Anthelmintic trails, conducted with albendazole, fenbendazole and ivermectin for efficacy against gastrointestinal nematodes, principally inhibited early fourth larval stages of Ostertagia ostertagi in naturally infected cattle. Cattle wee slaughtered seven to 20 days after treatment. O ostertagi was the predominant abomasal nematode recovered with occasional small numbers of Haemonchus species and Trichostrongylus axei. Control calves uniformly had very large O ostertagi infections, primarily early fourth stage larvae. Viable surviving worms and variable numbers of dead and degenerate worms were recovered in abomasal contents and washings. These O ostertagi larvae and adults were characterised by adherent debris or proteinaceous material, degenerated cuticles and distortion of internal structures. This study demonstrated the necessity for proper timing of slaughter for anthelmintic trial evaluation to allow clearance of dead nematodes, specifically O ostertagi larvae which are sequestered in the abomasal glandular tissue. Nematode collection within seven to 12 days after treatment will include dead and degenerate larval nematodes. The peripheral coating of larvae was suggestive of the Splendore-Hoeppli effect which has been associated with immunological responsiveness. The antigenic stimulus for this material and the lymphocyte and eosinophil infiltration was suspected to be early fourth stage O ostertagi larvae within the mucosa but was not identified definitively.

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.

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