Researchers in Argentina were able to restore the efficacy of fenbendazole (SafeGuard®) by replacing a resistant population of Haemonchus contortus with a susceptible one.
At the start of the experiment, the efficacy of fenbendazole was determined to be 0%. The resistant population of worms was reduced by anthelmintic treatments with efficacious drugs. Fecal egg counts were reduced from 2968 (300-7740) to 0 epg. Adult worm burdens of H. contortus were reduced from 2625 (800-5100) to 0.
At weaning, lambs were artificially dosed with a new susceptible population of worms. Fecal egg counts of up to 7275 (3240-13080) epg and adult worm burdens of up to 500 (200-800) of H. contortus were achieved. Lambs were left to graze the experimental pasture for 11 months. At 16 months post-population replacement, the absolute efficacy of benzimidazoles was determined to be 97.58%.
Source: Veterinary Parasitology, July 2019.