Fecal egg count-based parasite control, sometimes called surveillance deworming, not only saves money but also reduces the risk of resistance within the parasite population on your farm. The idea is to identify the horses in a herd who are shedding significant amounts of parasite eggs and treat only those horses with the appropriate products on a schedule dictated by their response to the treatment. Surveillance deworming is also fairly easy. Your veterinarian can help you establish your system, based on the following key steps.

1. Identify the high shedders. Research has shown that mature horses shed strongyle parasite eggs at different rates. In fact, in some herds, only a small number of horses may be responsible for most of the strongyle eggs that are shed onto the pasture.

The only way to identify which horses are high shedders is to conduct fecal egg counts—tests that analyze the manure to look for parasite eggs. If you identify high shedders in your herd, targeting your deworming treatments to those individuals will greatly reduce the number of eggs on your pasture. Low-shedding horses in your herd may require no more than twice-annual treatments to control large strongyles.

2. Choose the right deworming agent. Another benefit of fecal egg counts is that they will identify the types of parasites present on your farm. Then you can choose chemicals that are known to work against those worms.

Additionally, fecal egg counts provide a means of confirming those treatments selected are indeed effective: In a fecal egg count reduction test (FECRT), a veterinarian or laboratory will test one manure sample from the horse, treat that horse with a choosen dewormer and then test the horse’s manure again 10 to 14 days later. A 90 to 95 percent reduction in parasite eggs is considered a sign that the chemical is still effective against those parasites in that specific location. Experts generally recommend conducting FECRT once or twice a year.

3. Deliver the right dosage. Giving a horse too small a dose of a dewormer is one of the risk factors that can increase resistance. Calculate your horse’s weight in pounds using a tailor’s tape: Measure his girth circumference in inches, square it, then multiply by his body length (measured from the point of his shoulder to the point of his buttocks), then divide that number by 330.

4. Time treatments for best effect. Ironically, the greater the percentage of worms you kill in one deworming on your farm, the faster resistance will develop. That’s why it’s a good idea to deliver treatments when a higher percentage of parasites are in “refugia” or “out of reach.” That is, while the dewormer is working inside one horse, the chemicals would have no effect on eggs already on the pasture or in other low-shedding horses. Those eggs, for parasites that might be susceptible to the chemicals, could then be picked up and would “dilute” the number of resistance genes found in the next generation of worms. However, administering dewormers when the refugia population is low reduces this effect—for example, eggs do not survive long on the ground during very cold winters or in hot, dry summer weather.

Generally, the experts recommend that each horse in a herd be given two annual deworming treatments, with high shedders receiving an extra treatment. Your veterinarian can help you develop a deworming plan that will work best for your horses. 

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