608
The plague bacillus is an organism which is liable to sudden alterations 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.
All proofs we have at present show that climatic influences have absolutely nothing to do with the question. Plague epidemics occur in Siberia as well as in the equatorial regions. Sunshine, humidity and rain do not appear to have any determining influence on the occurrence of plague epidemics. The reports of the Acting Medical Officer of Health show this more or less clearly. We have to remember the constant presence of rat plague, and the occasional presence of human plague. Further it must be borne in mind that no rise in human plague is found unless it be preceded by a rise in rat plague.
Climatic influences in the production of human plague loose their significance by the establishment of the influences of rat plague. At the present day, few epidemiologists treat the climate seriously.
It is known that the plague bacillus may remain virulent for a considerable length of time at the temperature of 95-100° F.
By the continued passage of the plaque bacillus through Guinea pigs (cuta- neous inoculation), practically avirulent stocks of the B. pestis are obtained (KOLLE and MArtini).
This also obtains in rats, as I have found by experiment.
Such avirulent cultures produce chronic and marasmatic plaque.
KOLLE and MARTINI found that cultures of the B. pestis, which were highly virulent for Guinea pigs, would not kill rats with certainty.
I have found that rats occasionally show considerable resistance to infection
when fed upon the plague infected tissues of man and other animals.
Other facts in regard to the peculiar alteration in the virulence of the plague bacillus might be mentioned, but space will not permit of one going more deeply into the subject.
My researches lead me to the following conclusions. (Vide charts.)
Plaque is primary epizootic. Within a week or a fortnight it also becɔmes epidemic.
Throughout the epizootic period, the type of plugue amongst the rats is acute. Subsequent to the epizootic and epidemic plaque periods, acute rat plague is also met with, but chronic rat plague becomes predominant.
During the interval between plaque epidemics, rat plaque is present, but the type of the disease is chronic.
My results show that:-
Acute rat plague is followed by epidemic plague.
Chronic rat plague bridges over epidemic plague.
My results show further that the percentage number of live rats caught and found infected during the interval between two plague epidemics, is much greater than that found during plague epidemics.
The following is interesting:-
1903-% Live Rats found Infected.
January,
February,
March, April,. May, June,
July,
1 x 55
10
13
3
1
4
August,..
]
September,
13
October..
30
November......
13
December,
15