Rat-Bite Fever
Rat-bite fever is an acute zoonotic infection passed to humans through bites, scratches or contact with the secretions of rodents, especially rats and mice. Two distinct organisms cause it: the chain-forming Gram-negative bacillus Streptobacillus moniliformis (the streptobacillary form, common in Western countries) and the Gram-negative spiral bacterium Spirillum minus (the spirillary form, also called sodoku, more common in Asia). Even after the original bite has fully healed, the bacteria can keep multiplying quietly in the small blood vessels, then cause sudden high fever, a papular hemorrhagic rash on the palms and soles, and progressive polyarthritis.
At Haute-Derma, managing rat-bite fever means taking a careful exposure history, identifying the clinical form precisely, giving systemic bactericidal antibiotics and, once the acute phase is over, restoring skin tone and texture.
Pathophysiology and Transmission
Source of Infection and Route of Entry
- Rodent throat flora: Streptobacillus moniliformis and Spirillum minus live harmlessly in the nose and throat of healthy rats (found in 50–100% of wild and laboratory rats).
- Transmission: through bites and scratches, or through food and water contaminated with rat droppings and saliva (infection by swallowing S. moniliformis is known as Haverhill fever).
Effects on Small Vessels and Immune Evasion
- Bloodstream spread (bacteremia): once past the skin barrier, the bacteria enter the blood and spread through the body. Streptobacillus moniliformis can spontaneously switch to a cell wall-deficient form (L-form). This helps it hide from the immune system and makes it respond poorly to a single cell wall-targeting antibiotic given at too low a dose.
- Immune complexes in skin and joints: antigen-antibody complexes settle in the walls of small dermal vessels and in the joint lining, causing hemorrhagic small-vessel vasculitis (leukocytoclastic vasculitis) and acute synovitis.
Clinical Features by Type
Streptobacillary Form (Streptobacillus moniliformis)
- The bite heals on its own: the rat bite usually heals completely, with no scar and no local swelling or inflammation.
- Sudden onset: after an incubation period of 3–20 days (10 days on average), the patient abruptly develops high fever, shaking chills, headache, vomiting and severe muscle pain.
- Hemorrhagic skin rash: appears 2–4 days after the fever starts. It begins as slightly raised red papules that quickly turn into a petechial or purpuric rash, mainly on the limbs and especially the palms and soles.
- Migratory polyarthritis: about 50% of patients develop joint pain or flaring polyarthritis (often in the wrists, knees and ankles), with hot, red, swollen and very painful joints.
Spirillary Form / Sodoku (Spirillum minus)
- The bite flares up again: the incubation period is longer, 2–4 weeks. When the fever begins, the healed bite suddenly becomes red, swollen and hard, then breaks down into a necrotic ulcer with a sunken edge and a black crust.
- Swollen regional nodes and recurring fever: the draining lymph nodes become enlarged and painful. Without treatment, the fever comes and goes in cycles (relapsing fever) for weeks.
- Dark purple rash: the rash is usually made of dark purple or deep red patches, larger than in the streptobacillary form, and arthritis is uncommon.
Assessment and Diagnosis at Haute-Derma
Exposure History and Clinical Examination
- A detailed history of rat bites, keeping pet rats, laboratory work or contact with suspect water or food in the past month.
- Examination of the rash on the palms and soles, checking peripheral joints for pain and swelling, and dermoscopy of hemorrhagic skin lesions.
Microbiology Tests to Identify the Organism
- Darkfield microscopy, or Giemsa/Wright staining of ulcer smears or lymph node aspirates, to find the corkscrew-moving Spirillum minus.
- Blood or joint fluid culture in serum-enriched medium (specific for Streptobacillus moniliformis).
- PCR to detect bacteria-specific DNA, or serology looking for a rising antibody titer (a fourfold increase between two samples).
Medical Treatment and Skin Recovery at Haute-Derma
Systemic Antibiotic Treatment
- First-line: penicillin G 1.2 million units by IV infusion every 6 hours for 5–7 days. Once the fever has settled and the patient is improving, switch to oral penicillin V 500 mg four times daily (or amoxicillin 500 mg three times daily) to complete a 14-day course.
- Alternatives for patients allergic to penicillin:
- Doxycycline 100 mg orally twice daily for 14 days.
- Or tetracycline 500 mg orally four times daily for 14 days.
- Severe cases complicated by endocarditis: high-dose penicillin G combined with gentamicin 1 mg/kg IV every 8 hours for 4–6 weeks.
Restoring Skin Appearance and Post-Inflammatory Pigmentation
- Caring for acute skin lesions: creams with panthenol, zinc oxide and madecassoside to soothe hemorrhagic areas and protect the skin barrier.
- Treating post-inflammatory hyperpigmentation (PIH) and ulcer scars:
- Once the infection is fully controlled, serums with niacinamide and azelaic acid plus epidermal growth factor (EGF) help fade dark marks left by bleeding on the palms and soles.
- For sodoku with a hard, dark ulcer scar at the bite site: fractional CO2 laser combined with hyaluronic acid mesotherapy to release fibrosis, smooth the skin surface and even out pigmentation.
