Roundworms are the most significant internal parasites of your horse. Of the various roundworms that can infect your horse, Strongyles are the most significant. In the past, Large Strongyles (Bloodworms) were of primary concern but the more recent deworming products containing ivermectin or moxidectin have almost eliminated this parasite in horses that receive either of these products routinely. Small Strongyles are now the most significant parasite of concern and the main focus of parasite control programs. As Strongyles can only infect your horse while grazing on pasture, their control is only necessary in horses that are on pasture and in northern climates, only during the grazing seasons. Other roundworms of lesser concern are Ascarids (Large Roundworms) and Pinworms. These parasites are of greatest concern in foals and young horses, roughly up to three years of age. Both are effectively controlled by the products one would use to control Strongyles so are covered in routinely recommended parasite control programs.
Bots are also a common parasite of the horse but are of less significance then the roundworms. The Bot Fly lays its eggs on the front legs and chest of the horse and the horse ingests the eggs while grooming. The eggs hatch inside the horse and the larvae attach to the wall of the stomach where they overwinter. Treating a horse with the appropriate product in late fall will effectively control these parasites.
Tapeworms are the last on our list of significant horse parasites. They can also cause significant internal damage and the severity of the disease created is linked to the intensity of the infection. These parasites are also transmitted on pasture so the best time to treat your horse is late fall, at the end of the grazing season. As the products used for roundworm control do not affect tapeworms, specific products must be used for their control.