Immunity in plague: protection of the vervet (Cercopithecus aethips) against pneumonic plague by the oral administration of live attenuated Yersinia pestis.

Protection against pneumonic plague by the oral administration of a live, attenuated Yersinia pestis vaccine, EV76 (Paris) F, was evaluated in the vervet (Cercopithecus aethips). Six animals were vaccinated with a dose of 1.175 X 10(9) colony-forming units (cfu); all tolerated the dose and developed antibodies to Fraction I of Y. pestis. Three immunized animals were challenged with an inhaled dose of 3.2 X 10(6) cfu (160 LD50 [50% lethal dose]) of virulent Y. pestis strain 195/P and two survived; the other three received a challenge dose of 4.9 X 10(6) cfu (245 LD50) and one survived. The three surviving monkeys had high titers of antibody to Fraction I. At necropsy of the animals that succumbed to the challenge infection, almost complete consolidation of the lungs was apparent, and sizable effusions were present in the thoracic cavities. Indeed, the striking pathology indicated the severest form of pneumonic plague. Since it can be assumed that the animal surviving challenge with the larger inoculum would have survived the smaller dose, we conclude that the immunization procedure protected half of the vervets that were exposed to challenge.