This message is from: Jean Ernest <[EMAIL PROTECTED]> The article about ivermectin is in HORSE JOURNAL, November, 2003. http://www.horse-journal.com/issues/last/features/594-1.html I'm not sure that you can access it without a password and being a subscriber. If not, I can send yuou the article as an attachment.(I think) Here is part of it: ****** Deworming Protocols Are In Jeopardy
It may be time to seriously rethink how we are deworming our horses. When we heard of ivermectin resistance being documented in cows (see August 2002), we warned that the same problem could occur in horses. It appears it has. The Journal of the American Veterinary Medical Association reports the failure of ivermectin to eliminate roundworm infestation in 20 out of 37 foals treated with ivermectin. In some foals, the counts kept increasing between the time of treatment and a recheck 12 to 13 days later. The foals were residing on a large Thoroughbred breeding farm, where roundworm density was likely to be high. The ivermectin failures occurred mostly in foals that had been born on the farm and were regularly dewormed with ivermectin since birth, both suggesting the emergence of an ivermectin-resistant roundworm on this farm. The foals that did respond to the ivermectin were those that had not been lifetime residents of the farm and/or had been treated previously with dewormers other than ivermectin. Whether this meant those foals had not yet picked up heavy burdens of the ivermectin-resistant worm possibly unique to this farm, or the roundworms in those foals remained sensitive to ivermectin because they had not been intensively treated with ivermectin, was not clear. Either way, the possibility that ivermectin, which combines the broadest dewormer efficacy at the highest safety profile, might become widely ineffective is frightening. The thought of returning to the days of major strongyle damage is a sobering thought to anyone who remembers it. Theoretical Options [snip] The problem is most likely to surface on farms with a high density of horses and on breeding farms with many young horses where parasite density is high. Intensive use of ivermectin under those circumstances puts considerable pressure on the parasites to evolve to a resistant form. Its not known yet if rotational deworming using other agents would delay the resistance developing, but its one approach under consideration. Other experts contend that rotational deworming might contribute to the emergence of multiple resistant forms. More definitive research under farm conditions is needed. [snip] Do not overworm but, when you do deworm, be sure not to underdose the animal. Base dosage on an accurate body weight. Underdosage encourages the development of resistance. However, this isnt a call to overdose your horses either. Consider using one dewormer class for a year at a time, or until it becomes ineffective, as judged by a fecal worm-egg count. As a rule, parasites tend to develop resistance to all drugs within a class, but not between classes. Therefore, just changing to another drug within a major class wont help avoid resistance. You must change the actual class of drugs (see our chart at end of story). Isolate new animals, do a fecal test, deworm and retest. If parasites arent eliminated, deworm again with another class of drug until they are. Deworm only high-risk animals (pregnant, nursing, young, old or ill/debilitated animals) or animals with high fecal-egg counts. Not routinely treating animals with only low counts ensures that a number of nonresistant parasites will be present in the environment to crossbreed with any that may be developing resistance. This will prolong the interval to the development of drug resistance. Do not overworm but, when you do deworm, be sure not to underdose the animal. Base dosage on an accurate body weight. Underdosage encourages the development of resistance. However, this isnt a call to overdose your horses either. Consider using one dewormer class for a year at a time, or until it becomes ineffective, as judged by a fecal worm-egg count. As a rule, parasites tend to develop resistance to all drugs within a class, but not between classes. Therefore, just changing to another drug within a major class wont help avoid resistance. You must change the actual class of drugs (see our chart at end of story). Isolate new animals, do a fecal test, deworm and retest. If parasites arent eliminated, deworm again with another class of drug until they are. Deworm only high-risk animals (pregnant, nursing, young, old or ill/debilitated animals) or animals with high fecal-egg counts. Not routinely treating animals with only low counts ensures that a number of nonresistant parasites will be present in the environment to crossbreed with any that may be developing resistance. This will prolong the interval to the development of drug resistance. Ask your vet to do regular fecal examinations, both to determine if your dewormer is still effective and also to identify those animals that need to be dewormed. ************************************************************ Jean Ernest Fairbanks, Alaska mailto:[EMAIL PROTECTED]
