Prophylaxis of nematode infections in cattle with an indwelling rumino-reticular ivermectin sustained release bolus.

Ivermectin delivered continuously from a rumino-reticular sustained release device was prophylactically effective in preventing establishment of nine nematode parasite species in multiply-exposed cattle. Ivermectin dosages which permitted less than or equal to 1% of infected control calf worm populations to establish ranged from less than 2.5 micrograms kg-1 day-1, which totally prevented infection with Dictyocaulus viviparus and Oesophagostomum radiatum, to approximately equal to 30 micrograms kg-1 day-1 required to suppress Nematodirus helvetianus to the same extent. Between these extremes, in decreasing order of sensitivity to enterical sustained release ivermectin, were Ostertagia ostertagi, Trichostrongylus axei, Haemonchus placei, Cooperia punctata, C. oncophora and T. colubriformis which were maximally affected at less than or equal to 10 micrograms kg-1 day-1 of ivermectin.