OSU Sheep Team

By William P. Shulaw, Extension Veterinarian

(Originally Published in Sheep Team Newsletter October 2003)

In the April issue of the Sheep Team newsletter, we discussed drug resistant worms and the selection process that creates them in our sheep flocks. In that article we stressed the importance of knowing whether the products you are using are working for you and of monitoring the success of your program with quantitative fecal egg counting techniques. Since that article was published, I have become aware of three additional sheep flocks where ivermectin resistance is suspected. In one of these, we suspect that the source of resistant worms was goats that were purchased and commingled with the sheep flock. In August, the Journal of the AVMA published an article by Dr. Ray Kaplan of Georgia in which he describes ivermectin resistance in 17 of 18 goat flocks in Georgia. In three of these flocks, moxidectin resistance has also been detected. Anthelmintic resistance, including resistance to ivermectin, in goats from the south may be widespread.

In the newsletter article (http://sheep.osu.edu/2008/06/19/drug-resistant-worms-in-ohio-sheep-flocks/), I also highlighted the importance of being extremely careful of use of the avermectin/milbemycin class of dewormers (ivermectin, doramectin, eprinomectin) and moxidectin. With the exception of Ivomec Sheep Drench®, not only are these compounds NOT approved by the FDA for use in sheep, they are more persistent in the body. This includes ivermectin injection (approved for cattle). This characteristic has the potential to cause selection for resistance to develop, perhaps rather quickly. This is especially true for Hemonchus contortus, the blood-feeding worm that causes most losses in Ohio sheep. Once ivermectin resistance is present, the situation is set for moxidectin resistance as well (if it is used) because we believe that resistance to both compounds is by the same general mechanism in resistant worms. Unfortunately, once resistance to a chemical class is present, it is likely to be permanent.

Currently, the FDA has approved only three drugs for use in sheep. They are Ivomec Sheep Drench® (active ingredient is ivermectin – the avermectin/milbemycin class); Levasole® and Tramisol® soluble drench powder, and oblets, and PROHIBIT Soluble Drench Powder® (active ingredient is levamisole hydrochloride); and Valbazen® (active ingredient is albendazole – the benzimidazole class). This summer, some producers found it very difficult to obtain Levasole® and Tramisol® soluble drench powder (made by the same company). It appears that these two products may continue to be difficult to obtain for the foreseeable future. However, PROHIBIT Soluble Drench Powder contains the same active ingredient, levamisole hydrochloride, and is available from several suppliers in Ohio.

Although levamisole has been available as a sheep dewormer for a long time, it remains effective in many flocks. Research suggests that only H. contortus worms that are homozygous recessive for levamisole resistance are resistant. This is analogous to the spider lamb syndrome where mating of two carriers of the recessive gene results in spider lambs only one-fourth of the time and mating of a carrier with a normal animal results in 50% of the progeny being carriers but no spider lambs are produced. This may at least partially explain the reason that levamisole appears to remain effective in many flocks.

This is unlike the situation with ivermectin (and its chemical relatives) where we believe that resistance in H. contortus is expressed as a dominant trait making a worm with only one copy of the gene effectively resistant to ivermectin. In this situation, a mating between a resistant worm, with only one copy of the resistance gene, with a non-resistant worm still results in 50% of the progeny being effectively resistant to ivermectin. All progeny of a male or female worm homozygous for the resistance gene will be resistant. Australian researchers speculated as early as 1996 that resistance of H. contortus to ivermectin might become common rather quickly as a result of this mechanism of inheritance. Our observations in sheep and goats now seem to bear this observation out, and we must ask ourselves here in the USA to what extent the extra label use of this class of compounds has contributed to this problem.

It remains important for producers to know the compounds that they are using are effective. Currently, this can only be determined by using quantitative egg counting techniques and comparing the egg count reduction observed 10-14 days post-deworming with the level of shedding at the time of treatment. In addition, they should be very cautious about using dewormers in an extra label fashion; not only because of the potential for drug residues but also because of the potential for selection of worms for resistance.