There are several major internal parasites (worms) of horses, and control is aimed at reducing their numbers at certain stages in their life cycles. These stages will vary with the seasons, which, in turn, can vary with the geography and climate where you live. A seasonal approach to deworming is always more effective than routine intervals because parasites must return to the environment between successive generations.

Some horses develop natural resistance. While one horse has egg counts that aren’t problematic, another horse in the same pasture, same conditions, same age, may have a tremendous number of worms. Just deworm the wormy individuals. With resistance issues we now face, we should stop treating horses so frequently and only focus on treatments that are absolutely necessary.