Although all types of worms in any species of animal are problematic, few are as potentially deadly as the strongylus vulgaris or large strongyles. Not only do the worms and larva themselves cause damage but they can also trigger blood clots and serious internal bleeding that can lead to death over a very short period of time.

To understand the dangers of strongylus vulgaris it is important to take a closer look at their life cycle. Eggs of the adult worms are shed from the adult worms living in the horse's cecum or large intestine. These eggs are passed out of the horse's body with the fecal material, where they come in contact with the air and soil and grass of the pasture. In approximately three days in the warmer, spring, fall and summer weather the eggs hatch into tiny larvae. The larvae move to grass and vegetation and remain there until they are eaten by the unsuspecting horse. Once inside the horse's body the sheath or protective coating of the larvae comes off, allowing the very tiny larvae to move through the walls of the digestive system and into the blood stream. Once in the blood the larvae of the large strongyles travel to the main artery of the digestive system, known as the mesenteric artery. Most larva will continue to feed and grow in the mesenteric artery for three to four months, then they will migrate through the blood back to the large intestine and cecum where they will dig into the lining of the digestive tract and mature. This maturing process can take between six to eight months, at which time the now adult worms start shedding eggs to continue the cycle.

The danger for the horse from untreated infections with strongylus vulgaris is largely due to damage to the blood vessels and arteries where the larva are present. These larva actually burrow through the lining of the arteries, leaving rough areas that can trap blood cells and cause blood clots that can break off and travel through the rest of the circulatory system, leading to heart problems and blockages. Since the worse damage will be in the mesenteric artery, these blockages will prevent blood flow to the digestive system, resulting in poor health and poor cell metabolism. The weakened wall of the artery is also more prone to rupture, a condition that is always fatal for the horse.

The adult strongyles in the large intestine and cecum will literally tear chunks from the lining of the digestive tract. This will lead to mild to severe internal bleeding, noticeable blood in the fecal material, diarrhea, colic and anemia. Any of these conditions alone or in combination with other health problems can cause death.

Treatment for strongyle vulgaris is simple and easy. Worming pastes that come in pre-measured dosage are given to the horse typically every two months and most are safe for pregnant mares and foals. In areas where there is no winter freeze worming schedules may be slightly different. Carefully read and follow all worming medication recommendations and never combine worming products unless advised to do so by a vet.