Currently, the "small strongyle" group of parasitic worms represents the most important cause of gastrointestinal parasitism in North America. There are in excess of 100 different species of small strongyles that afflict horses. These miniature worms are so important, that today's modern parasite control programs and drug development efforts target small strongyles.
Some degree of small strongyle parasitism can be found in most adult horses. Unlike many of the parasites of the gastrointestinal tract of dogs and cats, the small strongyles actually cause damage by migrating inside the wall of the intestinal tract of horses. Horses acquire this type of parasite by eating pasture grass that has been contaminated by parasite larvae, which develop from eggs shed in horse manure. Pasture contamination by parasite eggs can be very severe and these eggs can survive for long periods of time on the pasture. Therefore the best way to reduce the chance of parasite problems is to combine methods that kill parasite eggs in conjunction with dewormers. Dewormers alone will not assure you that your horse will avoid parasite-related diseases.
Following ingestion of the infective larval-stage parasite at pasture, the parasite burrows into the wall of the horse's large intestine and elicits an inflammatory reaction. This inflammatory reaction, when sufficiently extensive, may interfere with the ability of the intestine to absorb nutrients and lead to colic and diarrhea. During its migration within the wall of the intestine, the parasites grow and mature.
The small strongyle parasite also has the option to enter a phase of dormancy in the intestinal wall at which time it is relatively resistant to deworming drugs.
These dormant parasites have one of the most sophisticated tricks found in nature. Not only are they resistant to drugs, but they can carefully and accurately time their awakening to coincide with optimal weather conditions for survival in the pasture. They predict the best weather for survival outside by sensing changes in the horse's circulating hormones, which change with the seasonal fluctuations in day length. This trick of nature prevents the death of eggs due to exposure to the elements, in particular very dry and warm conditions.
These parasites will also go into hibernation when the intestinal lining is already overcrowded with adult parasites. The hibernating stages, therefore, represent a reservoir of parasites that trickle down into the intestine when there is enough room, created by natural death of the adult worms, or deworming. Following adult deaths, many new mature larvae are able to "break out" into the intestinal lumen and reestablish a population of adult parasites and enable further egg production.
Clinical problems associated with the small strongyles range from the asymptomatic "carrier" to diarrhea, unthriftiness, poor growth, impaired performance and increased risk for colic.
There are only three different species of the large strongyles, the most important of which is Strongylus vulgaris. Before the advent of the new powerful deworming drugs, problems attributed to parasitism associated with Strongylus vulgaris were widespread. However, it is rare to see clinical problems due to this parasite in the modern age.
The large strongyles are recognized in the horse's manure by their red color (they contain blood); to this end, they are also commonly known as "blood worms" or "red worms." With an important single difference, the life cycle for Strongylus vulgaris is similar to that described for the small strongyles. However, instead of undergoing a period of arrested development in the lining of the large intestine, Strongylus vulgaris undergoes a migratory phase in the blood vessels that supply the intestinal tract. By migrating in the lining of the wall of this important artery, damage associated with Strongylus vulgaris includes interruption of the blood flow to the intestine.
Less commonly, larvae of large strongyles may be found in remote sites such as the brain, but, most typically, this aberrant migration is restricted to an intestinal artery and the lining of the intestinal wall. Most clinical problems associated with large strongyle parasitism include unthriftiness, diarrhea, poor growth, impaired performance, weight loss and increased risk for colic. Other problems may be reported if the parasite larvae invade other remote sites. Rarely, if parasite damage to the artery is sufficiently severe, death may occur as a result of internal hemorrhage into the abdomen.