The strain was isolated from the blood of a patient in Japan by serial passage through guineapigs in 1916 (Ido, Ito and Wani, 1918). In the original publication in Japanese in Nippon Naika Gakkai Zasshi, No. 5, 1917, the new leptospira is designated Spirochaeta nanukayaami (nanukaaami = seven day fever). In the publication of Ido et al. (1918) the designation hebdomadis appears for the first time, and was than generally adopted as the serovar name. As in neighter publication a strain name is mentioned, the serovar name was also used as the strain name. The strain was submitted to factor analysis by Kmety (1977), who suggested on the basis of the resuslts that it should be placed in a subgroup to be named Hebdomadis.