Quantum differences in oral susceptibility of voles, Microtus pennsylvanicus, to virulent Francisella tularensis type B, in drinking water: implications to epidemiology.

Shedding nephritis in voles with chronic tularemia is the probable source of frequent contamination of streams over wide areas of the northern hemisphere. However, voles inoculated parenterally with as few as one dex viable cells of Francisella tularensis palaearctica succumb uniformly to acute tularemia, whereas voles, beavers, and muskrats often thrive in waters contaminated with the organism. Nevertheless, water-borne epizootics do occur in these animals, and at these times streams are heavily contaminated. In attempts to determine the lethal dose, per os, of the bacteria to voles, it was found that there are stable, 'order-of-magnitude' differences in susceptibility of individual weaned voles reared under uniform conditions. Also, voles thus infected often became chronically infected with bacteriuria. It is proposed that the most susceptible voles are responsible for amplification of water contamination and thus for initiation of water-borne epizootics in hygrocolic mammals and for frequent mild infections in man.