How quickly do paste wormers work to eliminate worms and bots? How can I be sure that worms have been eliminated in my horse?

Different dewormers act in different ways. Some kill the parasites quickly, while others kill more slowly. Also, some dewormers control both adult and immature parasites, while others only control parasites in the intestinal tract. Only the avermectins (ivermectin and moxidectin) control bots.
To determine if the product used is working, have a fecal flotation exam done before deworming and again about 14 days after deworming. There should be at least an 80 to 90% drop in egg count. The reason for the time between deworming and the second fecal exam is to allow time for the parasites to be killed and passed out in the feces. Since resistance is becoming more common, fecal exams are recommended for all farms at least once a year.
There is no good test for bots. A fecal count does not work for bots since bots are immature parasites and do not mature/hatch until being passed in the feces. A decrease in bot flies demonstrates an effective bot control program.
When deworming, make sure to give enough product to cover the horse’s weight. It is a good practice to weigh the horse either with a scale or weight tape. The weight tape can be off slightly, so it is never a bad idea to add 200 lb to the weight tape weight to cover the error. This method helps deliver the proper amount of dewormer so that internal parasites will be killed.