ANTHELMINTIC RESISTANCE IN GASTROINTESTINAL NEMATODES OF SHEEP

Anthelmintics are used traditionally as an integral part of helminthic control strategies for grazing livestock to prevent production losses from parasitic infections. The continuous and indiscriminate use of the same anthelmintics over years together as the sole means of control are now failing due to the emergence of resistance strains of helminths. Resistance to the commonly used anthelmintics in gastrointestinal nematodes of sheep has become an increasingly wide spread problem throughout the world. The present study was aimed to determine the resistance against albendazole, fenbendazole, levamisole and closantel in gastrointestinal nematodes of sheep. Fifty five naturally infected Madras Red lambs of 6 - 12 months of age were selected and distributed randomly into five treatment groups of eleven animals each. Four groups were treated orally with albendazole (5mg/kg), fenbendazole (7mg/kg), levamisole (7.5m/kg) and closantel (10 mg/kg) respectively, while the fifth group served as untreated control.  Faecal samples were collected per rectum of each lamb just prior to treatment (pre treatment) and then on days 7, 14, 21 and 28 after treatment. The anthelmintic resistance was evaluated by in vivo faecal egg count reduction test (FECRT), post treatment larval culture and in vitro egg hatch assay. In the faecal egg count reduction test, albendazole reduced the faecal egg count by 86.50 per cent, 84.81 per cent, 85.28 per cent and 84.47 per cent respectively for four weeks after treatment. Faecal egg count reduction using fenbendazole was 92.64, 93.04, 90.80 and 90.06 per cent respectively for four weeks after treatment.  The per cent efficacy for levamisole and closantel was more than 95 per cent. The post treatment larval culture contained only Haemonchus contortus. In the in vitro egg hatch assay, the ED 50 value for benzimidazole was 0.299 µg thiabenazole/ml and levamisole showed an ED 50 value of 0.283 µg /ml.