DEVELOPMENT OF IMMUNITY TO LUNGWORM IN VACCINATED CALVES TREATED WITHAN IVERMECTIN SUSTAINED-RELEASE BOLUS OR AN OXFENDAZOLE PULSE RELEASEBOLUS AT TURNOUT

The ivermectin sustained release bolus (IVSRB), when used at turnout a s recommended, will provide season-long control of parasitic bronchiti s, thus obviating the need for use of a lungworm vaccine. However, som e concerns have been expressed that calves treated with an IVSRB will receive so little exposure to Dictyocaulus viviparus that it will comp romise their immunity in subsequent grazing seasons, which would be of particular importance in dairy herds. Although there is evidence that IVSRB-treated calves can develop immunity to D. viviparus when expose d to pasture infection, it was considered worthwhile to evaluate the c ompatibility of the IVSRB and lungworm vaccination to allow veterinary surgeons the option of using these products concurrently when they ha ve particular concerns about the long term immune status of replacemen t dairy heifers. Thirty-two dairy replacement heifers were vaccinated with two doses of lungworm vaccine and, at turnout, half the calves re ceived an IVSRB and the remainder an oxfendazole pulse release bolus ( OPRB). At the end of the grazing season four replicate bolus treated p airs and four parasite-naive calves were challenged with 1000 D. vivip arus infective larvae. At slaughter there was a 95% and 93% reduction in D. viviparus burdens in the IVSRB and OPRB treated calves respectiv ely, compared with the unvaccinated, untreated controls. These results indicate that where it is considered necessary to use lungworm vaccin ation in addition to an IVSRB or an OPRB, the compatibility of these p roducts with lungworm vaccine will allow development of a protective l evel of immunity to D. viviparus.