All horses are a victim of internal parasites, while horses can function normally with a small parasite burden, without our help, our horses worm burden can quickly escalate to a life threatening problem.

Internal parasites can cause or contribute to many respiratory, digestive, growth and performance problems as well as colic. 

The digestive tract is the most commonly affected area, along with migration through other tissues and organs such as heart, liver, lungs, and blood vessels. 

Strongyles are the most injurious, whereas ascarids, bots and pinworms generally are less harmful. A few parasites may be tolerated by the horse without apparent signs of ill effect but large numbers are quite apt to be harmful.

Young sucklings or weanlings and yearlings Horses are the most affected by parasites.
By the time a horse is 2 or 3 years old they have generally built up some resistance to ascarids and pinworm infection.
Strongyles and bots however effect horses of all ages.

STRONGYLES

Large Strongyles

Large strongyles - also known as bloodworms or redworms. 

The adult females lay thousands of eggs daily in the host (horse) and these pass out of the body in the manure.
The larvae require 6-1/2 to 8 months to reach maturity.
The adult parasites then migrate to the large intestine and remain there during their adult life.
In the right temperature the eggs hatch in approximately 20 hours.
The resulting free-living larvae develop into the infective stage in 5-6 days. They are then capable of infecting the horse when ingested and must be ingested to complete the life cycle. Freezing stops the hatching of the eggs and the development of the larvae but does not kill either.
Heat generated by composted manure kills both.
Larvae live about 3 months on the average but may live for a year or even longer in moist, cool climates.

Eggs in manure hatch into larvae that are consumed by the grazing horse. 
The larvae mature in the intestinal tract and burrow out into blood vessels where they migrate throughout various organs and eventually back to the intestine.
The larvae can cause extensive damage to the lining of blood vessels and mucus membranes.

Horses with large strongyle infestations may display weight loss, poor quality coat, anemia, or colic and are more susceptible to bacterial infections.
In extreme cases, the blood supply to the intestine may become completely blocked by the strongyles resulting in severe (and even fatal) colic. In heavily infested horses, blood vessels may become distended and may even rupture, leading to sudden death.

Small Strongyles

Small strongyles differ from large strongyles in several ways. 

First, small strongyles do not migrate through tissues as do large strongyles.

Second, small strongyle larvae may become encysted. This means that they burrow into the intestinal wall and lay dormant waiting for the proper conditions to emerge. During this encysted period, unlike adult parasites, small strongyle larvae are not susceptible to most dewormers.

If large numbers of small strongyles emerge from the intestinal wall simultaneously, severe damage to the intestinal lining may result. Colic and diarrhea may be seen. Other signs of small strongyle infestation include loss of condition, weight loss, poor coat condition and slowed growth.

Veterinarians diagnose strongyle infestation from microscopic observation of eggs in the faeces. Blood tests are often used to assess the seriousness of an infestation. 

ASCARIDS

Ascarids (large roundworms) infections are primarily a problem of young horses. They are primarily found in the small intestine.
The 6-to12-inch long worms can number in the hundreds in the horse’s small intestine and can adversely affect its nutrition. 

Infective eggs are taken in with contaminated feed and water.
The eggs hatch in the small intestine and the microscopic larvae penetrate the intestinal wall, enter the blood stream, and are carried to the liver and lungs within about a week.

From there, the young worms travel up the trachea to the mouth to be swallowed a second time. They mature in the intestine in 2 to 3 months, and then lay eggs that are passed in the feces and the cycle is repeated.
Female ascarids can lay up to 200,000 eggs per day.

While ascarids do not attach themselves to the intestinal wall as the strongyles, they do utilize a great deal of food, excrete toxic wastes, depress growth and development, cause digestive disturbances, and produce potbelly.

Colic, coughing and diarrhea are common clinical signs associated with ascarid infestation. In addition, ascarids may cause blockage of the intestine or migrate through the lungs causing pneumonia. Heavy numbers of larvae can cause severe inflammation and destruction in the liver and the lungs. 

The eggs pass out in the manure but do not hatch outside the host. They do embryonate and become infective in 10-14 days. The eggs are quite resistant, especially to drying and freezing, and can remain alive and infective for 5 years or longer. Heat is harmful to them and the hot, dry weather of summer or the heat generated in composting can destroy many. 

Larvae cause a summer cold syndrome and nasal discharge that may get better with antibiotics but return when antibiotics are discontinued.
If a heavy, severe infection is suspected, your veterinarian may suggest a de-wormer that kills the parasite slowly to give the foal's body a chance to eliminate the worms.

Piperazine, a fast ascarid kill product, should not be used in animals with a high ascarid load, which could result in an ascarid-impaction colic or even perforation of the small intestine. 

TAPEWORMS

Mites living in a horse pasture may consume tapeworm eggs from the faeces of infested horses. Grazing horses may then swallow the mites and become infested with tapeworms. 

Tapeworm infestation in horses can lead to varying degrees of colic. 

Because many deworming agents do not kill tapeworms, a specific product may need to be added to your deworming program. 

BOTS

Stomach bots are the larvae of the horse bot flies. There are more than one species and they differ primarily in the location on the horse, where the eggs are laid, and in the way in which the eggs hatch.

During late summer and early Autumn, adult botflies lay eggs on the hair of horses, particularly around the chest, forelegs, throat and nose.

Stimulated by the horse’s licking, the larvae hatch and enter the horse’s mouth, settling in the tissues of the gums, cheek and tongue. After a month, the larvae migrate and attach to the stomach lining causing irritation, interfering with digestion and obstructing the opening to the small intestine.
The larvae remain in the stomach for 8—10 months until they have completed their development. They detach themselves from the wall and are passed out in the manure and burrow into the ground, eventually maturing into adult flies and beginning the cycle again. 

The larvae pupate outside the host and the mature fly emerges in about one month. The flies mate and reproduce, thus completing the life cycle.

The female fly darts at the horse very quickly and repeatedly, attaching an egg to a hair each time. While the flies do not bite, they do annoy the horses, causing them to run or exhibit a restless condition.  Some horses get very upset by bot flys and can be dangerous in their attempts to run away from or kick at the fly.

Symptoms

Symptoms of worm parasitisms may be confused with symptoms of bacterial, viral, or other disease organisms - get your vet involved in the diagnosis and treatment of internal parasites. 
Especially in young animals the following symptoms may be observed when many worms are present: 

Unthriftiness. The colt grows slowly or older horses fail to maintain normal weight.

Rough coat. Hair appears to lie in an abnormal position, is variable in oiliness and seems to “stand up” even with relatively short coats during the summer.

Anemia. Membranes of the eyes, lips, nostrils, and tongue are a lighter pink than in uninfected horses.Anemia also may be caused by many bacterial or viral organisms as well as nutritional deficiencies.

Diarrhea. While this condition is highly variable it can be of diagnostic value in some horses at some stage of worm infection. However, it must be noted that some worms cause constipation for short periods in some horses.

Abnormal appetites. Horses may eat articles such as paper, matted hair, bark of trees and gnaw on wooden articles such as posts, trees, leather trappings. Sometimes this condition can be traced to mineral deficiencies, internal parasites or lack of total nutrients. 

“Potbellied” condition. Ascarid worms, with or without the presence of stronglid worms, frequently cause abnormally large abdominal girth, some muscle weakness and poor growth.

Prevention & Treatment 

Foals are born free of internal parasites, they get them from direct or indirect contact with older animals that are carrying the infections. 

All of the worm parasites discussed here use faeces or manure as the means of spreading the infections by contamination of feed and water supplies or the environment. Transfer stages of these worm parasites do not actively seek the host to complete the infection process. Instead, they rely on chance to be picked up and swallowed. Thus only a very small percentage actually complete this hazardous step in the life cycle. To compensate for this, large numbers of eggs are produced by the female worms to start the transfer process.

To reduce the chance of  your horse picking up parasites, rotate your pastures, clear them of manure and do not over-grazing paddocks.

Your vet can administer drugs via a stomach tube, pastes and pellets can be purchased from vets, feed stockists and saddleries and there are herbal remedies.

Which ever way you decide to control internal parasites in your horses it is important to always feed correct doses, at recommended intervals and ages.

Using Worm Pastes

When drenching with a paste, ensure the horse has an empty mouth or the paste will stick to any food in the mouth and the horse will spit it out.

Some drugs are not safe for young foals or mares in foal, others are dangerous if over-dosed, and in-effective if under-dosed.
Some drugs will work on some parasites but not others. investigate your options and discuss an effective parasite control plan with your vet.

Rotation of the type of worm treatment may be necessary, both to keep worm numbers down, and to prevent worms becoming resistant through the prolonged use of a worm paste which fails to remove all the worms.

Benzimidazole worm drenches have been very effective against most worms including the larval worm of the Strongylus species, particularly the ‘blood worm’ Strongylus vulgaris.
The introduction of Ivermectin and Doramectin ranges of drenches and pastes have shown a very effective action against all stages of all common horse worm cycles. They also have a prolonged action and horses may require drenching only every 12 – 14 weeks depending on the paste selected.

The following steps will increase your horses health and happiness and hopefully decrease your vet and feeding bills:

• Remove manure daily from stalls and weekly from pastures.

• Be sure pastures and paddocks are well-drained and not overpopulated.

• Compost manure rather than spreading it on fields where horses graze.

• Use a feeder for hay and grain and avoid ground feeding.

• Initiate effective fly control programs.

• Routinely examine horses for telltale signs of infestation.

• Establish a parasite prevention and monitoring program with your veterinarian.

Remember - internal parasite control is an ongoing job for horse owners.