Gastrointestinal Parasites: Deworming Demystified

Why fecal egg counts and targeted deworming will benefit your horse and maybe even your wallet:
Internal parasites can cause many problems in horses, such as colic, anemia, ill-thrift and diarrhea.  The goal of deworming your horse is to reduce the amount of eggs shed into their environment.  This is achieved by reducing the number of internal parasites in your horse as well as reducing environmental contamination.  For decades people  have managed worms using an interval deworming program in which horses are treated with a different drug every 6-8 weeks.  This equates to treating your horse as many as 8 times a year.  This system has many disadvantages, the first being that for many horses, deworming this frequently is unnecessary.  Approximately 50-60% of adult horses have an innate immunity to gastrointestinal parasites and don't require frequent treatment.  Secondly, frequent deworming is leading to drug resistance in parasites.  This means that the drugs (anthelmintics) are becoming less effective at killing the parasites and when they do work, they last a shorter period of time (egg reappearance period).  Resistance is concerning because drug companies are not currently developing new anthelmintics.  The bottom line is, as veterinarians and horse owners; we have to do what we can to slow down the development of resistance.

The current recommendation is to use a strategy called targeted or selective deworming.  Fecal egg counts (FEC) are used to identify horses with a high parasite burden (high egg shedders) and only those horses are treated.  Research has shown that the major benefit to this strategy is to slow down the progression of resistance in the worm population.  Targeted deworming is also better for the environment, prevents unnecessary medication of horses, and may even save you money in the long run.

For those 50-60% of adult horses with low egg counts (200 eggs per gram or less), twice yearly deworming with an ivermectin-praziquantel product is sufficient.  The main reason we recommend treating these horses is that tapeworms will not show up on fecal egg counts.  Praziquantel is effective against tapeworms. 

For the 25% of horses that shed a moderate amount of eggs, 3-4 treatments per year are recommended.

That leaves another 25% of horses that are high egg shedders.  These horses are responsible for about 80% of the total egg contamination on a farm and may need to be treated 4-5 times per year.

** Many of you use a daily feed through dewormers such as Strongid C.  We strongly recommend performing fecal egg counts on these horses as resistance to Strongid is well documented and often these horses have a high parasite burden.  Additionally, research suggests that daily dewormers are contributing to resistance in the worm population.

To determine the effectiveness of your deworming, serial fecal egg counts should be performed.  If your horse has a high fecal egg count, it should be treated with an appropriate deworming product and then a second FEC should be performed 10-14 days later.  The FEC should be close to zero at this point.  If it remains high, you have evidence that the worms in your horse have developed resistance to that particular drug and a dewormer from a different drug class should be used.  If this occurs, it is important to consult us, your veterinarian, so that we can work together to create a special deworming program for your horse.  Knowing the egg reappearance time for high egg shedders is also important information as it tells you if the drug is effective for a shorter than expected period of time.  This can be determined by performing a fecal egg count anywhere from 4 -12 weeks post-treatment (depending on the type of anthelmintic used).

Additional management practices to reduce the worm burden:
Deworming your horse will kill the worms currently living in them and decrease the amount of eggs shed into the environment.  However, it will not prevent reinfection from the eggs/larvae already in the environment.  Environmental hygiene is as important as the use of dewormers.
* Manure should be removed from stalls and small paddocks daily, and from larger paddocks on a weekly basis.
* If at all possible, dirt paddocks should be rotated in the summer months so that the eggs/ larvae die off without finding a host.
* Manure should be composted before spreading onto fields (the heat will kill the eggs/larvae).
* Weeds and grasses in paddocks should be kept short.
* Don't feed hay/grain out of buckets/wheelbarrows used for cleaning stalls.
* You wouldn't allow a new horse onto the farm without questioning its vaccination status.  Why should it be any different with internal parasites?  Perform FECs on new horses and deworm appropriately before turnout, especially if they are young or have an unknown deworming history.