Vaccination against bovine brucellosis with a low dose of strain 19 administered by the conjunctival route. I.--Protection demonstrated in guinea pigs.

Guinea pigs were vaccinated with different doses of strain Brucella abortus B 19, subcutaneously (SC) or by the conjunctival route (C), once or a second time three months later. Agglutination and complement fixation titers were determined on sera of guinea pigs taken at random at intervals from each vaccine group. Protection afforded by the vaccination was determined by intramuscular injection of the challenge strain, B. abortus 544 (5 X 10(3) bacteria) three months after the last vaccination. Forty days later the guinea pigs were killed and Brucella looked for and counted in the spleen and lymph nodes. After conjunctival vaccination with 10(6) or 10(7) bacteria, seriological responses were weak and transient even after the re-vaccination, and were lower than after subcutaneous vaccination with the 10(6) dose, and much lower than after the standard vaccination dose of 7 X 10(9) given subcutaneously. Protection, expressed as percentage of guinea pigs free of infection at autopsy (table 4) was 40 p. 100 in the standard vaccination group (SC, 7 X 10(9)) and 57 p. 100 in groups receiving successively 7 X 10(9) (SC) and 10(6) or 10(7) (C). Two successive conjunctival vaccinations with the 10(6) to 10(9) doses gave 47 to 60 p. 100 protection; two SC vaccinations with the 10(6) dose gave 75 p. 100 protection. All the control guinea pigs were heavily infected. The differences in protection rate among the groups vaccinated twice were not significant. Since vaccination through the conjunctiva with two successive instillations of 10(7) bacteria induced only a weak antibody response and protection equivalent to that of standard subcutaneous vaccination, the method appears worthy of application in cattle.