Use of anthelmintic combinations against multiple resistant Haemonchus contortus in Angora goats

Abstract

A fecal egg count reduction test (FECRT) was run in a flock of Angora goats naturally infected with a strain of ivermectin resistant Haemonchus contortus. Ivermectin (IVM) (0.45 mg kg −1), albendazole (ABZ) (40 mg kg −1), their combination (IVM + ABZ), fenbendazole (FBZ) (23 mg kg −1), levamisole (LEV) (16 mg kg −1), and their combination (FBZ + LEV) were tested. The Angora wethers, weighing between 18 and 27 kg, were divided into groups of 15 and treated as above. There were no untreated controls at the insistence of the owner. Worms were resistant to IVM, FBZ and LEV (efficacies based on FECRT of − 179%, 1% and 23%, respectively). The combination of FBZ and LEV demonstrated a synergistic effect by reducing eggs per gram counts by 62%. The combination of ABZ and IVM (97%) was not different from ABZ alone (91%). Geometric and arithmetic means were compared and a case was made for using arithmetic means to determine resistance. The only anthelmintic which had an acceptable FECRT in this flock was albendazole at 40 mg kg −1, four times the dosage used in cattle in the United States. The synergistic effect of drug combinations was superior to each individual compound but when resistance to multiple families of anthelmintics was already present the value of combinations is still likely to be disappointing.