Vaccination against bovine brucellosis with a low dose of strain 19 administered by the conjunctival route.

Eighteen 25 month old cows were vaccinated once either with 10(6) to 10(10) living bacteria of Brucella abortus B 19 by the conjunctival route, or with 2.5 to 5 X 10(10) formalin-killed bacteria of the same strain by subcutaneous injection. Fifteen days post-vaccination, low but definite agglutination titers were present in sera of cattle receiving 10(10) living bacteria by the conjunctival route, whereas high titers were observed in the control group given subcutaneous injection of killed bacteria (table I). Agglutination reactions were negative in sera from all cattle receiving less than 10(10) bacteria, and complement fixation tests were negative with all sera. Seventy-one 4 month old claves were vaccinated by the conjunctival route with different doses (10(6) to 10(10)) of living bacteria, either freshly prepared or lyophilized. At autopsy about 15 or about 30 days later, the parotid, submaxillary and retropharyngeal lymph nodes were removed and cultured on Brucella selective medium. Blood samples were taken at autopsy for testing by agglutination, complement fixation, Coombs test and (for some samples) by the Rose Bengal plate test. Colonization of at least one lymph node was observed in 26 of 34 calves given 10(9) bacteria, in all 9 calves given 5 X 10(9) bacteria and in all 12 calves given 10(10) bacteria. Serological responses were demonstrated rarely, and then only in agglutination and Coombs tests. The 5 X 10(9) dose, wich following conjunctival inoculation consistently colonized the regional lymph nodes without inducing significant serological response, would be the most suitable dose for the vaccination of calves and cows.