RESPONSE OF CATTLE TREATED WITH A FENBENDAZOLE SLOW-RELEASE BOLUS TO CHALLENGE FROM NEMATODES THE FOLLOWING SEASON

Nematode infection of cattle treated in their first year at pasture wi th the fenbendazole slow release bolus ('Bolus group') was compared du ring the second year with that of untreated cattle. Ostertagia was the most prevalent parasite associated with Cooperia. Except for the Dict yocaulus spp. which caused clinical signs of bronchitis in the 'Bolus' group, the infection during the second year resulted in a moderate re sponse of the cattle whatever the group. Rises in both pepsinogen and gastrin levels were correlated with the number of Ostertagia L(3) on h erbage. Damages in the abomasal mucosa were more frequent and severe i n the 'Bolus' group where more inflammatory signs were observed in spi te of a smaller number of worms. Nevertheless, the differences in tota l weight gains were not significant thanks to a compensatory effect du ring the second part of the grazing season in the 'Bolus' group. Hypot heses related to a minimum threshold of infection during the first yea r necessary to develop high enough protection during the second year a re discussed. The pathological effects of gastrointestinal nematodes s eem to vary more according to the inflammatory response than to the nu mber of worms.