VIRULENCE OF PASTEURELLA PESTIS AND IMMUNITY TO PLAGUE.

Abstract

With the introduction and development of efficient antibiotic therapy, improved methods of rodent control and potent insecticides it might well be considered that plague, as it has occurred in the past, is no longer a threat to humanity. This conclusion would be justified if it should so happen that conditions prevailing today continued unchanged into the future. The variability of biological systems in general would warn us that this is unlikely to be so and that we may expect, among other possibilities, the emergence of antibiotic resistant strains of Pasteurella pestis, of rodents refractory to present control methods and of insecticide-resistant vectors. Strains of Xenopsylla cheopis, resistant to DDT have in fact already appeared (Patel, Bhatia and Deobhankar 1960). The past six decades have witnessed a spectacular decline in the incidence of human plague throughout the world so that it is now a rare disease. Although its decline over this period most probably has resulted from the application of specific antiplague measures, we have no way of knowing to what extent plague would have declined in their absence. It is common knowledge that in past centuries plague, from being pandemic with enormous human consequences, has returned to a state of quiescence in the absence of antiplague measures as we now understand them, only later to reappear with undiminished severity. Since we are not in a position to say that plague could never recur in the modern world it would be prudent to assume the possibility and to investigate further all aspects of the disease whilst the opportunity exists. It is gratifying to know that such investigations are continuing. They should add to our knowledge and thereby improve our defence against plague in the future.