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Anthelmintic Resistance

2. Worldwide anthelmintic resistance situation

Although widely reported in sheep, AR appears less of a problem in cattle. This may be a reflection of the relative frequency of treatment and the differences in parasite population dynamics between the two hosts. It may also reflect the pro- longed survivability of free-living larval stages within the bovine faecal pat, thus ensuring a supply of sus-ceptible worms.

BZ-resistance has been described in Cooperia spp., Haemonchus spp., Ostertagia ostertagi and Trichostrongylus axei in Australia, New Zealand, USA, South Africa and parts of Europe. Reports of ML-resistance in cattle nematodes have been less common but have been described, mainly in Cooperia oncophora, but also Haemonchus spp., Trichostrongylus longispicularis in several of these countries, and more recently in O. ostertagi, in the USA. There have been a small number of reports from the USA, New Zealand and South America of cattle nematodes resistant to multiple anthelmintics.

Most reports of ML resistance in cattle have been reported in Cooperia species following the identification of positive FEC or FECRT after use of pour-on treatments. Poor absorption of pour-on ML anthelmintics and subsequent reduced efficacy against Cooperia species, which are the dose-limiting species for the ML group (Vercruysse and Rew 2002), provides a more likely explanation for positive post-treatment FEC than acquired resistance (McKenna 1995). However, in the longer term, shedding of Cooperia spp. eggs during the prepatent period following treatment with topical ML anthelmintics has been shown experimentally to select for AR (Van Zeveren and others 2007) and may lead to increasing AR reports in these species.

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