In this series, we ask some of the equine health questions you’ve wondered about but were too afraid to ask. Today, Dr. Chrissie Schneider, Senior Equine Professional Services Veterinarian with Merck Animal Health, tackles a few common questions about deworming horses.
How do dewormers work to fight parasites in horses?
Dr. Schneider: The drug compounds in horse dewormers target various metabolic processes in internal parasites. Malfunction of these metabolic processes causes the internal parasites to die. Importantly, the dewormers do not affect metabolic processes of the horse itself so they can safely be given to a horse (at manufacturer recommended doses) while causing death of the internal parasite.
How should an owner approach a horse with an unknown deworming history?
Dr. Schneider: A great place to start is to ask your veterinarian to perform a fecal egg count. A quantitative fecal egg count of a manure sample can provide a measurement in eggs per gram of the different internal parasites which are present. This information can guide decisions associated with frequency of deworming and determination of which deworming product is best to use. If the horse was dewormed recently (shortly before the current owner acquired the horse, for example) there may not be any parasite eggs in the sample. In that case, it would be wise to repeat the fecal egg count test in three months to reevaluate the horse’s dewormer needs.
Dewormer drug resistance is a concern in the horse industry. What can a horse owner do to impact this developing problem?
Dr. Schneider: Resistance of internal parasites to the available dewormer drugs for horses has become a problem in some areas. To slow down development of parasite resistance to dewormers, it is helpful for us to remember our deworming goal. Our goal is NOT to eradicate internal parasites from our horses completely. That isn’t possible and to try often means deworming horses very frequently – the opposite of what we want to do. Horses can and do live long, healthy lives with internal parasites present. Horses are dewormed to keep their burden of internal parasites low so that the horse does not develop illness related to their presence.
The current recommendation to slow down the development of parasite resistance to dewormer drugs is to employ strategic deworming for adult horses (those greater than three years of age). Strategic deworming means to deworm the horses shedding the most internal parasite eggs into the environment more frequently than we deworm the horses shedding very few parasite eggs. Your veterinarian can use fecal egg count tests to diagnose the shedding level for each horse on your farm and guide you in the appropriate deworming plan for them. This protocol decreases the overall internal parasite load on your farm while using the least amount of dewormer drugs possible. The more often parasites are exposed to dewormer drugs the more opportunities they have to evolve and acquire resistance to those drugs.
It’s important to keep in mind that foals and young horses (less than three years of age) have unique internal parasite challenges and require different deworming management than adult horses. Work with your veterinarian to be sure you’re appropriately deworming your young horses.
Are there non-drug ways to control internal parasites that horse owners can begin implementing?
Dr. Schneider: Yes! There are multiple farm management protocols that can help us control the burden of internal parasites in our horses. Employing these practices can contribute to a decrease in our horse’s exposure to parasite eggs in their environment while using less dewormer drugs to do it. A win-win!
Farm management practices to control internal parasites on your farm include:
- Avoid overstocking in pastures
- Do not spread fresh manure on pastures.
- Properly composted manure can be safely spread on pastures as composting kills parasite eggs.
- Remove manure from pastures frequently
- Cross-graze pastures with cattle and/or sheep
- Feed hay and grain up off the ground
- Do not mix age groups in pastures
How easy/difficult is it to develop new drugs to fight parasites in horses — is there a solution in sight to the drug resistance issue?
Dr. Schneider: It has proven to be very difficult to develop new drugs to deworm horses and no new dewormer drugs for horses are on the horizon. This makes it vital that we preserve the efficacy of the dewormer drugs we have so we can keep our horses healthy for generations to come.