Large strongyle larvae penetrate the large intestinal wall and migrate through different abdominal organs. There are 4 species of large strongyles, Strongyles vulgaris, Strongylus edntatus, Strongylus equinus, and Triodontophorus. Until around 1980 Strongylus vulgaris was the most harmful worm in horses. The larvae migrated through the abdominal organs and into the main artery that supplies the intestine known as the cranial mesenteric artery. Where this artery comes off the aorta these larvae seemed to gather. They caused considerable irritation to this area with the formation of blood clots and degeneration of the artery by disrupting the lining and weakening the artery wall. This occasionally caused an aneurysm of varying size to form, which allowed serum to leak from the artery and even occasionally was said to lead to rupture of the artery and sudden death of the horse. The blood clots that formed often were said to break off and travel down the artery obstructing the smaller braches of the artery thereby restricting the blood supply to a section intestine. Without proper blood supply the section of the intestine did not have the normal peristaltic movement and often the horse developed colic. This colic was named Thrombo-embolic colic, one of the few large terms I still remember from vet school. After this the larva travel back to the intestine and become adults and continue to cause the horse problems by attaching to the intestinal wall and sucking blood causing the horse to become anemic. This characteristic plus the fact the larvae travel in the blood stream gave this worm the name Blood Worms. When I first got out of vet school I dewormed a small herd of horses for a client in December. It snowed about 3 inches that night and the client called the next morning in a panic and claimed I had poisoned his horses. I rushed back to the farm to find that each pile of manure the horses had passed in the snow had very fine steaks of very bright red leading away from the pile a few inches to a foot. When examined carefully each streak had a dead worm at the end of it. The streak was due to the passage of the blood the worm had passed as it crawled away from the pile of manure. When Ivermectin came on the market around 1980 it was very effective against Strongylus vulgaris because it was absorbed from the intestine and would kill the larvae that were migrating through the horse. It was thought that the use Ivermectin would eventually eliminate worms; however the worms had a different idea and are still a threat to horses. Strongylus vulgaris is not nearly the threat it once was but still can be found if searched for. The deworming routine that was started back in the 70s and 80s may now be responsible for the development of the resistance to deworming medication we are seeing in worms to today.
The other large strongyles are less of a problem but can cause some significant harm if in large enough numbers. They have not been nearly as harmful as Strongylus vulgaris. Triodotophorus is unique in that is fits into the large strongyle size category but it does not migrate in its larval stages but becomes encysted as small strongyles do.
Between 30%-33% of the samples examined by Horsemen’s Laboratory are positive for worm eggs. 95% of the positive samples are positive for strongyle eggs. Research tells us likely only 5% of those eggs are from large strongyles. Most strongyle eggs appear very similar under a microscope. The eggs must hatch and the larvae must be examined to tell which species of strongyles are present. Most often it is a mixture of several different species.
The other large strongyles are less of a problem but can cause some significant harm if in large enough numbers. They have not been nearly as harmful as Strongylus vulgaris. Triodotophorus is unique in that is fits into the large strongyle size category but it does not migrate in its larval stages but becomes encysted as small strongyles do.
Between 30%-33% of the samples examined by Horsemen’s Laboratory are positive for worm eggs. 95% of the positive samples are positive for strongyle eggs. Research tells us likely only 5% of those eggs are from large strongyles. Most strongyle eggs appear very similar under a microscope. The eggs must hatch and the larvae must be examined to tell which species of strongyles are present. Most often it is a mixture of several different species.
