Streptobacillus moniliformis is a pleomorphic non-motile Gram-negative rod-shaped bacterium which can form thin branched filaments. It is a more frequent pathogen of rat bit fever than Spirillum minus. The bacterium was first isolated in 1916 and has been given various names in the course of history. The name Streptobacillus moniliformis refers to the necklace-like morphology exhibited by the bacteria (L. monile: necklace). The name Haverhill refers to a small town in Massachusetts where an epidemic broke out in 1926. The bacteria occur naturally in the nasopharynx of rats and are found in 50 to 100% of rats living in the wild. Infection may follow a rat bite or the consumption of water or milk contaminated by rat urine or faeces. Not only rats, but also other rodents such as mice, gerbils, squirrels or carnivores or omnivores which eat rodents (cats, dogs, pigs, weasels, ferrets) can transmit the bacteria. Streptobacillus moniliformis can also cause illness in animals. Eating infected animals (e.g. turkeys) may lead to infection in humans. People who work with animals (laboratory staff, some biologists) are at increased risk. The incubation time is 1 to 22 days, usually approximately 1 week. If there is a bite wound, this heals spontaneously. If infection is oral, there will be no local skin wound. After the wound has healed, intermittent chills, extreme fatigue, vomiting, diffuse muscle and joint pain and headache follow. In Haverhill fever there may be a variable asymmetrical non-purulent arthritis. This is not the case with sodoku. Generally the large joints are affected, such as the knees, ankles, elbows, wrists, shoulders and hips. Purulent arthritis is rare. If a patient is bitten on a finger, a neighbouring interphalangeal joint may exhibit impaired function. Approximately two to four days after the beginning of the fever a skin rash occurs. This may have a morbilliform, pustular or petechial character. The rash is most pronounced on the hands and feet. Desquamation may occur. Somewhat later the patient develops painful pharyngitis. In Haverhill fever there is no lymphadenopathy, unlike sodoku. After approximately five days spontaneous improvement is seen. The fever disappears and the other lesions improve over the course of a few weeks. After an irregular period of time there will be a relapse, however, which triggers a picture of fever of unknown origin. This recurrence may persist for two years. Complications may occur. These include ulcerative endocarditis, subacute myocarditis, pericarditis, meningitis, pneumonia, amnionitis and anaemia. Abscesses may occur in any organ. In epidemic Haverhill fever the name erythema arthriticum epidemicum is used

Differential diagnosis includes coxsackievirus (rash on hands and feet) or an aspecific viral exanthema, meningococcal septicaemia, leptospirosis, erythema multiforme, secondary syphilis, rickettsiosis (RMSF [Rocky Mountain spotted fever]), tularaemia, Bartonella henselae (cat scratch disease) and infections which typically occur after bites, such as Capnocytophaga canimorsus, Eikenella corrodens or Pasteurella multocida infections. If joint problems are prominent, Lyme disease, acute rheumatic fever, brucellosis, gonococcal infection, septic arthritis, infectious endocarditis and auto-immune disorders may have to be excluded.

The white blood cell count varies. Some patients have a normal blood count, while others have significant leukocytosis (to 30,000) with left shift. Up to a quarter of patients have a false positive syphilis test. A diagnosis may be reached clinically, but should be supported by cultures. The bacteria can be cultured on specially enriched media (micro-aerophilic medium with increased CO₂ ). Colonies with a "fried egg" morphology are sometimes observed and indicate the existence of L-phase variants (cell wall deficient bacteria). Recognition of the latter is important because beta-lactam antibiotics are not active against them. They are susceptible to doxycyclin. Biochemical profiling is difficult and time-consuming. Gas chromatography of cell wall lipids can be used.

Serology (ELISA) may be carried out in specialised laboratories. The treatment is based on tetracycline or penicillin, preferably given for 14 days. There may be a Jarisch-Herxheimer-like reaction at the beginning of treatment. Erythromycin and ceftriaxone are also effective.