to ascarids, and with fenbendazole, oxibendazole, and pyrantel to small strongyles. Development of new drugs takes many years, leading to the concern that worms could out-evolve the drugs currently available to treat them. As a result, most veterinarians now recommend deworming for small strongyles based on fecal egg counts to minimize the development of resistant parasite populations. Fecal egg count reduction tests can also be performed to identify which dewormers are effective on a particular farm. If a horse is heavily infested with parasites, dewormers must be given carefully. Small strongyles can form cysts embedded in the intestinal epithelium. A decrease in the active population of worms, as in the case of deworming, can cause larvae to emerge from the cysts (larval cyathostomiasis). Additionally, foals with a large load of ivermectin-susceptible ascarids in the small intestine may experience intestinal blockage or rupture after deworming. Thus, in heavily-infested animals, a veterinarian may recommend worming with a mild class of drugs, such as fenbendazole or a low-dose daily wormer for the first month or so, followed by periodic purge wormer treatments. Types of parasites found in equines Ascarids, also known as roundworms Pinworms, sometimes known as seatworms Tapeworms Strongyles – large and small, sometimes known as Redworm. Bots – fly larvae – bot eggs are laid on a horse's coat, and when accidentally ingested through the horse licking its coat, the larvae hatch in the tongue, migrate down the esophagus and mature in the stomach. Ringworm in horses is not actually a worm but a contagious fungal skin disease and is normally treated using an anti-fungal wash. There are several different brands of dewormer, using different types of active chemicals – which in turn kill different types of parasites. It is sometimes necessary to use a specific wormer at a certain time of year, depending on the life cycle of the parasites involved. In the past, horse owners rotated dewormers during the year, using different brands or formulations with different active chemicals, to combat drug-resistant parasites. However, this approach does not appear to prevent drug resistance, and many veterinarians now recommend individualized deworming plans dependent upon the horse's age and egg shedding status. Active chemicals found in different wormers The medications piperazine and thiabendazole are no longer commonly used as equine wormers; they have been replaced by the above drugs. Dental care A horse's teeth grow continuously throughout its life and can