Warning!
Parasite Resistance is on the Rise
By Kelsey Johns
The days of deworming routinely – every time your horse is trimmed, monthly, every six to eight weeks, whenever you remember, or when the other horses are dewormed – are over.
Why We Have Resistance
Resistance problems have been on the rise for the past 20 years and researchers believe this is due to the widespread misuse of common dewormers. The more frequently a horse is treated, the greater the potential for survival of the parasites that have become resistant. Deworming is not a “one size fits all program.” Twenty percent of the herd does not represent the entire population. Therefore, every horse should be treated individually.
Fecal Egg Counts
A fecal egg count test is a microscopic examination of parasites in fresh manure. Performing a fecal egg count (FEC) prior to deworming allows for deworming to be performed at a more knowledgeable level. FEC’s are effective in determining the number of eggs per gram of manure and are also required to determine if the horse is carrying a low, moderate, or high worm burden. To process an accurate FEC, a McMasters egg counting chamber is required. First, fresh fecals must be collected from each horse you wish to test. Collect the fecals about eight weeks after you last dewormed to see if the horse is dangerously infested with internal parasites. Collect another fecal a minimum of six to eight days after you last deworm. This test will determine if the dewormer used was effective in controlling the parasites (if egg count was reduced by 85-90%), or if resistance to that dewormer is developing.
Classes of Dewormers
Contrary to popular belief, there are technically only three major classes of dewormers. There is the Avermectin class, which includes ivermectin and moxidectin. This drug has been proven effective with small strongyles and bots. The Pyrantel class is effective against redworms, bloodworms, and roundworms. The most popular dewormers in this class are Strongid and Exodus. Fenbendazoles are the final class represented by Safe- Guard, Panacur and Anthelcide. This is the class used for Larvicidal dewormers, and have been proven effective against redworms, pinworms, and roundworms. Additionally, Praziquantel has been combined with the avermectins and is available as Zimecterin Gold, Equimax or Quest plus. Praziquantel is very effective against tapeworms. These drugs are very strong, therefore, should only be used on a correct rotation schedule as determined by a fecal egg count. In the average horse, Praziquantel should only be used once a year when the tapeworm is most suceptible. These drugs should only be used if parasites are detected in harmful levels. These classes of dewormers were very effective and strong for many years. There are no new dewormers on the horizon for at least the next twenty years making it even more important to perform fecal egg counts on a more regular basis and only deworming when absolutely necessary. We are all the stewards for care of our future generations of horses. We should now carefully, judiciously, and thoughtfully plan our deworming program correctly. Please involve your veterinarian in your new deworming plan.
