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A careful survey of both curves, namely, that of the epizootie and the epidemic, will enable one to draw their own conclusions. The rises and falls of the epizootic curve are followed only too closely by similar elevations and depressions in the incidence of human plague.
Although rat plague is in excess numerically, yet the tracking of the epidemic in the steps of the epizootic is only too well marked.
Such a following of one curve after another, one observes rarely, and I must confess that it was with much surprise that, subsequent to the preparation of the rat plague curve, the epidemic curve was found to show such similarity. The apices of both curves are found to correspond with the result obtained during 1902. Both reach their highest points within a fortnight of each other, the epizootic first.
The course of events, after the outbreaks have reached their highest figures, is interesting. The epidemic falls in numbers so rapidly, that pactically within a month, the number of cases of human plague has fallen to an insignificant figure. But what is even more remarkable, is the sudden drop in the epizootic. It maintains its elevated position for about a fortnight after the depression of the epidemic, and then, for some reason, suddenly drops.
The time occupied by the epizootic in falling from its maximum to its lowest level is about 15 weeks. The epidemic took about 13 weeks, namely, about a fortnight shorter.
The subsequent history of the curves is not so interesting. Rat plague never disappears. Human plague is practically absent.
This interval between the incidence of human and rat plague appears to be more or less constant. In 1902 the interval between the occurrence of both out- breaks was from 10 days to a fortnight The course of events during 1903 leads us to the same conclusion in regard to the time limit.
Compared with the epizootic the epidemic begins a fortnight later, it reaches its maximum a fortnight later, the maximum is maintained for a similar length of time and its fall occupis practically 14 days. Subsequently it disappears, whereas the epizootic maintains a low but certain level.
One would expect the fall in rat plague to occur somewhat previous to that of the epidemic. The condition of affair is, however, practically a repetition of what took place during 1902. A study of these outbreaks in each individual district teaches us that such is frequently found. Summing up these two curves, the conclusion which one must necessarily draw, are, that human plague com- mences immediately after the increased incident of the epizootic, and that va- riations in the latter occasion variations in the former. Such is true until the apices of both curves are reached. Subsequent to this period, the epidemic falls rapidly, and has reached a low 1-vel previous to great alterations in the incidence for about a fortnight after the great depression of the epidemic, then by means of two jumps suddenly drops to practically the same level from which it started at the commencement of the
year.
A close study of such a chart is hound to lead one to think of some relation between the two outbreaks. It may be said that both run independently of each other, only the epidemic appears about a fort- night later. It would be difficult to imagine such a regular system in the incidence of an infectious disease when compared with an epizootic.
We can imagine the epidemic of plague commencing practically at the same season during each year, but that it should regularly appear within a definite incubation period after the advent of the epizootic is difficult to get away from, unless we admit of the existence of a definite relationship. Again it is found that the plague epidemic does not commence at a definite period during each year. In 1902, it was not present until the middle of April. In 1903 it began in the middle of January or at least by the 1st of February. In 1904 its commencement was even later than during 1902.
Let one now compare the epizootic during these years. In 1902, the epizoo- tic began about the 1st of April. In 1903, rat plague began a fortnight earlier than human plague, namely, during the middle of January,
In 1904, it will be found that a similar condition of affairs was present.