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Internal Parasite Control in Horses

Establishing a regular and effective drenching program is essential for maintaining the health of your horse. At CVH, we stock a full range of equine paste drenches. These make it straightforward and easy to dose your horses against worms.


Talk to us about a drenching program to suit your horse’s situation. In general:

  • Start worming foals for roundworms at 6 weeks of age, then continue to drench every 6 to 8 weeks to 6 months of age.
  • From 6 months of age, drench every 6 to 8 weeks against strongyles and pinworms to 1 year.
  • Drench adult horses every 8 to 12 weeks for life.
  • In crowded multi-horse environments, monitor worms with faecal egg counts and if required, drench every 6 weeks.
  • Most paste drenches contain a boticide, but check to make sure you drench adults twice yearly against bot flies. In Crookwell, the winter drench should be given after the first frost.
  • Treat pregnant and lactating mares every 6 to 8 weeks against roundworms, bloodworms, redworms and pinworms.

How to be strategic with worm control

While it’s easy to keep up the drenches, make sure your program remains effective. Remember, treating the horse is just one part of worm control – the horse’s environment needs to be kept as free of worm eggs/larvae as possible (worm eggs can survive for months or years).

Here are some management tips for worm control:

  1. First, find out which worms your horse is carrying. The best way to monitor worm burdens is to bring in fresh droppings to CVH for faecal egg counts. Undertaking regular faecal egg counts is as important in horses as it is in sheep.
  2. Alternate anthelmintic (drench) chemical groups to help minimise drench resistance. Too often the same chemicals are used over long periods, increasing the potential for worms on your property to develop drench resistance. Talk to us about which drenches to use and when to alternate.
  3. When drenching, treat every horse on the property at the same time, and always drench new horses before they are introduced to your property.
  4. Rotate paddocks, making sure the empty paddocks are rested for 3 to 4 months before reintroducing horses – and drench horses before re-introduction. Where possible, it is best to immediately graze empty pastures with sheep, goats and/or cattle. These different animal species can safely consume the infective horse worm larvae, cleaning the paddocks and lowering the potential equine worm burden.
  5. Horse stalls and yards should be cleaned daily of manure. Dispose of safely by using as compost, or spread thinly over pastures not being grazed by horses.
  6. Harrowing (or mowing) paddocks every 2 weeks is one of the most effective ways to break down horse droppings, especially during warmer months. The harrowing helps to kill infective worm larvae by exposing them to sunlight and dry conditions.
  7. When feeding horses grain and/or hay, always place the feed in a rack or trough.
  8. Avoid overstocking.

Equine internal parasites

1. Bloodworms

Bloodworms (large strongyles) are the most dangerous of equine internal parasites. They are suspected of causing up to 90% of all colics – many of which are fatal. The most dangerous is Strongylus vulgaris, a pink-grey worm growing to 2cm long.

After worm larvae are picked up off pastures and swallowed by the horse they burrow through the large intestinal wall into arteries (blood vessels). Here they develop into adult worms over the next four months. The young adult worms migrate back to the large intestine where they attach to the walls, causing ulceration, bleeding and bouts of colic. The female worms lay eggs – up to 5,000 each day – and these pass out in the horse’s droppings onto pasture. In humid weather, eggs can become infective larvae in just 1 week and the cycle starts again.

Foals quickly begin to pick up worm eggs once they start nibbling pasture and once infected, damaging larvae can be migrating through the foal’s blood vessels for 6 months before eggs can be detected in droppings.

The larvae of bloodworms are the real killers. During their time in the blood vessels, they inflame, weaken and injure the vessel lining, causing blood clots and clusters of larvae to form. If a drenching/worm control program is instituted and re-infection prevented, the arterial lesions can heal.

In the most severe cases, the larvae totally block the blood vessels, cutting off blood supply to the intestine, and causing the death of that part of the gut. Major surgery to remove the inflamed and dying intestine is the only treatment option.

2. Redworms

These small strongyles are red-coloured worms just visible to the naked eye. While common, they are not as damaging as bloodworms, remaining in nodules in the intestine rather than migrating through blood vessels. Serious infestations can still cause weight loss, diarrhoea and colic.

3. Roundworms

The roundworm of horses (Parascaris equorum) is primarily a parasite of foals, and pregnant and lactating mares. After the age of 6-12 months, foals develop a strong immunity to these worms, often spontaneously expelling them in droppings. Pregnant and lactating mares act as a reservoir, resulting in worms being passed to their foals.

A white, round and long worm (up to 50cm), roundworms are easily observed in faeces. The worm egg is tough and sticky, adhering to feed bins and stables, and as the female worm can lay 200,000 eggs per day, it is very difficult to maintain a roundworm-free environment.

After a foal ingests worm eggs, these develop into larvae in the intestine and burrow through the gut wall, travel via the bloodstream to the liver, heart and lungs. They eventually reach the windpipe, are coughed up, swallowed and stay in the intestine as they grow into adult worms. This cycle takes 10-12 weeks.

Heavy roundworm burdens cause irritation to organs from larval migration, and adults in the gut interfere with nutrient absorption. Foals with roundworms have a poor growth rate, rough coat, potbelly and may have diarrhoea.

4. Pinworms

The common pinworm, Oxyuris equi, lives in the lower end of the horse’s large intestine. A greyish-white worm that grows to 10cm, it has a round body and sharply pointed tail making them easily recognisable in faeces.

Female adult worms travel to the rectum and anus where they lay their eggs on the surrounding skin, attaching them by a sticky and irritating substance. This causes the horse to rub its tail constantly against any available object. The eggs drop off, often into feedbins or water troughs where they are picked up and eaten. The cycle takes over 5 months.

As pinworm eggs are fragile and die quickly under dry conditions, pinworm infestations are most commonly seen in stabled horses. The worm causes little damage apart from irritation around the anus and a scruffy, rubbed tail.

5. Threadworms

Strongyloides westeri or threadworms cause diarrhoea in young foals. Larvae are swallowed or burrow through skin, travelling in blood vessels to the lungs where they are coughed up and swallowed, developing into adults in the small intestine. Larvae can penetrate the mammary glands of lactating mares and be ingested by foals as young as 4 days old. Foals develop immunity by 2-3 months, losing adults from the gut.

6. Bot flies

Bots (Gastrophilus species) are flies, not worms, and they use the horse as an intermediate host during their life cycle. Adult flies are dark yellow, hairy and look rather like bees. They are most active during summer and autumn. While the flies don’t bite horses, they cause irritation when laying their eggs on the horse’s coat hairs, mainly on the legs, neck, chest or face.

Removing the eggs is tedious and slow! This is done using a bot knife, by thorough and regular grooming, or by vigorously scrubbing the area with hot water. The hot water causes the eggs to hatch and the larvae die prematurely.

Once the horse has licked the eggs from its coat, the swallowed eggs develop into larvae in the horse’s mouth. They migrate in the mouth tissues and down the oesophagus to the stomach where they grow into the 1cm “grub” called the bot. These remain attached to the stomach lining for 10 months, causing little injury unless in large numbers. They drop off, pass out in droppings at the end of winter, burrow into the ground and pupate until warmer months when the adult fly emerges.

7. Tapeworms

The most common horse tapeworm (Anoplocephala perfoliata) is largely a problem in young adult horses. When present in large numbers it causes weight loss, a rough shaggy coat, diarrhoea and, worst case scenario, the worms can block the gut.

This tapeworm needs a particular grass mite as an intermediate host during the worm’s life cycle, then the worm completes its growth to adult worms in the horse’s intestines. The entire cycle takes 3 to 4 months.

It’s important to note that not all paste drenches treat for tapeworms. The infestation is best diagnosed by a faecal egg count. This allows us to identify the tapeworm eggs in your horse’s droppings and provide advice on the most effective drench.