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Making sure that your horse is dewormed correctly is essential to its overall health and for preventing diseases that result from severe parasitic infection. While the conventional wisdom is to deworm horses every 6-8 weeks, the understanding of the veterinary community regarding deworming has recently changed.


The problem with conventional deworming tactics is that parasites have begun to develop a resistance to anthelmintics (the drugs used in the deworming process). There is currently no known alternative to these treatments, so if they stop working entirely, there will be no way to effectively kill the parasites that can negatively affect horses. Using anthelmintics frequently, as was previously recommended, increases the likelihood of parasites developing resistance to them.


Instead of using these drugs liberally, veterinarians now recommend tailoring deworming treatments to the needs of your individual horse(s). Horses having a low parasitic load is natural and will not hurt them, so the focus should be on decreasing parasites in those horses with large numbers of parasites, called “high shedders.” You can determine the number of parasites in a horse by taking fecal samples for testing. High shedders will probably need more treatments than other horses; consult your veterinarian for the appropriate measures. 


Other horses should be treated for parasites bi-annually unless they are young or have a preexisting condition and depresses their immune system. The purpose of spacing the treatments out is to increase the number of weak parasites– called Refugia– in the population. Parasites gain resistance to anthelmintics by surviving deworming treatments; treating more frequently creates more opportunities for these resistant strains to occur. Refugia can be killed easily by anthelmintics; not introducing powerful drugs with regularity will result in parasite populations that can be easily eradicated, as they have no defense against the treatments.


The proper deworming procedure is an important preventative measure for ensuring that your horse(s) remain healthy. Always consult with your veterinarian to determine the optimal deworming regimen for each horse and to make sure you don’t over-medicate and risk creating anthelmintic resistant parasites. Following these new procedures will keep your horse healthy and prevent future infections