Spirillum minus is a small spiral-shaped bacterium and is usually classified as a spirochaete. The bacterium has flagella and moves quickly, unlike Streptobacillus moniliformis. The bacteria can be easily stained with methylene blue or Giemsa. The bacteria are present in healthy rats. The clinical picture triggered by this bacterium is known as sodoku (a Japanese name). Sodoku is found mainly in Asia, unlike the more widely distributed Haverhill fever. The bacterium is transmitted via the bite of a rat. Mice and guinea pigs may also become infected. Transmission via cats is rare. After an infected bite an incubation time of 5 to 30 days (one week on average) follows. There is local inflammation and even tissue necrosis. The infection may spread via the lymphatic system, resulting in regional lymphadenopathy. The body temperature rises progressively over the course of 3 days. Chills are accompanied by general malaise. Then the temperature diminishes again over three days, which is followed by a symptom-free period of 5 to 10 days. One or more relapses occur. They are accompanied by papular, nodular or roseola-like urticarial exanthema which sometimes has a purple tint. The rash occurs chiefly on the arms and thorax. Diffuse myalgia and arthralgia, hyperaesthesia and oedema of various parts of the body occur. Sometimes there is arthritis. During the fever there is moderate leukocytosis and sometimes eosinophilia, although the latter is questionable. A lumbar puncture may show increased pressure of the cerebrospinal fluid. The disease may persist for months and every attack is accompanied by repeated febrile episodes. In complications there may be delirium and coma. In rare fatal cases lesions of the liver and kidneys are found. Mortality is approximately 10%. Sodoku should be differentiated from recurrent fever (borreliosis from ticks or lice), trench fever (rickettsiosis), intermittent bacteraemia (e.g. in kidney infections, prostatitis, subacute bacterial endocarditis), brucellosis, sleeping sickness and malaria. The swollen face and the urticarial rash may simulate nephritis.

Diagnosis is not easy. Even if there has been a rat bite, this will not always be reported when taking the history. Nevertheless this detail will be an important guiding factor. The fluctuating fever, skin rash and swollen face suggest the diagnosis of sodoku. The bacteria can be demonstrated using dark-field microscopy of a little fluid from the site of the bite. Inoculation of test animals (xenodiagnosis) permits isolation of the bacteria. The bacterium cannot as yet be cultured in vitro. Serology is difficult. Treatment is based on IV penicillin G or oral penicillin. A swift recovery can be expected, even after a single injection. Nevertheless it is best to continue treatment for about ten days or two weeks. Streptomycin and tetracycline are also effective. Doxycycline is sometimes given after penicillin treatment due to the presence of L-phase variants.