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 Drug Level Time Fenbendazole Endectocides
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. Egg Hatch Test for BZ
Discriminating doses of Thiabendazole (TBZ) (relatively high solubility in water) used to determine % of hatch
Discriminating dose (DD) = dose that prevents hatching of 99% of susceptible eggs
DD for H. contortus, T. circumcincta, and T. colubriformis have been established for sheep

15. Egg Hatch Test for BZ % of eggs hatching = % of BZ R
Will probably not indicate eggs that are heterozygous for R unless genes for R are dominant
Discriminating dose not established for cattle

16. Microagar Larval Development Test
Age of eggs not important as long as not starting to hatch
Relies on development of eggs to L3 larvae
Speciated at the end of the test
Only reliable for BZ and LEV
Has not been adequately evaluated, e.g. discriminating doses, for bovine, to assure its efficacy

17. Molecular (PCR) Based Tests
BZ only
Only when diagnosis can be obtained from pooled larval DNA samples will routine testing take place
Testing of single stages too expensive

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

19. Facts
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

20. Facts
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

21. Facts 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

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

23. Combining FBZ and Avermectin
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)

24. Combining FBZ and Avermectin
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