Different horses have different deworming needs. The best treatment for any animal starts with a fecal egg count to determine if the horse is a high shedder, medium shedder or low shedder of eggs. This measurement helps determine how often a horse needs deworming and supports herd decisions.
Source: Horse Network, February 6, 2017.
For many years, horses were treated with a “one-size-fits-all” worming approach, rotating between a little of this every couple months, followed by a little of that, then back to this and one more shot of that, to fend off all types of harmful parasites. This worked well enough until the parasites began developing resistance to standard treatments, because that’s what parasites do.