Sunday, March 27, 2011

Equine Intestinal Parasites and DIY Fecal Counts

We've all heard about the problems with resistance to our commonly used dewormers for horses.  This problem is very real, and impacts all of us as horse owners.  I have been procrastinating on finishing and publishing this post, but it is an important issue that I care quite a bit about and thought it was time. 

For many years horses have co-existed with parasites.  These parasites come in many different forms, but the ones that are perhaps the most important in the health of our horses and parasitic resistance today are the common intestinal parasites- strongyles.  While a small worm load is acceptable, and even beneficial to a horse's immunity, large worm loads cause a lot of problems including anemia, weight loss, and of course can lead to impaction and colic.  To combat this, most horse owners  use a standard deworming plan that rotates between two or three classes of drugs used to rid the horse of these parasite burdens.  The idea behind the rotational deworming program is to use different types of medications each time to prevent worms from becoming resistant to them.  The standard deworming program involves rotation between three major drug classes- benzimidazoles (fenbendazole, oxibendazole), tetrahydropyrimidines (pyrantel pamoate, pyrantel tartrate), and macrocyclic lactones (ivermectin, moxidectin).  The fourth drug class includes the drug praziquantel, which is used to specifically target tapeworms, and is generally administered along with ivermectin or moxidectin.  This rotation is usually administered every 2-3 months, depending upon the climate, herd size, and veterinarian recommendations. 

However, over the past 30-40 years since this rotation was implemented, the worms have become much smarter, and our usual medications are not working as effectively as we would expect. The worms are surviving administrations of deworming medications and going on to become even stronger.  Two of the three major drug classes (the benzimidazoles and the tetrahydropyrimidines) have been found to have significant problems with resistance, yet they are continuing to be used in the same way as they have been for the past 40 years.  Part of this problem is lack of awareness on the part of horse owners administering the drugs, part of the problem is lack of resources in making a plan to change the way we use these medications, and a large part of the problems, scariest of all, is the lack of alternatives available to replace the drugs we currently use.

So what are we supposed to do about this problem of parasitic resistance?  Well that's a good question.  Of course there are several methods that can be used to keep parasitic infections to a minimum- this includes proper disposal of manure, pasture management (picking feces out of pastures being actively grazed), and judicious use of deworming medications.  One tool that horse owners can use is a quantitative fecal egg count.  This is a test that is readily available through most veterinarian labs.  But this is not a one-time, quick-fix answer.  Fecal egg counts (FEC) must be performed periodically, in a way to not only determine when deworming is needed, but also to assess the efficacy of our deworming medications.  By using FECs, one can estimate a horse's current worm-load and determine whether deworming is necessary or not, so our precious few deworming medications can be used judiciously, and only when needed.  We can also check the efficacy of our medications by performing a fecal egg count reduction test (FECRT) 10-14 days after administration of a deworming medication.  If the dewormer medication was effective, the strongyle FECRT should show at least 90% reduction in the egg count. 

FECs can be performed by your veterinarian lab, but they are expensive, usually ranging $20-40 per FEC.  If you are scientifically minded (and maybe at least half crazy like me), you can do your own FECs, and they're easy.  I started doing my own FECs a little over a year ago, and it has revolutionized the way I deworm my horses.  There is some learning involved in getting started, and some up-front cost to get set up with the tools necessary to perform these tests, but after the initial input of time and energy, I have found FECs to be quite easy, and relatively quick, with very little cost. 

The Horse ( has several excellent articles about equine parasites and resistance, and even a step by step tutorial on performing your own FECs.  The materials needed to get started include a microscope (I purchased a very basic microscope with a mechanical stage, monocular eyepiece, and the required 10x objective for $50 on ebay), counting slides with grids (I would recommend the Paracount-EPG kit from Chalex Corporation complete with two marked mixing vials, transfer syringes, two McMaster counting slides, and instructions- a very nice kit available for $50-60 from note, I would recommend purchasing the kit with green grid slides, much easier to see through the microscope) , and float solution ($10 for a gallon from my local vet's office).  So that's a total investment of $120 to be set up to do these tests for years to come- a small price to pay, in my opinion, for the health of our horses.  In reality, the supplies paid for themselves in four tests in comparison to what I pay my vet to do them.  The Paracount EPG kit is really nice and easy to use; it includes step by step instructions for setting up and interpreting the results of the FEC and FECRT, as well as information on the egg reappearance period for the commonly used drugs (basically how long each dewormer is effective), and pictures of the common parasites seen on FEC.  FECs are quite easy to perform with the help of this kit and the information available from The Horse. 

I have been doing FECs for Louie about every 2-3 months, and often after deworming for a FECRT.  The owner of the other two horses in Louie's pasture also has me do FECs for her two horses, so we get them all on the same page, and deworm them, when needed, on the same schedule.  In my own research, I have found ivermectin to be 100% effective (though there are reports of ivermectin resistance, which is very scary), fenbendazole to be only about 10% effective (and I haven't used it since, fenbendazole is the dewormer with the greatest problems with resistance), and pyrantel to be between 50 and 90% effective.  We generally rotate between pyrantel and ivermectin (+/- praziquantel- we use this twice a year to cover tapeworms, which do not routinely show up on FECs), but I am also going to try moxidectin (Quest) this spring, to increase the number of drug options used, though we are still only using two drug classes.  Perhaps fenbendazole will re-gain efficacy in the future if it is not used for a few years, but only time will tell. 

Last summer, my husband took a few photos of the FEC process (never mind the eye-patch headband, haha, I know it looks pretty silly, but I have to cover one eye in order to focus through the monocular microscope). 

Mixing the manure with the float solution in the measuring vial:

Drawing up a small amount of the mixture with the syringe to apply it to the slides.

Injecting the mixture into the counting slides.

Rinsing the tools, here you can see the set up, the measuring vial (I use a tongue depressor to stir), the syringe, and the loaded counting slides.

Looking through the microscope and counting the eggs (the number of eggs within the two grid areas on one slide are counted, then multiplied by 25 to reach a final eggs per gram count, administering dewormer at anything over 200 EPG)

And the end result- happy, healthy horses!


  1. what are your thoughts on vaccinations

  2. I'm a big proponent of vaccinations. I don't think every vaccine is as effective or as necessary for every horse, but I do believe every horse should be vaccinated against the basics- Tetanus, Rabies, sleeping sickness, flu and rhino at least once a year. I also think West Nile is important and should be included, but that could be because we live in the land of 12,000+ lakes. The other vaccinations are dependent upon the situation- are your horses at a large show barn with 50-100 horses leaving the property frequently for shows, or a small back yard barn with 2-3 horses that never leave the property or meet other horses? Do you live in a wet area? Has your horse had strangles? Etc. I don't vaccinate Louie against strangles because he had it when he was younger and its risk:benefit ratio is small given the lack of efficacy of the vaccine compared to the risk of reactions and still acquiring the infection after vaccination, but I do vaccinate our other two horses- simply because their boarding facility requires it. I have vaccinated all 3 for West Nile and Potomac. We have considered the EPM vaccine, but the vets in my area don't strongly recommend it because of questionable efficacy.

    All in all, I definitely strongly believe vaccinations are important and we should all be vaccinating our horses. Vaccines don't pose the same risk of resistance as dewormers do, they work on a different premise, and are given solely as a primary prevention, not once the horse is already infected like we do with dewormers. Sure, you can get a disease if you've been vaccinated for it, but chances are much less likely, and the chances that disease will pass through a lot of different animals to infect the whole herd is much much smaller if there is good herd immunity and all of the horses are vaccinated.

  3. Does resistance to dewormers make a quantam leap or is it limited to one localized herd where deworming has been used inappropriately?

  4. good question. While I'm not a vet and am not truly an expert, I would say that resistance to dewormers occurs slowly over time. The more dewormers are used, the more those worms are exposed to the medication, the more they have the opportunity to adapt and change and outsmart the drug. Especially when medications are given unnecessarily or at a lower dose than what would be effective (or another example is antibiotics taken for less than the recommended amount of time- that is a whole 'nother topic, but antibiotic resistance is similar and in the same vein as dewormer resistance), the bugs are exposed to the medication, but not killed by it, thereby giving it a chance to adapt and find ways to survive it. The more this happens, the more resistance we see. It happens gradually over time.

    We see it happening in herds throughout the country, as many of the same deworming practices are prevalent throughout the country (ie. the standard 3-way rotation). Dewormer resistance is not everywhere, but it is widespread. When I did FECRT's on my own horses in different herds at different stables (about 15 miles apart), there was a different amount of resistance, but there was resistance to the same drugs at both places. One horse was at a large boarding stable with a lot of turnover and poor manure and pasture management. Fenbendazole was only 10% effective there. Pyrantel was about 50% effective on that horse, in that herd, at that stable. I used Pyrantel on one of our other horses at another stable with decent pasture management and a much smaller and more stable herd, and it was 90% effective there. Quite a difference from one place to another, both tests run in moderate shedding 4 y/o horses.

    So, I guess to answer your very good question, resistance probably occurs slowly, gradually over time, and while there is resistance in most places in the USA, the amount of resistance is not the same everywhere. I hope that helps!

  5. by the way, worms and bacteria are way smarter/more adaptive than we give them credit for!