The recent increase in parasite resistance to commonly used deworming products has left the equine community concerned and in search of ways to slow down, if not reverse, the alarming trend. As parasites can pose a very serious threat to a horse’s health, including fatal situations, it is apparent that the commonly accepted approach to parasite control should be reconsidered. For example, the so called dewormer “rotation” is no longer an acceptable practice.
Strongyles, roundworms, bots, pinworms, and tapeworms are some of the parasites that veterinarians and horse owners deal with. Some of these can be identified easily, others not so much. That is why every treatment should start with analyzing fecal samples. The test is relatively inexpensive, and can provide a solid basis for choosing the right product that will ensure an effective deworming treatment. Every horse from the herd should be tested, but if the group is very large, then a proportionate test group may be used to gather samples from. The fecal test should be done once a year or at least every other year.
Statistically, horses can be categorized as follows:
|Number of eggs in a gram of fecal matter (EPG)||Category||Proportion relative to the herd|
|More than 600 (up to 3,000)||Heavy shedder||20%|
|200 - 500||Moderate shedder||30-40 %|
|0 - 100||Light shedder||40-50 %|
If you test the whole herd individually you may find that not all horses carry the same amount and/or type of parasites. Some horses will carry a lot of worms and others much less. This is due to individual immunity, genetics, exposure level, and a variety of other factors. It may be a good idea to group horses based on the parasite type of infestation they have. This may help prevent further transfer of eggs and larvae between individuals.
There is no longer any fixed time chart or an ideal schedule for equine deworming. Instead, a proper deworming program will be based on the level of infestation and on the climate in your area. It is strongly recommended to perform a treatment after the first killing frost in the late fall to control bots and another round in the spring with a praziquantel- containing dewormer, which will provide a solid tapeworm control program. Note that this is the minimal treatment for the so-called light shedders.
|Category||Recommended frequency of deworming per year|
|Light shedder||2 times per year (or no treatment at all) (late fall after the first killing frost and spring)|
|Moderate shedder||3 – 5 times per year (late fall after the first killing frost and spring , plus when needed, usually in the summer season in the warm environments) Will benefit from one additional treatment during the main season of pasture transmission –spring through autumn in the North; autumn through spring in the South*|
|Heavy shedder||5-6 times per year
(approximately every other month) Intensive treatment required all through the main seasons of transmission*
Always consult your veterinarian before treatment. Fecal testing can help determine the right product and the most effective way to treat your horses. The currently preferred drugs are pyrantel (not effective for tapeworm or bot treatment), ivermectin, moxidectin, and praziquantel (effective against tapeworm).
*Craig R. Reinemeyer, DVM, PhD, The Horse.com, “What is your Horse’s Fecal Egg Count Telling You?”, April 17, 2017