the problem of escalating resistance of haemonchus contortus to the modern anthelmintics in south africa.during the past decade in south africa there has been a continual increase in sheep of strains of gastrointestinal helminths resistant to the modern anthelmintics. five strains of haemonchus contortus are described in this paper. despite the fact that 2 of the 5 strains were tested for susceptibility only to ivermectin, a total of 10 instances of resistance were found. four of the 5 strains were resistant to ivermectin, 2 to closantel, 2 to rafoxanide and 2 to the benzimidazoles. one of these st ...19892726192
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