The bolus treats and prophylactically controls gastro-intestinal nematode infections in cattle caused by Ostertagia spp., Trichostrongylus spp., Cooperia spp., Oesophagostomum spp. and Haemonchus spp. The bolus controls parasitic bronchitis caused by Dictyocaulus viviparus. For use in ruminating cattle weighing between 100 kg and 300 kg on day of administration. When administered at turnout, the bolus controls parasitic gastroenteritis throughout the grazing season by reducing the build up of infective larvae on the pasture. Reduced pasture contamination in the Autumn lowers the risk of inhibited Ostertagia larvae accumulating in sufficient numbers to cause winter ostertagiasis. When administered later in the season, the bolus is effective in the treatment of established parasitic infections and continues to have a prophylactic effect up to 140 days after administration. This period may be reduced if cattle are moved to heavily infected pasture.