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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 REPORTS 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, namely, (1) those having mean midwinter temperatures 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.
3 These temperature groups were based on data contained in “The Climate of the Continents," by W.G. Kendrew. Oxford, 1922.
4 "Mean midwinter temperature" as used here would probably be more accurately stated as the mean January-February temperature for the Northern Hemisphere and the mean July-August temperature for 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 irly 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 I.-- Number of cases of bubonic plaque reported in seaport cities (except two) with mean midwinter temperatures 35° F. or below. (The principal cities of the world having such temperatures.) Reports received from January 1, 1899-December 31, 1922.
TABLE II.— Number of cases of bubonic plague reported in seaport cities principally
with mean midwinter temperatures 36° 10 45° F. (Only those cities in which plague has been reported are included.) Reports received from January 1, 1897–December 31, 1922.
1 The cities of Norfolk, Va., and Wilmington, N. C., belong in this temperature division,
2 It will be noted that 914 of the 1,403 cases in this group occurred in Kobe and Osaka, Japan. TABLE III.- Number of cises of bubonic plagie 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 31, 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 with mean mid
winter temperatures of 56° F. and over--Reports received from January 1, 1897December 31, 1922.
TABLE V.- Total number of cases of bubonic plaque reported from the four temperature
groups as given in Tables I, II, III, and IV, and percentage of cases occurring in
cach group. [Because of the fact that some of the cases reported from India occurred in the extrere northern part of
the country, where the mean midwinter temperaturcs are below 26° F., the percentages are computed both inclusive and exclusive of India.]
Table VI compares the percentages computed for the entire period of 26 years with those computed only for the last 154 years of that period.
Table VII shows the relation between the number of reported cases of bubonic plague and the mean midwinter temperatures in certain Chinese and Japanese cities. It is interesting to note that Peking, China, in 26 years has reported only two cases of plague, though tens of thousands of cases of the bubonic type occurred during that period in southeastern China and two disastrous epidemics of pneumonic plague visited Manchuria to the north during that time.
Table VIII shows the dates of occurrence or of beginning and ending of those scattered outbreaks of plague which occurred during the 26-year period in cities with a mean midwinter temperature of 36° to 45° F. It will be noted that with scarcely any exceptions these outbreaks have occurred in the late summer and fall. Being absent during the winter months, it seems that these human outbreaks occur only after plague has gradually increased in the rat population through the spring and early summer. The time of these outbreaks seems to coincide with the season in which fleas are known to be most plentiful. It is also noted that there is a disappearance of this disease not later than November or early December in nearly all cases reported. This coincides with the suggestion that I have advanced that this disease would disappear with the disappearance of the fleas at the beginning of the cold season--up to a certain temperature which remains to be determined.
TABLE VI.-Comparison of the percentages for the temperature groups given in Table
V (ercluding India), with percentages based on reports for the 154-year period, July 1, 1907-December 31, 1922.
It will be seen that the percentages are practically the same for each period, although more than 5,000,000 cases of bubonic plague were reported in the various countries between January 1, 1897, and June 30, 1907. If the case reported from India are included, the percentage of cases occurring in the highest temperature group is 99.8 in each period.
TABLE VII.-Relation between the number of reported cases of bubonic plague and
mean midwinter temperatures in certain cities of China and Japan.'
1 Factors such as difference in amount of shipping, suppressive measures employed, whether seaport or not (Peking an inland city), etc., are not considered here.
It may be said that this same relation between the number of reported cases of bubonic plague and mean midwinter temperature is markedly noticeable in India, although no compilation has been made separating the number of plague cases reported in India into temperature groups. However, of the 322,560 cases reported between October, 1919, and October, 1922, none was stated as occurring north of Delhi, at which place the mear midwinter temperature is 59.5° F. It has already been noted that cases of bubonic plague were reported in the northern part of India, but the number is relatively very small.