20 QUATERCENTENARY STUDIES IN PATHOLOGY 



and, thirdly, there is the fact that the epidemic of 1902 was a mild one ; 

 that of 1903 was severe, and during 1904 the epidemic was again slight. 

 Given these data, coupled with a careful scrutiny of the two charts, it 

 would appear that a malignant epizootic occurs towards the end of an 

 epidemic, and its continuance after the disappearance of the latter means 

 an early recurrence of human plague and a severe epidemic. On the 

 other hand, a mild epizootic during the interval between two epidemics 

 would appear to indicate a late recurrence of human plague and a mild 

 epidemic. The condition of affairs may be stated as follows : — 



1902. — Epidemic mild. ' 

 1902- 1903. — Epizootic severe. The epizootic 



1903. — Epidemic severe. - is 



1903-1904. — Epizootic mild. continuous. 



1904. — Epidemic mild. 



It has now been clearly shown that in an endemic plague centre like 

 Hong-Kong, rat plague is present, in varying amount, throughout the 

 whole year. It becomes epizootic at certain times of the year, and the 

 reasons for this would not appear, at first sight, to be obvious. 



In order to explain this regular appearance of the epizootic, I believe 

 that a consideration of the question of the variability in virulence of the 

 B. pestis, and the period when rats are most prolific, will simplify matters 

 greatly. The B. pestis is an organism which is liable to sudden altera- 

 tions in its virulence. Two races of the bacillus may be cultivated under 

 the same conditions for months. One may retain its virulence. The 

 other may lose its virulence, either partially or completely. This loss of 

 virulence may take place suddenly. It is stated that the plague bacillus 

 may remain virulent for a considerable length of time at the temperature 

 of 95°- 100° F. 



Further, the continued passage of a strain of the B. pestis through 

 rats, leads eventually to the production of practically avirulent stocks of 

 the organism. 



Such cultures are found by experiment to produce in rats and other 

 animals a condition of chronic and marasmic plague. 



Further, chronic rat plague is predominant during the intervals 

 between epidemics of plague, and from such cases an avirulent strain of 

 the B. pestis may be obtained. 



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