It is inevitable that every grazing horse will pick up different types of parasitic worms from their environment. Under normal conditions, the horse’s immune system keeps the worm burden in the gut low – and it is important to understand that it is healthy and normal for a horse to carry a small worm burden as that helps them to develop immunity.
However, it is also important to monitor this burden and treat horses that have above recommended levels of intestinal parasites if horses are to maintain good health. If the horse’s immune system fails, or if the horse is exposed to high numbers of worms, the intestinal worm burden can become excessive, which can become life-threatening. Worm-related gut damage can lead to colic, diarrhoea and other significant on-going and long-term problems for the horse, even after the worms have been treated.
While young horses tend to be more susceptible to worm-related problems, horses of all ages can be affected.
Types of horse worms
Internal equine parasites that are cause for concern include:
- Small strongyles (small redworms)
- Large strongyles (large redworm)
- Roundworms (ascarids)
There are many different species of small redworm. They can be either red or white in colour and measure 0.5cm to 2.5cm in length, which means they can actually be a similar size to large redworms.
The adult worms live in the horse’s hindgut (caecum and colon) where they can cause inflammation and ulceration. The adults lay numerous eggs in the gut, which pass out of the horse in faeces. Once on the ground they hatch into larvae in the droppings and transfer on to pasture, where horses eat them while grazing. In warm, moist conditions, it can take as little as one to three weeks for eggs to develop into larvae outside the horse. Once back inside the horse the larvae burrow into the wall of the large intestine where they develop further before emerging as mature egg-laying small redworms six to 12 weeks later and the cycle continues. Larvae that remain on grazing land can survive a mild UK winter, although they will be killed by heavy frosts.
In autumn some small redworm larvae will enter an encysted phase, which means they remain dormant in the gut wall for a longer period, before emerging en mass in late winter or early spring. This condition is called larval cyathostominosis. This causes severe diarrhoea and colic, and has a mortality rate of up to 50%. A horse with larval cyathostominosis will require urgent treatment in the form of fluids, medication to control the diarrhoea and steroids to reduce gut inflammation, as well as a suitable deworming product to remove the encysted larvae. One challenge is that most dewormers are not effective in treating these encysted larvae, with treatments containing moxidectin thought to be the most efficacious. Horses that survive may take two to three months to fully recover. If the horse is sharing its pasture with other (particularly young) horses, then you should contact your vet to discuss their treatment as aggressive deworming could trigger the disease in more animals.
Adult large redworm (Strongylus vulgaris) are red in colour and measure 1.4cm to 2.5cm in length. These are now believe to be rare.
They live in the horse’s large intestine, attached to the gut wall. The adult worms lay eggs that pass out of the horse and develop into larvae, which grazing horses then eat. Once back inside the horse, the larvae burrow through the gut wall into the arteries of the intestines. Some species migrate to the liver and the abdominal cavity. They then return to the large intestine where they develop into egg-laying adults. The life cycle takes around six months. Large redworm larvae can survive winter outside the horse.
The damage caused by adult large redworms living in the hind gut can lead to loss of blood and protein, while the chemicals the worms release also affect the natural movement of the gut. The migrating larvae can cause inflammation within the blood vessels and lead to the formation of blood clots, which can affect the blood supply to sections of the intestine, leading to serious colic and death in some cases. Migrating larvae can also cause hepatitis and peritonitis.
Tapeworms (Anoplocephala perfoliata) can be yellow, grey, green, cream or white in colour and the adults can be up to 8cm by 1.2cm in size.
The adult tapeworms live where the small and large intestine meet in the horse. They cause inflammation and can decrease the natural gut movement. The adult tapeworms release segments full of eggs that pass out of the horse on to pasture. These are eaten by the forage mite before the eggs hatch and larvae start to develop inside the mite. Horses then ingest the mites while grazing and the larvae develop into adults over the next six to 10 weeks while inside the horse. The complete life cycle of the tapeworm is around five to six months.
While some horses will carry tapeworms while showing no signs, large burdens can cause problems, including spasmodic colic, particularly in young horses. A very heavy burden can block the area at the end of the small intestine, leading to a potentially fatal rupture of the gut wall.
Roundworms or ascarids (Parascaris equorum) are large white worms that can grow up to 50cm in length. They live in the small intestine of young horses (typically foals, yearlings and two-year-olds).
These worms produce vast numbers of eggs that travel through the gut on to pasture. The microscopic eggs can also stick to a mare’s udder and be ingested by foals while nursing. Larvae develop inside the eggs, then once inside the horse’s stomach they hatch and burrow through the wall of the small intestine. They enter the hepatic vein and travel to the liver, where they remain for seven to 10 days before progressing to the lungs, where they develop for a further two to three weeks. While damage done to the liver is rarely serious because it can regenerate, damage caused to the lungs by these parasites can be more serious as the lungs produce scar tissue in the healing process. Once the larvae have matured, they move into the air sacks of the lungs, where they are coughed up and swallowed back into the gut. When they reach the small intestine they develop into adult roundworms. This cycle takes around three months, while eggs can remain viable outside the horse for a number of years.
A heavy roundworm burden can cause a blockage of the gut, which may require emergency surgery to resolve. Such a blockage may occur, when a horse with a large infestation is treated and the worms die internally.
Pinworms (Oxyuris equi) are white in colour and the females can be up to 150mm long with a long thin pin-like tail (hence the name). Males are much smaller at just 9-12mm in length.
Pinworms have a much less complicated life cycle than most other equine parasites. They live in the horse’s colon and the females crawl out of the horse’s anus then deposit eggs in a sticky fluid on to the skin. The eggs then drop off into the horse’s stable or field or stick to nearby surfaces such as stable walls, managers and field fences. If eaten by a horse they develop into larvae and then adult worms as they pass through the horse’s gut before settling in the colon.
Other internal equine parasites include:
- Threadworm (Strongyloides westeri) – this is a very small worm that can cause diarrheoa in young foals
- Bots (Gasterophilus intestinalis and Gasterophilus nasalis) – the adult flies lay eggs on the horse’s skin during the summer months
- Lungworm (Dictyocaulus arnfieldi) – mainly found in donkeys
- Neck threadworm (Oncocerca spp) – a very thin worm that lives in the nuchal ligament in the horse’s neck
Signs of worms in horses
A horse with a significant worm burden may sometimes, but not always, appear ‘poor’ with a pot-bellied appearance and may have a rough coat that is not easily shed in the spring. Weight loss, loose droppings, diarrhoea and colic can all be signs that a horse is suffering from worms.
Young horses are more susceptible to worms and may not grow as well as expected if parasite levels are not controlled. In cases of roundworm, young horses may cough and have nasal discharge in addition to the other common signs.
Testing for horse worms
Equine parasites are becoming increasingly resistant to the active ingredients used in equine deworming products (anthelmintics), so it is now strongly recommended that owners test for the presence of worms before deworming their horses, rather than simply treating for particular worms at set times of the year. By testing and then only treating horses when necessary, owners can help prevent more worms from developing resistance to the limited range of deworming products we have available to us.
A faecal worm egg count (FEC) test will show the presence of many adult worms, but not tapeworm, encysted small redworm, pinworm or migrating roundworm larvae. Despite this the test does give a clear indication of the level of infection. The test involves a sample of a horse’s droppings being mixed with saline and examined under a microscope where the number and type of eggs are counted and identified. The result is given as number of eggs per gram of faeces (EPG).
Because worm eggs are not equally distributed among droppings, samples should be taken from multiple areas or balls of a fresh (ideally no more than four hours old) dropping and placed in a leak-proof container with the air squeezed out for transit. If you are sending your sample by post, it is recommended to avoid doing so on a Friday or over the weekend as any delay in the test could affect the accuracy of the result. Most equine vets will provide this service and then can advise with the local knowledge as to how best to manage parasites effectively.
Repeated FEC test can be used to assess worm resistance to an anthelmintic. The FEC reduction test involves doing a worm egg count immediately before administering an anthelmintic, and then another 10 to 14 days later. If the wormer is working, the second count should have reduced by at least 95%; if there are resistant worms, the count will not have reduced as much. If you think you may have resistant worms on your premises then do discuss the benefits of running this test with your vet.
Tapeworm can be tested for with either a blood test via your vet or a saliva test that owners can do themselves and send off to get the results. A period of four months should be left between any treatment for tapeworm and testing.
A new small redworm blood test is now available to identify burdens of small redworm, included those at the encysted stage in their life cycle. Speak to your vet if you like more information. If you are unable to run this test, it is recommended horses are treated to remove encysted small redworm in late autumn or early winter each year.
Treating horse worms
We should not attempt to eliminate every worm from every horse. It is healthier for the horse and all his herd mates to maintain a low level of worms in the gut, and important to maintain a population of worms that are still susceptible to the different anthelmintic drugs. Aiming for negative worm counts can result in increased resistance among the worm population.
Foals: All foals pick up worms, and it is appropriate to give anthelmintics to youngsters up to the age of 18 months at three- to four-monthly periods (be aware that moxidectin should not be used in a foal less than four months old).
FECs are useful in foals to determine whether roundworms or small strongyles are the predominant worms present; this helps determine which anthelmintic to use due to differing resistance patterns (for example, fenbendazole to treat ascarids; pyrantel, ivermectin or moxidectin to treat small redworms).
Adult horses: With older horses, worming should be undertaken only when tests show a horse is carrying a specific worm burden of a level that requires action. Horses with a low worm burden do not necessarily require treatment – it is no longer recommended to treat all horses with worms as a low worm burden may help the horse to gain immunity. Your vet or a suitably qualified professional (SQP) will be able to advise you on what levels are acceptable and which require action to be taken.
New arrivals: When a new horse joins your establishment, a faecal worm egg count test should be done immediately to find out what burden they are carrying. If their burden exceeds 200 to 300 eggs per gram, then they should be dewormed using appropriate anthelmintics, in accordance with your vet’s advice.
Pinworm and roundworm can be problematic to treat and have a tendency to recur, so it is best to ask your vet, who will have local knowledge regarding the levels of resistance in your area, for advice.
Other important considerations
Before deworming your horse, use a weight tape or weigh bridge to find out your horse’s actual weight. Do not guess. It is important that you give the correct amount of a relevant dewormer to treat the worms you are looking to target. If you guess your horse’s weight you risk under-dosing, which could lead to the worms currently affecting your horse developing a resistance to the active ingredients, making it more difficult to remove them in future.
The regular (at least weekly) removal of droppings from turnout paddocks and grazing land is a highly effective way of preventing the infective stages of worms passing from a fresh pile of droppings on to the surrounding grass and thereby on to other horses. As the medications become less effective, this time-consuming task is recognised as an increasingly beneficial part of worm control.
Avoiding overstocking paddocks with large numbers of horses; rotating and resting paddocks to allow grass to recover; and alternating horses with grazing cattle and sheep will all help reduce worm infestations on grassland.
Horse wormers: active ingredients
Not all dewormers (anthelmintics) are effective against all types of worms so if your horse does need worming make sure the brand of wormer you choose contains the correct active ingredients for the worms you are looking to treat, and the likelihood of resistance in the worms being carried by the animals you are treating.
Fenbendazole/Mebendazole: these are less commonly used for worming as there is confirmed resistance to them. A five-day course of a fenbendazole-based wormer can be given between October and March to remove small redworm encysted larvae.
Pyrantel: this ingredient is effective against most adult roundworms and needs to be administered at twice the standard dose when being used for the treatment of tapeworm.
Ivermectin: lvermectin-based wormers are effective against both larval and adult stages of small redworms as well as pinworms, intestinal threadworms, stomach worms, lungworms, neck threadworms, ascarids and bots. Unfortunately there are increasing reports of worms being resistant to Ivermectin.
Moxidectin: this ingredient is recommended as more effective against encysted developing small redworm larvae as well as large redworm, pinworms, intestinal threadworms, ascarids (adult and larval stages) and will also treat bots.
Praziquantel: this ingredient is effective against all three known species of equine tapeworm in a single dose. It is not effective against other worm types.
Equine deworming products can be purchased from vets, via online pharmacies and over the counter at equestrian retailers that have an AMTRA SQP [Animal Medicines Training Regulatory Authority suitably qualified person] on the staff. You may be asked to show your horse’s passport when purchasing deworming products.
How did worms become resistant to dewormers?
The first modern anthelmintics were developed in the 1970s, with the introduction of fenbendazole. This was followed by pyrantel, ivermectin and moxidectin, among others, leading to the development of paste dewormers which owners could administer themselves.
Thanks to this, worm control was much easier, and, for a time, more effective. Worms have fought back, however, by developing resistance to anthelmintics. Like all living things, worms are subject to natural selection – so-called survival of the fittest. In a population there will be many that will be killed by the anthelmintic, but also a small group that are naturally resistant to its effects.
Over time, only resistant worms survive the repeated deworming treatments and these contaminate the pasture with worm eggs that give rise to yet more resistant worms. By regularly administering anthelmintics, we have created a situation where there is a rapidly growing population of worms that are resistant to many of the drugs.
With no new types of anthelmintic drugs for horses on the horizon, the drugs we use are becoming less and less effective – and we are seeing a rise in worm-related diseases as a result. As with climate change and antibiotic resistance, we need to act now to slow the tsunami of anthelmintic resistance that is heading our way.
Additional reporting by Dr Tim Mair FRCVS. See Vet Clinic published in Horse & Hound magazine (17 June 2021)
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