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Although no doubt as much care as circumstances will afford is exercised in this diagnosis of plague in rats, it is quite impossible for the Government Bacter- iologist to definitely decide by microscopical examination alone in the short time at his disposal whether any given rat is infected with plague, and not with some other disease shewing similar post-mortem changes and bacterial morphology.
To illustrate this I may say that the same difficulty exists. in deciding differ- entially between plague and each of the following diseases, namely, the septicemia hæmorrhagica of cattle, swine fever and chicken cholera.
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Each of these three diseases exists in this Colony, and I think no one would undertake to say from post-mortem changes and microscopic examination alone, that given a chance of a particular animal being infected with plague, it had not died from one of the three above mentioned diseases.
The fact, however, remains that rats are susceptible to plague and if experi- ments are occasionally and systematically undertaken throughout the year to decide whether certain suspected rats are definitely infected with plague, the rat return from the Public Mortuary will be a great help in our defensive measures.
For the purpose of the following charts shewing the curves for rat and human plague from July 31st, 1903, to August 20th, 1904, I have divided the City of Victoria into six sections. Health Districts I, II and II are taken separately for the following reasons. H. D. I. is more or less topographically separated from H. D. II by the grounds of the Royal Naval Hospital, Morrison Hill and the Cemeteries at Happy Valley. H. D. III is largely European or Foreign in its residents.
Health Districts IV, V, VI, VI! and VIII are taken together as representing the central part of the City which is densely populated and almost wholly by Chinese.
Health District IX needs separate consideration, as in 1904 many more plague cases occurred there than in any other district of the City.
The curves are constructed on the following principle:--
(1.) The numbers of rats reported as plague infected each week are ex- pressed as a percentage on the total number caught each week, and the result is shewn in the upper curve.
(2.) The actual number of plague cases for each corresponding week are
shewn in the lower curve.
The reason for taking a percentage curve for rats is that by this method only can one graphically shew the rise and fall of plague amongst those animals. Some- times a comparatively large number of rats from a given district are reported as infected out of a small total number of rats and sometimes the converse is the case. As the total number of rats inhabiting each district and also the total number of those infected each week is unknown, it is absolutely necessary to take a percentage of those reported infected on the total caught.
With regard to the lower or human plague curve it will be seen that if the population be assumed to be constant from week to week a percentage curve will be precisely similar to, but on a smaller scale than, the curve as I have given it. As the percentages of plague cases would be such small fractions, I ha preferred to adopt the curve as I have given it.
The chief difficulties in making any curves to represent the relationship of rat plague to human plague and also the relationship of one week's incidence to that of the previous and subsequent weeks are:-
(1) That in the case of human beings it may not be taken for granted
that the population of any district remains constant.
(2) That we do not get information of every case of plague in human
beings in the Colony.
(3) That in the case of rats we have not even an estimated population. (4) That it is by no means probable that even the majority of rats
infected are recorded.
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