How often should I deworm my horse and what should I use?

This question appears simple enough, but the answer is not.  Modern deworming products first became available about fifty years ago.  Most of the original products were suspensions with a volume so large they were administered by stomach tube twice each year.  Over the past twenty years newer products have become available in smaller volume paste formulations.  Over the years many of these products have become ineffective as parasites have developed resistance to these products.  Rotating deworming products was initiated as a strategy for dealing with resistant strains of parasites but this practice has become less successful as well.
Much of the resistance problem lies in the fact that the practice of regular administration of deworming products to horses whether they have parasites or not tends to select for resistant strains.  An analogy would be to periodically give an antibiotic injection just in case a disease is brewing.  Veterinary parasitologists are extremely concerned about parasite resistance since only two new deworming products have been introduced in the past twenty years and no others are on the horizon.  We are being encouraged to deworm strategically, or in other words, when they need it.
Fecal egg counts are the best way to determine the parasite burden of the horse.  One or two fecal balls from each horse are tested to determine how many parasite eggs are in the sample.  The cost of the test is slightly more than the cost of deworming but over time can be performed less frequently once a pattern has been established for your horse(s).

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