THE DEVELOPMENT OF PROTECTIVE IMMUNITY AGAINST GASTROINTESTINAL NEMATODE AND LUNGWORM INFECTIONS AFTER USE OF AN IVERMECTIN BOLUS IN FIRST-YEAR GRAZING CALVES

Interactions between treatment with an ivermectin bolus at turnout and immunity to bovine parasitic gastroenteritis and bronchitis were exam ined, Immunity related parameters, i.e. development of clinical diseas e, parasite development and stimulation of parasite specific antibodie s were examined for two grazing seasons and compared with untreated se cond season cattle (immune control) and untreated parasite naive cattl e (nonimmune control). With respect to gastrointestinal nematodes, cli nical condition, body weight development faecal egg counts and pepsino gen concentrations of the bolus treated animals were not significantly different from the respective values of untreated immune cattle, show ing a considerable degree of resistance in both groups compared to the parasite naive cattle. With respect to lungworms, untreated immune ca ttle were protected against clinical disease, but two of eight animals shed larvae again. Bolus treated animals transiently showed mild clin ical symptoms and six of seven animals shed low numbers of larvae agai n, whereas all parasite naive cattle shed high numbers of lungworm lar vae and showed signs of disease during the whole grazing season. In sp ite of the effective treatment with an ivermectin bolus during the fir st year a considerable resistance to gastrointestinal nematode and lun gworm infection was present in the second grazing season.