Vertebrate Pest Conference Proceedings collection


Date of this Version

March 1964


Bubonic plague no longer is regarded as the dreaded black death of the middle ages. The last great plague pandemic has come to an end. In California, human cases have averaged no more than one every two years for several decades. With modern antibiotics properly administered, recovery is assured. At the same time, there is no doubt that plague is firmly entrenched in the rodent fauna and we should not be lulled even by years of quiescence into assuming that massive epizootics will not break out in the future. There is the un-measurable risk of a quick pneumonic outbreak with tragic results, or the chance that infection may transfer to urban rat populations and thus pose a markedly greater hazard. No formula can determine the magnitude of these potential hazards, or the intensity of control efforts that should be applied. Reason, both biological and fiscal, dictates that the program should be a modest one. Having reached this conclusion, it behooves us to be sure that this modest effort is applied with the fullest knowledge and understanding to secure the greatest possible benefit. Let us review briefly the history of sylvatic plague suppression in California. Early in this century, massive epizootics in California ground squirrels were discovered in the San Francisco Bay area. A great campaign of squirrel control was launched, with the purpose of eradicating the infection before it could spread. At one point an official claimed that this goal had been achieved. But new outbreaks appeared, and gradually plague was found to be present throughout most of the west. Ground squirrels of several species, chipmunks, and marmots were the animals consistently and conspicuously identified in epizootics. Not surprisingly, they were regarded as the reservoirs of plague. Occasional plague-infected mice were thought of as incidental victims. Sites where plague was found were regarded as "foci". Survey workers returned to these sites again and again in succeeding years and often found infected animals or fleas, thus reinforcing the original assumption. It naturally followed that controlling squirrels in these "foci" would control the disease itself. This led to designating a series of plague areas, subject to constant ground squirrel suppression, many of which continue to be observed as geographical control units. Current knowledge of plague ecology reveals a much different picture. Recent plague studies throughout the world show that the persistent reservoirs of infection are not those susceptible rodent species which Suffer periodic violent epizootics. Instead, they are rodents capable of maintaining a quiet state of infection, with little or no mortality. It is becoming clear that plague persists in relatively small pockets where suitable climate, flea vectors and rodent hosts occur, characteristically in cold mountainous or high plateau regions, or coastal fog belts. From these pockets of enzootic infection, plague may spread periodically through susceptible host populations in epizootic form. Even when such hosts sustain the disease for a number of years, it must follow an ever shifting epizootic path to survive. Presumably a rather high degree of crowding is required for an epizootic to be sustained.