Infectious Disease and Microbiology – Spirillum minus

Overview

Spirillum minus, historically also called Spirillum minor, is a spiral-shaped Gram-negative bacterium associated with one form of rat-bite fever, known as sodoku.

The infection occurs worldwide but is reported more frequently in Asia, particularly Japan, and is relatively uncommon in the United States. Disease typically follows a rat bite or other exposure to infected rodents and is characterized by recurrent or relapsing fever, inflammation at the bite site, regional lymphadenopathy, rash, and musculoskeletal symptoms.

Classification

Genus: Spirillum

Species: Spirillum minus

Historical synonym: Spirillum minor

Disease: Rat-bite fever (sodoku)

Microbiologic Characteristics

S. minus is a:

• Gram-negative organism

• Spiral or helical bacterium

• Motile organism

• Traditionally described as aerobic

• Fastidious organism that is extremely difficult to cultivate using routine laboratory methods

Its spiral morphology is reflected in the name:

Spirillum

High-Yield Microbiology Pattern

Spiral Gram-negative bacterium

• ●

Rat exposure

• ●

Relapsing fever

• ●

Rash and inflammatory bite lesion

→ Think Spirillum minus

Incubation Period

The incubation period ranges from approximately:

2 days to 3 weeks

Symptoms therefore may not appear immediately after the rodent exposure.

Epidemiology

S. minus infection has a:

Worldwide distribution

However, it is:

Rare in the United States

and has historically been reported more commonly in:

Asia, especially Japan

Rodent Association

The major epidemiologic association is:

RATS

Rodents can carry the organism without necessarily appearing ill.

Human infection generally occurs after exposure to infected rodent secretions through:

Broken skin or a bite wound

Rat-Bite Fever

S. minus causes:

SODOKU

Sodoku is one of the two classic forms of:

Rat-bite fever

The other major form is caused by:

Streptobacillus moniliformis

High-Yield Association

Spirillum minus

→ Sodoku

Streptobacillus moniliformis

→ Streptobacillary rat-bite fever

Clinical Course

Following inoculation:

Rat bite

↓

Initial wound may begin to heal

↓

Local inflammatory lesion may subsequently develop or recur

↓

Regional lymphadenopathy

↓

Fever

↓

Relapsing episodes

↓

Possible rash, myalgia, and arthralgia

This relapsing pattern is particularly characteristic of:

Sodoku

Relapsing Fever

A major clinical feature is:

RECURRENT OR RELAPSING FEVER

Patients may experience episodes of:

Fever → improvement → recurrent fever

rather than a single continuous febrile illness.

Bite-Site Lesion

The original bite site may become:

• Erythematous

• Swollen

• Painful

• Indurated

• Ulcerated in some cases

A particularly useful clue is:

Reactivation or inflammation of a previously healing rat-bite wound.

Regional Lymphadenopathy

Local infection may be accompanied by:

Regional lymph-node enlargement

and sometimes:

Lymphangitis

This local lymphatic involvement is a useful distinction from some other forms of rat-bite fever.

Rash

The source describes:

Maculopapular rash

as a possible manifestation.

The eruption accompanies the systemic febrile illness and should be interpreted in the context of:

Rodent exposure + relapsing fever

Musculoskeletal Manifestations

Patients may experience:

• Myalgia

• Arthralgia

• Polyarthritis

Thus, the clinical syndrome may resemble other systemic bacterial or inflammatory illnesses.

High-Yield Clinical Pattern

Rat bite

• ●

2 days–3 weeks incubation

• ●

Recurrent fever

• ●

Inflamed bite site

• ●

Regional lymphadenopathy

• ●

Maculopapular rash

→ Think SODOKU due to Spirillum minus

Severity

Untreated infection can become serious.

The source reports an untreated case-fatality rate of approximately:

10%

This emphasizes the importance of recognizing the disease and initiating appropriate antimicrobial therapy.

Diagnosis

Diagnosis can be challenging because S. minus is:

Extremely fastidious

and is not readily detected using ordinary bacterial culture techniques.

The source emphasizes that the:

Microbiology laboratory should be notified before specimens are collected

when Spirillum infection is suspected.

Culture

Historically, specialized techniques have been required for microbiologic detection.

The source describes:

Special media

and prolonged incubation of approximately:

2–3 weeks

However, S. minus is exceptionally difficult to cultivate, and routine clinical culture should not be expected to reliably recover the organism.

Serology

The source states:

No serologic test is available

for routine diagnosis.

Therefore, diagnosis depends heavily on:

Clinical suspicion + exposure history + specialized organism detection

Molecular Diagnosis

Molecular techniques such as:

PCR

can assist in identifying the organism in specialized settings.

These approaches are particularly useful because conventional cultivation is difficult.

Laboratory Communication

A major practical principle is:

Tell the microbiology laboratory when rat-bite fever is suspected.

This helps ensure that:

• Appropriate specimens are collected

• Specialized diagnostic approaches are considered

• Routine negative cultures are not incorrectly interpreted as excluding infection

Spirillum minus vs. Streptobacillus moniliformis

This is the most important comparison.

Spirillum minus

→ Causes sodoku

→ Historically more associated with Asia

→ Spiral organism

→ Incubation can extend to several weeks

→ Relapsing fever prominent

→ Bite-site inflammation may recur

→ Regional lymphadenopathy may occur

→ Very difficult to culture

Streptobacillus moniliformis

→ Major cause of rat-bite fever in North America

→ Pleomorphic filamentous Gram-negative bacillus

→ Fever, rash, and migratory polyarthralgia/polyarthritis

→ Bite wound may already have healed

→ Can also cause Haverhill fever after ingestion of contaminated food or water

Exam Comparison

Rat bite + relapsing fever + recurrent bite-site inflammation + lymphadenopathy + Asia/Japan

→ Spirillum minus

Rat exposure + fever + rash + migratory polyarthritis, especially in North America

→ Streptobacillus moniliformis

Treatment

The source recommends:

PENICILLIN

as the primary treatment.

S. minus rat-bite fever generally responds well to appropriate antimicrobial therapy when recognized promptly.

Additional Treatment Options

The source lists:

• Doxycycline

• Ampicillin

• Azithromycin

• Streptomycin

Choice of therapy should take into account:

Disease severity + allergies + patient factors + clinical response

Complicated Infection

Patients with severe or complicated disease require more intensive evaluation.

Rat-bite fever can occasionally be associated with serious complications involving sites such as:

• Heart valves

• Joints

• CNS

• Other internal organs

Persistent bacteremia or compatible cardiac findings should therefore raise concern for:

Endocarditis

Prevention

Prevention centers on reducing exposure to:

Rodent bites and secretions

Important measures include:

• Appropriate rodent control

• Protective handling of laboratory or pet rodents

• Avoiding direct contact with wild rats

• Prompt cleansing of rodent bites and scratches

• Seeking medical evaluation when systemic symptoms develop after rodent exposure

High-Yield Memory Aid

S = Spirillum

S = Sodoku

S = Spiral

S = Several relapses

Spirillum minus → Sodoku with relapsing fever after a rat bite

High-Yield Clinical Pattern

Rat bite

• ●

Asia/Japan

• ●

Relapsing fever

• ●

Recurrent inflammation at bite site

• ●

Regional lymphadenopathy

• ●

Maculopapular rash and arthralgia

→ Think SPIRILLUM MINUS

Exam Essentials

Genus: Spirillum

Species: S. minus

Historical name: S. minor

Organism: Spiral Gram-negative bacterium

Disease: Rat-bite fever (sodoku)

Incubation: 2 days–3 weeks

Distribution: Worldwide

Geographic clue: More commonly reported historically in Asia, especially Japan

US occurrence: Rare

Transmission: Primarily rodent bite/exposure

Classic clinical feature: Relapsing fever

Local clue: Recurrent inflammation at the bite site with regional lymphadenopathy

Other manifestations: Maculopapular rash, myalgia, arthralgia, and polyarthritis

Untreated mortality in source: Approximately 10%

Diagnosis: Difficult; specialized microbiologic/molecular methods may be required

Routine culture: Poor diagnostic method because the organism is extremely fastidious

Serology: No routine serologic test

Important practical step: Notify the microbiology laboratory when infection is suspected

Primary source treatment: Penicillin

Additional source treatments: Doxycycline, ampicillin, azithromycin, and streptomycin

Major differential: Streptobacillus moniliformis rat-bite fever

Key clinical pearl: Spirillum minus causes the sodoku form of rat-bite fever and is classically associated with Asia, particularly Japan. The most useful examination pattern is a rat bite followed days to weeks later by recurrent inflammation at the bite site, regional lymphadenopathy, and relapsing episodes of fever with rash, myalgia, or arthralgia. The organism is exceptionally difficult to cultivate, so diagnosis requires strong clinical suspicion and specialized laboratory evaluation; penicillin is the classic treatment.