It used to be simple to deworm horses. You went to the tack store, bought the dewormer that was on sale and gave one tube to your horse every 60 days. That plan has gotten us in a lot of trouble. First, severe resistance has developed to most dewormers. 80 to 100% of parasites are immune to one or two classes of available dewormer. We have to get smarter about how we deworm or we are going to be left with no effective drugs to use on our horses.
The plan now is to have a plan; that is, to develop an individualized plan for each horse based on population density, herd stability, individual resistance, exposure and lab testing. Some horses will continue to require frequent deworming and others can safely get by on once a year treatment. Identifying which horses need which treatment interval and which dewormer is the new plan.
The key to it all is fecal testing. This will identify which horses have a parasite load, how heavy it is, if there are resistant parasites and which is the best drug to use. Unfortunately, there are no longer simple rules of thumb or a chart to hang on the wall that applies to all (or even most) horses. To perform a fecal test, a small, fresh sample of manure (about the size of a golf ball) is collected and submitted to a lab. Once the results are known (usually within 24 hours), we can advise you on either which dewormer to use and when, or when to retest (usually in 4 months). After a couple of fecal tests, we can figure out each horse’s situation and devise a long term program. The main thing to avoid is random deworming with random products. You could be using a product that is completely ineffective against your horse’s parasite load, administering drugs when none are needed or contributing to an already serious resistance problem. The new method requires a consultation with the veterinarian, a little more planning and some lab testing, but will safer and more effective for your horse.