The Horse

APR 2019

The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.

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26 April 2019 The Horse | of modern dewormers, worm control isn't about eliminating parasites com- pletely, Nielsen says. That's proven to be impossible. "Horse worms such as cyathostomins (small strongyles, which primarily affect the equine large intestine) are ubiquitous and affect all grazing horses," he says. "But they only cause disease extremely rarely and only when infections reach extremely high levels." Frequent anthelmintic treatments are not needed to keep adult horses healthy. "What is needed," says Nielsen, "are prop- erly timed treatments with effective anthel- mintics administered at the appropriate time of the year, which correspond to the parasite life cycles and the levels of para- site egg shedding in individual horses." Counts Are Key Fecal egg counts (FEC) will help you check the efficacy of your dewormers, de- termine which of your adult horses shed the most strongyle eggs, and monitor ascarid presence in foals and yearlings. To conduct an FEC, collect a fresh manure sample from each horse to be tested. About three "nuggets" of manure will suffice. Label, date, and refrigerate the samples. Ideally, have them tested by a veterinarian or an animal health labora- tory within 24 hours and no later than seven days after collection. Seek advice from your veterinarian about collecting samples and where to send them. The FEC results enable you to catego- rize adult horses as low (0-200 eggs per gram [EPG]), medium (200-500 EPG), or heavy (>500 EPG) strongyle shedders. By identifying the horses that shed the most strongyle eggs in their feces and treating them with effective dewormers, you can effectively reduce strongyle egg- shedding in your pasture. "One study illustrated that if effective drugs are used, treating all adult horses exceeding an FEC of 200 EPG only leads to treating about 50% of the adult horse population, but still provides about 95% reduction of the overall strongyle egg shedding," Nielsen says. He points out that this applies only to adult horses and that owners must take a different approach when addressing parasites in foals and yearlings. "In foals and young horses (starting around 5 months of age), FECs show whether parasite burdens are primarily ascarids, strongyles, or both," he says. Nielsen says knowing the type of worm infecting foals is crucial because some drugs that work against ascarids don't against strongyles and vice versa. Know- ing what you're treating helps you choose the correct drug. He suggests treating all yearlings as high strongyle shedders. But Fecals Don't Show Everything Fecal egg counts do not distinguish large strongyle (the most dangerous but least common parasite in horses) eggs from small strongyle eggs. Equine para- sitologists recognize this as one of the limitations of FECs. Other constraints the American Association of Equine Practi- tioners (AAEP) lists include: ■ They do not accurately reflect the horse's total adult strongyle or ascarid burden; a higher egg count just means more eggs, not necessarily more worms; ■ They do not detect immature or larval stages of parasites, including migrating large strongyles and ascarids, and/or encysted cyathostomins (small stron- gyles embedded as cysts in the large intestine wall); ■ Standard fecal techniques often un- derestimate tapeworm infections, so a modified FEC, as well as serum and saliva tests, exist for detecting these parasites; and ■ They usually miss pinworm eggs, which adhere in packets around the anus rather than shedding in feces. Veterinarians can use several tech- niques to overcome these limitations—for example, they can use PCR and ELISA testing to detect bloodworms (large stron- gyles) and an ultrasound technique to quantify ascarid burdens, among others. Despite its limitations, the FEC is the only effective means of determining RISK & REALITY Use fecal egg counts to determine your horse's shedding status, as well as dewormer efficacy. LAURA PALAZZOLO Deworm only the horses you've identified as having high fecal egg counts, then conduct another FEC two weeks later in those animals to determine drug efficacy. SHELLEY PAULSON

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