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About 75 per cent. of the rats caught alive and found infected, suffer from chronic plague.
An explanation of the cecurrence of chronic rat plague, as a sequel to an epizootic of acute rat plague is by no means easy. From what has already been said, the continued passage of the B. pestis through the body of the rat tends to render the organism less virulent. A strain of the bacillus which excites acute rat plague during the early passages, will only cause chronic rat plague during the later passages.
In the plague bacillus we are dealing with an organism of the group of hæmorrhagic septicæmias. Such bacteria when introduce into the bodies of certain animals are known to progressively lose their virulence (DANYSZ). They give rise to outbreaks of spontaneons disease in animal. On the recrudescence of the epizootic the virulence again becomes increased. During the natural evolution of the epizootic, we find the microbe becomes attenuated and either finally disappears or, as in plague, persists in chronic form.
Recrudescence of rat plague in acute form would appear to depnd upon the natural infection of "fresh" generations of rats, the bodies of which are highly suscep- tible to plague infection.
My observations and knowledge of the habits of rats lead me to the conclusion that the obstinate clinging of plague to any particular area and its periodical recrudescence is occasioned through chronic rat plague. The recrudescence of the epizootic occurs at definite seasons of the year, namely, during the period of maximal rat prolificacy.
WILLIAM HUNTER,
The History of the Course and Relations of Epizootic and Epidemic Plague in the Health Districts
of Hongkong during the year 1902.
Introductory Remarks.-Previous to my arrival in Hongkong on the 27th February, 1902, no attempt had been made to thorouhly investigate the course of epizootic plague in rats. From time to time a few rats were examined for the presence of plague infection at the Public Mortuary by the Medical Officer in charge, but these examinations, amounting to a few hundreds only, cannot be regarded as of much value, beyond establishing the fact that such a disease as "Kat Plague" existed in the Colony.
In London, I learned from Professor SIMPSON, who was about to leave for Hongkong, that inuch was to be gained by regular and systematic examinations of all rats, dead or alive, and further, his experience of plague in South Africa, pointed to an intimate association between the rat epizootic and the human epidemic.
On my arival in Hongkong, arrangements were. made by Professor SIMPSON and Dr. CLARK, the Medical Officer of Health, to have as many rats as possible collected from the various Health Districts of the Colony and forwarded to me for examination.
With the consent of the Principal Civil Medical Officer, I assumed charge of the Government Public Mortuary on the 20th March, 1902.
The rats caught alive or found dead were sent daily to the Public Mortuary. Exact details as to the place where each rat was found was furnished by the Sanitary Department. During the months of March, April, and part of May, these bacteriological examinations were systematically carried out by myself.
It was evident from the very commencement, however, that if such a research was to be prosecuted on an extensive scale, and in a systematic manner, with uniform result, the accommodation at the Public Mortuary would have to be increased, and extra assistance provided.