Resistance is real. Equine parasite resistance has been reported in 21 countries, including the United States, and no new deworming drugs are currently in development. Keeping the effectiveness of the drugs we have for as long as possible is therefore very important. Some European countries are so concerned about parasite resistance, in fact, that dewormers are once again only available through a veterinarian or by prescription.
Preventing transmission is key. Delaying parasite resistance and preventing clinical disease due to parasites are our primary goals. Today’s deworming program should focus on preventing pasture contamination and transmission of infective larvae in large numbers.
Identify high shedders. Conventional deworming strategies over-treat many horses, but under-treat high shedders. High-shedding horses have less natural immunity to internal parasites than others. These horses shed many more eggs than those with higher immunity (known as low shedders) because between deworming treatments, their body allows more adult worms to survive and produce eggs. While high shedders only account for 20% of the horse population, they produce 80% of the parasite eggs on the pasture. High shedders need to be dewormed more often than their low shedding herdmates to prevent contamination of the pasture with large numbers of parasite eggs. Low shedding horses may only require deworming twice a year, depending on their management situation. A simple test called a Fecal Egg Count will identify high shedding vs. low shedding horses. This test is best performed at least 3 months after the last deworming treatment. In our climate, spring is a convenient time to perform the test because most horses are dewormed less in the winter.
Check for resistance. Small strongyle resistance to the benzimidazole (Panacur, Anthelcide) and pyrantel pamoate (Strongid) dewormer classes and ascarid resistance to macrocyclic lactone (Ivermectin, Quest) dewormer class have been reported in the United States. With this in mind, it is important to check for resistance periodically in your herd. A Fecal Egg Count Reduction Test is the preferred method for testing for resistance. This test is basically two Fecal Egg Counts; one done before deworming and another two weeks after deworming.
A few worms are OK. The goal of previous deworming strategies was to eliminate all parasites from the horse. In an effort to delay widespread resistance, however, today’s deworming strategies should be focused on reducing transmission of parasites, keeping parasite burdens below harmful levels, and treating clinically affected horses. In this manner, a population of susceptible parasites is maintained to dilute the resistant genes in the population and thereby keeping our deworming medications effective as long as possible.
Young horses require special consideration.
The primary parasite species of concern in young horses is the ascarid, or round worm. Most horses develop complete immunity to this parasite by the time they are 18 months old, but it causes significant damage in the meantime if left unaddressed. Unfortunately, ascarids have developed a high prevalence of ivermectin/moxidectin resistance on some breeding farms. Since resistance to the benzimidazoles has been seen in small strongyles, a species of concern in both young and adult horses, treatment with both classes of dewormers is sometimes necessary in young horses. For this reason, Fecal Egg Count Reduction Tests to check for resistance in foals and young horses are extremely important.
Deworming Recommendations
Adults
High Shedders: April 1: Fecal Egg Count, treat with Ivermectin June 1: Treat with Quest Sept 1: Fecal Egg Count, treat with oxibendazole (Anthelcide), need Fecal Egg Count
Reduction Test the first year to test for resistance October or November, after a hard frost: treat with Ivermectin with Praziquantel
Low Shedders in High Density situation: April 1: Fecal Egg Count, treat with Ivermectin June 1: Treat with Quest Sept 1: Fecal Egg Count, treat if egg count is high October or November, after a hard frost: treat with Ivermectin with Praziquantel
Low Shedders in Low Density situation: April 1: Fecal Egg Count, treat with Ivermectin Sept 1: Fecal Egg Count, treat if egg count is high October or November, after a hard frost: treat with Quest Plus
Foals
Fecal Egg Count Reduction Tests (FECRT) should be performed with all deworming medications used in foals to detect resistance common in ascarids and small strongyles. If no resistance is detected, adhere to the following plan. If resistance is detected, contact your veterinarian for advice. Once resistance pattern on farm is established, FECRT may only need to be done every few years.
2 months of age: Pyrantel Pamoate (Strongid) treatment (too young for FECRT) 4 months of age: FECRT in conjunction with Ivermectin treatment. 6 months of age: FECRT in conjunction with Pyrantel Pamoate (Strongid) treatment 8 months of age: FECRT in conjunction with Oxibendazole (Anthelcide) treatment 10 months of age: Ivermectin treatment 12 months of age: Start on Adult program, but monitor Fecal Egg Counts to detect Ascarid infection due to late-developing immunity
Identify high shedders. Conventional deworming strategies over-treat many horses, but under-treat high shedders. High-shedding horses have less natural immunity to internal parasites than others. These horses shed many more eggs than those with higher immunity (known as low shedders) because between deworming treatments, their body allows more adult worms to survive and produce eggs. While high shedders only account for 20% of the horse population, they produce 80% of the parasite eggs on the pasture. High shedders need to be dewormed more often than their low shedding herdmates to prevent contamination of the pasture with large numbers of parasite eggs. Low shedding horses may only require deworming twice a year, depending on their management situation. A simple test called a Fecal Egg Count will identify high shedding vs. low shedding horses. This test is best performed at least 3 months after the last deworming treatment. In our climate, spring is a convenient time to perform the test because most horses are dewormed less in the winter. 
