Intestinal worms become a problem when some or all of the following are found:

  • A lot of young horses (4 years old and under) on a premises. Young animals are more susceptible.
  • The pastures are grazed by horses only. Co-grazing by other species ‘vacuum cleans’ the paddocks of horse worm larvae.
  • Droppings are not collected.
  • The paddocks are densely stocked and closely grazed.
  • Worm control is erratic, or not practised on all horses in a group.
  • Inappropriate worming drugs are used.
  • Worming drug resistance has developed.

Redworms (Strongyles)

The adult worms live in the large intestine and suck blood, hence the red colour. They lay eggs that pass out in the droppings, develop into infective larvae, which are then ingested by the horse during grazing. These develop into adult worms that lay eggs and so on.

There are two types of red worms in horses:

Large redworms

There are three species, the most important of which has a developmental phase in the blood vessels supplying the digestive system. These vessels can become blocked, causing colic. Modem wormers have virtually eradicated these species.

Small redworms (Cyathostomins)

There are 51 species of which 5-10 occur commonly. These are only a few millimetres long and just visible to the naked eye. Larvae taken in during grazing develop in the wall of the large intestine and emerge when mature to form adult worms. If ingested in the late autumn some may become inhibited in the wall and then emerge in waves in spring.

The clinical signs of redworm infestation include: colic, diarrhoea, failure to thrive, poor coat and anaemia.

Ascarids

Ascarids are parasites of foals and weanlings; older horses become naturally immune. The eggs are taken in while grazing, hatch in the stomach and the larvae travel via the liver and lungs before settling down in the small intestine and developing into adult worms, which may be very large. The adults lay eggs and the cycle continues.

The clinical signs are failure to grow, poor body condition, coughing (due to migration of worms through the lungs), colic due to small intestinal obstruction.

Bots

Bots are reddish grubs that attach to the wall of the stomach and small intestine during winter. They detach in spring and are passed in the faeces, where they pupate and develop into bot flies, which lay their eggs on the horse’s legs. The eggs are licked off by the horse and swallowed and develop into grubs.

The clinical signs are annoyance in grazing horses due to bot flies attempting to lay eggs and, rarely, colic, principally caused by the species that attaches to the small intestine.

Tapeworms

These small flat worms live in the small intestine. Eggs are passed in the faeces and ingested by a small mite on the grass. Further development occurs in the mite, which is in turn inadvertently eaten by the grazing horse. Adult tapeworms then develop in the horse.

The clinical signs of tapeworm infection are spasmodic colic (about 20% of spasmodic cases are believed to be tapeworm-associated) and impaction of the lower small intestine, causing colic that requires surgery.

Pinworms

These worms live in the rectum of horses and cause tail rubbing and anal irritation, but are otherwise harmless. They have an elongated ‘tail’ and can sometimes be seen in the droppings.

Diagnosing Worms in Horses

It is often possible to make a diagnosis if the grazing and worming histories are known.

Faecal worm egg counts (WEC)

Often erroneously called ‘worm counts’, these are of limited value in diagnosing whether a horse has a significant worm burden, because:

  • The correlation between the number of worm eggs in the faeces and the number of adult worms in the intestine is poor.
  • Tapeworm eggs are not demonstrated by conventional counting techniques.
  • In the case of small redworms, most of the damage is caused by larvae rather than by egg-laying adult worms. A horse may have a severe small redworm colitis but have a negative faecal WEC.

However WECs are very useful in worm control because control relies on ensuring very few eggs are passed onto the pasture.

Blood tests

A blood test is available that demonstrates the approximate size of the tapeworm burden a horse carries. Blood tests may also give indirect evidence of

parasite infection. For example, there may be evidence of anaemia, inflammation and low blood protein levels.

Intestinal Parasite Control

What follows is general advice as equine premises vary hugely and it is important to obtain tailored veterinary advice. What is appropriate for a small stud, for example, may be inappropriate for a large livery yard with a changing horse population.

Redworm control

Because resistance to worming drugs is becoming widespread, it has become necessary to challenge much of the dogma relating to worm control that has been promulgated over the last 20 years.

Key facts

  • 20% of the horses carry 80% of the worms. Many horses (around 80-90%) are naturally resistant and produce very few worm eggs.
  • Young horses (4 years old or younger) are more susceptible and produce the most worm eggs.
  • Control relies on reducing worm uptake from the pasture by:
  • using worming drugs to kill the egg-laying adults in the horse, thereby reducing the number of eggs in the droppings
  • physically removing the eggs by collecting droppings every 1-2 days
  • co-grazing with other species, especially sheep; the worm larvae will be harmlessly destroyed in the other species.

Worming Methods

lnterval dosing

This is the traditional approach. The horses are wormed at regular intervals (depending on the drug used) during the high-risk summer grazing period. The aim is to suppress completely egg output in the droppings, thereby reducing worm larval uptake when grazing. After dosing the worm egg count will fall to zero, and then gradually rise again as new adult worms develop and further eggs are laid. Dosing is not necessary during the low-risk winter period or it the horse is stabled for most of the day.

The dosing interval is based on the egg re-appearance time for each drug. These are: Ivermectin 6-8 weeks; Moxidectin 13 weeks; Pyrantel 4-6 weeks.

Interval dosing has been criticised because a lot of the drug is used, horses are treated indiscriminately although many of them have very few eggs in their droppings due to natural resistance and every generation of the worm receives exposure to the drug, favouring the selection of resistant strains.

Targeted strategic dosing (TSD)

This is currently favoured. The aim is to suppress pasture egg contamination at critical times of the year. All horses have WECs performed first and only those with significant adult parasite burdens are treated. This may be only a small minority of the horses tested.

How is TSD done?

Faeces are obtained from all horses in the group (see chart) and WECs performed. Only horses with WECs above 150-200 eggs per gram are treated.

The drugs to be used are based on veterinary advice. In addition all young horses must receive a larvicidal dose of moxidectin in the autumn to remove inhibited larvae.

Targeted dosing will not control tapeworms as tapeworm eggs are not counted in routine WECs.

Benefits of TSD:

  • Fewer horses are treated.
  • Cost savings in worming drug usage outweigh the costs of the WECs.
  • Resistance is avoided or delayed.

Worming drugs

The following worming drugs are available for treating horses. (‘Herbal’ wormers have not been scientifically evaluated so they cannot be recommended.)

DRUGBOTSREDWORMSASCARIDSTAPEWORMSPINWORMS
IvermectinYESYES (1)YES (4)NOYES
Moxidectin (8)YESYES (2)YESNOYES
PyrantelNOYES (3)YESYES (5)YES
Praziquantel (6)NONONOYESNO
FenbendazoleNOPOOR (7)YESNOYES

YES: Effective, NO: Not effective or completely ineffective.

Notes

  1. No activity against inhibited small redworms in the intestinal wall
  2. Currently the only drug consistently effective against inhibited small redworms
  3. No activity against inhibited larvae, and resistance has been reported
  4. Resistance of ascarids to ivermectin has been reported in some countries.
  5. For tapeworm treatment a double dose must be given.
  6. This drug is available on its own or combined with ivermectin or moxidectin.
  7. Redworm resistance is widespread to this drug. No longer commonly recommended for equine use.
  8. Do not use moxidectin in foals under 4 months of age.

Ascarids

The eggs last for years on pasture. Foals and weanlings should be treated from eight weeks (moxidectin cannot be given under four months old).

Bots

Remove the yellow eggs from the coat using a special bot comb. In addition between December and February, kill the bot larvae in the stomach by treating the horse with an ivermectin or moxidectin wormer.

Tapeworms

Twice yearly dosing with either a double dose of pyrantel or a standard dose of praziquantel is recommended. Praziquantel is available either on its own or combined with ivermectin or moxidectin in a single syringe. Praziquantel is the more effective of the two.

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