Worming Your Horse 

Why should I worm my horse?

Horses which have large burdens of worms are at risk of developing problems such as colic, weight loss and diarrhoea. In severe cases, a high worm burden can prove fatal. While horses of any age can suffer from worm infestation, it is usually the young and old that are most susceptible.

What parasites are we trying to control?

There are two main groups of worms which we need to consider when constructing a worming programme: Nematodes (roundworms) and Cestodes (tapeworms). The nematode group can be subdivided into: ascarids, large strongyles, small strongyles and the pin worm (Oxyuris equi).

Ascarids are rarely seen in horses over five years old, due to immunity developing during the early years of their life. Most commonly, they cause clinical problems in horses under 12 months old. However, clinical problems caused by this worm have also been reported in horses up to five years old. Ascarids live in the small intestine where an increase in numbers may lead to small intestinal obstruction, which may require surgical correction. This highlights the importance of an appropriate and effective worming protocol from day one.

Figure 1: Ascarid worms causing intestinal obstruction

Figure 2: Small strongyles (red worm)

Small strongyles (cyathostomins) are the main parasite group against which anthelmintic therapy (a group of antiparasitic drugs) has been targeted towards in recent years. There are around 50 different species of cyathostomins, with 5-10 common species affecting the horse. It is well documented that resistance of cyathostomins to wormers (anthelmintics) is becoming increasingly common. This is a major concern, because no new products are expected on the market in the near future.  Resistance was first reported against the benzimadazole-based products in the USA as far back as the mid-1960s and since then there have been many other reports of resistance to the other groups of wormer, including the more modern macrocyclic lactones.

Small strongyles have a relatively simple life cycle with no migration or intermediate hosts; however they do penetrate the wall of the large colon and caecum as third-stage larvae. At this location, they may remain dormant for months to years. This is known as hypobiosis and is a little like hibernation. As such, it can be very difficult to accurately assess the true overall worm burden, as a high dormant larval burden may not be reflected in a high worm egg count in the faeces.


Cyathostome life-cycle


Figure 3: Life cycle of cyathostomins


Large Strongyles (S. vulgaris, S. edentatus, S. equinus) infestation used to account for a large proportion of the colics observed in the equine population. This is because the immature stages of the worm migrate through the gut wall and then along the arteries, which supply blood to the gut wall. The larvae damage the walls of the arteries, causing small blood clots to form, which can impede blood flow and therefore oxygen supply to that particular part of the gut. In turn this leads to damage to that part of the gut. The larvae then move back to inside the gut where they develop into adults. The adult worms produce eggs, which contaminate the pasture, and so the life cycle continues.

The use of effective anthelmintics against the migratory stages of these helminths over the last twenty years has virtually removed these worms from the equine population. Consequently, it is now rare to see colic caused by these worms.

Large StrongylesFigure 4: Large strongyles

pin worm

Figure 5: Pinworm infected horse - notice hair loss due to rubbing

Pin worm (Oxyuris equi) infestation can result in anal pruritis (itching) causing the horse irritation and discomfort. So this worm literally can be a pain in the back side! The adult worms live in the colon and the females migrate to the anus and surrounding area to lay their eggs. Affected horses subsequently rub against walls and fences depositing the eggs, which can then be transferred to a new host. The eggs are able to live out with a host for up to one month.
Cestodes (tapeworms). The most common species affecting equidae is Anoplocephala perfoliata. This tapeworm has been associated with both spasmodic colic and ileal impactions.  Anoplocephala perfoliata live within the caecum, attaching to the wall at the junction of the small intestine (ileocaecal junction) and have a migratory life cycle and an intermediate host (the Oribatid mite).

Tape Worm

Figure 6: Tape worm


Quantifying Worm Burdens

Worm Egg Count

Figure 7: A worm egg observed as part of a faecal worm egg count

Determining a horse’s worm burden is relatively simple and inexpensive as far as round worms (nematodes) are concerned. Faecal worm egg counts (FWEC’s) can be conducted on faecal samples and here at Central Equine Vets, we can process the samples in our “in-house” laboratory. This test is reasonably reflective of the egg-laying adult burden within the bowel, however, as previously mentioned, it may not reflect a high cyathostomin larval burden residing within the colon wall.

FWEC’s are not able to reliably detect or quantify a tapeworm infestation owing to the low egg output by these parasites. As such, a blood test, which detects the antibodies against currently or recently resident tapeworms, is commonly used. This blood test has proved reliable for detecting high tapeworm infestations in clinical and subclinical cases. As this test measures the horse’s antibodies, detectable levels of antibody can remain present for a variable period of time (around 5 months) following treatment.

Thus, it can be difficult to ascertain whether the horse has a current infestation or has had one, which has been treated within the previous 5 months. As such, it is important that the horse’s worming history is considered along side the blood test results.

Available Worming Products

There are a large number of worming products available. However, these can be divided into several classes of drugs:

  • Benzimadazoles (e.g. Panacur Equine Guard)
  • Pyrantel salts (e.g. Strongid P)
  • Macrocyclic lactones (Noromectin, Equest)
  • Praziquantel (Equitape)

When deciding on which wormer to use it is important to consider which worms you are looking to kill, which stage of the worms’ life cycle you are looking to target, what time of year it is (usually in relation to the worm’s life cycle) and what type of a worming programe your horse is on.  So as you can see, deciding on which wormer to use can be a difficult choice! As such we recommend that you contact us at Central Equine Vets to discuss your horses’ worming needs. We’re always happy to help!

What worming programme should I use?

Unfortunately there is not a “one size fits all” policy when it comes to worming horses and there are several worming strategies that can be used to control worm burdens in your horse(s). There are advantages and disadvantages to each worming strategy and it may be that one particular strategy suits your needs best. As such, it is important to discuss your horse’s worming needs with your vet.

Some commonly used worming strategies are:

Interval worming:  This strategy is commonly used on large livery yards purely for ease of management, because it means that all the horses are wormed at once. It involves worming the horses based on the egg reappearance time after treatment with a specific wormer. For example if moxidectin (Equest) is used the horse(s) need to be wormed every 13 weeks, as this is how long it takes for the eggs to reappear in the faeces following treatment with this particular drug. As different drugs have different egg reappearance times, it is important to make sure that you adhere to the manufacturers’ dosing intervals i.e. moxidectin (Equest) = 13 weeks, ivermectin (e.g. Noromectin) = 8 weeks, pyrantel (e.g. Strongid –P) = 6-8 weeks and fenbendazole (Panacur Equine Guard) = 4-6 weeks.

Although interval worming is an easy worming strategy to use, the major disadvantage it has is that it can encourage resistance to the wormers used. This is because 80% of the worms are found in just 20% of the horses. As such, on this programme, some horses will be wormed unnecessarily.

Strategic worming: This strategy involves worming horses at certain times of the year when worm burdens are thought to be at their highest, for example, using a larvicidal dose of wormer in the winter in order to treat the encysted cyathostomin larvae which reside in the gut wall or using a tapewormer in the Autumn and Spring time. While this strategy is less likely to lead to resistance of the worms to a particular wormer, problems can arise when abnormal weather patterns (e.g. an unusually mild winter or unusually cold spring) can lead to early or late high pasture counts as the horses may be wormed at the wrong time in relation to the worms’ life cycle, which can be influenced by climatic conditions. Similarly, if horses with high worm burdens are added to the population, this may interfere with the efficacy of this strategy.

Targeted strategic worming: This strategy involves worming horses based on the results of faecal worm egg counts (FWECs) taken every 3-4 months and worming them at set times of the year in relation to the worms’ life cycle and environmental factors (such as pasture management). This means that a specific worming programme is tailored according to each individual yard.

The major advantage of this particular programme is that FWECs are used to monitor the worm burdens in each horse, so that only the horses with worm egg counts  > 200 eggs per gram are wormed (targeted part of programme). As such some horses are left untreated thereby reducing the amount of wormers used on a property often by as much as 50%. In turn, this should mean that resistance to wormers becomes less of a problem.  However, it is important to remember that FWECs only detect adult worms and therefore a larvicidal product should be used once per year (usually in the winter time), regardless of FWECs (strategic part of programme). Similarly horses should be treated for tapeworm twice a year (Autumn and Spring).

Pasture Management: It is important to remember that as well as using an appropriate worming strategy good pasture management is essential to help minimize worm burdens in your horse.

Some important rules to follow are:

  • Do not overstock paddock
  • Remove faeces regularly from pasture (minimum of twice a week)
  • When worming, dose according to weight (avoid under dosing as best as possible by using weigh tapes or weigh bridges)
  • Quarantine all new arrivals for 14 days, and during this period worm with moxidectin and praziquantel (Equest Pramox)
  • Co-graze with ruminants (sheep and cattle). These animals act as biological hoovers and eat up any worms. The worms cannot live in theses species (dead end hosts) and subsequently die
  • Do not mix young animals with old – have paddocks of similar ages

How is the efficacy of a worming programme assessed?

If despite worming horses at the appropriate times FWECs remain high it is possible that there may be a resistance problem to a particular wormer. Suspect anthelmintic resistance can be assessed by performing a faecal egg count reduction test (FECRT). This involves taking faecal samples from a minimum of 6 horses on day 0 (prior to worming) and performing a FWEC. The wormer in question is then administered at the correct dose and the FWEC is repeated.  The percentage of faecal egg count reduction is then calculated as follows:

%FECR= ([FECpre-FECpost]/FECpre) x 100

In order to assess the likelihood of resistance the following guidelines tend to be used:

Benzimadazoles:  <90% reduction -  resistance likely

Pyrantel salts: <90% reduction -  resistance likely

Ivermectin: <95% reduction – resistance likely

Moxidectin: <95% reduction – resistance likley

To find out more about worming your horse(s) please contact Central Equine Vets on 0131 664 5606 or e-mail us at: info@centralequinevets. We’re always happy to help!

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