Parascaris equorum, aka ascarids or roundworms, are a dangerous parasite of foals. Most healthy adults have a strong immunity, but this can be lost when the animal is ill or injured and as the horse ages. Since the infective eggs of this parasite are resistant to environmental factors, horses are exposed for years after a property is contaminated.
Studies have confirmed there are now strains of this parasite resistant to one of our hardest-hitting dewormers, ivermectin and its chemical cousin, abamectin. Fortunately, the ivermectin strains can be eliminated by double-dose pyrantel pamoate (Strongid), 13.2 mg/kg of bodyweight. Double-dose pyrantel pamoate is also effective against tapeworms.
Ivermectin remains an excellent dewormer choice for horses of all ages, and we don’t know yet how widespread these resistant roundworm strains may be. If ivermectin is still working well for you, don’t change it unless you’re seeing signs of parasitism (distended abdomen, poor hair coat, poor growth in young animals, or weight loss) despite adequate treatments. If you are, it would be wise to have a fecal checked to determine if ivermectin-resistant roundworms are the problem. Alternatively, try a deworming with double-dose pyrantel pamoate. Adult roundworms are usually expelled intact. This large parasite can be seen in manure.