Anthelmintic efficacy of albendazole against inhibited larvae of Ostertagia ostertagi.

Albendazole was evaluated for its efficacy against inhibited early 4th-stage larvae of Ostertagia ostertagi for the 3rd consecutive year (April 1979) in naturally infected yearling beef cattle. Mean numbers of O ostertagi developmental stages in 10 nontreated controls were: adults, 11,485; developing 4th-stage larvae, 1,370; and early 4th-stage larvae, 51,500. The mean proportion of inhibited early 4th-stage larvae was 76.8%. Small numbers of Haemonchus sp and Trichostrongylus axei were uniformly present in the controls. At a dosage level of 7.5 mg/kg of body weight in a group of 10 cattle, the percentage of reduction in comparison with controls was: O ostertagi adults, 93.5; developing 4th-stage larvae, 53.4; developing 4th-stage larvae, 30.8. Percentage of reduction against Haemonchus sp and T axei adults was 94.5 and 100.0, respectively. At the larger dosage of 10.0 mg/kg in 10 cattle, percentage of reduction was: O ostertagi adults, 94.8; developing 4th-stage larvae, 18.6. Percentage of reduction against Haemonchus sp and T axei adults was 100.0 for both species. Considerable variability of efficacy against inhibited larvae, as well as developing 4th-stage larvae in half of the animals of each treatment group, was responsible for the poor reductions. Clinical signs of toxicosis were not observed after administration of albendazole.