Abstract
Since the second large epidemic of Japanese encephalitis in Japan in 1935, animal experiments have been very actively adopted for the studies on this species of virus. However, in most cases, younger rabbits were selected as test animals. Originally, it is generally stated that, unlike mice, inoculations into rabbits do not necessarily secure infection with typical symptoms each time. The authors performed the following experiments in order to investigate the influences of the blockade of the reticuloendothelial system with various colloidal substances on the inoculation of the Japanese encephalitis virus into adult rabbit.
Rabbits are preliminary blockaded with carbon cinnabar or carmine fluid and inoculated with the supernatant fluid obtained by centrifuge for 15 minutes at 3, 000 r.p.m. of 1: 10 emulsion of brain taken from the mouse infected with Japanese encephalitis virus (Nakayama strain) and manifesting the symptoms. The rabbits were inoculated with 0.7 cc of the supernatant intravenously and 0.1 cc of the same supernatant fluid into the parenchyme of the brain tissue.
The rabbits, which had preliminary treatment and were inoculated with the virus demonstrated various symptoms such as poor appetite, slow movement, ataxia, crouching, ruffled fur, excitement, slackness of the tension of the abdominal wall etc. in various grades since the 6th or 7th day from the inoculation. The above symptoms were seen in more than 50% of 40 rabbits, and some of them demonstrated remittent fever. Approximately on the 12th day from the inoculation, they were sacrificed and the sterility of their brain and spleen was confirmed by bacteriological technique, and pathological histological examinations were performed, after which return inoculations were performed with part of the brain. In some of the cases, the return inoculations were successful. Generally, in the the rabbits of the treated group, changes resembling the findings observed in the human cases of Japanese encephalitis were recognized. In other words, necrosis, demyelinisation, proliferation of glial cells in a limited area and formation of small nodules were seen in the brain of rabbit in which the infection had been generally considered difficult in the past. However, these findings were not recognized in the blockaded control cases, therefore, these findings were evidently considered to have been induced by direct influence of the virus. Other principal findings observed were degeneration of ganglion cells, neuronophagia, hyperemia, hemorrhage, loosening of the tissue, perivascular infiltration, dilatation and fibrosis of Virchow-Robin cavity, diffuse proliferation of glial cells etc. These findings were recognized in a high degree in the cerebral cortex, in the gray matter of thalamencephalon and mesencephalon. By synthesizing the symptoms, rise of temperature, histological findings in the brain tissue of the rabbit, the results of the return inoculation on mice and their histological findings, 82.5% of the rabbits inoculated were regarded as successful On the 12 or 13th day from the inoculation, these findings were observed most intensly showing much superior to the results of the inoculated control group with preliminary treatment.
As stated in the above, it was considered that the blockade of the reticuloendothelial system of adult rabbits furnishes a favourable conditions to the infection of the virus and makes the invasion and propagation of the virus easier by reducing the resistance of the rabbits to the virus.
