Comparative efficacy of ivermectin pour-on, albendazole, oxfendazole and fenbendazole against Ostertagia ostertagi inhibited larvae, other gastrointestinal nematodes and lungworm of cattle.

An experiment was conducted to evaluate the current efficacy of albendazole (ABZ), oxfendazole (OXF) and fenbendazole (FBZ) compared with ivermectin pour-on (IVM-PO) against inhibited early fourth-stage larvae (IEL4) of Ostertagia ostertagi, other gastrointestinal nematodes and lungworm of cattle during spring in Louisiana. Twenty-five crossbred beef heifer calves of 235 kg average weight and 10-12 months of age were acquired in late winter and grazed for 9 weeks on pasture contaminated with O. ostertagi and other nematodes until May 15. The cattle were weighed and randomly allotted into 5 groups of 5 calves on May 16 (day 0) and treatments were as follows: group 1, nontreated controls (CONT); group 2, IVM-PO on mid-backline at 500 micrograms/kg; group 3, ABZ suspension (oral) at 10 mg/kg; group 4, OXF suspension (oral) at 4.5 mg/kg; group 5, FBZ suspension (oral) at 5 mg/kg. After treatment and confinement in separate pens for each group, approximately equal numbers of cattle from each group were necropsied daily between days 29-31. Mean numbers of O. ostertagi developmental stages present in CONT were: adult, 5234; developing (DL4), 3130; IEL4, 44,077. The mean percentage of IEL4 was 84.1. Cooperia spp. were the second most prevalent in CONT (20,307) and smaller numbers of abomasal and intestinal species and Dictyocaulus viviparus were present in nearly all CONT. Percent reductions for the four compounds against O. ostertagi adult, DL4 and IEL4, respectively, were IVM-PO: 99.7, 98.3, 98.1; ABZ: 74.1, 76.5, 75.3; OXF: 78.5, 42.1, 32.0; FBZ: 63.6, 17.7, 39.7. Efficacy of IVM-PO was greater (P < 0.05) against all O. ostertagi stages than the benzimidazole (BZ) drugs, except for ABZ (DL4). There were no significant differences in group means (except for C. punctata adult males, P < 0.05 lower for IVM-PO) or wide variation in reduction percentages for other abomasal and intestinal species and D. viviparus between IVM-PO and BZ drugs. The low efficacy of all three BZ drugs for O. ostertagi, but especially for OXF and FBZ, under conditions of this experiment, were suggestive of drug tolerance to dosages employed or resistance. However, problems of drug availability/concentration in the abomasum, and the factor of IEL4 and DL4 replacing adult worms after treatment cannot be excluded. Efficacy of the BZ drugs against all other species were essentially similar to that of IVM-PO.