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In most of Europe and at least one of the Canadian provinces it is already necessary to get a vet to administer anthelmintics to horses because of resistance. Here's the problem: a commercial wormer, administered under perfect conditions, may eliminate 95-100% of worms. That low percentage that are not eliminated will reproduce, repopulate in the animal and then the new population is less susceptible to that medication than the earlier ones were. Here's another issue: studies regularly show that only a few individuals in a group of animals will be shedding most of the parasitic eggs. The implication is that most individuals in the group have some innate ability to fight off parasitic infection. Indeed, some poultry breeds have been shown to be more resistant to worm infection than others which suggests a genetic component to infection resistance. Indiscriminate and regular worming will eventually lead to worms that do not respond to anthelmintic treatments. The current recommendation by parasitologists is to determine which individuals in a group have the worms and target those individuals with the medication, rather than the entire group. Unfortunately, the costs with getting a fecal count on all of a flock of chickens would be cost prohibitive. If I suspected an individual bird or two of having worms, I'd have those tested before administering medication to the whole group.