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A multi-species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems

Dobson, R.J., Barnes, E.H., Tyrrell, K.L., Hosking, B.C., Larsen, J.W.A., Besier, R.B., Love, S., Rolfe, P.F. and Bailey, J.N. (2011) A multi-species model to assess the effect of refugia on worm control and anthelmintic resistance in sheep grazing systems. Australian Veterinary Journal, 89 (6). pp. 200-208.

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    Link to Published Version: http://dx.doi.org/10.1111/j.1751-0813.2011.00719.x
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    Abstract

    Develop a computer simulation model that uses daily meteorological data and farm management practices to predict populations of Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia (Ostertagia) circumcincta and the evolution of anthelmintic resistance within a sheep flock. Use the model to explore if increased refugia, provided by leaving some adult sheep untreated, would delay development of anthelmintic resistance without compromising nematode control.

    Procedures
    Compare model predictions with field observations from a breeding flock in Armidale, NSW. Simulate the impact of leaving 1-10% of adult sheep untreated in diverse sheep-grazing systems.

    Results
    Predicted populations of Tr. colubriformis and T. circumcincta were less than those observed in the field, attributed to nutritional stress experienced by the sheep during drought and not accounted for by the model. Observed variation in faecal egg counts explained by the model (R2) for these species was 40-50%. The H. contortus populations and R2 were both low. Leaving some sheep untreated worked best in situations where animals were already grazing or were moved onto pastures with low populations of infective larvae. In those cases, anthelmintic resistance was delayed and nematode control was maintained when 1-4% of adult stock remained untreated.

    Conclusions
    In general, the model predicted that leaving more than 4% of adults untreated did not sufficiently delay the development of anthelmintic resistance to justify the increased production risk from such a strategy. The choice of a drug rotation strategy had an equal or larger effect on nematode control, and selection for resistance, than leaving 1-10% of adults untreated.

    Publication Type: Journal Article
    Murdoch Affiliation: School of Veterinary and Biomedical Sciences
    Publisher: Wiley-Blackwell
    Copyright: 2011 The Authors
    Notes: ‘The definitive version is available at www.interscience.wiley.com’
    URI: http://researchrepository.murdoch.edu.au/id/eprint/4464
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