Susceptibility and Antibody Response of Rattus Species to Experimental Plague

The susceptibility and immune response to experimental plague in laboratory-reared white rats and wild rats were studied. Some albino rats, wild rats from plague-free areas, and wild rats from plague foci survived a standard sc-administered infective dose of 5,550–12,600 Yersinia pestis. Of the survivors, about 33%–76% produced both CF and HA antibodies (Y. pestis-specific) in peripheral blood 20–24 days after infection; 24%–67% were nonseroreactive but resisted reinfection. A varying percentage of nonseroreactive rodents became seropositive for the pathogen when reinfected. Most rats with serum antibodies and a few nonseroreactors (naturally resistant) withstood an overwhelming superinfective dose of an attenuated, toxic strain. Thus early reports of natural resistance to plague are now supported by experimental proof that plague-resistant rats consist of two groups: one that resists minor infections and produces antibodies (acquired resistance), and another that possesses a nonspecific defense mechanism (natural resistance). The various possible heterogeneities among susceptible, partially resistant, and fully resistant strains in field populations may account for quiescent periods interspersed with epizootics of plague, since the proportions of naturally resistant and susceptible strains are constantly changing.