Why Too Much Deworming Can be a Bad Thing

Question:  Can too much deworming actually be a bad thing?  

Answer:  While it is unlikely a horse will become ill or suffer harmful effects from being dewormed too often, in the long term, all horses’ health can be compromised by the development of parasite resistance to dewormers.¹,²

When deworming strategies were developed in the 1960s, the protocol was simple – treat every horse on an eight-week schedule with the newly available benzimidazole anthelmintics (an anthelmintic is a medication causing parasitic intestinal worms to be expelled or killed). A dramatic reduction in mortality from parasitic disease resulted.¹ During the next two decades, as new anthelmintics became available, veterinarians recommended rotating between classes of products, but still treating every horse the same.¹

Parasites, however, responded to the chemical challenge by developing resistance. In the case of small strongyles, identified as the most prevalent parasite in adult horses today,³ there is evidence of their widespread resistance to two of the three major dewormer classes – benzimidazoles and pyrantels.^3-6 Contributing to the development of small strongyle resistance is the common practice of rotating drugs, some of which are still effective against this parasite and some of which are not.^3-5,⁷

Experts say it’s time to throw out these outdated deworming practices. We now know only 20 to 30 percent of horses in a herd shed about 80 percent of the worm eggs.¹ Thus, it doesn’t make sense to treat every horse with the same eight-week frequency. Once you’ve determined how often each horse needs to be treated, it’s important to make sure you’re using products that are actually working against the target parasites on your farm. These practices are often called “strategic deworming” and are a better way to manage parasites and help avoid contributing to the development of resistance1 on your farm.  

How does a strategic deworming program work? With the help of your veterinarian, the first step is to conduct a fecal egg count (FEC) on each horse, which will identify which parasites are present and which of the horses are high, medium and low shedders. Based on the results, the veterinarian will recommend how often each horse needs to be treated. Your veterinarian will also likely follow up with fecal egg count reduction tests (FECRT), used to then determine whether specific products are still effective against the parasites on your farm. Ultimately, you may find it is appropriate to discontinue the use of some products that were on your rotation calendar.  

By investing time to develop a strategic deworming program and incorporating a broad-spectrum product like ZIMECTERIN® Gold (ivermectin/praziquantel), you will be able to manage the parasite challenge in your horse or horses. In large-scale studies, ZIMECTERIN Gold has been proven to be effective against benzimidazole-resistant small strongyles.^3,8 ZIMECTERIN Gold was also the first dewormer approved by the FDA to effectively control tapeworms,* which have been recognized as a significant threat to the health of horses.⁹