oxepins and Haemonchiasis

Drug resistance in gastrointestinal nematodes is a severe problem for sheep farmers. With the recent introduction of monepantel (Zolvix®) and of derquantel plus abamectin (Startect®) in New Zealand, two new anthelmintic classes will be available to control gastrointestinal nematodes. While monepantel covers a broad spectrum of nematodes, the efficacy of derquantel is mid-spectrum and limited to a smaller number of species and stages. The combination of derquantel and abamectin allows to enlarge the spectrum and to cover most parasitic nematodes in sheep. However, the question remained open, if the efficacy of the new anthelmintics can be maintained in the presence of severe anthelmintic resistance. The present study investigated the efficacy against adult stages of a multi-resistant Haemonchus contortus isolate. While monepantel resulted in 100 % elimination, derquantel in combination with abamectin resulted in efficacies <95 % (faecal egg counts and worm counts).

Topics: Aminoacetonitrile; Animals; Anthelmintics; Disease Models, Animal; Feces; Haemonchiasis; Haemonchus; Indoles; Ivermectin; Oxepins; Parasite Egg Count; Sheep; Treatment Outcome

Early in 2015, sheep in a summer rainfall area of NSW, Australia, displayed signs of haemonchosis despite treatment with monepantel. A faecal egg count reduction test (FECRT) was performed on yearlings with natural field infections using various anthelmintics. Only a four-way combination drench achieved a reduction in faecal egg count (FEC) greater than 95%. The combination contained abamectin, albendazole, levamisole and closantel. Treatments with a derquantel/abamectin combination, monepantel and moxidectin reduced FECs by 93, 31, and 30% respectively. Sheep treated with abamectin displayed an increase in FEC of 22%. Larval differentiation counts conducted 10days post-treatment showed that 100% of survivors were Haemonchus sp. This result confirms for the first time monepantel resistant Haemonchus in sheep in NSW, and is amongst the first of the Australian cases in sheep not associated with goats. A second FECRT was performed using sheep from the moxidectin and abamectin treatment groups in the first FECRT. In this second FECRT, monepantel treatment reduced FECs by 51% and 29% in the sheep previously treated with moxidectin and abamectin respectively. This suggests monepantel, in combination with moxidectin, may give some control against severely abamectin resistant Haemonchus.

Topics: Albendazole; Aminoacetonitrile; Animals; Anthelmintics; Australia; Drug Therapy, Combination; Feces; Haemonchiasis; Haemonchus; Indoles; Ivermectin; Macrolides; Male; Oxepins; Parasite Egg Count; Random Allocation; Salicylanilides; Sheep; Sheep Diseases; Treatment Failure