Histopathological changes of mice injected with plague live vaccine strain EV.

The virulence and immunogenicity (protectivity) of an attenuated live strain of Paeteurella pestis, EV, were examined in mice. When injected subcutaneously at a dose of 106 bacilli, the organisms survived at the injection site for as long as one week though the population decreased gradually. A small number of EV was recovered from the spleen, liver and kidneys from the second to the fifth days after injection. The optimal doses for immunization ranged 106-7 bacilli; a smaller or larger dose resulted in a lower protection. Various organs of mice inoculated subcutaneously with the live vaccine strain EV at a dose of 6.0•~106 organisms were subjected to histopathological examination every day. Pathological changes developed at first in the liver on the next day after the injection, and then the lungs and spleen were involved from the 2nd to the 18th days. The changes in the liver were characterized by perivascular and parenchymal infiltration of cells, granuloma and necrosis. The perivascular changes were characteristic to EV strain, in contrast to the sole parenchymal changes caused by virulent strains. The changes in the spleen were hyperplasia of follicles, reticulum cell proliferation in the germinal centers, infiltration of plasma cells and of polymorphonuclear leucocytes and