Dewormer Dilemma: Creating a Dewormer Program

September 22, 2016
By Deb M. Eldredge, DVM

The process of deworming a horse has evolved from a difficult veterinary procedure to a quick and easy at-home treatment.  However, there are a number of dewormers on the market and it can be confusing to horse owners in knowing which one to buy and how they differ.  So… how do you choose?

These days, horse owners can do their deworming by themselves using a deworming plan customized by their veterinarian for their horses and situation.  This method is the most effective and economical treatment plan for your horse and is quick, easy and safe for all involved.


Identifying the Parasites

In order to customize a deworming plan for your horse, the first step is to determine which parasites may be present.  To do this, take a fecal sample and give it to your veterinarian for analysis.  You don’t need a wheelbarrow full of manure, just one fecal ball is generally plenty.  Two different evaluations will then be conducted as part of the fecal egg count exam: qualitative and quantitative. 

Qualitative Testing

Qualitative testing identifies which types of parasites are present in your horse.  Note: these tests determine “which,” not “if,” your horse has parasites.  All horses have some parasites, and it’s important to identify which parasites are affecting your horse because different parasites require different active ingredients.  There is no sense in wasting your time and money on products that won’t address your horse’s situation. 


Quantitative Testing

Quantitative testing estimates the parasite loads that your horse has.  A low level of parasites may require less frequent deworming, while a horse with a heavy load may require deworming at more frequent intervals.  Even horses grazing on the same pasture, with the same living arrangement, may have different parasite levels.


Understanding the Active Ingredients

Once you know the types of parasites that you’re dealing with, your veterinarian can help you plan your deworming schedule.  Keep an eye out for the most damaging parasites in horses: large strongyles and small strongyles, and ascarids (aka roundworms) which are found primarily in young horses.  The ideal deworming regimen will address these three and attack not only adult worms in the intestines, but migrating or encysted larvae as well.  This is a lot to ask of one dewormer, so it’s likely that your deworming program will rotate ingredient chemical classes.

There are four classes of deworming ingredients:  pyrimidines, benzimidazoles, macrocyclic lactones and isoquinolones.  In order for a deworming program to work correctly, horse owners should rotate chemical classes of dewormers, not brand name.



In the pyrimidine class are pyrantel pamoate and pyrantel tartrate.  Pyrantel pamoate has the quickest action among the other major chemical classes, but because this drug only kills the adult worms, the results are short-lived; fecal egg counts are suppressed for about 4 weeks.  Pyrantel pamoate is great for killing roundworms, and may even kill tapeworms if given in a double dose.  It has some effect on large strongyles, but not on larvae of any type.  Pyrantel pamoate is available in paste dewormers used for purging.  Pyrantel tartrate is the active in continuous dewormers – a pellet form that gets fed daily for prevention of strongyles and ascarids.  Note: even if you feed a continuous dewormer, you may still need to supplement with purge dewormers periodically.  



The benzimidazole category includes oxibendazole, fenbendazole and mebendazole.  Benzimidazoles are often recommended for a foal’s first deworming to ensure efficacy against ascarids.  They are also effective against large strongyles, roundworms and pinworms, but aren’t great for fighting small strongyles.  Oxibendazole is the newest drug in this family, while fenbendazole is the “old standby.”  These actives can be found in paste and granule forms. 


Macrocyclic Lactones

Macrocyclic lactones are the newest chemical class to be used against worms, killing at a fraction of the dosage of other dewormer chemical classes.  The two types of macrocyclic lactones are moxidectin and ivermectin.  Moxidectin acts on large and small strongyles, roundworms, encysted and migrating larvae, pinworms and bots; however, is not effective against tapeworms.  Due to possible safety concerns, moxidectin is not recommended for use on foals younger than 6 months of age. 

Many deworming schedules are based on the use of ivermectin.  It is one of the most effective and broad spectrum dewormers available today.  It’s highly effective for most parasites, such as large and small strongyles, ascarids, pinworms and bots.  Although as with all dewormer compounds, it has its weaknesses:  it’s ineffective against encysted small strongyles and tapeworms.  Ivermectin dewormers, such as Horse Health™ (ivermectin paste) 1.87%, can be used on horses of all ages, including broodmares, stallions and foals.



The only ingredient in this chemical class is praziquantel.  Praziquantel is only effective against tapeworms, so you will usually see it combined with one of the macrocyclic lactones.  Since it is difficult to test a horse for tapeworms, the only thing horse owners can do is decide whether or not they would like to include a dewormer with praziquantel into their rotational program.

Although these products can be used on horses, it is important to note that ivermectin and other macrocyclic lactones can be toxic to dogs if ingested.  Manure from horses who are dewormed with macrocyclic lactones will contain significant drug levels for about a week after the horse is dewormed, so be sure to clean up the manure immediately, as well as any paste that may have dropped to the floor, to avoid any accidental ingestion by barn dogs.

Creating the Program

Once you’ve identified which parasites are affecting your horse and what chemical classes are appropriate to use, you and your veterinarian can come up with a rational deworming program.  In addition to rotating dewormer chemical classes, talk to your veterinarian about the frequency of your deworming program.  Horse owners may choose to deworm their horses every 3 months, 6 weeks or use a daily prevention program depending on their horses’ situation. 


The deworming process has come a long way since the days of nasogastric tubes and visits from the veterinarian.  With a thorough understanding of the parasites effecting your horse and the chemical classes used to treat them, you can confidently deworm your horse with success.



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