Two field trials, one with suckling Merino ewe lambs and the other with yearling Dohne Merino rams, are described. In these the anthelmintic efficacy of febantel (a benzimidazole), ivermectin, levamisole and morantel are compared, using the first stage larval reduction test. The mean natural log (+ 1 for zero values) of the post treatment larval counts of the treated groups was compared with that of the untreated controls and the percentage reduction used to assess anthelmintic efficacy.
Febantel was only 87,4% effective against Teladorsagia in suckling lambs but the other anthelmintics were more than 99% effective against this genus. Efficacy against Haemonchus and Trichostrongylus ranged from 93,2%-100% for all 4 compounds. In the rams all compounds were 100% effective against Trichostrongylus, with the exception of morantel which was only 87,5% effective. None of the compounds were effective against Teladorsagia, particularly morantel, animals treated with which having more larvae than the controls.
The interpretation of anthelmintic efficacy; the advantages of the first stage larval reduction test, compared with the faecal egg count reduction test; and the importance of incubating cultures at 30°C for 24 h, in order to harvest first stage larvae, are discussed.
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