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The lungs are fibrous. or indurated and may contain capsulated areas of dead tissue.
Similar pathological appearances are found in the liver, spleen, and kidneys. Plague bacilli are found in the various organs and tissues. They are, how- ever, few in number. Such dead tissues when emulsified and administered to other healthy rats are able to reproduce the disease, even in acute form.
The spleen contains the largest number of plague bacilli. This organ much enlarged. In one or two occasions, several prominent greyish white areas of necrosis have been found, resembling closely the condition met with in tuber- culosis.
occur.
Latent Plague in Rats.-Theoretically there is no reason why this should not
So far no extended series of observations has been made in this direction. KOLLE, from certain laboratory experiments, is a strong believer in the exist ence of latent plague amongst rats. He is supported in his conclusions by GOTSCHLICH.
My own observations are strongly suggestive of latency in rat plague.
In a favourable opportunity presenting itself, I propose to carry out a series of experiments with a view of determining as exactly as possible, the course and duration of chronic rat plague, its relation to acute rat plague, the question of latent rat plague, seasonal variations, prolific influence on rat plague, and the factors upon which the appearance of the different types of the disease depend.
Further details in regard to latent rat plague will be found in a subsequent part of this research.
General Modes of Infection in Rats.-Rats may become plague infected either naturally or artificially.
In order to define as clearly as possible the avenues of spontaneous infection it will be more convenient to discuss the artificial methods of induction of the disease in these animals, and then draw conclusions as to the probable natural means of conveyance of the infection to the rat.
(1.) The Artificially Induced Disease.
(a.) By subcutaneous inoculation.
(b.) By cutaneous inoculation.
(c.) By intra-peritoneal inoculation.
(d.) By inhalation.
(e) By contact with uninjured mucous membranes.
(F) By feeding.
(a.) By Subcutaneous Inoculation.-Experimentally this is one of the most effectual methods of producing plague infection in the rat. The type of disease induced, is usually acute rat plague.
At the same time, it would appear that all rats do not react equally to subcutaneous inoculation: The same degree of susceptibility is not shown by different rats to a definite culture or strain of the B. pestis.
In some cases, a mere prick with the point of an infected needle is sufficient to occasion the infection. (German Plague Commission.) In other cases, however. a very sensible quantity of a culture of the B. pestis must be introduced in order to call forth the disease. (Indian Plague Commission.) It is said that occasion- ally rats are found which give no reaction to inoculation of considerable quantities of fully virulent cultures. (Austrian Plague Commission.) In my opinion, the latter must be indeed rare, in regard to the variability of rats in their degree of susceptibility to plague infection. Professor SIMPSON and I had an experience similar to that of the Indian Plague Commissioners. We found that occasionally it was a difficult matter to produce plague in certain rats. Some were easily infected. Others only succumbed to the infection after most severe tests had been applied.