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It was from the foregoing points of view that I took up the subject over two years ago. I felt compelled to investigate the question thoroughly, and determine how far one was justified in becoming a disciple of those content with the simple theory expressed in the word "Rat."
The accompanying remarks, with charts, tables, etc., are the outcome of nearly 2 years' systematic research into the question from every point of view which appeared to me to be of importance. In Hongkong, far removed from the centres of medical science, I have experienced considerable difficulty in keeping abreast with the status of our knowledge f this subject. A more or less constant communication has been kept up by me with the more important scientific schools of Europe, the directors of which have greatly assisted me in the task of obtaining accurate information in regard to the problems of the task which was before me.
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There does not appear any necessity to discuss the question at length. Remarks, charts, tables, etc. are appended which are, in my opinion, sufficiently clear, and their interpretation is left to the readers' judgment.
I shall content myself here with concentrating the results, and applying them to the various questions which are constantly being plied by those who still appear sceptical as to the significance of the words "infected plague rats.”
Observations in regard to the beliefs of our ancestors have been made in another section of this Report. Our forefathers noted that rat epizootics constantly preceded or accompanied plague epidemics,
Within the last decade, much evidence has been brought forward in regard to the relation of the epidemic to the epizootic.
The experience gained by the outbreaks of plague in the sea-ports of Oporto, Alexandria, Sydney, Kobe, etc., goes far to establish some connection between rat and human plague.
In these ports, as well as in others, the disease clung fast to the harbour town. The neighbouring cities and villages were not or only slightly affected by the disease.
Again, in the grossly overcrowded and filthy cities of the Orient, the introduc- tion of human plague has scarcely ever been followed by the establishment of an actual endemic plague focus. In such cities or ports, in which plague broke out epidemically, these epidemics would not appear to depend on the arrival of plague infected human beings, but upon the introduction of plague infected rats.
In different plague epidemics, grain stores, in which rats preferably lodge, have frequently become the central points of the outbreak. In Bombay, plague broke out first among the Banniahs. (German Plague Commission).
In Oporto, the food stores appeared to be the centres from which the epidemic spread. (KOSSEL and FROSCH. Klin Jahr. Bd. 7, 1900.) Of great importance are rats on board ship. Examples of infected ship rats have abready been cited. (Vide Hamburg case of ship trom Smyrna, cited by KOSSEL and NOCHT.) Another instance of the same was found in Bristol in 1901. Again, the experience gained from the condition of certain passenger ships visiting Hongkong, adds further evidence in the chain connecting rat-plague with the epidemic. (Vide SIMPSON'S Report on Plague in Hongkong.)
Plague rats may be present on board ships. without the advent of human plague. Further plague rats or the bodies of plague rats are probably of greater importance than a plague infected man.
The reports of English writers, during the past few years, admit the importance of the rat in disseminating plague infection, yet the majority of their writers show that considerable difficulty is experienced in perceiving how the infection can be conveyed from the rat to man.
Snow's observations contain no suggestions that the rat is the important cause of epidemic plague.
HANKIN (Past. Anul., 1898) infers that the incidence of plague, in a neigh- bourhood, stood in relation to its accessibility to infected rats and not to any of the commonly relied up causes as filth, overcrowding, etc. He was inclined to
the opinion that some insect acted as the intermediary host of the B. pestis.
In the same year SIMOND stated “that the epidemicity of plague was due to migrations of plague rats and not to human intercourse".