If you are only using ivermectin compounds when deworming your horses, you may not be doing a good job. Ivermectin is an effective broad spectrum dewormer. In fact, many horse farms have used ivermectin as their only dewormer for years. However, there have been some reports that equine roundworms may be developing resistance to ivermectin. Because a large number of horses move from farm to farm for breeding or training, resistant worms can be spread to other farms. Roundworms cause the most signs in young foals. The larvae migrate through the lungs, and the adult worms cause impactions in the intestinal tract. These young foals may have a cough and/or nasal discharge, and they may colic. Some may just appear unthrifty.

To determine if ivermectin resistant roundworms have developed, parasitologists at Texas A&M performed a study on foals born in 2004 and 2005 on a quarter horse farm in Texas. This farm had used only ivermectin as a dewormer for the last 20 years. Results indicated ivermectin was not effective in treating roundworms until the foals were 8 months old. Another dewormer, pyrantel pamoate, was also used in a group of these foals beginning in 2002. You may recognize the drug pyrantel by the trade name Strongid. At regular doses, pyrantel was also ineffective in killing roundworms but was effective when a double dose was used. The bottom line is you can not only use ivermectin to treat your horses for parasites, but may need to use a combination of drugs. Also, you should have your vet perform fecal egg counts to determine which dewormers are effective on your farm. Horses of different ages require different dewormers.