A new species of spirochete which we have called Spirochæta hebdomadis has been described as the specific etiological agent of seven day fever, a disease prevailing in the autumn in Fukuoka and other parts of Japan.
This spirochete is distinguishable from Spirochæta icterohæmorrhagiæ to which it presents certain similarities.
Young guinea pigs are susceptible to inoculation with the blood of patients and to pure cultures of the spirochete, and those developing infection exhibit definite symptoms suggestive of those of seven day fever in man.
The blood serum of convalescents from seven day fever contains specific immune bodies acting spirochetolytically and spirocheticidally against the specific spirochetes, but not against Spirochæta icterohæmorrhagiæ.
The field mouse (Microtus montebelli) is the normal host of the spirochetes, which have been detected in the kidneys and urine of 3.3 per cent of the animals examined.
The endemic area of prevalence of seven day fever corresponds with the region in which field mice abound.