[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/) › Rat-bite fever agents

# Rat-bite fever agents

- Bacterium

## Gram-negative?

Thin wall with an outer membrane: stains pink.

- For completeness

Bacteria carried by healthy rats that cause fever, rash on the hands and feet and painful joints after a bite, a scratch or contaminated food.

Updated Nov 13, 2026

Also called: Streptobacillus moniliformis, Spirillum minus, Haverhill fever, sodoku

## Quick facts

**Type**
Bacterium · [Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/)

**Gram / shape**
Gram-negative · S

**Spread**
Bite or scratch of a rat or other rodent

**Causes**
Rat-bite fever · Septic or reactive arthritis · Sodoku

**Treatment**
Penicillin is the drug of choice

On this page

- The basics Key points
- At a glance
- Morphology
- Transmission
- Memory aid
- What the colours mean

- Clinical Virulence factors
- Diseases it causes
- Clinical picture
- Diagnosis

- Treatment Treatment
- Prevention

- Study Test yourself

- Connections Connections

## Key points

- Rat bite + fever + rash on hands and feet + migratory arthritis: Streptobacillus moniliformis.
- Same illness after milk or water contaminated by rats: Haverhill fever.
- Sodoku (Spirillum minus): cannot be cultured; bite re-ulcerates; Asia.
- Penicillin is the drug of choice; endocarditis is the deadliest complication.

## At a glance

**Gram**
Gram-negative

**Shape**
S. moniliformis: pleomorphic rods with bead-like swellings; Spirillum minus: short, tightly coiled spiral

**Oxygen**
S. moniliformis: facultative anaerobe, fastidious

**Family**
Leptotrichiaceae (Streptobacillus); Spirillaceae (Spirillum)

**Stain**
S. minus is seen with Giemsa, Wright or dark-field microscopy; it cannot be cultured

## Morphology

- S. moniliformis: Gram-negative rods that form chains and filaments like a string of beads
- Needs media enriched with blood or serum; inhibited by the anticoagulant in many blood-culture bottles (sodium polyanethol sulfonate)
- Can form wall-deficient (L-form) variants

## Transmission

- Bite or scratch of a rat or other rodent
- Contact with rodent saliva or urine, including handling or kissing pet rats
- Ingestion of contaminated milk, water or food (Haverhill fever)
- Not spread from person to person

**Reservoir:** Healthy rats, mice and other rodents; animals that catch rodents can carry it

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- Spreads through the blood to skin, joints and heart valves
- L-forms may help it persist despite treatment

## Diseases it causes

- Rat-bite fever: fever, rash and migratory polyarthralgia (S. moniliformis)
- Septic or reactive arthritis [Septic Arthritis of the Knee →](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- Endocarditis, the most serious complication [Infective endocarditis →](https://durgical.com/en/conditions/infective-endocarditis/)
- Sodoku (S. minus, mainly Asia): relapsing fever with ulceration at the bite and swollen lymph nodes

## Clinical picture

- Abrupt fever, chills, vomiting, headache and muscle pain usually 3–10 days after contact
- Rash on the hands and feet, sometimes with petechiae or pustules
- The bite usually heals fast in S. moniliformis infection; in sodoku it ulcerates again

## Diagnosis

- Ask about rodent contact and tell the laboratory
- Blood or joint fluid culture on enriched media without the inhibitory anticoagulant
- PCR or 16S rRNA sequencing; MALDI-TOF mass spectrometry on isolates
- S. minus: direct microscopy of blood or lesion material

## Treatment

Penicillin is the drug of choice; doxycycline or streptomycin are options for penicillin allergy, and endocarditis is treated with penicillin plus an aminoglycoside. Early treatment prevents most deaths.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

- [Benzylpenicillin (penicillin G)](https://durgical.com/en/pharmacology/benzylpenicillin/)
- [Doxycycline](https://durgical.com/en/pharmacology/doxycycline/)
- [Streptomycin](https://durgical.com/en/pharmacology/streptomycin/)

## Prevention

- Avoid contact with rodents; rodent control at home and work
- Gloves and hand washing when handling pet or laboratory rodents
- Clean bites promptly and seek care
- Pasteurised milk and safe water

## Memory aid

«Beads, bite and bumps»: a bead-chain rod, a rat bite, and spots on palms and soles with sore joints.

## Test yourself

Think of the answer, then tap the question to see it.

## Which bacterium causes rat-bite fever in the Americas?

Streptobacillus moniliformis.

## What is Haverhill fever?

Rat-bite fever acquired by eating or drinking food, milk or water contaminated by rats.

## Why may S. moniliformis fail to grow in routine blood-culture bottles?

The anticoagulant sodium polyanethol sulfonate inhibits it, and it needs enriched media.

## Which antibiotic is the drug of choice for rat-bite fever?

Penicillin.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/rat-bite-fever-agents/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

## Connections

## Related 4

- [Leptospira interrogans](https://durgical.com/en/microbiology/leptospira-interrogans/)
- [Borrelia recurrentis and tick-borne relapsing fever borreliae](https://durgical.com/en/microbiology/borrelia-recurrentis-and-tick-borne-relapsing-fever-borreliae/)
- [Pasteurella multocida](https://durgical.com/en/microbiology/pasteurella-multocida/)
- [Capnocytophaga canimorsus](https://durgical.com/en/microbiology/capnocytophaga-canimorsus/)

## Related conditions 2

- [Septic Arthritis of the Knee](https://durgical.com/en/conditions/septic-arthritis-of-the-knee/)
- [Infective endocarditis](https://durgical.com/en/conditions/infective-endocarditis/)

[← Back to the group: Other clinically important bacteria](https://durgical.com/en/microbiology/groups/other-clinically-important-bacteria/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [About Rat Bite Fever (RBF) | Rat-bite Fever (RBF) | CDC](https://www.cdc.gov/rat-bite-fever/about/index.html) (CDC, 2024); [Rat Bite Fever and Streptobacillus moniliformis](https://pmc.ncbi.nlm.nih.gov/articles/PMC1797630/) (Clinical Microbiology Reviews (ASM), 2007).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/rat-bite-fever-agents/