Your horse's age and health can determine which wormers you should use. You also must consider your location and your horse's pasture situation---for example, how many horses are grouped together. Three types of worming programs are available.
Oral Broad-Spectrum Worming
These products are designed to be given orally to your horse, usually in paste form, and kill a wide variety of parasites in different stages of their life cycles. If you decide to use this option, you should worm your horse three times a year, beginning in the spring. For example, you could worm your horse in April, then in August, and again in December. Ivermectin is the key ingredient used in most broad-spectrum wormers. This is a simple program to use, and can be effective for healthy horses in low-risk situations. A horse that does not graze with a large number of horses each day, and whose pastures are rotated regularly and kept clean would be a candidate for this treatment.
This program will use different forms of oral wormers, which are then given to the horse in intervals throughout the year. It is effective in treating multiple types of worms, and can target certain worms---such as tapeworms---in order to make sure they are taken care of. This method also discourages the parasites from building up an immunity to any one type of wormer, and is effective at keeping all types of worms at bay.
If you decide to use this method, you can cycle two or three types of wormers---either an Ivermectin-based wormer with a pyrantel pamoate-based wormer, or those two with a fenbendazole-based product added. Your horse will get wormed every other month. For example, you may give it the Ivermectin-based product in April, August and December, and then give it the pyrantel pamoate-based product in February, June and October. If you decide to use all three wormers, you may give it the Ivermectin-based wormer in April and October, the pyrantel-pamoate-based wormer in February and August, and the fenbendazole-based wormer in June and December.
You may also use a different type of wormer---Fenbendazole-based, Ivermectin-based, Pyrantel Pamoate-based, oxibendazole-based, and moxidectin-based---and alternate them throughout the year, using a different type of wormer every other month.
You could feed your horse a daily wormer. Simply add it to its daily feed. This type of wormer usually comes in pellet form, and is an easy way to keep up with parasite control. However, you should still give your horse an oral broad-spectrum wormer twice a year to ensure that all parasites are being taken care of.
If you believe your horse is already infected with worms, talk to your veterinarian before beginning a worming regimen. This is important because ridding your horse's system of too many worms at one time can be harmful to the animal. Do not use a moxidectin-based wormer on horses that are either young or weak; an overdose can be very harmful to them.
Talk to your veterinarian and, if you board your horse, the stable owner or manager before choosing a worming program.