Reactivation of a residual Brucella abortus 19 vaccine infection in mice by a virulent challenge or by injection of brucellin or of Brucella lipopolysaccharide.

Facultative intracellular bacteria localize preferentially in reticulo endothelial system organs where they can either multiply or be destroyed, concomitantly or successively. Immunity may thus be estimated by counts of viable (surviving) bacteria at convenient time(s) after a standard challenge. When mice vaccinated with the living attenuated Brucella abortus strain 19 were intravenously challenged with the virulent B abortus strain 544, some mice exhibited unexpected high spleen counts. The vaccinal strain surviving at low level in some mice 30 days after a subcutaneous vaccination was reactivated by the virulent challenge as evidenced by a fast but temporary count increase. Reactivation was stronger in outbred OF 1 mice than in outbred CD-1 mice. Reactivation did not alter the normal course of the virulent infection as shown by comparison between mice already cured of the vaccinal infection or not, at time of challenge. Because reactivation was also induced by intravenous injection of either the brucellin allergen or the brucella lipopolysaccharide antigen, hypersensitivity reactions occurring inside the vaccinal granuloma foci may expel the surviving bacteria into the surrounding extracellular environment where a secondary growth may develop.