Streptobacillus moniliformis

• Causes rat-bite fever

Background

Microbiology

• Pleomorphic, non-motile, non-sporulating, non-encapsulated filamentous Gram-negative bacillus
  • Often long, but can even be coccoid
  • Lateral bulbar swellings
• Anaerobic, requires 8-10% CO2 to grow in culture, and requires an enriched medium
  • Blood cultures often negative because it is inhibited by SPS in blood culture bottles
• Nonhemolytic cotton-like colonies on blood agar
• Forms small flocculent puffballs in liquid media
• Can form a penicillin-resistant, cell-wall-deficient L-phase variant
  • "Fried egg" appearance on plate

Epidemiology

• Transmitted by the bite or scratch (or sometimes merely handling) of a rat, mouse, guinea pigs, or squirrel, or of an animal that eats these (e.g. dog, cat, pig, ferret, weasel, or python)
  • Oral flora of these animals
• Can have occupational exposures in a labs or pet stores, household exposure due to rat infestation, or exposure from pet rats
  • 50% of cases are in children
  • Occurs in about 10% of rat bites, though only 200 cases reported in the US
• Can also be acquired by ingestion of turkey, or milk or water contaminated with rat feces
  • Milk outbreak in Haverhill, MA, gives it its name of Haverhill fever

Pathophysiology

• Likely gains access through a broken skin (or mucosal) barrier
• Relapses may be related to spontaneously changing to and from the L variant
• Autopsy shows erythrophagocytosis, hepatosplenomegaly, interstitial pneumonia, and lymph node sinus hyperplasia

Clinical Manifestations

• Incubation period 10 days (range 1 to 22 days)
• Fever, chills, headache, nausea/vomiting, severe migratory arthralgias and myalgias
  • Wound has often healed by presentation
• Significant leukocytosis
• Can have false-positive non-treponemal syphilis serology
• After 2 to 4 days, develops a nonpruritic rash that can be maculopapular, morbilliform, petechial, vesicular, or pustular, on extremities
  • May become purpuric and confluent
  • Involves palms and soles
• Arthralgias progress to arthritis
• Fever resolves within days, and the other symptoms within about 2 weeks
• However, can develop a relapsing-remitting course over months, and arthritis can persist for years

Haverhill Fever

• When acquired from oral ingestion, can present with pharyngitis and severe vomiting, then progress to the usual

Rare Sequelae

• Endocarditis, myocarditis, pericarditis, sepsis, systemic vasculitis, meningitis, pneumonia, hepatitis, septic arthritis, amnionitis, and anemia
• Abscesses in any organ, including brain, liver, spleen, kidney, epidural space, bone, skin, muscle, and genital tract
• Diarrhea and weight loss in children
• Mortality as high as 13% if untreated

Diagnosis

• Often on the differential with leptospirosis, and can mimic RMSF, secondary syphilis, gonococcal infection, Lyme disease, brucellosis, septic arthritis, and other
• On microscopy, the bacilli stain with Giemsa, acridine orange, or Gram stain
• Must be cultured with specific enriched media
• ELISA exists for antibodies, and PCR exists

Treatment

• Penicillin G 12 million units total daily dose IV, step down to oral after a week if well
• Alternative: tetracycline 500 mg po q6h, streptomycin 7.5 mg/kg IM q12h
• Duration 10 to 14 days

Prevention

• Rat bites should be cleansed thoroughly
• Tetanus prophylaxis if appropriate
• Can do 3 days of prophylactic penicillin 2 g daily divided

Further Reading

• Rat Bite Fever and Streptobacillus moniliformis. Clin Microbiol Rev. 2007;20(1):13-22. doi: 10.1128/CMR.00016-06