Development of immunity to lungworm in vaccinated calves treated with an ivermectin sustained release bolus or an oxfendazole pulse release bolus at turnout.

The ivermectin sustained release bolus (IVSRB), when used at turnout as recommended, will provide season-long control of parasitic bronchitis, thus obviating the need for use of a lungworm vaccine. However, some concerns have been expressed that calves treated with an IVSRB will receive so little exposure to Dictyocaulus viviparus that it will compromise 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 exposed to pasture infection, it was considered worthwhile to evaluate the compatibility of the IVSRB and lungworm vaccination to allow veterinary surgeons the option of using these products concurrently when they have particular concerns about the long term immune status of replacement dairy heifers. Thirty-two dairy replacement heifers were vaccinated with two doses of lungworm vaccine and, at turnout, half the calves received an IVSRB and the remainder an oxfendazole pulse release bolus (OPRB). At the end of the grazing season four replicate bolus treated pairs and four parasite-naive calves were challenged with 1000 D. viviparus infective larvae. At slaughter there was a 95% and 93% reduction in D. viviparus burdens in the IVSRB and OPRB treated calves respectively, compared with the unvaccinated, untreated controls. These results indicate that where it is considered necessary to use lungworm vaccination in addition to an IVSRB or an OPRB, the compatibility of these products with lungworm vaccine will allow development of a protective level of immunity to D. viviparus.