Parasites And Pastures

Pastures and internal parasites. You can’t have one without the other. If you have  pastures, your horses will have internal parasites. These pesky little creatures that can erode a horse’s good health have been successful in thwarting the best efforts of science to destroy them completely. Parasites can be controlled, but as long as there are horses, there very likely will be parasites that will attack them.

Unfortunately, one of the internal parasite’s favorite breeding grounds is also the healthiest place for a horse to be when it is not being ridden or involved in a training program--the pasture. One can grasp the magnitude of the problem when it is realized that there are more than 150 types of internal parasites that can afflict horses.

The number one culprit in the parasite world right now is the small strongyle, says Craig Reinemeyer, DVM, PhD, associate professor at the College of Veterinary Medicine at the University of Tennessee.

Ascarids from a horse

Ascarids or roundworms primarily are a problem for foals and growing horses. (Photo courtesy Farnam Co.)

"Small strongyles are found universally wherever horses are pastured," he says. "Virtually all horses on pasture are infected with them. Although severe clinical infection with obvious symptoms that we associate with a ‘wormy’ horse may occur, most horses infected with small strongyles do not display symptoms. It is difficult to document, but subclinical effects still occur."

The small strongyle has taken over the position of Public Parasite Enemy Number One from the large strongyle, also known as blood worm or red worm. Until the mid-1980s, Reinemeyer says, large strongyles infected more than 50% of all adult horses.

Then along came a potent weapon--ivermectin. Well-managed horse herds using ivermectin or moxidectin as a dewormer are able to eradicate large strongyles almost completely, according to Reinemeyer. With the removal of large strongyles as the main internal parasite problem, the distinction of being the most common and economically devastating internal parasite of adult horses fell to the small strongyle.

Reinemeyer describes the life cycle of the small strongyle thusly:

The cycle begins when horses swallow infective or third-stage larvae on grass contaminated with feces. After the third-stage larvae are ingested, they burrow into the wall of the large bowel, where they develop into fourth-stage larvae, referred to as "encysted cyathostomes" by parasitologists and veterinarians.

The encysted cyathostomes can remain in the tissue of the mucosa in an inactive state from several weeks up to two years or longer.

While they are in the encysted state, small strongyles appear to cause very little inflammatory reaction in the horse. However, severe intestinal problems can occur if large numbers of encysted cyathostomes begin emerging from the gut wall at once.

"When encysted cyathostomes emerge," Reinemeyer said, "the mucosal lining of the gut becomes inflamed because the horse is being exposed to all of the metabolic excretory products the larvae have been producing within the cyst. The level of damage and disease varies with the level of infection and can include mild to severe colic, low blood protein, severe diarrhea, and weight loss."

The timing of the emergence of encysted cyathostomes from the gut wall is something of a mystery.

"We don’t know enough about the timing of emergence of encysted cyathostomes, or why large numbers tend to emerge all at once," says Reinemeyer. "We do know that in the northern United States, encysted cyathostomes tend to emerge in the late winter or early spring. In the southern United States, the reverse might be true, with large numbers of encysted cyathostomes emerging during summer and fall."

After encysted cyathostomes emerge, they mature into adults and produce eggs that are excreted by the horse in feces. After the eggs hatch, the larvae go through two molts and become third-stage larvae, which are presnt on grass and are ingested by grazing horses. The cycle of infection continues as long as horses remain on pasture.

By combining good pasture management with a planned deworming program, Public Parasite Enemy Number One can be controlled.

Reinemeyer says that "control" is the key word, rather than elimination.

"A low or mild level of infection is actually desirable," he says, "because it helps the horse maintain acquired immunity."

Science has produced a potent new deworming agent that can help control small strongyles, and Reinemeyer has some suggestions about pasture management that can help reduce contamination.

Before we get into that discussion, however, we should be aware of some of the other common internal parasites that use horse pastures as an aid to their survival. They might not hold the number one position, but they can be equally damaging to a horse’s health and well-being. Only by understanding the life cycles of these parasites can we understand the type of pasture management and deworming programs required to keep the pests in check.

We will find with all of these parasites that overpopulation of horses in a field is one of their chief allies in keeping the life cycle going. When many horses occupy a small area, the chances for infection with internal parasites go up astronomically.

We need only look to wild horses for comparison. It is rare that wild horses, which are allowed to range over thousands of acres of land, are seriously impacted by internal parasites even though they will live their entire lives without a dewormer ever being administered.

The prime reason is that feces and grass combine to produce infestations. Wild horses will graze in an area for a time, then move on, leaving their feces behind. By the time they return to the area or another band comes through, the life cycle of the parasite, in many instances, has been broken.

Compare this with domesticated settings where horses are confined to a specific pasture and can’t leave their feces behind.

The climate in which domestic horses live and are pastured can also have a bearing. Horses in areas of warm, moist weather will be more at risk from parasites than those in hot, dry climes.

Ascarids Or Roundworms

These parasites are primarily a problem for foals and growing horses. As the horse matures, it often develops an immunity to roundworms. This means that older horses normally don’t have a problem with roundworms. On the other hand, the problem can be severe for foals and young, growing horses.

The roundworm’s life cycle begins when the horse swallows eggs that are located on the food it is eating or the water it is drinking. As part of this parasite’s insidious nature, infective eggs can remain viable for 10 years in contaminated soil. The principal source of contamination generally is from eggs that have been deposited in pastures, stalls, or paddocks the previous year.

The first stage of the life cycle after ingestion is the hatching of the eggs. The resultant larvae burrow into the wall of the small intestine and from there, migrate into the veins. The flow of blood through the horse’s body provides the larvae with a transportation system that carries them through the liver and heart before eventually depositing them in the lungs.

While in the lungs, the larvae make their way from lung tissue into the air sacs. They then are coughed up by the horse and reswallowed, putting them back to where they started as eggs--in the small intestine. It is in this location that the worm grows to maturity. If left unchecked, that growth can reach such proportions that they cause intestinal blockage. At best, the adult worms will compete with the young horse for digested nutrients.

Now comes an important part of the roundworm’s life cycle. While in the small intestines, it lays eggs that are passed out with the feces. In about two weeks, larvae form within the egg. It then is infective and ready to start the vicious cycle all over again. One female roundworm is capable of laying 200,000 eggs per day.

It takes the roundworm about three months to complete its life cycle. One of the problems involved in breaking up the roundworm’s life cycle is that its eggs are resistant to many adverse weather conditions. This means that the eggs might lie in unoccupied pastures or paddocks for long periods and still remain infective.

If the cycle isn’t broken, one year’s foal crop can leave behind a deadly legacy of potential infestation for the next year’s crop.

Large Strongyles

We already have discussed small strongyles, but we also should be aware of large strongyles, even though they are being conquered in the equine population. There are three major species of large strongyles. The smallest of the species is up to 25 millimeters in length, the middle-sized species is up to 40 millimeters in length, and the largest of the three is up to 50 millimeters in length.

Under favorable conditions, the larvae develop into the infective stage within one to two weeks after the eggs are passed. As with small strongyles, the horse ingests the eggs by eating grass or other forms of vegetation to which they are clinging. Once in the large intestine, the larvae shed their protective sheath and begin a migratory route.

The smallest of the three species--Strongylus vulgaris--larvae are the most dangerous and destructive. They burrow into small arteries in the gut wall and use them as a pathway to the anterior mesenteric artery, which is the main blood supply to the digestive tract.

Their migration causes a disruption in blood flow by formation of blood clots in the artery. The larvae remain in the anterior mesenteric artery for approximately 120 days, during which time they continue to grow and develop.

When they reach a certain stage of development, the larvae retrace their route through the arteries until they arrive once again in the large intestine, where they grow to maturity. The female large strongyle is capable of laying several thousand eggs per day. The eggs are passed out in the manure, and hatch into larvae which adhere to pasture grass, and are positioned to start the life cycle over again.


These little parasites make use of pastures as part of their life cycle, but perhaps are the least insidious in their effect on horses. Their life cycle is relatively simple. The adults are found primarily in the colon and rectum. They lay their eggs around the anus. The eggs then drop off and contaminate pastures, water, bedding, and feeding areas. The horse consumes the eggs when grazing, eating its feed, or drinking water that contains pinworm eggs.

About the most serious damage caused by pinworms is irritation around the tail. This often causes the horse to rub the tail and can result in a chafed and inflamed area where the rubbing occurs.


Tapeworms can afflict horses young and old. It is an internal parasite that needs an intermediate host. That host is the oribatid mite, which exists in a free-living form on pastures. The infection process begins when mites inges tapeworm eggs in manure. The future tapeworms changes into an infective from within the mite, and the cycle is completed when the horse ingests the mites. In two to four months after ingestion, the tapeworm reaches maturity.

Large numbers of tapeworms pose a threat to a horse’s well-being by causing ulceration in the large intestine and cecum, colic, and a severe form of intestinal blockage.

As mentioned earlier, although there are many other internal parasites, those listed above are the most common culprits that use pastures and paddocks as middlegrounds.

Parasite Management

Breaking this cycle with proper pasture management begins with deworming. Pasture management is important, but it becomes much more effective when combined with an appropriate deworming program.

Most veterinarians will recommend that horses be dewormed at least four to six times a year. Many vets recommend deworming foals and weanlings every 30 to 60 days during the first year of life.

The timing of deworming treatment during the year is critical, Reinemeyer says. He believes that deworming programs work best if treatments are concentrated during the period when climatic conditions are favorable for the hatching of eggs and development of larvae.

My early horse years were spent in Minnesota, and we complacently told ourselves that the parasite cycle was broken each winter by bitterly cold temperatures.

Reinemeyer punches a hole in that little balloon. Cold, he says, has little effect on the survival of strongyle larvae. However, he adds, heat can be very destructive to them.

Internal parasite eggs, he said, are similar to plant seeds in that they hatch at temperatures that range between 45° and 85° Fahrenheit. When temperatures get higher than 85°, he says, the larvae often are destroyed. In the southern U.S. this means that the emphasis on controlling parasites with a deworming program should begin in early autumn and continue through February or March.

Earlier, we mentioned that there is a potent compound available that can be a big assist in controlling small strongyles. That ingredient is moxidectin. It was first developed and introduced on the worldwide market in the 1990s by American Cyanamid. Fort Dodge Animal Health acquired American Cyanamid in 1994 and is now responsible for international sales and distribution of moxidectin.

Moxidectin is capable of killing small strongyles in the encysted state prior to emergence. It also can delay egg reappearance 84 days after treatment, thus breaking up the life cycle for a relatively long period. In 1998, a regimen of Panacur (fenbendazole) at 10 mg/kg daily for five days was approved for larvicidal therapy against encysted small strongyles.

There are a number of other effective dewormers to combat the parasites listed above on the market . The wise horse owner will consult with a veterinarian in his or her specific area in order to establish a deworming program based on geography, climate, parasites particularly endemic to that area, and equine population on the premises.

Most of the following recommendations are only applicable for the southern United States, with hot summers when daily temperatures frequently reach or exceed 85°F. In the northern U.S., pastures remain infective throughout the year, with the annual low levels during the month of June. The main break from parasite exposure for the northern horses comes if they are stabled during the winter months. Strategic programs for northern horses involve treatments around mid to late April followed by suppressive treatments until, at least, July. One addition treatment might be beneficial when given just prior to winter confinement.

Fortunately, the safest time for a horse to be on pasture, from a parasite infestation point of view, is the same time that the grass is richest and most edible--during the warm summer months. The hotter the weather, the less likely parasite eggs will survive when passed with the manure.

Horses, themselves, lend a hand in keeping free of internal parasites. They will, whenever possible, graze on the "lawns" of a pasture rather than the "roughs." The "lawns" are uncontaminated areas and the "roughs" are areas containing piles of manure.

However, as the summer wanes and cool weather arrives, a two-fold problem develops. First, the cooler temperatures do not have a killing effect on parasite larvae, and they will survive in volume. Second, as heavy grazing causes the "lawns" to disappear, the horses will graze closer and closer to the "roughs" and, in the process, could wind up consuming copious quantities of infective larvae.

One way to rectify the problem is to clear the pastures of manure on a regular basis. To be truly effective, Reinemeyer says, manure should be removed from pastures twice a week.

Some horsemen use an approach in hot weather that can be helpful under the right circumstances. They use a tractor to pull a steel harrow through the pastures. While this does not remove the fecal material, it breaks it up and exposes the larvae to the hot sun. However, there are a couple of problems with this procedure. First, the manure remains in the pasture and this means there is the continued threat of internal parasite egg infestation. Second, if a cool snap sets in rather than temperatures above 85°, one might have effectively spread infective eggs from the "roughs" to the "lawns."

If one is to use this approach, it should be done only when pastures are unoccupied. Horses should only be returned to the harrowed pasture two or more weeks after harrowing and only after a hot sun has had a chance to destroy the parasite eggs.

Mowing pastures falls into the same category. It can be effective in disturbing the "roughs," but should be done only to unoccupied pastures.

Although laborious, the best approach remains removing the manure from the pastures.

In the case of small strongyles, rotating pastures between horses and sheep or cattle can be effective in breaking the life cycle. Neither sheep nor cattle are infected by small strongyles. Thus, grazing them on pasture that has been inhabited by horses could remove stage-three larvae when the sheep and cattle ingest the grass to which they are clinging. Sheep and cattle also will more inclined to graze on "roughs" than will horses.

How one handles a new horse which arrives on the farm also can play a role in parasite control. Because one doesn’t know for certain the type of deworming or management program in which the newcomer was involved, it is wise to isolate the animal and deworm it with a larvicidal product before turning it out to pasture with other horses.

Horses also should be pastured by age groups. For example, pasturing very young horses with older animals in a concentrated area can expose the youngsters to heavy larval populations. A good place to start, says Reinemeyer, is to pasture mares and foals away from other horses which are less than two years of age.

Yearlings should be managed differently than broodmares because they are affected by different types of parasites, Reinemeyer points out. They also might need to be on a different deworming schedule. It is much more difficult to reduce parasite infection if all ages and categories of horses are present in the same pasture or paddock.

The number of horses per acre is highly important in controlling internal parasites. The more horses in any given pasture or paddock, the greater the opportunity for parasite infestation.

How we feed horses in the cool spring and fall days also can have an effect on parasite control. If the grass is not yet of a sufficient height or quality in the spring or has been cropped down in the fall, the horse owner often will supplement by feeding hay. If the hay is fed on infected grass, the horse could ingest parasite eggs along with its food.

It is better, under those circumstances, to provide hay bunks or mangers rather than tossing the hay on the ground. It also is a good idea to clean and disinfect the hay bunks periodically. The same is true of water troughs, tanks, and buckets. They should be kept free of fecal material to prevent contamination of the water supply.

It has become obvious that manure is the number one culprit in horses becoming infected with internal parasites whether in the pasture, paddock, or box stall. Some horsemen compound the problem by spreading contaminated manure on the horse pastures, thus effectively increasing the parasite egg population by untold millions. It is not necessarily wrong to spread manure on pastures, but it should be done only after the manure has been composted. Composting allows temperatures to rise to the point that parasite eggs are destroyed.

If the manure is not composted, it should be kept from horse pastures in use and spread on either cropland or ungrazed pastures. There is no universal approach to controlling internal parasites. Each situation at any given farm is different, with such things as climate, season of the year, amount of rainfall, age of the horses and the number of horses in a given pasture or paddock having a profound effect.

Each horse owner should have a planned deworming and management approach that fits her or his area and horse population. This is best done with the advice of a veterinarian. Even with the best management plan and the soundest deworming program, we will not eliminate internal equine parasites. However, we can hold them in check to the point where they will not compromise the good health status of our horses.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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