Nasty Little Parasites - An Update on Deworming


The staff of Burwash Equine Services, Ltd. enjoyed an evening of education last night courtesy of Vetoquinol and Dr. Ela Misuno.  Dr. Misuno is a board certified internist with an interest in parasitology and the problem of emerging resistance in this field.  She gave a fantastic presentation with all of the latest updates and recommendations for deworming our equine friends.  I have summarized her recommendations below.

Over the last decade, the nature of parasites in horses has seen a population shift.  In years past, the parasites of most concern were those known as Bloodworms (Large Strongyles) that were responsible for many cases of colic and verminous arteritis and were a serious threat to the health of the horse.  With modern dewormers and deworming protocols, the incidence of this type of parasite has been dramatically reduced and the parasites of biggest concern these days are the small Strongyles (or Cyathostomes).  These nasty little parasites have learned to develop resistance mechanisms to the commonly used dewormers, making them increasingly difficult to control.  When infected with these parasites, a horse will often show signs of weight loss, colic, diarrhea and edema (or swelling) of the lower abdomen and head.  While present in low numbers in the horse population in Alberta, tapeworms are also a type of parasite that can infect horses and need to be addressed.  They tend to live at the junction of the colon and cecum and are a known cause of intussusceptions (where one portion of bowel works its way inside out into another portion of bowel and gets trapped, resulting in an acutely colicky horse).  In foals, we also have to be concerned about the parasites known as Roundworms (or Ascarids).  These worms are the largest of the equine parasites and make their home in not only the smallest of our equine friends (the foals), but also the smallest of the intestine in these little friends.  It does not take very many of these parasites to get tangled up in the bowel and cause an impaction (in other words, a blockage of the gut).

There are other parasites that will infect horses, such as the well-known Bots (the tiny yellow eggs of which we have likely all seen attached to the hair on our horses legs, chest and sides) and Pinworms (who lay their eggs around the rectum of the horse, causing the characteristic tail rubbing most people associate with parasites).  While unpleasant to the average horse owner, these parasites cause little harm to the horse and therefore, are not at the top of our list of concerns when we talk about parasites in horses.


The single most important reason for changing our deworming protocols is because of the EMERGING RESISTANCE of small Strongyles and Ascarids to the deworming products that we have available for horses.  Currently on the market, there are only 4 classes of dewormers for use in horses that can kill these species of parasites.  These are fenbendazole, pyrantel, ivermectin, and moxidectin.  When you are picking out a dewormer for your horse, it is important to pay attention to the fine print below the brand name as this will be where you find what class of dewormer it is.  There are numerous products on the market for each of these classes of dewormers.  Unfortunately, we are seeing increasing resistance to all 4 classes of dewormers that are available, and there are no new products coming down the pipeline.  This means that if the parasites develop enough resistance that the drug no longer works to kill them, we will have a huge problem on our hands!  Sadly, the southern states and some areas of Ontario are already having problems with this and are now faced with species of parasites that are resistant to all of the dewormers available – let’s be proactive and not let this happen in Alberta!

Over the past 5 years, Burwash Equine Services has been implementing these concerns into our recommendations for performing a Fecal Egg Count (FEC) on all horses prior to deworming.  The FEC allows us to count the number of parasite eggs per gram of your horse’s manure and classify them into one of three groups based on their parasite burden.  Horses are now classified as low shedders (<200 eggs per gram), moderate shedders (200-500 eggs per gram) and high shedders (>500 eggs per gram).  The best time to perform these FEC tests in our area is late April or May and again in July or August.  Fall and winter samples are not recommended because the parasites go into a period of hiding during this time where they are not producing eggs, so the sample reads as being falsely low or negative - and this is not necessarily the case.  Once a horse has been identified as a low shedder (and this may take 2-3 years to be confident about), he/she can then be treated far less frequently than in the past.  Based on the latest research in this field, we now know that these horses only need to be treated once yearly after the first hard frost in the fall.  The moderate and high shedders are targeted more aggressively.  The timing and products used for these dewormings varies and a program needs to be developed for each herd individually.


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Once the horses in your herd have been classified into their appropriate group, the next step is determining which class of dewormer is going to be effective for the parasites in that herd.  The degree of resistance varies dramatically from herd to herd, so the dewormer that will be effective in each herd will also vary.  The effectiveness of a drug can be tested by performing fecal egg count reduction tests (FECRT’s) on some of the high shedders in the herd.  A fecal egg count is performed before deworming and again 2 weeks after the dewormer is given.  The sample after deworming should be free of parasite eggs if the deworming medication administered was effective.  If the sample is not negative at that time, it indicates that the parasites in your herd are resistant to that medication and so another class of drug needs to be tried and the process repeated.    The goal is to find a dewormer that is 100% effective in each herd and to stick with that drug and only rotate if the efficacy of the new product is equally as effective.  The FECRT’s should be repeated every 3 years to ensure that the dewormer continues to be effective.  Any new horses that are introduced to the herd should be treated on arrival with the dewormer that is known to be effective in that herd and tested in 2 weeks to be sure that the drug was effective for the parasites that that horse might have brought with him.  During this time, the new horse should be isolated from the rest of the herd to prevent introducing any new parasites to the other horses.  If the FEC is not negative after 2 weeks, the horse should be dewormed with another class of drugs, isolated for an additional 2 weeks and retested at that time.


One additional aspect of parasite control that we as horse owners have absolute control over and that should not be overlooked is environmental management.  All parasites require a time spent outside of the horse to complete their lifecycle and re-infect the horse.  This gives us a distinct opportunity to interrupt that life cycle and thereby prevent re-infection of our horses.  Accomplishing this is as simple as picking up the manure containing all of these parasite eggs from the horses paddock once or twice a week.  In the words of Dr. Misuno…”love stinks” and the best way to protect your horse is to be on poop patrol.  In the past, the practice of harrowing the pastures to break up the manure and expose the parasites to air to dry them out has been commonly performed.  Unfortunately, most of these parasite eggs and larvae will survive this practice quite easily as they are only killed if the temperature remains hot (above 30 degrees Celsius) for several days in a row – an uncommon occurrence in central Alberta!  So most of the time, harrowing the pasture simply serves to spread all of the parasite eggs and larvae evenly across the entire pasture and your horse’s favorite grazing spot!  We understand that the task of removing all of the manure from your horses pen can be somewhat daunting, so start by simply removing the biggest piles and manure from the highest traffic areas where your horse spends most of its time eating, such as around feeders and in shelters – every little bit helps.


Foals are a bit of a different story.  As mentioned previously, the most worrisome parasite in young foals are Roundworms (Ascarids).  These parasites are not only much larger than other parasites, but they also have the ability to produce a MUCH larger number of eggs than all of the other species and they can survive in the environment for a MUCH longer time.  Our goal is to kill these parasites before they have the opportunity to produce all these eggs and as a result, we are unable to use fecal egg counts to determine if they are infected and how many of these worms they may have.  Having said that, we can sometimes test older weanlings or yearlings on the same farm to check for these parasites and to perform fecal egg count reduction tests to help us find the dewormer that is most effective.  In general, because of the nasty complications associated with these parasites, we tend to treat all foals as though they have a high number of these parasites starting at 2 months of age.

In summary, there have been some dramatic changes to the way we approach deworming over the last decade.   Despite your best intentions, you may not be doing the best thing for your horse anymore by indiscriminately deworming.  In order to establish an effective deworming program, we must start by first identifying the high shedders in a herd, and then determining which class of dewormer is the most appropriate to use for that herd.  We challenge you all to start (or to continue!) to perform fecal egg counts on the horses in your herd this spring!