Understanding Parasite Resistance

Understanding Parasite Resistance


Resistance is the ability of worms in a population to survive treatment by a particular deworming chemical. Resistance grows when parasites survive treatment, and then pass the ability to survive on to their offspring.

Factors that influence resistance growth1:

          • Under dosing.

          • Deworming frequency.

          • Using dewormers that do not kill resistant parasites.

Resistance can be reduced by:

          • Use the proper dose and ensure the horse ingests the full dose.

          • Deworm only when needed.

          • Use dewormers that are effective. Studies have shown certain deworming classes, e.g. benzimidazoles have a high rate of resistance2.
 
80% of horses in a recent study3 whose weights were set visually were underestimated.  Optimally, weigh your horse.  This can be done when trailering, weigh your vehicle with your horse and without (scales can be found at feed mills, truck stops etc).  If a scale is unavailable, weight tapes use a combination of a horse’s girth and height to estimate weight.  Use care, weight tapes can be inaccurate for foals, miniatures and heavily muscled and high withered horses.

Know how many pounds the dewormer treats.  This varies by brand, by as much as 20%.  Round up to the nearest measurement and securely lock the syringe mechanism in place for that dosage.

Be sure once administered the horse ingests the full dose.  This can be aided by raising the horses head and stroking under their jaw.  Apple flavored dewormers can increase the ease of dosing, this can help reduce spit outs and other activities that prevent a full dose from being ingested.

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[1]  Sangster NC. Pharmacology of anthelmintics resistance in cyathostomes: will it occur with the avermectin/milbemycins? Vet. Parasitol. 1999; 85: 189-204.
[2]  Kaplan RM. Anthelmintic resistance in nematodes of horses. Vet. Res. 2002; 33: 491-507. 
[3]  Asquith, R. Johnson, E. Kivipelto, J. and Depew, C. (1990). Erroneous weight estimation of horses. Proceedings of the annual convention of the American 
       Association of equine practitioners. 599-607.