|Clinical features||Streptobacillary form||Spirillary form|
|Incubation period||2–10 days||7–21 days|
|Indurated bite wound||–||+++|
|Irregular periodicity||Regular periodicity|
The incubation period of streptobacillary rat-bite fever is 2 to 10 days; however, onset usually occurs within 3 days of exposure. Clinical signs develop despite rapid healing of the bite wound, presumably as a result of bacteremia and septicemia. Illness of sudden onset is characterized by remittent chills, fever, headache, and myalgia, and results from direct infectious as well as immune-mediated mechanisms. A morbilliform or petechial rash, which may be a result of leukocytoclastic vasculitis, develops in 75% of patients, frequently within days of onset, on either the lateral or the extensor surfaces of the extremities, occasionally involving the palms and the soles. Infrequently, the rash may be generalized and/or present with pustules, desquamation, and purpura. Simultaneous with rash development, approximately 50% of patients have severe arthralgia or frank arthritis of at least one, but frequently more than one, large joint. The arthritis may be suppurative or nonsuppurative; monoarticular, or migrating and polyarticular; and rarely occurs without other manifestations. Untreated, the course is biphasic, with fever and symptoms diminishing 2 to 5 days after onset and recurring several days later. Arthritis, endocarditis, myocarditis, pericarditis, hepatitis, pancreatitis, parotiditis, prostatitis, pneumonia, nephritis, meningitis, metastatic abscessation, septicemia, and chorioamnionitis are reported complications. Relapsing fever with return of constitutional symptoms of 1 to 6 days’ duration is not uncommon. Afebrile cases have been described.
In the United States, the spirillary form is considerably less common than streptobacillary disease and is rarely associated with infection acquired from laboratory rats. Illness follows an incubation period, usually 7 to 21 days but sometimes as short as 2 days or as long as months. There is initial healing of the bite wound. Subsequently, an indurated chancre or eschar develops at the wound site and is accompanied by a regional lymphadenitis and lymphangitis, fever, rigors, myalgia, and, in about 50% of the cases, an erythematous maculopapular rash originating from the wound. Arthritis is uncommon. Untreated, fevers and other symptoms resolve but then recur regularly, and mortality is estimated between 7% and 13%.
Diagnosis is suggested by a rat bite or rat contact, or exposure to rat-infested areas or contaminated materials, and clinical presentation. Patients may present without a history of rat bite or after a prolonged disease course. For infections caused by S. moniliformis, definitive diagnosis depends on isolation of the organism by microbiologic culture and/or identification of the bacterium in culture or blood, fluid, or tissue samples by amplifying part of the 16S RNA gene using a generic primer set, followed by sequencing, using polymerase chain reaction (PCR). A S. moniliformis-specific PCR has also been described using primers based on human and rodent strains. Detection by PCR in blood is more difficult than tissues because the copy number is lower, hemoglobin is inhibitory, and clearance of dead organisms is quicker after antibiotic treatment is initiated. There are no reliable serologic tests currently available in humans for either organism. A high index of suspicion in the laboratory is frequently necessary, as these organisms are extremely difficult to isolate.
S. moniliformis is a fastidious, facultatively anaerobic, highly pleomorphic, asporogenous, gram-negative rod, measuring less than 1 × 1 to 5 μ