THE EFFECT OF TIME BETWEEN DOSES ON SEROLOGICAL RESPONSE TO A RECOMBINANT MULTIVALENT PILUS VACCINE AGAINST FOOTROT IN SHEEP

The response of sheep to a recombinant multivalent footrot vaccine con taining pilus antigens was examined after the administration of two do ses of vaccine at intervals ranging from 2 to 52 weeks. Agglutinating antibody titres were measured 3 weeks after the second vaccination and showed that lengthening of the interdose interval results in higher a gglutinin titres. The capability of sheep to mount an increasingly str ong immune response as the interval between doses is increased provide s an opportunity to maximise the usefulness of vaccination by administ ering the first dose well before an expected footrot transmission peri od. This advantage of increasing the interdose interval has not been r eported for traditional, whole-cell footrot vaccines, and use of the n ew pilus vaccine in this manner may improve prospects for disease cont rol. Furthermore, sheep given a third dose either 6 or 12 months after their initial two-dose vaccination program achieved significantly hig her titres than those elicited after the second dose, suggesting the l ikelihood of further improvement in disease control in successive seas ons.