1. Since our first report on the discovery of the cause of rat-bite fever, we have been able to prove the existence of the same spirochete in five out of six more cases which have come under our observation.
2. The clinical symptoms of rat-bite fever are inflammation of the bitten parts, paroxysms of fever of the relapsing type, swelling of the lymph glands, and eruption of the skin, all occurring after an incubation period usually of from 10 to 22 days, or longer.
3. Our spirochete is present in the swollen local lesion of the skin and the enlarged lymph glands. But as the spirochetes are so few in number it is exceedingly difficult to discover them directly in material taken from patients. It is therefore better to inoculate the material into a mouse. In some cases the organism is found in the blood of the inoculated animal after a lapse of 5 to 14 days, or at the latest 4 weeks.
4. Generally speaking, the spirochetes present thick and short forms of about 2 to 5 µ and have flagella at both ends. Including the flagella, they measure 6 to 10 µ in length. Some forms in the cultures reach 12 to 19 µ excluding the flagella. The curves are regular, and the majority have one curve in 1 µ. Smaller ones are found in the blood and larger ones in the tissues.
5. The spirochetes stain easily. With Giemsa's stain they take a deep violet-red; they also stain with ordinary aniline dyes. The flagella, too, take Giemsa's stain.
6. The movements of our spirochetes are very rapid, resembling those of a vibrio, and distinguish them from all other kinds of spirochetes. When, however, the movements become a little sluggish, they begin to present movements characteristic of ordinary spirochetes.
7. For experimental purposes, mice, house rats, white rats, and monkeys are the most suitable animals. Monkeys have intermittent fever after infection, and spirochetes can be found in their blood, but they are not so numerous as in the blood of mice. Mice are the most suitable animals for these experiments, and they appear, as a rule, to escape fatal consequences.
8. The spirochete is markedly affected by salvarsan.
9. The organism is not present in the blood of all rats, and there is no relation between the species of the rat and the ratio of infection. We have never found the spirochete in healthy guinea pigs or mice. By permitting a rat infected with the spirochete to bite a guinea pig, the latter develops the disease.
10. We have succeeded in cultivating the spirochete in Shimamine's medium.
11. Among the spirochetes described in the literature or discovered in the blood of rats and mice, there may be some resembling our spirochete, but none of the descriptions agree with it fully. Hence we have named our organism Spirochæta morsus muris and regard it as belonging to the Spironemacea (Gross) of the nature of treponema.
12. The spirochete can be detected in the bodies of patients. In seven cases out of eight, it disappears on recovery, only to reappear during the relapse.
13. The spirochete can be detected in about 3 per cent of house rats. These facts enable us to identify the cause of the disease.
14. There may be other causes than the spirochete for diseases following the bite of a rat. The cause, however, of rat-bite fever in the form most common in Japan is, we believe, the spirochete which we have described.