Ivermectin Is Resistance-Free

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In our October 2008 article on dewormers, we reported on a study performed by the Gluck Equine Research Center in Kentucky that showed fecal small strongyle egg counts becoming positive again as quickly as four weeks after deworming with ivermectin. The study hinted at resistance, although egg counts for the first few weeks after treatment were negative. We questioned whether this was truly a resistance issue.

To clarify the situation, the same research group, headed by Dr. Eugene Lyons, did serial fecal egg counts and ultimately necropsy studies on four yearlings after treatment with ivermectin. The results were reported in the October 2008 issue of Parasitology Research. It turned out that the return of positive egg counts was due to young L4 small strongyle larvae, remaining in the colon after ivermectin treatment, maturing to the egg-laying stage. Ivermectin is not labeled for the removal of L4 larval stages. The small strongyle species identified as reaching sexual maturity in this four-week period was Cylicocyclus insigne, and it was also the dominant species of small strongyle found in the four yearlings on this farm.

This study confirms that ivermectin was 99+% effective in killing mature, egg-laying small strongyles of all species. Since ivermectin has never been claimed to be effective against L4 stages of small strongyles, this study confirms there is no new resistance to ivermectin.

Bottom Line

Ivermectin remains effective against adult small strongyles. However, deworming at the usual interval of six weeks may encourage buildup of parasite species that become sexually mature quickly and begin to lay eggs within that six-week window. Young and heavily parasitized horses should either be dewormed at three- to four-week intervals or have a fecal exam checked at four weeks to ensure this is not a problem in your area.

With this confirmation of ivermectin’s resistance-free standing, our recommendations are:

• For regular deworming, we like ivermectin and ivermectin-praziquantel (spring and fall for tapeworms) drugs for your deworming needs. Pay attention to recommended retreatment periods.

• Consider targeted deworming, which means you deworm only when necessary based upon fecal egg counts, especially for healthy adult horses at low risk of parasite infestation. This saves you money and may help combat resistance developing.

• If you choose to use a traditional drug-rotational system, time treatments based on recommended retreatment times. Remember that most drugs used in a rotation system have documented resistance studies.