Figure 1.

Brown Norway rats are highly susceptible to pneumonic plague but only sometimes generate immunity after sublethal challenge. Groups of four to six Brown Norway rats were challenged with increasing doses of Y. pestis CO92 delivered by intranasal instillation. A: Animals succumbed to disease between 60 and 108 hours after infection, with a mean 50% lethal dose (LD50) of 171 cfu. Data shown are representative of three independent experiments. B: Rats that survived sublethal infection (R1 to R5) did not consistently develop antibodies to dominant plague antigens, CaF1 and LcrV, because only three of five survivors (R1, R2, and R4) from the experiment in A had seroconverted by day 14 after challenge.