Sessional_Paper_1904 — Page 759

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The Course and Relation of Epizootic and Epidemic Plague during the Intervals 1902, 1903, 1904.

In almost all plague infected countries, the epidemic is found to be most prevalent during certain seasons of the year. In Hongkong, our epidemics range from March to July, the month of May or even that of June, supplying the largest percentage of cases. In other countries, plague appears during the colder seasons of the year, and vice versa.

The reasons for such a seasonal recurrence of plague are by no means obvious. A consultation of plague literature helps one but little.

The layman's notion that climate has to do with the recurrence of plague or infectious disease in general must be pigeon-holed along with many other superstitions belonging to the pre-epidemiological days.

All that can be said of climate, is that it may exercise an indirect influence on the course of infection.

Under the present heading, it is not intended to discuss the factors which possibly assist in bridging over epidemics of plague. These will require con- sideration under a special heading. All that is intended to show at present is the persistence of rat plague from one year to another.

During the interval between the end of one epidemic and first cases of the succeeding epidemic, human plague is to all intents and purposes non-existent. A few cases occur, but these are of no practical importance apart from the fact that they help us to remember that with the decline of the last epidemic, our Sanatarians have in all probability not yet succeeded in stamping out the disease. Further, the reasons for the outbreak of erratic cases of human plague, are by no means obirous.

Little or nothing is known in regard to the history of epizootic plague in rats through a number of years.

Many authorities believe that rat plague dies out on the decline of human plague.

The preparation of charts showing the relations existing between epizootic and epidemic plague during the periods July, 1902, to June, 1903, inclusive, and July, 1903, to May, 1904, inclusive, is found to show us clearly the condition of affairs which one obtains in an endemic plague centre like Hongkong.

In both charts the epidemic vanishes. Isolated cases occur periodically but are of no practical importance.

With a fall in epidemic plague, a decline in the epizootic is also observed. The latter falls to nearly the same figure as human plague.

Almost immediately afterwards, however, within a week or a fortnight the rat plague exacerbates and again reaches a high level. This level means a goodly number of cases of rat plague per week, but nothing in comparison to the numbers found during the proper plague season.

The epizootic curve shows a considerable oxillation but is more regular in its course than during epidemic periods. Little change takes place in regard to the curve, until suddenly during the early part of the following year, the curve shoots upwards. Immediately this occurs, plague breaks out severely in man and be- comes epidemic.

From these curves one would rightly conclude that rat plague is epizootic continuously.

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Human plague is epidemic only during the acute exacerbations of the epizootic. Another point of importance in regard to these two charts is the following

A comparison of the epizootic of 1902-03 with that of 1903-04, shows

us that the former was much more severe.

The amount of rat plague at the end of 1902 was in excess.

A general

survey of the curve even shows its tendency to rise gradually after its sudden fall in September of 1902.