Treatment of Rat Bite Fever Without Fever
Treat rat bite fever with intravenous penicillin G (12-20 million units/day in divided doses every 4-6 hours) for 5-7 days, followed by oral penicillin or amoxicillin to complete a total of 14 days of therapy, regardless of whether fever is present. 1, 2
Why Absence of Fever Does Not Change Management
Rat bite fever can present without fever in a significant minority of cases, making diagnosis particularly challenging but not altering the treatment approach 3. The disease is caused by Streptobacillus moniliformis in the United States and remains potentially lethal with a 10-13% mortality rate if untreated, but is easily treatable with appropriate antibiotics 4, 3, 2.
Clinical Recognition Without Fever
Look for these key diagnostic features even when fever is absent:
- Polyarthritis or migratory polyarthralgias (occurs in up to 50% of cases) 3, 5
- Maculopapular or petechial rash (may be pustular) 4, 5
- History of rat exposure or bite (though many patients lack this history) 4, 6
- Leukocytosis and elevated C-reactive protein 5
- Asymmetric joint involvement that can mimic rheumatologic disease 5
Definitive Treatment Regimen
First-Line Therapy
Intravenous penicillin G potassium: 12-20 million units/day administered in equally divided doses every 4-6 hours for 5-7 days 1, 2
Transition to Oral Therapy
After clinical improvement with IV therapy, transition to oral penicillin V or amoxicillin to complete a total duration of 14 days 2
Alternative for Penicillin Allergy
If penicillin-allergic, use doxycycline or ceftriaxone as alternatives 2
Special Considerations for Complicated Disease
Septic Arthritis
When rat bite fever presents with septic arthritis (as in afebrile cases), extend treatment to 4 weeks of antibiotics 5. Some cases may require:
- Arthrotomy and joint debridement for persistent joint involvement 4
- Prolonged antibiotic therapy until complete clinical resolution 4
Osteomyelitis or Discitis
If vertebral osteomyelitis or discitis is present, treat with 6 weeks of antibiotic therapy (cephalexin has been used successfully) 6
Critical Diagnostic Pitfalls
Blood cultures are notoriously difficult because S. moniliformis is a fastidious, pleomorphic Gram-negative bacillus that requires enriched media and prolonged incubation 2. Alert the microbiology laboratory to your clinical suspicion to optimize culture conditions 2.
The absence of fever combined with polyarthritis frequently leads to misdiagnosis as a rheumatologic condition, especially when anti-cyclic citrullinated peptide (anti-CCP) antibodies are positive 5. Do not be misled by positive rheumatologic markers—maintain rat bite fever in your differential when there is rodent exposure history 5.
Monitoring and Follow-Up
- Continue antibiotics for at least 48-72 hours after the patient becomes asymptomatic 1
- Monitor for complications including endocarditis, which would require 4 weeks of IV therapy 1
- Reassess if symptoms persist beyond 5 days of appropriate therapy 4
The key principle: fever is not required for diagnosis or treatment of rat bite fever, and the standard antibiotic regimen remains unchanged regardless of temperature. 3, 2