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Determining Anthelmintic Resistance on Sheep Farms in the Southeastern US

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Presentation given at Annual Meeting and Professional Improvement Conference of National Association of County Agricultural Agents

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Determining Anthelmintic Resistance on Sheep Farms in the Southeastern US

  1. 1. Determining anthelmintic resistance on sheep farms in the southeastern US S. SCHOENIAN1, D. O’BRIEN2 AND N. WHITLEY3 1UNIVERSITY OF MARYLAND, 2VIRGINIA STATE UNIVERSITY, 3FORT VALLEY STATE UNIVERSITY This study was made possible by funding from the Let’s Grow committee of the American Sheep Industry Association (ASI).
  2. 2. Introduction • Gastro-intestinal nematodes are the primary health problem affecting sheep in warm, moist climates and during periods of summer rainfall. • Haemonchus contortus (barber pole worm) is the most pathogenic parasite species, having high morbidity and mortality. • H. contortus is usually the most numerous species in small ruminant parasitic infections.
  3. 3. Worms have developed varying degrees of resistance to all dewormers and dewormer “chemical” classes. 1 2 3 Benzimidazoles Macrocylic lactones Nicotonic agonists imidazothiazolesAvermectins Milbimycins Fenbendazole SafeGuard® Ivermectin Ivomec®* Moxidectin Cydectin®* Quest® Levamisole Prohibit®* Leva-Med®* Albendazole Valbazen®* Doramectin Dectomax® Oxfendazole Synanthic® Eprinomectin Eprinex® *FDA-approved for use in sheep.anthelmintic = dewormer
  4. 4. Two methods for determining anthelmintic resistance FECAL EGG COUNT REDUCTION TEST (FECRT) “Gold standard” • Old protocol Compare fecal egg counts from animals that have and have not been treated. • New protocol Compare pre-and post-treatment fecal egg counts. • Requires individual or pooled composite samples from 15 animals with minimum FEC of 250 epg. • Large number of animals required if multiple anthelmintics are to be tested. • Can be labor intensive and expensive. DRENCHRITE® TEST LARVAL DEVELOPMENT ASSAY (LDA) • In-vitro laboratory test that determines resistance to all dewormer “chemical” classes simultaneously from a single pooled fecal sample from 10-15 animals with a minimum FEC of 500 epg. • Larvae also identified, e.g. % HC • $450/sample • Done exclusively by Dr. Ray Kaplan’s lab at the University of Georgia.
  5. 5. Materials and Methods • In 2016, the American Sheep Industry Association’s Let’s Grow Program funded a project to determine anthelmintic resistance on 30 commercial sheep farms in the southeastern US. 1. Maryland, via University of Maryland (1-10) 2. Virginia, via Virginia State University (11-16) 3. Georgia, via Fort Valley State University (17-26) • The project cost-shared the DrenchRite® test for producers and facilitated the collection and shipment of samples. • $225 paid by Let’s Grow funds • $225 paid by producer
  6. 6. DrenchRite® larval development assay • Eggs are isolated from sample and cultured to L3s in specialized assay plates in the presence of varying (doubling) concentrations of the drugs. • The critical well is the number of the well in which 50% of the eggs are inhibited from developing to L3. • Critical well values which correlate to fecal egg count reductions are used to determine resistance. • Resistance is present if the percentage reduction in fecal egg count is less than 95%. As the percentage declines, the drug becomes less effective as the sole treatment. It is not effective below 50 percent. • Data from ivermectin wells is used to determine moxidectin resistance, since both drugs are in the same class.
  7. 7. Results (n=26): 82.1 ± 3.8% Haemonchus contortus 500 10850 8800 3900 6400 7950 1650 2650 1000 7100 1500 6500 500 2850 5000 600 4200 2750 6250 1150 3450 900 6300 3600 7850 500 53 100 99 93 93 97 96 84 94 85 54 90 38 72 97 52 92 92 88 97 91 91 91 59 96 42 0 2000 4000 6000 8000 10000 12000 0 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 FEC % HC
  8. 8. Results Percent farms with anthelmintic resistance (<95% FECR) 100% 92.3% 42.3%84.6% 0 10 20 30 40 50 60 70 80 90 100 Benzimidazoles Ivermectin Moxidectin Levamisole Maryland Virginia Georgia
  9. 9. Interpretation of test results Depending upon level of resistance, actual efficacy may vary from 0% to as high as 95%. Farm #5
  10. 10. BENZIMIDAZOLES BZ 1. Fenbendazole (SafeGuard®, Panacur®) 2. Albendazole Valbazen®
  11. 11. IVERMECTIN IVM 1. Ivermectin Ivomec® 2. Doramectin Dectomax® 3. Eprinomectin Eprinex®
  12. 12. MOXIDECTIN MOX DD Cydectin®
  13. 13. LEVAMISOLE LEV Prohibit® Leva-med®
  14. 14. Conclusions • All farms had resistance to one or more dewormers and dewormer classes. • Some farms have resistance to all dewormers and dewormer classes. • Resistance varies by geographic region and farm; worse the further south you go. • Farmers should test for anthelmintic resistance to determine resistance levels. • Many farms still have efficacy of one or more dewormers and need to take steps to maintain their effectiveness. • Due to widespread resistance, combination treatments are now recommended for clinically-parasitized animals.
  15. 15. This study was made possible by funding from the Let’s Grow committee of the American Sheep Industry Association (ASI).

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