A Flea Survey

In the last 26 years, bubonic plague has spread east and west from India in a broad belt which now encircles the globe on both sides of the equator, roughly bounded by the 35th parallels of latitude. In Europe it is prevalent as far north as the 45th parallel, but in the Western Hemisphere it has appeared sporadically only in the large cities of the Gulf and Pacific Coasts. It is essentially a disease of the Tropics. Within this belt no preventive measures have been able to stamp it out. Further to the north or south it has failed to spread, whether efforts were made to bar it or not. Where the mean midWinter temperature is 45 degrees F. or below, the plague is temporary, accidental and self-limited.

These are some of the findings of Surgeon H. M. G. Robertson, of the U. S. Public Health Service, who has been making a special study of the bubonic problem. The three factors in the epidemiological circle of the disease are believed to be the rat, the man, the flea. The flea is the only factor that can be considered seasonally variable. Studies by the Indian Plague Commission and the U. S. Bureau of Entomology have led to the conclusion that the adult flea does not usually live through the Winter in cool climates. The species is prevented from dying out by the ability of the larvae to exist for long periods in a sort of hibernation. Dr. Robertson advocates a flea survey of the cities of the Atlantic Coast to verify the details of the life cycle of Bacillus pestis, the causative organism. If fleas are relatively abundant on rats at all seasons of the year in this region, the absence of plague must be wholly accidental. But if there are few or no fleas during the cold months, theories regarding the carriers of plague would seem to be on the wrong track, and some other explanation must be found.