Horse worms and the expert way to deworm a horse

Horse worms include redworms shown here in horse droppings — red worms on horse poo

(Image credit: Horse & Hound/TI Media)

This article has been edited and approved by Karen Coumbe MRCVS, H&H’s veterinary advisor since 1991.

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 be 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.

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.

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.

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.

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.

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.

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

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 tests can be used to assess worm resistance to an anthelmintic (deworming product). The FEC reduction test involves doing a worm egg count before administering an anthelmintic (to confirm it is required), and then another 10 to 14 days later. If the dewormer 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.

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 early winter each year, ideally after a period of consecutive days when the outside temperatures have dropped below zero.

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.

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.

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 any population there will be many that will be killed by the anthelmintic, but also a small group that are naturally resistant to its effects.

TOPICS

Horse & Hound’s essential guide to worms and deworming

Types of horse worms

Signs of worms in horses

Testing for horse worms

Treating horse worms

Other important considerations

Horse wormers: active ingredients

How did worms become resistant to dewormers?

Additional reporting by Dr Tim Mair FRCVS. See Vet Clinic published in Horse & Hound magazine (17 June 2021)

You may also be interested in…

Westgate Labs Horse Worm Egg Count Kit H&H Approved

EquiSal Tapeworm Test review H&H Approved

Horse & Hound’s definitive guide to equine vaccinations

Lameness in horses: what every owner needs to know

Winter hoof care tips: simple steps to help maintain healthy hooves