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20 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 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 Philadelphia, 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.
The question as to whether human plague will appear in a locality in which rat plague must disappear at the beginning of each winter can be answered only after consideration of several factors. Among these are the following: The number of fleas per rat in the months during which fleas are present; the number of months in which fleas are found present; and the relative number of rats in the locality under consideration. The character of the communication with active plague centers might also be taken into account. As regards the number of fleas per rat, this knowledge will be valuable only when compared with a like knowledge of similar conditions in localities where bubonic plague has prevailed throughout the entire year, as, for example, some cities in India. The number of months during which fleas may be found on rats should be fewer the farther north the locality is situated. In regard to communication with plague centers, we have noted that Great Britain, though in intimate maritime association with India, has not suffered seriously.
In order to have some definite data based on reported occurrence, which might sustain my belief that the absence or relative absence of fleas during the cold months determined the absence of bubonic plague from certain countries, a rather detailed study of the reports of cases of plague as given in the Public Health Reports for the past 26 years was undertaken. A table was prepared giving the names of cities and countries in which plague had been reported in the Public Health Repouts for the years 1897 to 1922, inclusive. Each year was divided into two periods, corresponding to the semiannual summaries given in the last issues of the Public Health Reports for the months of June and December, respectively. The total figures for the 52 six-month periods have been used in preparing the tables which follow. The cities and countries in which the reports showed the occurrence of plague, and other principal cities, have been combined into four temperature groups,3 namely, (1) those having mean midwinter temperatures'1 of 35° F. or below; (2) those having mean midwinter temperatures of 36°-45° F.; (3) those having mean midwinter temperatures of 46°-55° F.; and (4) those countries having mean midwinter temperatures of 65° F. and higher (Tables I, II, III, and IV).
In this way it was possible to show at a glance where plague had actually occurred, how extensively, and what was its seeming relation to different temperatures. These temperature periods also explain the apparent exceptions to the spread of plague north of parallel 35° north.
• These temperature groups were based on data contained in "The Climate of the Continents," by W. O. Kcndrew. Oxford, 1922.
< " Mean midwinter temperature " as used here would probably be more accurately stated as the moan January-February temperature for the Northern Hemisphere and the mean July-August temperature lor the Southern Hemisphere.
No question is raised in this statistical study of the accuracy and completeness of the reports. It is obvious that in some instances in certain countries they are far from complete, yet there is no reason to suspect that any reported cases are incorrect. It is more likely that fairly complete returns have been made for those cities and countries in Tables I and II than for those in Tables III and IV, as the cities given in the former tables are those which have for years paid much attention to sanitary matters and from which correct reports of all kinds might be expected.
Table I lists the principal seaports of the world in which the mean midwinter temperature is 35° F. or below. In only four of these cities has plague been reported, the others being given because of the rather prevalent belief that plague had already spread to the principal seaports of the world. Two inland cities are also included because they have reported plague.
Table II shows those cities of the world having a mean midwinter temperature of 36° to 45° F., in which plague has been reported, and the number of cases in each.
Table III shows those cities and countries in which plague has been reported, in which the mean midwinter temperature is between 46° and 55° F.
Table IV shows those countries having a mean midwinter temperature of 56° F. and over, in all of which bubonic plague has been reported during the 26 years under consideration.
Table V shows the total number of cases of bubonic plague occurring in each of the four temperature groups, and the percentage of cases reported for each one of those temperature divisions. The percentages are given exclusive and inclusive of the cases reported from India.
Table II.—Number nf cases of bubonic plague reported in seaport cities principally with mean midwinter temperatures S6° to 45° F.1 (Only those cities in which plague has been reported ate included.) Reports received from January 1, 1897-December 31,
1 The cities of Norfolk, Va.. and Wilmington, N. C, belong in this temperature division.
a It will be noted that 914 of the 1,409 cases in this group occurred in Kobe and Osaka, Japan.
Table III.—Number of ease* of bubonic plague reported in seacoast countries and cities with mean midwinter temperatures 46° to 55° F. (Only those in which bubonic plague has been reported are included.) Reports received from January 1, 1897-December SI, 1922.
1 Nearly two-thirds of these cases were reported from Bagdad and vicinity and from South Africa.
Table IV.—Number of cases of bubonic plague reported in countries uilh mean midwinter temperatures of 56° F. and over—Reports received from January 1, 1897December SI, 1922.