The general rationale has been that all parasites are terrible and horse owners must do everything that they can to ensure horses are completely parasite-free at all times, writes David Ramey, DVM. Even with anthelmintics, ‘parasite-free’ is for all practical matters an oxymoron.
The most important thing for you to do is to test your horses and monitor them. Horse owners need to develop effective parasite control programs and not just deworm willy-nilly.” – David Ramey, DVM
Source: The Horse Network, May 21, 2021. Link. No matter how many times you deworm your horse, and no matter what product you use, it’s not going to make a horse completely parasite free.