As responsible livestock managers, we all know that the more time you spend taking care of your animals the better off they will be. But be careful translating logic like this into your worming practices – you might regret taking such good care of you herd!
Like people getting a booster on their flu shot, parasites can become immune to wormers – the more you worm, the more immune they get. Now you’re probably thinking: “How can they become immune if the wormer is killing them?” Well, it comes down to a numbers game.
In order for a wormer to be considered effective, it must reduce the fecal egg count of the treated animal by 90% - anything less means you’re building immunity. In almost any set of worms, there will be a few that are not affected by the wormer – it doesn’t matter how much you pump through, those worms are gonna stay alive. Wormer is designed to get rid of the other 90%. But if you kill all the worms that are not immune to the wormer (thereby eliminating the "refugia") then, as the parasite population rebuilds, it will be built only on the genetics of the parasites that survived because they were immune.
This is where things get tricky. In order to prevent the worm population from becoming immune to the wormer, you must allow the worms that can be killed to mate with the ones that cannot be killed. By doing this, you allow the genetics from the immune worms to be “corrupted” by the non-immune parasites. But if you worm the entire herd every month, the worm population will very quickly dwindle down until the only worms that are left cannot be affected by wormer – they’re immune.
So don’t worm unless you have to – and even then, remember that 80% of your worms come from 20% of your herd. If you worm the bottom 20%, it will do almost as much to cut back on the parasites as worming every animal on the place – but you’re less likely to build immunity.
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