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PLAGUE IN COLD COUNTRIES. By H. MCG. ROBERTSON, Surgeon, United States Public Health Service. A study of the reports of plague occurrence as published in the PUBLIC HEALTH REPORTS ? shows that in the past 26 years bubonic plague has spread in an east and west direction from India until, at the present time, it may be found encircling the globe in a rather broad band, bounded, roughly, by the thirty-fifth degree parallel north and south of the Equator. This plague circle of the globe was completed as far back as 1914, when the disease appeared for the first time in one of the cities of our Southern States. Bubonic plague has appeared north of the thirty-fifth parallel north latitude rather extensively in southern Europe; in fact, it may be said that in that part of the world the forty-fifth latitude north more probably describes the northern boundary. It is also noted that in our own country this disease, appearing in San Francisco, is north of the thirty-fifth parallel. Reference to climatic conditions along the Mediterranean coasts and in San Francisco will show that the temperatures are markedly modified in these localities as compared with the temperatures in other parts of the United States and in other countries of this same latitude. This, of course, is due in great part to ocean currents and to the warm waters of the Mediterranean.

While bubonic plague has spread around the globe during this 26year period, there has been no such corresponding spread to the north and south of parallels 35, with one notable exception, namely, the British Isles. And here we see again the climate modified to a great extent, in this instance by the Gulf Stream. Notwithstanding this modification of climate, and in spite of the fact that bubonic plague has appeared in different places in Great Britain as often as twenty times during the past 26 years, there has been no general spread of the disease in Great Britain during this time, nor any spread at any time from city to city; and during the year 1922 and up to May, 1923, no cases of or deaths from plague have been reported in Great Britain. In

1 EDITORIAL NOTE.-The article here published is valuable as a summary of the history of plague during the periods studied; but, taking into consideration the past history of this disease, it is believed that the period of observation is much too short to justify definite conclusions. The Public Health Service has begun flea surieys at one or more North Atlantic ports.

* These reports are received from medical officers of the Public Health Service, American consuls, health authorities of foreign countries, and other sources.--Ed.



26 years there have been only 26 deaths from plague in all parts of Great Britain, and this, in spite of daily maritime contact with the plague centers of the world and, until recently, without any efforts being made at quarantine restriction or any extensive effort on the part of local authorities to suppress these outbreaks.

It is seen from this study of the PUBLIC HEALTH REPORTS that during this term of years bubonic plague has spread alike to countries which endeavored to prevent its entrance and to those which made no such efforts. It is seen, too, that this disease has failed to spread alike to colder countries which made no effort to prevent its entrance as well as to those which, like our own North Atlantic seacoast, have made strenuous effort to prevent its ingress. In other words, it appears that the efforts of man to control the world-wide spread of this disease have been of little or no avail. It would seem, too, that in this extended period of time bubonic plague would have spread to northern countries also, were it epidemiologically possible for it to have done so. That it has not so spread is not due, apparently, to efforts on the part of man, nor is it explained satisfactorily by considering it accidental. Plague has never been reported north of the sixtieth degree of latitude north; in the Western Hemisphere it has never been reported north of the fortieth degree of latitude, except in Seattle, Wash. The existence of the disease north of latitude 35 north has never, in any part of the world, constituted a serious sanitary problem, except in those warmer sections previously mentioned or in ages past under conditions with which we can not now be entirely familiar. What, then, is the explanation of the failure of this disease to spread to countries where the climate is cold or relatively cold ? .

In considering bubonic plague in its relation to human beings it is necessary, or usual, at this period of the world's history, to consider the disease in the rat as antecedent and necessary to its spread to the human population. The time may have been when man, dressing in skins and furs and living with lower animals in insanitary surroundings, contracted bubonic plague through fleas coming directly from a member of his own race. Such a transfer of this infection is not now of sanitary significance, and the three factors-man, the rat, and the flea--are taken together in any epidemiological study of bubonic plague.

We note that in the tropical sections of the world human plague is or has been reported in all months of the year, and for numbers of years in succession. The inference is (and this inference is supported by studies of the Indian Plague Commission) that all three factors are present throughout the entire year. The flea is the only factor which might be considered variable; but this insect is found at all seasons, though more plentiful during certain months. Man

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and the rat must be active at all seasons in order to live. They are
relatively long lived; they do not hibernate in any climate; and their
young do not pass through the winter in the immature state. Neither
the human nor the rat population changes extensively in any given
area during short periods. The adult flea, on the other hand, lives
usually not many months; and in a cold or cool climate the adult
rarely passes through the winter. The flea, as a species, is prevented
from dying out in cold countries by the ability of the immature forms
to exist for periods which may range to more than a year. These
long pupal stages are seen only in cool or cold weather. These facts
are borne out by the studies of the Indian Plague Commission and
by work done by investigators of the Bureau of Entomology, United
States Department of Agriculture.

My own observations in regard to the absence of fleas in adult
form in the winter months have been confined to cats. During two
recent winters I was unable to find fleas on cats between December
15 and March 15, although the animals in question spent the entire
winter months indoors in warm places, and the basements where they
slept had been heavily infested with fleas during the previous
summers. These observations were made in the vicinity of Phila-
delphia, Pa., at which place the mean midwinter temperature is
about 31.5° F.

It was this observation that led me to speculate as to the probable absence of fleas from their rat hosts during the cold months of the year, and further led me to suspect that the freedom of certain countries from outbreaks of human cases of bubonic plague could be explained by the absence of fleas. During the past year I have advocated making a flea survey of the cities of our Atlantic coast in order to determine definitely whether I was correct in the above explanation.

Should this flea survey show that fleas are relatively abundant on rats at all seasons of the year, the absence of bubonic plague would, of necessity, be accidental, as all three factors needed in its spread would be present. If, on the other hand, it were shown that during certain cold months no fleas, or only a few fleas, occasionally, were found, an explanation of the absence of human plague would be at hand. Rat plague introduced into a locality where adult fleas are present would spread among the rats, the rapidity of this spread being dependent on the abundance of fleas and the number of rats. If the fleas should be present throughout the entire year, as in the Tropics, the infection among the rats would become so extensive as eventually to result in the appearance of human cases. Rat plague introduced into a community where fleas are absent for certain months must, of necessity, die out when the flea disappears, as one factor in the spread of the disease will be absent with their disappearance.

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