Targeted Deworming Strategies for Weanlings and Yearlings

University of Kentucky held the third session of its UK Equine Research Showcase Feb. 2. The virtual research session included three expert talks with information about causes of mortality, vaccination immunology and parasitology in young horses. Presenting sponsors for the event included BET, Kentucky Performance Products, McCauley’s, Merck, Rood & Riddle Equine Hospital and Tribute Equine Nutrition.

Martin Nielsen, DVM, PhD, Dipl. ACVM, Schlaikjer professor of Equine Infectious Disease and associate professor at the Gluck Equine Research Center, concluded the program with a presentation about parasitology relating to young horses, specifically weanlings and yearlings.

During the presentation, Nielsen discussed drug resistance in several species of parasites, including ascarids, small strongyles and large strongyles. Understanding the types of parasites present and what anthelmintic drugs are best for treating them is key to proper deworming. He also spoke about the types of parasites that are present in young horses at certain times of their life and times of the year.

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Nielsen began by sharing information about a herd of horses on UK’s farm, a herd that is part of the university’s “historic herd,” a group of horses established in 1979 by the late Harold Drudge, PhD, and the late Gene Lyons, PhD. This herd has not been dewormed since it was created. Studying the parasite loads in this group has led to significant research understanding and developments.

“We have foals hitting the ground every year, so we move through this cycle of parasites that a horse encounters on its path through life,” Nielsen said regarding the importance of the research herd.

“There are a lot of drug-resistant worms out there; and that should not be a surprise to you,” Nielsen said.

Nielsen reviewed a few common parasites and the frequency of the resistance to common anthelmintic (antiparasitic) drugs. Ascarids (roundworms) are known to have a wide-spread resistance to both ivermectin and moxidectin, and small strongyles are starting to develop a resistance to these drugs. Ascarids have minimal to no resistance to benzimidazoles (fenbendazole and oxibendazole), and small strongyles are known to have a widespread resistance to this class of dewormer. Ascarids have minimal resistance to Pyrantel, while small strongyles have wide-spread resistance to this class. All dewormer classes are still effective for use against large strongyles (bloodworms).

According to Nielsen, intensive treatment for large strongyles is what led to widespread resistance to anthelmintic drugs amongst ascarids and small strongyles. Discussing a famous benzimidazole resistance study carried out by the legendary Dr. Gene Lyons, Nielsen shared that drug resistance remains even after several decades of withdrawal from the specific drug.

Nielsen also recapped the parasites present in foals and the typical timeline in which they are present. Threadworms (S. westeri) are highly present in the first months, followed by ascarids (Parascaris spp.), followed in the later months of the first year by small strongyles and tapeworms (Anoplocephala spp.).

“We can’t know by looking at a foal, what is going on inside that foal. It is really crucial that we have a good idea of what’s going on, and so testing is the way for it, there’s no other way to know this,” Nielsen said.

As foals move into their yearling stage, parasites are more often coordinated with the time of year. Eliminating and monitoring ascarids early on, typically January or February, prevents their frequency throughout the rest of the year. Strongyles will always be present, but this is less of a concern because they are only mild pathogens. Tapeworms are a seasonal parasite, where infections will occur typically early fall to late spring.

Nielsen recommends that deworming drugs being used should be tested for treatment efficacy. A simple fecal egg count reduction test can determine how effective the drug is. A fecal egg count reduction test should be performed on a minimum of six horses, at 14 days apart, to be completely accurate.

Fecal egg count amounts are relative to the type of parasite and the age of the horse. Where a strongyle egg count of 700 would be a moderate or normal amount for foals and yearlings, it would be considered a somewhat high count for an adult. Therefore, the threshold for parasite egg counts in younger horses is much higher than it is in adult horses.

“A higher egg count does not mean more worms,” Nielsen said in reference to the misconception that there is a correlation between amounts of eggs being shed and number of worms. He said that the purpose of a fecal egg count is to figure out what an individual horse is contributing to contaminating the pastures. Counts also lead us to make more informed decisions on what drugs to use for parasite control.

Nielsen then discussed his New Zealand study on deworming protocols for foals. One group of foals was heavily treated with anthelmintics every month while the other group was only dewormed twice prior to weaning. Amongst the 93 foals, there were no discernable health observations in either group. Additionally, weight gains were identical in the two groups.

“There were no differences in any health parameter between these foals despite the vastly, vastly different number of treatments that were administered,” Nielsen said. “In all, are we deworming too much? Yes, I think so.”