Regimens to prevent the development of anthelmintic resistance (AR) have been promoted for many years. All reflect some appreciation of the selection pressure that different anthelmintic treatment policies apply to nematodes but AR is clearly more complicated than this for reasons we still do not yet understand sufficiently well. Whilst managing a refugia population on a farm is important to limit the development of AR there are many aspects that are not understood. Why is AR more common in some species than others, especially for some such as Dictyocaulus where there is clearly a short-lived refugia population and heavy selection pressure yet no recorded AR. The cost of AR to farmers has slowly escalated over the years and using a New Zealand example it can be estimated for a typical sheep farm that the cost of using a single active of an older anthelmintic such as levamisole compared to a newer anthelmintic such as monepantel is about NZ$3.27 per lamb raised. Diagnosis of AR continues to largely rely on the use of faecal egg count reduction tests. These are a very crude tool but generally acceptable for sheep. However, for cattle and farmed deer it is difficult to get egg counts high enough to conduct a meaningful test. Clearly more effort is needed to understand the mechanisms associated with AR and to develop better diagnostic tests so that farmers with their advisors are better able to achieve sustainable helminth control.
Proceedings of the New Zealand Society of Animal Production, Volume 78, Lincoln, 105-108, 2018
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