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How to improve Drenchrite

Dobson, R.J., Stein, P.A. and Craven, J.H. (1998) How to improve Drenchrite. In: Australian Sheep Veterinary Society AVA conference, 17 - 22 May, Sydney, Australia pp. 16-20.

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Understanding the strengths and weaknesses of DrenchRite and how it relates to faecal egg count reduction tests (FECRT) will lead to clearer interpretation of DrenchRite results and a better perspective on both tests. In previous studies efficacy was estimated for worms resistant to BZ, LEV and BZILEV combinations by FECRT, DrenchRite and post mortem worm counts. Correlations of in vivo efficacy with FECRT determined by them were: Trichostrongylus colubriformis 88%, Ostertagia circumcincta 56% and Haemonchus contortus 69%. The low correlation for O. circumcincta was associated with the variability in estimating species composition when O. circumcincta was present at low levels. Correlations of DrenchRite with in vivo efficacy were: O. circumcincta 87%, T. colubriformis 82% and H. contortus 80%. The correlation between resistance determined by Larval Development Assay (LDA) and FECRT was not high at 61% (averaged over drugs and species). Similar data for Macrocyclic Lactones (ML) resistant worms are not yet available.

Previous studies have shown for some LDA systems that eggs are more likely to be inhibited in development to L3s early (2-3 weeks post infection (PI)) and late (beyond 10 weeks PI) after infection and are most resilient to the effect of drugs at about 7 weeks PI. No systematic changes in DrenchRite results were observed over age of worm infection when tested weekly for 2 to 14 week old worms.

Large bowel worms of sheep have not generally been examined under LDA conditions so a study was undertaken to determine the LD50s of susceptible Oesophagostomum columbianum (nodule worm) and Chabertia ovina (large mouthed bowel worm) on DrenchRite plates. LD50s for BZ and LEV were approximately 0.04 and 0.8 uM respectively for both worms. For the ML O. columbianum had a much higher LD50 (45 nM) than C. ovina (8 nM). In relation to the other common sheep worms, development characteristics of C. ovina in LDA is like H. contortus while O. columbianum is more like O. circumcincta. Both large bowel worms were very sensitive to BZILEV combinations, with LD50s 4 to 8 fold lower than those observed for H. contortus and O. circumcincta.

Item Type: Conference Paper
Notes: In: Australian Sheep Veterinary Society AVA conference proceedings : reaching new heights, 17-22 May 1998, Sydney pp 16-20.
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