Horses may only need to be dewormed two to three times per year. The active ingredient in commercial dewormers need to be rotated to prevent resistance. The best approach for worm control in horses is to utilize fecal egg counts (FEC) to determine which horses merit treatment, and rotate products with different active ingredients.
Generally, most horse owners deworm their horses every eight to ten weeks based on the old recommendations. This practice has led to the development of drug resistance in worm (nematode) populations. We have a limited number of dewormer products to use. To keep these dewormers effective, we must employ new strategies to prevent resistance.
In general, 20% of the horse population harbors 80% of the worm population. The other 80% of our horses may only need deworming twice per year. Not only will this help prevent resistance, but also save you money.
New recommendations for adult horses call for 3 fecal egg counts per year and deworming based on the results of these tests. A detailed suggested worm control program can be found at: http://animal.ifas.ufl.edu/extension/equine/documents/2009equineinstit/kaplanwormcon.pdf