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Comparison of calculation methods used for the determination of anthelmintic resistance in sheep in a temperate continental climate

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An Erratum to this article was published on 14 February 2015

Abstract

This study compared results obtained with five different fecal egg count reduction (FECR) calculation methods for defining resistance to ivermectin, fenbendazole, and levamisole in gastrointestinal nematodes of sheep in a temperate continental climate: FECR1 and FECR2 used pre- and posttreatment fecal egg count (FEC) means from both treated and control animals, but FECR1 used arithmetic means, whereas FECR2 used geometric means; FECR3 used arithmetic means for pre- and posttreatment FECs from treated animals only; FECR4 was calculated using only arithmetic means for posttreatment FECs from treated and control animals; and FECR5 was calculated using mean FEC estimates from a general linear mixed model. The classification of farm anthelmintic resistance (AR) status varied, depending on which FECR calculation method was used and whether a bias correction term (BCT, i.e., half the minimum detection limit) was added to the zeroes or not. Overall, agreement between all methods was higher when a BCT was used, particularly when levels of resistance were low. FECR4 showed the highest agreement with all the other FECR methods. We therefore recommend that small ruminant clinicians use the FECR4 formula with a BCT for AR determination, as this would reduce the cost of the FECRT, while still minimizing bias and allowing for comparisons between different farms. For researchers, we recommend the use of FECR1 or FECR2, as the inclusion of both pre- and posttreatment FECs and use of randomly allocated animals in treatment and control groups makes these methods mathematically more likely to estimate the true anthelmintic efficacy.

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Acknowledgments

This research was supported by the Ontario Ministry of Agriculture, Food and Rural Affairs-University of Guelph agreement through the Animal Health Strategic Investment fund, with additional in-kind assistance from Merial. The authors are very grateful to Krishna Shakya and Jacob Avula for their assistance in the field and laboratory. We especially acknowledge the sheep producers that participated in the study.

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Author notes

  1. L. C. Falzon

    Present address: Veterinary Public Health Institute, Vetsuisse Faculty, University of Bern, Schwarzenburgstrasse 155, 3097, Liebefeld, Switzerland

Authors and Affiliations

  1. Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, ON, N1G 2 W1, Canada

    L. C. Falzon, P. I. Menzies, A. Jones-Bitton & W. Sears

  2. Centre for Veterinary Epidemiological Research, Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PE, C1A 4P3, Canada

    J. van Leeuwen

  3. Veterinary Science and Policy, Ontario Ministry of Agriculture and Food, Elora, ON, N0B 1S0, Canada

    J. T. Jansen

  4. Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario, N1G 2 W1, Canada

    A. S. Peregrine

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  1. L. C. Falzon
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  2. J. van Leeuwen
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Correspondence to L. C. Falzon.

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Falzon, L.C., van Leeuwen, J., Menzies, P.I. et al. Comparison of calculation methods used for the determination of anthelmintic resistance in sheep in a temperate continental climate. Parasitol Res 113, 2311–2322 (2014). https://doi.org/10.1007/s00436-014-3886-9

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