Age and the ability of calves to respond to a leptospiral vaccine.

The serological responses of calves at two different ages to a commercial hardjo/pomona vaccine were examined. Microscopic agglutination test (MAT) titres of a group of ten three-month-old calves and three groups of 11,12 and 12 six-month-old calves were monitored over a period of 28 weeks. The calves vaccinated at three months of age had a poorer serological response rate to vaccination (30% responded to pomona and 40% responded to hardjo) compared with those vaccinated at six months of age (91-100% responded to pomona and 83-91% responded to hardjo). Those three-month-old calves which did respond also produced a lower level of-antibody than six-month-old calves. It was concluded that in order to achieve adequate protection using commercially available leptospiral bacterins at the recommended dose, vaccination should not be undertaken routinely with animals less than six months of age. Where circumstances arise in which earlier vaccination is necessary, repeat vaccination for continued protection at six months or older is suggested.