Services/Equine/Worm Control in Horses

Every year our vets treat horses affected by worm related problems. Most problems we see are caused by Cyathostomes (small red worm) and Pinworm with some cases suspected of colic due to tapeworms.

Cyathostome (Small red worm) larvae 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 can cause 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.

Worm Control in Horses

Pinworm (Oxyuris) is in itself quite harmless, the main problem is itchiness around the back end of the horse and some individuals are very sensitive to them.

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 the life cycle of the Cyathostome:
The adult worms in the horse’s gut lay eggs which are deposited on the pasture in the faeces- which mainly occurs from early spring until the autumn. Generally the number of eggs in the droppings decreases during the winter months. The eggs develop into infective larvae in warm damp conditions so are more of a problem through the summer. Once on the pasture the eggs and larvae remain infectious for at least five months. They can survive over the winter. 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 can then emerge from the gut and it is this emergence of larvae which results in the symptoms of Larval Cyathostomiasis.

Worm control

There are two main worm control strategies:

  1. the ‘traditional’ and
  2. the ‘targeted’ or ‘intelligent worming’ program.

For either worming program to be effective 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 helpful as the larvae are not infectious to other species and therefore are ‘vacuumed up’.

Close stocking of horses and ponies will lead to a higher risk of problems developing.

Chain harrowing rather than poo picking will just spread the worms out more widely on the pasture.

Traditional worming program

This is based on using wormers all year round whether or not the horse has a worm burden needing treatment. We would strongly recommend you consider the intelligent worming program instead.

The problem with the traditional approach is that worms are building up resistance over time to the worming drugs we have; eventually the wormers will become less effective. We are very aware we have resistant worms here in Orkney and strongly advise especially against use of Panacur for routine worming – this should be reserved for foals, sick horses and special cases. There are no new drugs under development nor will there be any in the foreseeable future; so we have to use what we have at the moment carefully to minimise resistance.

The only wormers effective against encysted /hibernating small red worms in the gut wall are Moxidectin (Equest) and 5-days of Fenbendazole (Panacur) -if no resistance is occurring.

The only wormers effective against tapeworm are ones containing Praziquantel or double dose Pyrantel (Strongid-P).

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

A good traditional worming schedule would be:

  • November – Equest Pramox
  • February – Ivermectin or pyrantel
  • May – Equest, Ivermectin or pyrantel
  • August – Ivermectin or pyrantel

Intelligent worming program

Due to concerns about the development of resistance by worms to some of the wormers currently used, a program 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 a 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) and then keep off pasture for 3 days.

Intelligent worming program

  • March/April – Do a Worm Egg Count (WEC) : – Positive (over 200 eggs/gram) – worm – (which wormer on the advice of the vet). Negative; no action needed.
  • June– Worm Egg Count: – Positive (over 200 eggs/gram) → use wormer as advised. Negative; no action needed.
  • Aug/Sept – Worm Egg Count: – Positive (over 200 eggs/gram) → use wormer as advised. Negative; no action needed.
  • October/Nov – Tapeworm test on blood or saliva can be done and tapewormer only used if present.
  • Obligatory Equest or Equest Pramox (also active against tapeworm) must be given in October or November to deal with any inhibited larvae that could be present in the gut.

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.

If several horses are sharing a pasture we advise individual worm egg counts are fine on each horse. Some horses are just much more prone to have a worm burden than others. Youngsters are generally more prone than adult horses. Often about 20% of horses are found to contribute up to 80% of the worm burden on a pasture.

Only individual horses with a WEC of 200 epg or higher should be wormed. Other horses sharing the pasture with very low or negative WEC should not be wormed.

If there are problems reducing WEC results an extra WEC taken 2 weeks after a worm dose may be advised in case ineffective worming due to underdosing or resistance has occurred.

We have a HORSE WORM PROGRAM available for £30 + VAT per horse or pony, which covers:

  • Three worm egg counts (WEC) – we will remind you when to bring samples in
  • 10% off wormers
  • If you have three horses on the program a fourth can be added for free.
  • Measuring tape for body weight estimation for worming.

Taking poo samples for a Worm Egg Count

Take large punches from at least 3 balls of fresh poo from each horse and place in an individual plastic bag. Total amount from each horse to be approximately size of a golf ball. Label the bag with your name and horse name and date and keep in fridge. Take to the surgery within 2 days