Fighting the parasite problem

By Jennifer Kunz, AnimalForum.com staff

There are more than 150 internal parasites known to infect horses. Keeping your horse safe from all of them is a year-round job. If parasites are not controlled, the horse they infest can become unhealthy, will be prone to colic, and can suffer permanent damage to its intestinal structures and other internal organs. Carrying a heavy load of internal parasites also means a horse is getting only partial value from his food. A regular worming program as well as careful management of environment will assist horse owners in keeping their horses healthy.

The most significant parasites are ascarids (roundworms), strongyles (bloodworms), bots, threadworms, and pinworms.

Ascarids, or roundworms, are primarily a problem for foals and younger horses. Older horses usually don't have a problem with this parasite as they develop resistance to it as they mature. The life cycle of the ascarid begins when the horse swallows eggs located on the food or water it is ingesting. The eggs can remain viable for years in soil and can infect horses for years after the initial deposit. After the eggs hatch, the larvae burrow into the wall of the small intestine and then into the veins. The blood in the veins carries the larvae to the liver and heart, and then to the lungs where they move into the air sacs. They are then coughed up by the horse and reswallowed and deposited into the small intestine again, this time in larva rather than egg form.

They grow and mature, reaching lengths that range from 5 to 22 inches. If allowed to grow unchecked, they cause intestinal blockage that can result in death. In mild cases, the worms will compromise the horse's ability to digest its food. Once mature, the roundworms lay eggs that are passed out with manure. In two weeks, larvae form within the eggs. At this point they are infective and ready to start the whole cycle all over again. One female roundworm is capable of laying 200,000 eggs per day.

Signs of ascarid infection in foals or young horses include a rough coat, an unthrifty appearance, pot belly, depression, and in a number of cases, a cough and nasal discharge that don't respond to antibiotics.

Strongyles, or bloodworms, are perhaps the most common of internal horse parasites. They can infect horses of all ages except newborn foals and are found all over the world. There are three types of large and one type of small strongyle. Large strongyles infect the large intestine of the horse. The largest of the large burrow into small arteries in the gut wall and use them as pathways to the main blood supply artery of the digestive tract. Their migration causes disruption in blood flow by formation of blood clots in the artery. The eggs are usually found on vegetation rather than in manure, so transmission occurs mainly outdoors rather than in stalls and barns. The smaller two of the large type of strongyles do not cause as much damage as the large one. They migrate within the liver and are capable of causing damage to that organ in the process, then return to the large intestine.

Small strongyles are found in the cecum and colon and they do not migrate. Most of the small strongyles feed on the intestinal mucus and the larvae are capable of lying dormant in the intestinal wall for prolonged periods of time. Toward spring in temperate climates, there can be a mass emergence from the intestinal wall, causing the afflicted horse to suffer diarrhea that sometimes reaches the severe stage.

Stomach bots are the larvae of the bot fly. Female bot flies lay eggs on the horse's coat. The eggs are small and yellow and very sticky. When the horse licks the eggs, the larvae emerge and attach to the tongue and are swallowed. Eggs that aren't licked hatch eventually too and the larvae travel to the mouth on their own to be ingested. Once in the stomach, the larvae attach to the stomach wall and remain there for about nine weeks. They are passed through the horse's digestive system and expelled in manure where they will hatch into bot flies and start the cycle once again. The larvae do not cause serious damage to the stomach, but they do damage the mouth and gums. Bots are active from spring until the first killing frost in the fall so treatment is usually carried our once at mid-summer and then again after the frost. Bot eggs can be removed from the horse's coat with special bot scrapers, a warm damp cloth, or hair can be clipped.

Threadworms mainly infect young foals. They are ingested through the dam's milk or they can penetrate the foals skin if they exist on bedding. Once they have penetrated they will migrate through the lungs and the small intestine, causing damage. The life cycle of this parasite is short, only two weeks, but foals will be seriously ill during those two weeks. They quickly develop an immunity, but can suffer from severe diarrhea during the migration.

Pinworms have a relatively simple life cycle and are the least harmful of the major parasites. 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 and the life cycle begins again. The most serious damage caused by pinworms is irritation around the tail. This often causes the horse to rub his tail, sometimes until it is chafed and inflamed.

Most veterinarians recommend deworming a horse four to six times a year, or every eight to twelve weeks. Foals should be dosed every thirty to sixty days for the first year of their life. There are many brands and types of dewormer on the equine market and choosing the one that is right for your horse in his particular situation can be difficult at times. Dewormers can be purchased in paste, pellet, or gel form and administered by the horse owner quite easily. Weather conditions and climate do affect the needs of horses in a particular area and their parasite prevention program should take them into account. The best approach to selecting a dewormer or beginning a parasite control program is to consult your vet and discuss the various options and requirements for your area.

Managing your horse's environment is another crucial part of parasite control. Manure removal from stalls, paddocks, and even pastures will greatly assist in breaking parasite life cycles. If it isn't possible to remove manure completely from pastures, harrowing or raking them is another alternative. Harrowing should be done on hot, dry days as spreading manure while the weather is cool only spreads eggs around and infects a wider area of the field. Don't overcrowd paddocks or pastures with horses as the more horses present, the more manure produced and the greater the exposure to parasites. Feed bins, water buckets, automatic waterers, and hay racks should be regularly cleaned to prevent any sort of habitat developing that could harbor parasites. Horses should not be fed off the ground, this increases the likelihood of infection from parasites living on ground vegetation or manure piles.

A sound deworming and management program is the best way to keep your horse free from harmful parasites. Speak to your veterinarian about the options that will work best for you and your horse in your particular climate.