Vaccination against bovine brucellosis with a low dose of strain 19 administered by the conjunctival route. III. -- Serological response and immunity in the pregnant cow.

Seventy-four heifers, 7 to 12 months old, were randomized in four groups: group A, 8 heifers as controls; group B, 19 heifers vaccinated subcutaneously with 9 X 10(10) Brucella abortus strain B 19; group C, 19 heifers vaccinated as in group B, then revaccinated by the conjunctival route 6 to 8 months later with 5 X 10(9) bacteria; group D, 28 heifers vaccinated twice by the conjunctival route with the same dose and time intervals as in group C. Serological responses in agglutination, complement fixation and Rose Bengal tests were typical of those following standard vaccination with Strain B 19 in group B. Iu group C after the booster vaccination, there was a transient rise in titers which lasted about 3 months. Iu group D, titers were infrequent, low, and lasted no more than 8 weeks, after both primary and secondary vaccination. Fifty of the heifers, when 4 1/2 to 6 1/2 months pregnant, were challenged by the conjunctival route, with 16 X 10(6) B. abortus strain 544. Calves were born at full term (greater than or equal to 264 days) to 1/7 heifers in group A, 6/12 in group B, 8/II in group C and 14/19 in group D. Serological tests every two weeks after challenge; bacteriological examination of vaginal mucus, colostrum, foetuses and dead calves; bacterial enumeration of ten mixed samples of lymph nodes and organs taken at slaughter about 6 weeks after parturition, were made to determine the infection status of the heifers. Brucella was isolated at some time from 7/7 heifers in group A, II/I2 in group B, 6/I2 in group C and I4/I9 in group D. Five heifers (2 in B, I in C, 2 in D) cleared themselves of infection between parturition and slaughter, The average degree of infection per group at slaughter, expressed as a logarithmic index of the number of Brucella isolated from the ten samples, was significantly lower in the three vaccinated groups than in the controls, and in groups C and D than in groups B, and it was not significantly different in group C and D. For field vaccination, a booster vaccination by the conjunctival route, as in group C, would provide more protection than the standard vaccination without serious interference in routine diagnostic tests. Two vaccinations by the conjunctival route, as in group D, would be simpler, more economical and at least as effective as the standard system of vaccination, and would have the advantage that vaccination could be done at nay age without risk of serological response.