Dr. Colleen Lewis / December 14, 2016
As a veterinarian, I am unable to look at your horse from over the fence and give you an idea of his worm burden. There are just not any reliable visual factors directly associated with adult worm numbers by looking or listening from the outside. Adult parasites, or worms that reside in your horse’s digestive tract replicate by laying eggs: sometimes thousands of eggs per day. Examining a gram of fresh fecal material allows us to quantify the number of eggs found; the fecal egg count is expressed as “per gram” of feces.
A fecal egg count will help to determine the worm burden which is simply the type and relative number of parasites affecting an animal. A solution is added to the feces to loosen and extract the eggs using a combination of floatation and centrifugation. The eggs are lighter than the resulting solution and float to the top, where they are affixed to a slide and placed onto a microscope for identification and quantity. This process can be used for a group of horses, called pooled samples, or for an individual horse.
Example case #1: The first fecal test reveals roundworms and small strongyles. Once you have determined the worm burden, select a specific dewormer (anthelmintic) for treatment. Two weeks after treatment, repeat the egg counts to determine if the product worked. Case #1 results: The second test is clean. Consider that the appropriate product was used and the parasites were susceptible and killed off. The process of running the two tests sequentially is called a fecal egg count reduction test (FECRT). The FECRT can be very helpful in determining if your deworming program is working or if parasite resistance is present. Resistance occurs when a parasite is not killed by a dewormer and the adult worms left behind will continue to cause gastrointestinal damage. The parasites will also continue to lay their eggs and reproduce offspring.
Example case #2: The first fecal test reveals roundworms and large strongyles. After deworming with a product labeled for both roundworms and large strongyles, the second fecal examination reveals that 75% of the large strongyles are still present. The results are indicating resistance with the product used on large strongyles. Another class of dewormer needs to be selected and other environmental prevention techniques can be used to reduce parasite numbers. It is important to work with your veterinarian to ensure that fecal collection, handling and testing are done properly, to generate informed, deworming protocols for your horses.
Unfortunately, not all equine parasites are readily identified via a fecal egg count. The bot lays its eggs on the outside of the horse, attaching to the hairs of the legs, chest and neck. Tapeworms lay their eggs in packets (proglottids) that do not float as well as other equine parasite eggs. Your anthelminthic program should seasonally account for bot (winter) and tapeworm (fall and spring) treatment in endemic areas as needed.
About the Author
Dr. Colleen Lewis is a 1996 graduate of Kansas State University, College of Veterinary Medicine. Her career has taken her to many places as a practice owner, consultant, embryologist, and mentor. She enjoys mixed animal practice, teaching, traveling, farming and high school sports with her husband, Andrew and their three boys.
