Anthelmintic resistance and management of nematode parasites on beef cattle-rearing farms in the North Island of New Zealand

Abstract

It is clear that in practice anthelmintic resistance is a secondary consideration to obtaining productivity advantages from the use of anthelmintics in beef cattle. Farmers' opinions were divided on many issues and the overall impression was of confused and diverse thinking regarding the principles of the use of anthelmintics. The overall outlook regarding anthelmintic resistance in cattle is bleak unless the need for integrated and long-term research activities is acted upon soon.

Anthelmintic resistance in cattle nematodes on 13 south‐west Victorian properties

“…on dry summer pastures) become the major source of future populations unless sufficient non‐resistant worms to dilute them survive in ‘refugia’ from the anthelmintics. In a New Zealand survey, only 1 in 5 of the properties with breeder cows drenched the mature age cows, 19 whereas 7 of 11 properties in the present study drenched their adult cows at least once annually. It has been suggested worms in ‘refugia’ in non‐treated older animals play an important part in delaying the development of resistance 19 …”

“…and the longer drier summers means anthelmintic treatments may be more selective for O. ostertagi resistance than in New Zealand. The interaction between the season of treatment and the external environment is now accepted as a major factor explaining the development of anthelmintic resistance in sheep 17,18 and cattle nematodes 19 . Worms that survive treatments given during periods of minimal re‐infection with worm larvae (e.g.…”

“…Given the differences in the environments and management of the various properties in the two studies it is difficult to make direct comparisons. Furthermore, a farmer survey conducted at the same time as the New Zealand trials failed to identify significant factors associated with farms that had resistance, 19 so comparisons of the factors involved are not possible.…”

“…The interaction between the season of treatment and the external environment is now accepted as a major factor explaining the development of anthelmintic resistance in sheep 17,18 and cattle nematodes. 19 Worms that survive treatments given during periods of minimal re-infection with worm larvae (e.g. on dry summer pastures) become the major source of future populations unless sufficient non-resistant worms to dilute them survive in 'refugia' from the anthelmintics.…”

“…It has been suggested worms in 'refugia' in non-treated older animals play an important part in delaying the development of resistance. 19 The New Zealand survey was conducted from June 2004 to June 2005, 4 whereas the current study was conducted from 2006 to 2009, with most herds tested in 2008 or 2009. ML was only available 12 months earlier in New Zealand than in Australia.…”