Worm Control in Horses

Every year vets from our practice treat many horses affected by worm related problems. Most problems we see are caused by Cyathostomes, Tapeworm and Pinworm.

Cyathostome larvae (small red worm) migrating through the gut of horses can cause severe damage to the large colon resulting in weight loss often accompanied by diarrhoea. This disease is known as ‘larval Cyathostomiasis’. Irreversible damage can lead to permanent ill thrift and only 50% of badly affected horses make a full recovery. Horses of all ages can be affected but the disease is most common in horses less than six years of age.

Tapeworm causes problems by attaching in clumps to the large intestine in a specific location; the transition from caecum to colon. The inflammatory reaction that follows can have serious effects on gut motility. The main presenting problem in a horse with a significant tapeworm infection is colic which can be more or less severe. These worms DO NOT cause weight loss.

Pinworm (Oxyuris) is in itself quite harmless, the main problem is itchiness around the back end of the horse.

Because of the severe damage to the intestines that can be caused by worms it is very important that an effective worm control strategy should be carried out.

When considering a worming programme it is necessary to understand a little about the life cycle of the Cyathostome. The adult worms within the gut lay eggs which are deposited on the pasture in faeces, this occurs from early spring until the autumn. Generally the number of eggs in the droppings decreases during the winter months. Once on the pasture the eggs remain infectious for up to five months. When eaten by a horse the worm larvae burrow into the gut where many encyst (hibernate) and can remain there for many months or even years. Large numbers of these encysted larvae then emerge from the gut during the spring and summer months, and it is this emergence of larvae which results in the symptoms of larval Cyathostomiasis. It is important to note that due to the long grazing season of horses in the U.K. (virtually 12 months) that the disease can occur at any time of year, not just in the spring and summer.


Worm control strategies

There are two main worm control strategies; the ‘traditional’ and the ‘intelligent worming’ system. For either worming strategy to be effective on the long-term, good field management is of utmost importance; regular poo-picking can never be replaced by wormers. To prevent larvae spreading onto the pasture, droppings have to be collected at least twice weekly. Alternating the grazing of pasture with cattle and sheep can be useful as the larvae are not infectious to other species and therefore are ‘vacuumed up’. Fields should not be over-stocked.


Traditional worming strategy

This strategy is based on using wormers all year round. The problem with this approach is that worms build up resistance over time; eventually the wormer will become less effective. There are no new drugs under development nor will there be any in the foreseeable future; we have to do with what we have at the moment.

The only wormers effective against hibernating small red worms are Moxidectin and 5-day Fenbendazole; the active ingredients in Equest and Panacur.

The only wormers effective against tapeworm are double dose Pyrantel and Praziquantel; the active ingredients in Strongid-P and Equitape.

Equest Pramox contains both Moxidectin and Praziquantel making it a very powerful combination.


A good traditional worming schedule would be:

November – Equest Pramox

February – Equest

May – Equest or Equest Pramox

August – Equest


Intelligent worming system

Due to concerns about the development of resistance by worms to some of the wormers currently used, a system has been devised to reduce the use of anthelmintics (wormers). Under the traditional system many horses are wormed regularly even if they have no significant worm infection. The intelligent worming system relies on regularly monitoring the number of eggs in every horse sharing pasture. A small number of worms is allowed to remain (refugia) as they will on the long run produce offspring with no resistance against the wormers we use.


Pasture management

Intelligent worming is not effective if not combined with this advice:

  • Poo pick fields at least twice weekly.
  • You can use rotational grazing with ruminants (sheep/cows) to decrease worm egg infection of the field.
  • Worm horses new to the yard and with uncertain worming history with an Equest Pramox (moxidectin/praziquantel). 

    Intelligent worming schedule

  • Feb – Worm Egg Count: – Positive (over 200 eggs/gram) → use Equest.- Negative; no action needed.
  • May – Worm Egg Count: – Positive (over 200 eggs/gram) → use Equest.- Negative; no action needed.
  • Aug – Worm Egg Count: – Positive (over 200 eggs/gram) → use Equest.- Negative; no action needed.
  • November 5th – obligatory Equest Pramox (also active against tapeworm). 
  • If samples repeatedly come back negative on a yard with little change in horses present, the number of worm egg counts may be adjusted to twice yearly. 

    Advantages of ‘Intelligent worming’ are:

    • Fewer wormers are used; this will hopefully delay the development of resistance. This should mean that the existing wormers will be available to treat worm infection.
    • Although the initial set up costs will be higher than the traditional system, eventually there should be a saving on the purchase of wormers.
    • It’s a ‘green’ system. Fewer drugs are administered to the horses.
    • Monitoring of the faecal egg counts gives assurance that the worm control strategy is working. 

    Worming foals

    Foals are particularly sensitive to worms and will start to pick up eggs from the environment and the mare’s milk from the day they are born. Under high infection pressure this can result in infection from 2 weeks of age. In most foals however no significant burden is present until 3 months of age. The most important worms in foals are the threadworm (Strongyloides westeri) and the large roundworm (Parascaris equorum) which can cause diarrhoea, slower growth, weight loss, rough hair coat, colic and respiratory disease. In adult horses these worms do not usually cause problems as they build up immunity.

    • Taking this into consideration we have devised our worming strategy for foals:
    • Before 3 months of age only worm if infection pressure high (history of problems on yard) or if foal has potentially worm related problems (diarrhoea, coughing, poor doer).
    • At 3 months: worm with Pyrantel (Strongid-P).
    • At 4 ½ months: worm with Ivermectin (Noromectin).
    • At 6 months: join mature horses on yard with Worm Egg Counts.We advise not to use Equest Pramox in foals <8 months and Equest in foals <6 months.

    Worming the pregnant/lactating mare

  • Worm according to normal worming schedule, if the infection pressure is high (history of problems, many horses on fields, no regular poo-picking); worm with Equest 2 weeks after foaling.
  • Donkeys and lungworm

    Lungworm in horses (Dictyocaulus arnfieldi) is quite an uncommon problem in the UK but should not be forgotten about. Donkeys can harbour lungworms without showing any signs and transfer the infection to horses. Horses are much more sensitive to the effects of a lungworm infection and show signs very similar to Recurrent Airway Obstruction (RAO, see other topic); a chronic cough, nasal discharge and shortness of breath. It can be difficult to diagnose in horses as the larvae do not always show in a fecal worm egg count. This technique is more reliable in donkeys. Endoscopic examination of the airways including taking a fluid sample is helpful in many cases but not always. Therefore, if a horse has been kept with Donkeys and shows signs of chronic airway disease, a wormer should be administered. Ivermectin is safe to use in Donkeys and effective against lungworm. In horses both Moxidectin and Ivermectin can be used safely.

    Pinworm- Oxyuris Equi

    Oxyuris is a roundworm that colonises the rectum (the very last part of the intestine before the anus) of an affected horse. The worms are white-grey in colour. The male worms are small but the females may reach 4 inches long and have a long tail tapering to a point, hence the name “pinworm”. Sometimes this tail can be seen sticking out of the anus as the worm is laying eggs. The presence of the worms and eggs causes severe itchiness. As a consequence, affected horses will rub the area around the anus repeatedly. In effect, the signs very closely resemble sweet itch. To further confuse matters, Oxyuris eggs can be transferred to other parts of its body, often the mane and poll, causing irritation there.

    How is Oxyuris spread?
    An adult female can produce huge numbers of eggs (up to 6,000 eggs a day) and horses become infested when they ingest eggs in the process of grooming an affected horse or by swallowing eggs that have dropped on to pasture, feed or drinking water. Many horses are quite resistant to infection however and outbreaks never seem to occur, in most cases we see a single individual is affected.

    Treatment and Prevention

    The treatment of Oxyuris is best carried out at three levels; treating worms present in the intestine, treating eggs present on the skin and treating the environment to prevent re-infection or spread to other horses.

    • Administer 5 day panacur guard (full 5 day course).
    • At the same time start once daily application of pinworm solution (propylene glycol and ivermectin) to the area around the anus (wearing rubber gloves, pour the liquid into the palm of your hand and apply). Continue for at least one week and until itching has stopped.
    • Remove all bedding from the stable, power hose the walls and floor, put fresh bedding down.
    • Poo pick paddocks regularly, at least twice weekly.
    • Daily grooming starting at the head and progressing to hind end before discarding brush for thorough cleaning/disinfection.

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    Pool House Vets