Intestinal parasites are a common problem in horses. Intestinal parasites can have a negative effect on horse's health, and deworming medications that have been used for many decades to kill intestinal parasites have done a really good job. In fact, the dewormers have really been too effective and have been used too often over the years, which has led to resistance. Many of the dewormers that were effective 30 years ago are basically ineffective at this time against small strongyles, which is the most important equine intestinal parasite in horses today.
When these dewormers first came on the market, the recommendation was to deworm all horses every 8 weeks. Unfortunately, this recommendation allowed some parasites to become resistant and using dewormers every 8 weeks only killed the susceptible parasites and allowed the resistant ones to increase. For this reason, the only deworming products that are effective at this time against small strongyles are ivermectin and moxidectin, or Quest. When these products were first released in the 80s and 90s, parasite eggs did not reappear in the feces after deworming for up to 13 weeks for ivermectin and up to 22 weeks for Quest. Now, due to horse owners continuing to deworm their horses with ineffective products and deworming too often, the reappearance of eggs after ivermectin has dropped from 13 weeks to 6 weeks and for Quest from 22 weeks to 8 weeks. For this reason, horses should not be dewormed unless their fecal samples are checked by your vet once a year. Instead of going to the feed store and buying a dewormer, have your vet check your horses and recommend which products to buy. Your veterinarian will also discuss when to use them to decrease future resistance of worms on your farm because no new dewormers are being developed.