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.

Efficacy of ivermectin against inhibited larvae of Ostertagia ostertagi.

The efficacy of ivermectin against inhibited early 4th-stage larvae of ostertagia ostertagi and other nematodes of the abomasum and intestinal tract was determined in naturally infected yearling beef cattle. The time when large numbers of inhibited larvae were acquired was determined by monthly slaughter of monitor cattle, beginning in January. In April, 12 animals were removed from pasture and maintained free of further helminth exposure until slaughter (21 days). At 9 days after the cattle were removed from pasture, ivermectin was administered to the principals by subcutaneous injection (200 micrograms/kg); the other 6 animals were given subcutaneous injections of the ivermectin vehicle. both groups were klled and necropsied at 12 days after treatment. Mean numbers of O ostertagi in the 6 controls were: adults, 41,906; developing 4th stage, 73,813; and early 4th stage, 334,965. The mean proportion of early 4th-stage larvae was 73.7%. In the 6 principals (treated with ivermectin), the following reductions were observed: O ostertagi adults, 100%; developing 4th stage, 99.8%; and early 4th stage, 99,9%. Small numbers of dead and degenerated O ostertagi of all developmental stages were recovered from abomasal washings before fixation; few viable worms were recovered.

Further studies on the efficacy of fenbendazole against inhibited larvae of Ostertagia ostertagi.

Fenbendazole, at low dosage levels, was evaluated for its efficacy against inhibited early fourth stage larvae of Ostertagia ostertagi in naturally infected, yearling beef cattle in April. The mean number of inhibited larvae in 10 untreated control cattle was 51,500. At a dosage level of 1 mg per kg (pelleted fenbendazole) per day in feed over five days in 10 cattle, the percentage reduction in comparison to controls was: adults, 85.9 per cent; developing stages, 79.0 per cent; inhibited early fourth stage, 47.6 per cent; other abomasal genera (Haemonchus species and Trichostrongylus axei, 100 per cent). At a dosage level of 5 mg per kg (10 per cent liquid suspension) administered as an oral drench to 10 cattle, the percentage reduction in comparison to controls was: adults, 96.9 per cent; developing stages, 82.9 per cent; inhibited early fourth stage, 74.7 per cent; other genera, 98.6 per cent. Efficacy against parasite genera of the intestinal tract (primarily Cooperia species) was in excess of 94 per cent at both dosage levels. Some variability of efficacy was observed, in that large numbers of surviving inhibited O ostertagi larvae were recovered from two or three animals in treated groups.