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Worming Explained

Every horse alive will have a small population of parasitic worms in its body, which cause the horse no ill-effects whatsoever; in fact, a small worm burden is an excellent ‘booster’ to a horse’s immune system and its ability to fight infection. However some horses develop heavy worm burdens which, if left untreated, can prove fatal. Domestication and the subsequent management of horses has increased the likelihood of heavy worm burdens developing, so it is vital that we, as horse owners, do all we can to keep our equine friends parasite free. Equine parasite control can be very confusing if you are unsure of which parasite you need to treat for, so we have put together some vital information to help you understand the goings on in your horse’s gut and how best to keep it under control! If you need any help formulating a worming plan, please don't hesitate to contact our in-house SQP, Hannah who would be more than happy to help you.

Common equine parasites and the problems they cause

Lifecycle Small Redworms (Cyathostomins): With over 50 different species, small redworms are the most common horse parasite. Adults inhabit the small intestine and lay eggs which are passed out in the faeces. The eggs hatch and develop through three stages, with the third stage being infective. If a horse eats the infective stage the larva will migrate to the lining of the intestine where it can remain dormant for long periods of time, this is known as ‘encysting’. There is the potential for large numbers of encysted larva to build up in the gut wall. Eventually, the larva will ‘re-activate’ and emerge out of the lining, which can cause extensive damage. In order to prevent this, treatment with moxidectin is recommended in winter.

Large Redworm (Strongylus vulgaris and Strongylus edentates): Despite similar names, the large redworm and small redworm differ in life cycle, as well as size. In the case of the large redworm, horses become infected by ingesting larvae as they are grazing. These larvae are an important cause of disease; they migrate through the blood vessels to the arteries of the intestinal tract where they can cause severe damage. They remain in the arteries for around six months, after which time they return to the intestine as adult worms. Adults are between 1.5 and 5cm in length and can cause a number of different problems if present in large numbers. Treatment for large redworm is recommended in the summer months, with an ivermectin-based wormer.

Large Roundworm (Parascaris equorum): Roundworms, or ascarids, are usually only problematic for foals and younger horses. Roundworm eggs can remain viable in the soil for many years and are ingested by horses as they graze. The eggs hatch inside the intestine and the emerging larvae migrate through the walls of the small intestine and into the veins. The larvae are then transported via the blood to the lungs where they are coughed up, re-swallowed and passed into the small intestine. Once in the small intestine the larvae develop into adult roundworm. Adults can grow up to about 50cm long and lay eggs which are passed out in the faeces.

Tapeworm (Anoplocephala perfoliata): The equine tapeworm is flat and triangular and has a small, round head which attaches to the lining of the gut with four suckers. Adult tapeworm can grow to approximately 8cm long by around 1.5cm wide. It lives in the mid-part of the gut at the junction between the small and large intestine known as the ileo-caecal junction. Eggs on the pasture are eaten by tiny oribatid mites, of which there are thousands in every square metre of grass. Inside the mite, the eggs hatch and develop into an infective stage and when the mites are eaten by a horse, the larvae will infect the horse and continue to develop into adult tapeworm in clusters at the ileo-caecal junction. Treatment for tapeworm should be every 6-12 months, depending on the horse’s circumstances. Praziquantel is the ideal drug to treat tapeworm.

Pinworm (Oxyuris equi): Pinworms are less common than some of the other worms and are not thought to be harmful, but can cause itching and irritation around the tail. Pinworms inhabit the large and small colon and have a simple life cycle: the females lay their eggs around the anus of the horse using a sticky substance, which is irritating to the horse. The eggs are dislodged as droppings are passed onto the pasture, where they are ingested by horses.

Bots (Gasterophilus intestinalis): Small, sticky, yellow eggs are deposited on the horses coat by the female bot fly usually in late summer. The eggs hatch and the larvae get into the horse’s mouth when the horse licks its coat. The larvae are then swallowed. Once in the gut, the bot larvae attach themselves to the gut lining. They are eventually passed through the horse’s digestive system and are expelled in the droppings. Bots can cause mild damage to the mouth, gums and lining of the stomach.

Equine parasites would not be a problem if they lived in perfect harmony with their hosts. At low levels of infection, worms are well tolerated however some horses are susceptible to developing large worm burdens, where disease caused by the parasites may occur. The diseases caused by a heavy worm burden can be divided into two categories:

  • Colic
  • Diarrhoea

Colic: Heavy burdens of tapeworm, large strongyles, cyanthastomins and ascarids have all been associated with colic in the horse. Some of these colic episodes may be minor, whilst others are very severe and can lead to death. As with all colic, a vet should be called as soon as possible on noticing the symptoms.

Research has shown that there is an association between spasmodic colic and a heavy tapeworm burden; a study using data from 100 horses with spasmodic colic and 200 horses living in close proximity to the colic cases demonstrated a significantly increased risk of colic in horses infected with tapeworm. Further studies by vets and parasitologists in the USA investigated the incidence of colic at five different stable yards, each using a different worming programme. It was concluded that the better the worming programme, the lower the incidence of colic.

Diarrhoea: Many horses that suffer from diarrhoea, particularly young horses in the spring time, do so because of small redworm infection. During the grazing season, horses can accumulate tens of thousands of larvae within their gut walls, which encyst and do not reappear until the following spring, when they erupt out of the gut wall. If all the larvae do this at the same time it causes massive damage to the gut wall, leading to diarrhoea, weight loss and sometimes death. What’s more, this condition is particularly difficult for vets to treat, for the following reasons:

  • It is difficult to diagnose
  • The drugs available are not effective at treating the condition
  • A significant proportion of horses with this condition will die in spite of intensive veterinary care.

Some horses with heavy small redworm burdens will suffer from weight loss, poor condition, reduced performance, lethargy and failure to grow without developing diarrhoea. From this information, it is clear to see that a correct worming programme that is followed accurately is definitely better than a cure!

Year-round worm control

The above information is proof that the only way forward to control equine parasites is through the use of well-planned and executed worming programmes and good management of horses and the environment in which they live.

Worming programme: Previously most horse owners opted for “interval dosing” whereby wormers are administered to the horse at regular intervals throughout the year, regardless of whether a worm burden was present or not. Unfortunately, this strategy is no longer thought to be the best way to control the equine parasite population. In fact, it is actually assisting the parasites, by allowing them to evolve to become resistant to the drugs being used, making this treatment plan less effective. Vets and SQP’s are now encouraging horse owners to adopt the “targeted dosing” method of parasite control. This involves only treating those horses that have been identified as having worm burdens and dosing for specific parasites at certain times of the year. This is when the information above, regarding parasite life-cycles, is of great use: A targeted worming programme can be devised, which allows minimal unnecessary drug use and maximum worm control, by targeting certain types of parasite with specific drugs at set times of the year. Below is an example of one such worming programme.

Worming plan 


Environmental management: Horses out to grass, especially in groups, are likely to encounter many infective parasite larvae, which they will inadvertently ingest as they graze. There are several factors which will increase the levels of infective larvae ingested by a grazing horse; they are:

  •  Large numbers of horses on a small amount of grazing
  •  Pasture that does not have time to ‘rest’
  •  Use of the same grazing by multiple horses
  •   Horses with high faecal egg counts out to grass
  •  Young horses within the grazing herd
  •  Warm, damp weather.

 Fortunately, there are a number of steps that we, as horse owners, can take to decrease the force of infection by minimising contamination of the pasture:

  •  Poo picking – Removing droppings from the field just twice weekly has been shown to be an extremely effective method of suppressing faecal egg counts in grazing horses. This is often overlooked as a necessity in horse management however it is vital to control parasite infection in our four-legged friends
  •  Grazing rotation – Moving horses onto a new field to allow an old one to ‘rest’ allows infective eggs and larvae to die before they are able to infect another horse. The minimum rest period of three months is recommended.
  •  Mixed species grazing – Cattle and sheep will act as “biological vacuum cleaners” eating the infective equine parasite larvae, which are unable to survive in any other species.


Faecal Egg Counts (FECs)

 A faecal egg count (also known as a worm egg count or a worm count) is a count of the number of large redworm eggs per gram of faeces. It will also identify the eggs of ascarids. Performed on a small sample of fresh droppings, this test is useful for routine monitoring of parasite status, identification of horses with high burdens that will require treatment and investigation of the parasite status of a new horse. An FEC can also be useful in the investigation of unexplained colic or weight loss. The number of eggs in a sample is measured as eggs per gram (epg). Usually, horses with 200 epg and over are wormed and those with a lower count would be untreated and retested at the next FEC. There are several benefits to choosing FEC’s over routine worming, including:

  • The ability to identify horses with large worm burdens and targeting treatment at these animals
  • The ability to recognise and treat a heavy worm burden before the horse presents with a parasite associated disease
  • The ability to evaluate the efficacy of the worming programme
  • The ability to not treat those horses with low worm counts, which is more economical for you, better for the horse and better for the environment.

It is worth noting that, whilst FECs definitely have a place in a worming programme, they do have some limitations. The most important to remember is that this test does not detect the presence of tapeworm, therefore it is essential that horses are either treated for tapeworm every six months with a combination wormer (ivermectin and praziquantel) or a tapeworm only treatment (praziquantel), or they are tested for tapeworm antibodies (see paragraph below). FECs are also unable to detect levels of encysted small redworm in the horse, so it is recommended that routine treatment with either moxidectin or a short course of fenbendazole is undertaken in winter (there is now widespread resistance to fenbendazole, so it is recommended that this only be used if resistance is not present). To buy a Faecal Egg Count kit, please click here.

Saliva Tapeworm Test (EquiSal)

The EquiSal Tapeworm Test is a revolutionary horse saliva test to tell whether your horse has tapeworm. Over the last two years the EquiSal team, consisting of four experienced scientists (one of whom was an inventor of the the Clear Blue pregnancy test) have developed this innovative test for tapeworm using horse saliva that you collect yourself, without the need for a vet. The test measures antibody in saliva which is specific to tapeworm, using a combination of two laboratory tests (called ELISAs). Each sample is analysed under very carefully defined conditions to account for variations in saliva flow and impacts of diet. The saliva samples are handled by an automated liquid handling system to ensure very high accuracy, as well as high-fidelity sample tracking throughout the test procedure. A complex algorithm is applied to integrate and de-convolute data from the two different tests to deliver an EquiSal diagnosis of tapeworm burden.

How accurate is the EquiSal tapeworm test?

The EquiSal Tapeworm Test was proven to be accurate by testing saliva samples taken from horses in which the number of tapeworms present had been counted at post-mortem. The test identifies horses with a low burden, a borderline result or a moderate/high burden, and treatment is recommended for any borderline or moderate/high results. In scientific terms, the EquiSal Tapeworm Test has both high sensitivity and specificity, which is important for correctly identifying horses with tapeworm burdens.

The majority of horses that were identified with more than one tapeworm at post-mortem were correctly identified. The remaining few were diagnosed as being negative but these horses had burdens considered by vets to be mild, amounting to no more than 20 tapeworms. Experts consider that a burden of up to 20 tapeworms is not pathogenic (capable of producing disease). This is similar to the current guidelines for redworm faecal egg counts (FEC), where less than 200 eggs/gram is not considered pathogenic.

This means that the EquiSal Tapeworm Test can be relied upon to:

  • correctly identify the vast majority of horses with a moderate/high burden
  • correctly identify all horses with pathogenic burdens.


How many horses have a tapeworm burden?

Current EquiSal data shows that approximately 20% of horses tested for tapeworm have a burden that needs treatment. This finding suggests that a huge number of horses (around 80%) are receiving routine tapeworm treatment when they don't need it.

How long after worming can a horse be tested?

We recommend that a horse has not been wormed for 4 months before testing with the EquiSal Tapeworm Test. This is similar to faecal egg counts, where, for example, you should not worm with Equest for 13 weeks prior to carrying out a faecal egg count. With faecal egg counts, if there is a worm burden, you can carry out a faecal egg count reduction test to check whether the worming has been effective. EquiSal hope to offer a simillar worming efficiency service and are investigating this in detail. Early research indicates that salivary tapeworm-specific antibody levels are very low 2-3 months after worming, therefore re-testing at this time would confirm whether the worming strategy was successful. Profiles from various horses show reduction of salivary tapeworm-specific antibody levels quite soon after worming, so it may be possible in the future to determine relatively quickly if the wormer is working or if there is resistance or under-dosing.

How often should horses be tested for tapeworm?

We recommend that you test your horse twice a year for tapeworm. The best time to do the test is in late winter/early spring and autumn/early winter, as these are considered to be the ideal times of year to worm for tapeworm. To buy an Equisal tapeworm test please click here.


How much wormer does my horse need?

As previously mentioned, resistance to wormers is on the rise and the way to combat this is to ensure that we administer the correct dose of anthelmintic to our horses and to only worm them when necessary, with the use of FECs and tapeworm tests. The only way to ensure that our horse receives the correct amount of wormer (once it had been determined that he needs worming) is to accurately determine our horse’s weight and treat accordingly. There are several ways to do this, which will be discussed below.

Resistance to wormers, the facts!

  • Resistance to the benzimadazole group of drugshas been widely reported worldwide and resistance to pyrantel embonated has been recorded in the USA and parts of Europe
  • Fenbendazole resistance was found to be widespread in the south of England in the early 1990’s. Recent investigations in Yorkshire have shown that a significant number of horses are infected with fenbendazole-resistant worms. Of particular concern is the finding that treatment with a short course of fenbendazole (rather than one treatment) did not overcome resistance.
  • The egg reappearance period following worming with ivermectin was 9-10 weeks when it was first introduced in the 1980’s. This has now fallen to six weeks. Currently there is no known resistance to ivermectin or moxidectin however this information shows that, unless horse owners nationally change the way they worm their horses, resistance to these drugs will soon appear.


How to calculate your horse's weight: 

A good alrounderWeigh bridge: This is by far the most accurate way of determining your horse’s weight, as the guesswork has been completely removed, however this isn’t always possible. Some vets and feed companies have weigh bridges, so contacting them would be the best way to get your horse weighed.

Calculation: If you do not have access to a weigh bridge, the best alternative is to use this simple calculation and a tape measure:

  • Place the tape measure around the horse’s heart girth and note the length in centimetres
  • Then measure the length of the horse from the point of shoulder to the point of buttock, keeping the tape measure against the contour of the horse’s side
  • The horse’s weight is then calculated using the following formula:


Weight Formula 


Weigh tape: Weigh tapes are fairly inaccurate, however regular measurement with one of these will allow you to determine if your horse is gaining or losing weight. If you are using a weigh tape to determine weight to worm a horse, it is often better to slightly “over worm” for the measurement taken, rather than “under worm,” as giving the horse slightly more wormer than required will not cause harm, however under-worming may significantly increase the chances of parasite resistance. Click here to buy a weigh tape.


How to worm my horse

 Once the weight of the horse has been determined, the ring on the plunger should be set to administer the correct amount of wormer. Standing just in front of your horse’s shoulder, with one arm over his nose, place the syringe between the front and back teeth and deposit the paste at the base of the horse’s tongue. It is often useful to elevate the horse’s head for a few seconds to ensure that the wormer is swallowed. It is important to note that horse wormers are highly toxic to dogs and cats, so it is vital that they are locked away when worming your horse and that any spillages are properly cleaned up.


 Sample worming programme

 If it is absolutely necessary to continue with interval dosing with your horses (on large yards where there are lots of different horses coming and going) it is important that you stick to a regular worming programme and change the class of drug you use every two years. In simple terms, this involves you worming with ivermectin and praziquantel for years one and two, then worming with single and double doses of pyrantel embonate for years three and four and then repeating the sequence. It is highly recommended that faecal egg counts and tapeworm tests be used if at all possible.

 Sample Worming Plan

If at all possible, the strategic approach to worming should be used, which involves a routine faecal egg count every 2-3 months from March to October, along with regular tapeworm testing and a treatment for encysted redworm in winter. Where “broad-spectrum” is stated, this refers to a combination wormer (ivermectin or moxidectin plus praziquantel) “Tapewormer only” is a praziquantel-based product and the choice given for winter is moxidectin which is generally preferred, or a short course of fenbendazole

Please note that these worming programmes are only examples and the correct worming programme for your horse may differ from these. If you would like a worming programme tailor made for your horse, please do not hesitate to contact our SQP


Worming donkeys

Donkeys need a regular worming programme that includes FEC’s and routine worming for tapeworm, just like horses. However, unlike horses, it is vital that donkeys are wormed for lungworm, as it is estimated that between 4 and 100% of donkeys in the UK carry this worm, often asymptomatically. This becomes even more important if the donkeys are mixing with horses, as the lungworm can cause horses to cough. There are only a small number of wormers licensed to treat donkeys for worm infections, so it is vital that they are only wormed when necessary. There are only a small number of wormers that are licensed for use in donkeys, however vets can prescibe off-license if there is a particular wormer you require. It is worth noting that none of these wormers will be effective against tapeworm in a single dose, so when treating a donkey for tapeworm, a double dose of Pyrantel Embonate should be used. The treatment of choice for lungworm in Donkeys is Ivermectin

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