222
During the months of March, April and of May these examinations were systematically carried out by myself. In May the services of 4 Japanese Doctors were obtained, and their researches, supervised by myself, were carried on until the 13th October, when they returned to Japan. From the latter date onwards, the work has been done by myself assisted by Chinese Qualified Doctors and Students of the College of Medicine in Hongkong. These examinations were gone on with even during the hottest weather, the daily number of rats examined--- conting six days to a week-averaged nearly 400. This reaches approximately between 2.000 or 3,000 rats examined per week.
THE METHODS EMPLOYED.
Exact details as to the place where each rat was found was furnished by the Sanitary Department. The post-mortem on each rat was under antiseptic precau- tions, and smears both of the heart blood and spleen pulp were made on micros- copic glass slides. These were dried, fixed and stained by the usual tinctorial methods and examined microscopically. In almost all cases plague infection in rats is one most pronounced, the stained smears usually containing millions of typically ovalbipolar plague bacilli. If any doubt existed as to the nature of the organisms present GRAM's method of decolorisation was employed as a counter test. Seeing that so enormus a number of rats were examined by such a limited staff, these methods were the only possible. Subsequent to the post-mortem examina- tions, all the rats are cremated in an apparatus erected in the immediate neigh- bourhood.
THE INTERPRETATION OF THE RESULTS.
The post-mortem and bacteriological examination of large numbers of living and dead rats is a research which must be carried out with considerable cure. The diagnosis of the presence of plague bacilli in any tissue or organ by means of the inicroscope alone is frequently one of extreme difficulty, and this is all the more so because in the tissues of man and animals micro-organisms occur which by the microscope alone cannot under any conditions be distinguished from the plague bacillus. The research is even complicated to a much greater degree in regard to the diagnosis of the presence of plague bacilli in rats. Rats suffer from a large number of septicemic diseases. These diseases frequently break out amongst them in epidemic form with a heavy mortality. Among the many micro-organisms causing these epidemic diseases may be mentioned B. DANYsz and B. v. SCHILLING, etc. Other micro-organisms such as B. of fowl cholera, B. swine plague, etc., are also pathogenic for rats. Morphologically and tinctorially all the above mentioned micro-organisms resemble the B. Pestis. Therefore it is evident that even though plague is prevalent among rats in any particular city, one has to be on guard for the possible occurrence of other epidemic diseases which might account for an increased death-rate at any time among them.
The rat is one of the most susceptible animals to plague infection, the gray as well as the white rat probably sharing this almost equally.
The paths of infection in the rat are numerous.
The skin, the mere rubbing of plague material on the shaved abdomen of a rat is sufficient.
The mouth, the throat, and the nose are frequently sources of infection. In these cases one frequently has evidence of infection of the nearest lympathic gland, e.g., of the neck. The glands are sowllen, edematous and hæmorrhagic, and are full of plague bacilli. This condition may be met with in rats found alive, but pre- senting symptoms of disease.
In other cases it is not difficult to trace the infection by way of the alimentary canal.
The stomach and small intestine show marked swelling, adema and hæmorrhagic infiltration. The intestinal follicles and mesenteric glands are the seat of great ædema and blood extravasation. The glands may even reach the size of a pea, and they contain enormous masses of plague bacilli.
Further infection may take place by way of the lungs setting up what is called an Aspiration Pneumonia. This may either be patchy or affect whole lobe of a hung. and the consolidated patches always show great hæmorrhagic infiltration. The parts are always choked with plague bacilli.