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Jennifer Lowry, DVM *Spring 2012*


Recent studies have shown that parasite resistance to the dewormers that we routinely use is much more common than we suspected.  In spite of our best efforts to take care of our horses we have actually contributed to the problem.  By using dewormers with a greater frequency than is needed in many horses we have encouraged parasite resistance.  An example of the devastating effects of this trend can be seen in sheep and goats where a deadly parasite has become over 90% resistant to the only dewormer available to treat it.  Current deworming practices must be changed to prevent this from happening in our equine population.  With any new equine dewormers still years away from market availability, we must use the only ones we have wisely. 


1)  Determine which horses require more or less frequent treatment by performing a fecal egg count (FEC).  Who are the high and low ?shedders? of parasite eggs in each group?

2)  Evaluate the overall success of the worm control program by monitoring the FEC of each horse at regular intervals.


This is a lab test that requires submitting a sample of feces from each horse.  Place one fresh fecal ball in a ziplock bag and label it with the horse?s name.  The sample may be collected the night before submission and stored in the refrigerator.  We will call you to discuss the FEC results and deworming plan after determining which horses shed a high, moderate or low amount of eggs. By performing routine FECs, we can decrease the frequency of deworming to as little as 2-3 times per year for most horses and avoid medicating horses that may not need it.  Performing a FEC in the spring and fall is the best way to effectively monitor your parasite control program.


* 20-30% of the horse herd is responsible for harboring about 80% of the worms.  These horses are more susceptible to parasite infection and tend to remain this way for life.  It is important to identify them with FECs and deworm accordingly.

* Temperature matters: cold winter months and hot, dry summer months do not support parasite larvae development.  Horses that are identified with low worm burdens on a FEC do not need to be dewormed during these months.  Spring and fall are optimal times for parasite development, and treatments should be focused on these months for all horses. 

* Harrowing or dragging pastures kills worm eggs by exposing them to the dry heat of the summer.  After the temperatures become cooler in the fall (below 80 degrees), harrowing actually helps the worms multiply by spreading their eggs, and should not be performed. 


(adult horses over 1 year old)

March 1 to April 1:  (when days are consistently 60 degrees)

Start with a fecal egg count (FEC) for each horse. 

Deworm each horse with Quest Plus after you are called with the FEC results.   

If a horse has a high FEC, you will be asked to submit a second fecal 2 weeks after deworming. 

June 1

Strongid paste (optional, use if horse had a high worm burden when checked in March)

September 1 to October 1: (when days are consistently 60 degrees)

Check a FEC on any horse that had a high FEC in March.

Deworm all horses with either Zimecterin Gold or Equimax

* Please call for recommendations for a new horse with an unknown deworming history. 



-         Moxidectin (adult AND larval strongyles) + Praziquantel (tapeworms)

-         the only dewormer available that effectively removes the larval form of the strongyle

-         the larval form causes the most problems?GI inflammation, nutrient loss & weight loss. The larval form will not show up on a fecal egg count as this test measures eggs shed by adults only.

-         effective for up to 12 weeks, the drug with the least parasite resistance


-  Ivermectin (adult strongyles) + Praziquantel (tapeworms)

     -  effective for up to 8 weeks on most farms


-  targets adult strongyles and weakly targets the larval form

      -  overall an ineffective dewormer due to high resistance problems

-  these are the safest products to start giving an emaciated horse with an unknown 

   deworming history


      -  targets adult strongyles

      -  the daily use of this product is highly discouraged by veterinary parasitologists as it

         promotes parasite resistance  


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