other to potentially produce a higher proportion of resistant worms and a -smaller population of susceptible worms within the population. Successive doses of the same wormer, or wormers from the same drug class, can lead to an established resistant population.

Frequent, unnecessary worming can also increase the potential for resistance, by actively selecting for worms that are resistant and killing out susceptible worms before they can reach sexual maturity and produce off spring. Once resistance is present in a worm population, the health, welfare and performance of those horses infested with resistant worms will be compromised. It is also impossible to revert back to a susceptible population.

Reducing & Preventing Resistance

The key to reducing the likelihood of resistance developing starts with the identification of those horses which need to be treated. This can be achieved by testing individual horses, thereby identifying those animals with a significant worm burden, and then using the correct wormer to treat them - calculating the correct dose and time to worm. Worm egg counts help to ensure that wormers are only given when they are needed and therefore may reduce the likelihood of resistance developing. This helps your individual horse and the equine community as a whole.

As part of this more targeted approach to worming, industry experts are calling for horse owners to make better use of Worm Egg Counts (WECs). A WEC is a microscopic examination of a dung sample from a horse to detect and count the number of roundworm eggs present. The egg count is expressed as eggs per gram (epg), and in most cases if the count is greater than 200epg then worming should be considered (some foals and horses, especially where there is no previous worming history, may still require treatment where the worm egg count is less than 200 epg; consult your vet for further advice). A WEC therefore helps to identify those horses that have an excessive worm burden and would benefit from a treatment. It also identifies the main species of worms with the exception of tapeworms, and immature and encysted worms.

It is known that approximately 80% of worms are carried by only 20% of horses. The regime of regular worming at fixed intervals, a system that has traditionally been popular on large yards, has provided an effective means of limiting worm burdens, but it is possible that many horses are being wormed unnecessarily, which encourages the development of resistance. With the consensus of expert opinion on the future of worming firmly behind the use of targeted programmes incorporating WECs, there has never been a better time to make worm egg counts a regular part of your worming programme.

Keeping track of treatments and WEC results may seem like an onerous task, however Merial’s online SMART planner reminds you when to test and worm accordingly, helping you to keep track of every horse in your yard if required.

Find out more here. 

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