Worm Prevention

Signs of worm infestation in horses can be extremely variable, ranging from a barely detectable reduction in growth or performance through to a severe disease, colic, and sometimes death.

Signs of worms:

* Poor growth

* Weight loss

* Tail rubbing

* Scouring

* Coughing in young foals

* Colic

* Death

There are a number of worms that infest horses; the most important are large red worms (also known as large strongyles), small red worms (also known as small strongyles or cyathostomes), threadworms, pinworms and large roundworms.

Large and small red worms produce eggs which are passed in the manure. These eggs hatch and develop within the manure into immature worms known as ‘infective larvae’. The speed at which these worm eggs develop into infective larvae is dependent upon temperature and moisture. In warm, moist conditions development is rapid, whereas in cool, dry conditions development is slower. Eggs and larvae can survive for considerable periods in cool weather with adequate moisture. Often the infective larvae move away from the manure to nearby pasture, ready to be eaten by a horse. A horse becomes worm-infested by ingesting infective larvae. In favourable climates, where conditions are cool and moist, pasture contamination can increase steadily; however, heat and dryness can reduce larvae numbers.

Large roundworms produce large numbers of eggs which tend to stick around the anal area of the horse, as well as to objects in the horse’s environment. Large roundworm eggs develop into an infective stage, and when ingested, hatch and continue development inside the horse. Horses usually develop an immunity to large roundworms at approximately 6 to 9 months of age, so these worms are generally only a problem in foals. Foals usually ingest infective eggs while suckling from their dam.

The more horses in a given area, and the heavier the worm burden, the greater will be the level of environmental contamination

Due to the variety of conditions under which horses are used and housed, horse owners should develop a worm control program for their own situation in consultation with their veterinarian.

Worm control program

An effective worm control program consists of drenching to remove worms from the horse; limiting reinfestation by removing manure, harrowing paddocks, or grazing management; and monitoring worm burdens using worm tests.


A drench is a chemical or mixture of chemicals that is capable of killing the worms inside the horse’s body without harming the horse. There are many different drenches on the market today. The three main chemical groups are benzimidazoles (white drench), morantel or pyrantel (clear drench) and macrocyclic lactones. It is recommended that horse wormer rotation of chemical groups be done every 12 months to delay a build-up of resistance in worms to a particular drench chemical.


Foals should be drenched from 6 weeks of age every 4 weeks until 6 months of age.

Pregnant mares

Pregnant mares should be treated just before foaling to control roundworm. Make sure the drench is safe to use in pregnant mares.

Other horses

Drenching may be required as often as every 6 to 8 weeks, or as little as twice a year, depending on the drench used and whether reinfestation is being controlled. Use a worm test to determine when drenching is necessary.

Paste and gel drenches

Horse worming pastes and gels are convenient and easy to use. Dosage is easy to adjust. Always ensure the horse’s mouth is empty of food, and deposit the drench as far back in the mouth as possible so the horse does not spit it out.

Powder and liquid drenches

Some powder and liquid drenches are sufficiently palatable to be accepted when mixed with feed, particularly if molasses is used to disguise them. Liquid drenches are preferably administered into the mouth using a drenching syringe. Frequent loss of drench is a problem with oral administration of liquid drenches.

Granules and pellets

Drenches available as granules and pellets have been specifically formulated for mixing in the feed. Usual recommendations are to give a smaller than normal feed and to dose each horse separately. Molasses is useful in disguising drench taste.

Tube drenches

Stomach tubing is done by a veterinarian. The drench is delivered directly into the horse’s stomach, so dosage is accurate and there are no problems with large volumes or unpleasant drench odour or taste. A wider variety and/or mixture of chemicals can be used.

* Remove manure regularly from stables, yards and paddocks – daily to every 3 days.

* Harrow the paddock and spell it during hot, dry conditions for 6 to 8 weeks.

* Alternate grazing with another species such as cattle or sheep.

* Wash the perineum and udder of pregnant mares prior to foaling.

* Clean and disinfect foaling boxes and stables.

* Avoid feeding horses on the ground; use feed bins and hay nets or racks.

Limiting reinfestation

The number of worm eggs or infective larvae on the pasture or in the horse’s environment can be physically reduced by several management practices.Foals and young horses should be given priority for low-worm pasture, as they are most susceptible to significant disease from worms. If possible, segregate horses by age. Do not put mares and foals on paddocks previously grazed by weanlings or yearlings, as these paddocks are likely to be high-risk for worms.

Stronglyes in horses

Internal parasites are small organisms that live a portion of their life cycle in a host animal. They live in internal organs, body cavities, and tissues while gaining their nutritive source by feeding on the host animal. The horse is affected by many different species of parasites. The nature and extent of damage varies with the parasite.

Parasite infestation causes loss of nutrients or blood from the host, resulting in serious medical problems. Horses heavily burdened with parasites will have a loss of condition due to a depletion of nutrients and blood, decreased growth, and reduced reproductive and athletic performance.

Numerous internal parasites infect horses, but there are only a few that commonly cause significant health problems. To establish an effective parasite control program, it is important to first understand the life cycle of parasites. Successful prevention and control programs are effective because they interrupt the life cycles of parasites. In some areas of the country, the primary class of internal parasites that cause health problems for horses are nematodes, such as large and small strongyles, ascarids, and tapeworms. Other internal parasites perhaps of lesser significance, such as pinworms and botfly larvae, are often considered when designing a parasite control program.


Strongyles are grouped as either large or small. The three primary species of large strongyles that infect the horse are Strongylus vulgaris, Strongylus endentatus, and Strongylus equinus.

The adult form of all strongyles (large or small) live in the large intestine. Adult strongyles produce eggs that are passed out in the feces into the horse’s environment. These eggs then develop into infective larvae that exist on the pasture vegetation or in stalls. The horse is infected when it consumes grass, feed, or water contaminated with infective larvae. These larvae are very resistant to harsh environmental conditions because of a protective sheath. Strongyles can survive in a freezing environment, but a hot and dry environment will often kill them. The infective larvae survive up to 31 weeks at winter temperatures, compared to up to seven weeks at summer temperatures.

The larvae of large strongyles migrate through various parts of the body. Strongylus vulgaris, the bloodworm, will burrow into and migrate in the walls of the arteries that are the primary blood supplier to the small and large intestines. This migration can result in the formation of blood clots, which can disrupt the blood flow to the intestines and cause scar tissue formation in affected arteries. After approximately 120 days, the larvae move to the lumen of the large intestine, where maturation is completed. As adults, these parasites will lay several thousand eggs each day, completing the life cycle. The entire life cycle takes six to seven months.

The other two large strongyles (Strongylus endentatus and Strongylus equinus) have similar life cycles, but their larval migration is primarily through the liver. This migration results in damage to the organ but not nearly to the extent of the S. vulgaris migration through the intestinal blood supply. S. endentatus and S. equinus larvae also return to the large intestines, where they mature into adults. Their life cycle is approximately eight to 11 months. The use of effective anthelmintic (antiparasitic) compounds has reduced the prevalence of large strongyles, which, in the past, have caused the most damage to horses. The small strongyle is considered the most common internal parasite of horses. Horses can be infected with small strongyles and show no overt signs of disease. However, severely infected horses may exhibit clinical signs, such as sudden onset of diarrhea and colic. Small strongyles have been implicated in causing decreased feed efficiency, rate of gain, and performance.

The life cycle of the small strongyle (cyathostomes) is very similar to large strongyles except the larvae do not migrate beyond the wall of the intestines. The larvae burrow in or encyst in the wall of the large colon. Horses with heavy infestations will show clinical signs of diarrhea and negative growth performance. These clinical signs usually occur when large numbers of the encysted cyathostomes emerge from the gut wall, resulting in inflammation. The severity of clinical signs is related to the degree of damage to the intestines, which varies with the level of infection. At present, it is unclear why the cyathostomes tend to emerge all at once. It is postulated that the occurrence of larval cyathostomosis may be associated with:

1: seasonal factors (winter/spring in northern areas, spring/summer in southern areas)

2: antiparasitic treatment within two weeks

3: young horses (less than six years).



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