Originally Posted by Ryle
Over deworming will most certainly build resistance. No dewormer kills 100% of the parasites in their different life stages in the horse so with every dose of dewormer you have some parasites who are not killed but have been exposed. This is one of the reasons that resistance develops.
While it is true that when deworming a horse you do not kill ever parasite. It is also true that each one that remains has been exposed to the Dewormer. However most people under medicate their horse. This is more detrimental to parasite resistance then over medicating. By give 2 instead of one dewormers what happens is that you get more of the worms so there are less left to become resistant. Lets say you have 100 parasites (I know there would be a lot more but that is an easy number for people to wrap their heads around) you deworm with only one dewormer. You kill 75 of the 100 parasites (75%). You now have 25 that have been exposed but not killed. Now take that same 100. You kill 75 with the first dose so now you give a second dose normally done the next day. Of the remaining 25 left you kill 20 of them. Now you only have 5 that have been exposed and not killed. Most vets will tell you if you are going to hit the horse hard to repeat this for 3-5 days. Also known as a power pack. Most vets will tell you to do this at least one time a year.
So which are you better off with? 25 worms left from a single dose or 5?? Both numbers have been exposed to the dewormer. Both number b/c of that can become resistant. Which would you rather have 25 resistant worms or 5?? This is also why you test. So you are only deworming for the worms that are present and for the number you have. If the number is not high enough you are better off no deworming and letting it go a bit longer.
This info has come from OSU and several other vets from Ky where the resistance in worms is the biggest area with a problem at this time.