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Bill Burdett, D.V.M., Intervet Inc.

Bovine Nematode Resistance. Bill Burdett, D.V.M., Intervet Inc. An Overview. Today’s Discussion. Resistance (R) – what is it? Major nematodes of cattle Classes of dewormers Methods of R detection Characteristics of nematode infection Fecal sampling Case histories of R

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Bill Burdett, D.V.M., Intervet Inc.

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  1. Bovine Nematode Resistance Bill Burdett, D.V.M., Intervet Inc. An Overview

  2. Today’s Discussion Resistance (R) – what is it? Major nematodes of cattle Classes of dewormers Methods of R detection Characteristics of nematode infection Fecal sampling Case histories of R Using combinations of dewormers

  3. Defining Resistance What R is not: “It is not the development of superworms.” (Anthelmintic Resistance Roundtable, 2005) Some disagreement as to whether or not it is truly mutation per se How does it occur? “We are simply creating an environment where that particular genetic makeup survives.” (Dr. Cliff Monahan, OhSU, Anthelmintic Resistance Roundtable, 2005)

  4. Defining Resistance “You are not changing something in the worm. By giving dewormers, we are not inducing resistance – we are simply providing an environment for the resistant parasites to flourish and multiply.” (Dr. Dwight Bowman, Cornell U., Anthelmintic Resistance Roundtable, 2005) <90% worm egg reduction following use of a specific dewormer = resistance

  5. Scope of the Problem Huge in sheep and goats Becoming a major issue in cattle, particularly to the endectocides (macrocyclic lactones - avermectins and milbemycins – ivermectin, doramectin, eprinomectin, and moxidectin)

  6. Risk Factors for Resistance Frequent use of same drug class Subtherapeutic drug levels from under dosing and/or persistance in the environment at low levels

  7. Parasite Resistance Fenbendazole Endectocides Drug Level Lethal Dose 70 – 80 days 24 hours Time CONCEPT ILLUSTRATION

  8. Major Cattle Nematodes Predominant genera = Ostertagia and Cooperia Less predominant genera = Haemonchus, Trichostrongylus, Oesophagostomum, Strongyloides, Trichuris, Nematodirus, and Monezia

  9. Broad Spectrum Nematode Dewormers Group 1 = benzimadazoles (BZ) (attack parasite’s metabolic system) Group 2 = imidazoles (levamisole, LEV) and hydropyrimidines (pyrantel/morantel) Group 3 = macrocyclic lactones (avermectins and milbemycins, ML) (attack parasite’s nervous system)

  10. Methods of Resistance (R) Detection Fecal Egg Reduction Count Test (FERCT) for R to any dewormer (only test available for macrocyclic lactones) Egg hatch test for BZ R Microagar larval development test for BZ and LEV R

  11. Methods of Resistance (R) Detection PCR based test for BZ R in strongyles of sheep and goats, cattle, and horses Real time PCR for quantitative determination of gene frequency Controlled efficacy test – gold standard

  12. FERCT Nematode eggs counted on initial fecal exam Eggs counted again at defined times following treatment, e.g. 14 days % reduction determined Only reliable if > 25% of worms resistant

  13. FERCT Original product pour on? Confirmation of R using an injectable or oral to eliminate possible problems with malabsorption Cattle Ideal to use animals with minimum count of 100 epg (higher in sheep) ≥17, e.g. 20 cattle per each anthelmintic tested ML’s persistant activity (label) second sample at X days post-treatment depending on species recommended combined with larval cultures (for appropriate species)

  14. Pertinent Things to Remember And Some Examples of Combination Use and Resistance

  15. Calves 3-4 months of age and grazing for at least 2 weeks most susceptible and likely to have most eggs Moisture and warm temperatures (>40°F) Nematodirus – few eggs but very detrimental to calves Facts

  16. Small number of animals typically shed most of the eggs, e.g. the 80/20 rule, particularly as animals age Recommended minimum number of samples = 20 Sampling too few animals in a group will likely under represent the small percentage of high egg shedding animals Standard deviation exceeds the mean Facts

  17. Heritability of egg shedding = .3 - .4 Nematodes drive a Th2 immune response which tends to counter a Th 1 response Th1 response necessary for good immunity to intracellular pathogens such as viruses Single most significant factor of economic loss from internal parasite infection = appetite suppression Facts

  18. Physiological responses to GI nematodes Anorexia Increased flow rate of feedstuffs through gut Increases muscle mass in intestinal tract Decreased fluid absorption Facts

  19. First feedyard trial (moderate egg counts,e.g. in the 30’s per gram) demonstrated extra 18 lbs. gain for FBZ + Ivomec pour on versus Ivomec pour on alone (+$20.08) Second feedyard trial (low egg counts, 7-9 per gram) showed extra 20 lbs. FBZ + Ivomec pour on versus Dectomax injectable alone ($9.31) Combining FBZ and Avermectin

  20. Third feedyard trial (moderate egg counts, 150’s per 3 grams) demonstrated reduced egg counts favoring FBZ or FBZ + Ivermectin injectable versus Ivermectin injectable but no difference in economic return Dr. Tom Elam economic evaluation of the three trials showed an advantage of using both FBZ and an avermectin of $22.19, $9.01, and $4.20, respectively Combining FBZ and Avermectin

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