Parasites are becoming immune to our deworming drugs.

Why Test


Parasite Resistance

Resistance develops when the recommended dose of dewormer does not kill 100% of the parasites inhabiting an animal. This process of selection allows resistant survivors (those not killed by the dewormer) to mate with other resistant worms and subsequently pass resistant genes onto their offspring. Resistance becomes a clinical problem when there are a high number of resistant worms within the treated population leading to treatment failure and clinical disease. The rate at which modification to the parasite gene pool occurs within a given population varies depending upon a number of factors including: the frequency of exposure to a particular class of anthelmintic, the immune status of the individual horse and the size of the parasite population in refugia (the population that has never be exposed to anthelmintics).

The following is a comprehensive compilation of peer reviewed information regarding Fecal Egg Count interpretations and treatment regimens from the leading equine parasitologists.


A successful deworming program must combine the following:

  • Epidemiological principles of nematode control.
  • Determining which drugs remain effective on each farm.
  • Using the correct drug for the correct parasite at the correct time of the year.
  • Determining which horses require less or more frequent treatment by performing Fecal Egg Counts.
  • Evaluating the overall success of the worm control program by monitoring the FECs of all horses on the property at regular intervals.

“Think about it this way: You would never randomly use an antibiotic to treat a suspected infection. It wastes money, it wastes therapeutic time just when speed is of the essence and it increases the risk for developing resistant pathogens. That same school of thought must now be applied to equine dewormers.”

~ Frank Hurtig, DVM, MBA, director, Merial Large Animal Veterinary Services

General Recommendations:

  • Deworm horses shedding greater than 250 EPG before spring turnout and as needed during the grazing season based on post treatment Fecal Egg Counts. Our Equine Deworming Guide.
  • Treat for tapeworms (with praziquantel) before going on pasture in the spring. If tapeworms are a big issue, another treatment can be done in the fall. Treat for bots after first freeze in the fall.
  • The concept of zero tolerance for small strongyles should be abandoned, as some degree of parasitism is necessary to stimulate an immune response.
  • Individual horses vary greatly in their susceptibility to internal parasites; some limit infection almost entirely even without deworming drugs, while others carry very high parasite loads even with regular deworming. Thus, we can also reduce anthelmintic use by only deworming the horses that really need it.
  • Anthelmintics are not the only control measures for internal parasites. For example, removing feces from the environment every few days ensures that infective larvae are hauled off in the wheelbarrow. Implementation of such programs requires close work with a veterinarian.