The recovery of fenbendazole efficacy on Haemonchus contortus by refugia management and worm population replacement

MUCHIUT, SEBASTIÁN; FERNÁNDEZ, ALICIA SILVINA; STEFFAN, PEDRO; LLOBERAS, MERCEDES; LUQUE, SONIA; CARDOZO, PATRICIA; BERNAT, GISELE; RIVA, ELIANA; FIEL, CÉSAR

Belem

Congreso; XIX Brazilian Congress of Veterinary Parasitology; 2016

Objective: To recover the high efficacy of fenbendazole (FBZ) on Haemonchus contortus resistant populations by means of management of worm refugia and introduction of benzimidazole-susceptible nematodes.. Methodology: The study was carried out over 2.5 years in an experimental farm located in the southeast of Buenos Aires province (Argentina), where clinical efficacy and absolute efficacy of FBZ against H. contortus had shown to be 82.18% and 0%, respectively. In the first year 40 Corriedale and Texel lambs on a pasture naturally infected with benzimidazole-resistant H. contortus were used. An anthelmintic treatment program to diminish larval populationon herbage was carried out based on faecal egg counts and FAMACHA performed fortnightly. In the second year 42 new weaned lambs were inoculated with 3000 infective larvae (L3) of H. contortus fully susceptible to benzimidazole and transferred onto the pasture used the previous year.. A nematode control program was carried out as described above, allowing the benzimidazole-susceptible strain to be established as refugia over 16 months. , A fecal egg count reduction test (FECRT) and a controlled efficacy test (CET) were performed at the end of the experiment to evaluate both, the clinical and absolute efficacy of FBZ against H. contortus. Results and Conclusions: After a 16 months period from the introduction of the benzimidazole-susceptible strain of H. contortus, the FECRT showed a clinical efficacy of 50.61% while the CET showed an absolute efficacy of 97.78% after treatingthe infected lambs with FBZ orally at a dose rate of 7.5 mg/kbw. At the end of the study the intestinal worms Nematodirus spathiger and Trichostrongylus colubriformis remained resistant to FBZ.. The present work shows that it might be possible to mitigate the phenomenon of anthelmintic resistance through refugia management and replacement of resistant worm population by susceptible ones. Partial financing of this study: FONCYT, PICT 2012-1048.