Legionella pneumonia

A 65-year-old man with chronic obstructive pulmonary disease presents with fever, cough, and shortness of breath for several days. He also reports fatigue, malaise, myalgias, headache, confusion, and diarrhea. He recently returned from cruise ship travel. Vital signs show a blood pressure of 110/70 mm Hg, pulse rate of 58 beats per minute, respiratory rate of 18 breaths per minute, oxygen saturation of 92% on room air, and temperature of 101.2°F. Laboratory studies show leukocytosis, hyponatremia, and elevated AST and ALT. Urinary Legionella antigen testing is positive, and chest X-ray shows diffuse pulmonary infiltrates. Diagnosis?

The diagnosis is Legionnaires' disease (Legionella pneumonia).

1. Definition

Legionnaires' disease is a potentially severe pneumonia caused by Legionella species, most commonly Legionella pneumophila. Legionella species are gram-negative facultative intracellular bacteria associated primarily with human-made water systems.

2. Etiology and Risk Factors

1.      Infection usually occurs through inhalation of aerosolized contaminated water or, less commonly, aspiration of contaminated water

2.      Important exposure sources include:

o    Cooling towers

o    Showers and faucets

o    Hot tubs

o    Decorative fountains

o    Large building water systems

3.      Recent travel, including hotels and cruise ships

4.      Age ≥50 years

5.      Current or former smoking

6.      Chronic lung disease, including COPD

7.      Immunocompromised state

8.      Person-to-person transmission is extremely rare

3. Pathophysiology

1.      Contaminated aerosols are inhaled into the lower respiratory tract

2.      Legionella infects alveolar macrophages

3.      The organism interferes with normal intracellular phagolysosomal processing

4.      It undergoes intracellular replication

5.      Macrophage injury and the host inflammatory response produce pneumonia and systemic manifestations

4. Clinical Features

1.      Fever

2.      Cough, which may be dry or productive

3.      Dyspnea

4.      Gastrointestinal symptoms

o    Diarrhea

o    Nausea or abdominal symptoms

5.      Neurologic manifestations

o    Headache

o    Confusion

6.      Myalgias and marked constitutional symptoms

7.      Relative bradycardia may occur but is neither sensitive nor specific

8.      Legionnaires' disease cannot be reliably distinguished from other causes of pneumonia by clinical features or chest imaging alone

5. Diagnosis

5.1 Urinary Antigen Test

1.      Rapid and highly specific

2.      Primarily detects Legionella pneumophila serogroup 1

3.      A negative urinary antigen test does not exclude infection with other Legionella species or serogroups

5.2 Lower Respiratory Testing

1.      Culture or molecular testing of a lower respiratory specimen should be performed alongside urinary antigen testing when Legionnaires' disease is suspected

2.      PCR or other molecular testing can detect a broader range of Legionella species and serogroups

3.      Culture

o    Requires specialized media such as buffered charcoal yeast extract (BCYE) agar

o    Can detect species and serogroups not identified by urinary antigen testing

o    Provides an isolate that can be compared with environmental samples during outbreak investigations

5.3 Laboratory Findings

Common supportive abnormalities include:

1.      Leukocytosis

2.      Hyponatremia

o    Commonly associated with Legionnaires' disease

o    Should not automatically be attributed to SIADH, because the mechanism may be multifactorial and can include renal salt wasting

3.      Elevated AST and ALT

4.      Other abnormalities may include elevated inflammatory markers, creatine kinase, or renal dysfunction in severe disease

5.4 Chest Imaging

1.      May show focal, patchy, multilobar, or diffuse infiltrates

2.      Pulmonary infiltrates can progress despite early appropriate therapy

3.      Imaging findings are not specific for Legionella

6. Management

6.1 Antibiotic Therapy

1.      First-line agents with good intracellular activity include:

o    Azithromycin

o    Levofloxacin

o    Moxifloxacin as another respiratory fluoroquinolone option

2.      Appropriate antibiotic therapy should be started promptly when Legionnaires' disease is suspected

3.      Beta-lactam monotherapy is inadequate for Legionnaires' disease because it does not provide reliable clinical activity against intracellular Legionella

6.2 Severe or Hospitalized Disease

1.      IV azithromycin or levofloxacin is commonly used for severe disease

2.      Routine combination therapy has not been proven superior to appropriate monotherapy

3.      Monotherapy with azithromycin or a respiratory fluoroquinolone is appropriate for most patients

4.      Combination therapy with a macrolide plus a fluoroquinolone may be considered in selected patients with:

o    Shock requiring vasoactive support

o    Significant immunocompromise

o    Failure of appropriate monotherapy

6.3 Duration

1.      The optimal duration of Legionella-specific therapy is not firmly established, because Legionella-specific randomized duration trials are lacking

2.      Non-severe disease: approximately 3 to 5 days, provided the patient has achieved clinical stability

3.      Severe disease: approximately 10 to 14 days, individualized according to clinical response and complications

4.      Immunocompromised patients: generally require prolonged therapy, commonly 10 to 14 days or longer, depending on disease severity, complications, antimicrobial choice, and clinical response

6.4 Supportive Care

1.      Supplemental oxygen when indicated

2.      Appropriate fluid and electrolyte management

3.      Management of sepsis or respiratory failure when present

4.      Patients with travel-associated disease should be evaluated for potential public health reporting and outbreak investigation according to local requirements

7. Complications

1.      Acute hypoxemic respiratory failure

2.      Sepsis and septic shock

3.      Acute kidney injury

4.      Multiorgan dysfunction

5.      Rhabdomyolysis, in some severe cases

8. Key Clinical Insight

Pneumonia accompanied by diarrhea, confusion, hyponatremia, elevated liver enzymes, relative bradycardia, and recent cruise ship travel or another potential exposure to aerosolized water systems strongly suggests Legionnaires' disease. In this patient, the positive urinary Legionella antigen test provides confirmatory laboratory evidence for L. pneumophila serogroup 1 infection.

References

1.      Centers for Disease Control and Prevention. Laboratory Testing for Legionella. CDC. Updated May 13, 2026.

2.      Dinh A, Saliba M, Maulin L, Welker Y. Treatment duration of community-acquired pneumonia due to Legionella pneumophila: where do we stand? Infect Dis Now. 2026;56(5):105299. doi:10.1016/j.idnow.2026.105299.

3.      Rello J, Allam C, Ruiz-Spinelli A, Jarraud S. Severe Legionnaires' disease. Ann Intensive Care. 2024;14:51. doi:10.1186/s13613-024-01252-y.

Clostridioides difficile infection

A 55-year-old woman presents with a 5-day history of abdominal cramping, watery diarrhea, and low-grade fever. She was recently treated with ciprofloxacin for cystitis. She denies recent travel or contaminated food exposure. Examination shows mild abdominal tenderness and hyperactive bowel sounds. Vital signs show a blood pressure of 120/80 mm Hg, pulse rate of 108 beats per minute, respiratory rate of 18 breaths per minute, oxygen saturation of 92% on room air, and temperature of 100.2°F. Laboratory studies show leukocytosis, and stool PCR is positive for Clostridioides difficile toxin genes. Diagnosis?

The diagnosis is Clostridioides difficile infection following recent antibiotic exposure.

1. Definition

Clostridioides difficile infection (CDI) is a toxin-mediated inflammatory disease of the colon caused by toxigenic C. difficile, usually after disruption of the normal intestinal microbiota. Disease ranges from mild diarrhea to severe colitis, pseudomembranous colitis, and fulminant colitis.

2. Etiology and Risk Factors

1.      Caused by Clostridioides difficile, a gram-positive, spore-forming, anaerobic bacterium

2.      The most important modifiable risk factor is recent antibiotic exposure

3.      Antibiotics strongly associated with CDI include:

o    Clindamycin

o    Fluoroquinolones

o    Cephalosporins

o    Broad-spectrum penicillins

4.      Other important risk factors include:

o    Older age, particularly ≥65 years

o    Recent hospitalization or healthcare exposure

o    Previous CDI

o    Immunosuppression

o    Significant comorbidity

5.      Proton pump inhibitor use is associated with CDI, although unnecessary acid-suppressive therapy should be discontinued rather than stopping therapy that has a valid indication

3. Pathophysiology

1.      Antibiotic exposure disrupts the normal intestinal microbiota

2.      Ingested C. difficile spores can germinate and permit colonization and bacterial overgrowth

3.      Toxigenic strains produce:

o    Toxin A

o    Toxin B

4.      These toxins glucosylate and inactivate Rho-family GTPases

5.      This causes:

o    Cytoskeletal disruption

o    Loss of epithelial barrier integrity

o    Fluid secretion

o    Neutrophilic inflammation and colonic injury

6.      Severe mucosal injury can result in pseudomembrane formation

4. Clinical Features

1.      Watery diarrhea, typically ≥3 unformed stools within 24 hours

2.      Abdominal pain and cramping

3.      Fever

4.      Leukocytosis

5.      Nausea and anorexia may occur

6.      Severe or fulminant disease may cause:

o    Marked abdominal pain or distension

o    Ileus

o    Toxic megacolon

o    Hypotension or shock

o    Colonic perforation

o    Sepsis

5. Diagnosis

Diagnosis requires compatible clinical symptoms together with laboratory evidence of toxigenic C. difficile.

5.1 Who Should Be Tested

1.      Test patients with unexplained new-onset diarrhea, generally defined as ≥3 unformed stools in 24 hours

2.      Test only unformed stool, except in selected patients with suspected ileus or toxic megacolon

3.      Do not routinely test asymptomatic patients

4.      Do not perform a test of cure, because patients may remain colonized after clinical recovery

5.2 Stool Testing

1.      Nucleic acid amplification testing (NAAT/PCR)

o    Detects genes encoding C. difficile toxins

o    Highly sensitive

o    Does not directly demonstrate free toxin production

2.      A positive NAAT can occur with asymptomatic colonization, so results must be interpreted in the appropriate clinical context

3.      A multistep testing algorithm is often preferred for diagnostic accuracy, such as:

o    GDH antigen plus toxin assay

o    GDH plus toxin assay with NAAT arbitration

o    NAAT plus toxin assay

4.      NAAT alone may be used when strict preagreed stool-submission criteria are applied

5.3 Endoscopy

1.      Colonoscopy is not routinely required

2.      It may demonstrate characteristic yellow-white pseudomembranes

3.      Endoscopy may be considered when the diagnosis remains uncertain and direct visualization is clinically necessary

6. Severity Classification

6.1 Nonsevere CDI

1.      WBC ≤15,000 cells/µL

2.      Serum creatinine ≤1.5 times baseline

6.2 Severe CDI

1.      WBC >15,000 cells/µL, and/or

2.      Serum creatinine >1.5 times baseline

6.3 Fulminant CDI

Characterized by one or more of the following:

1.      Hypotension or shock

2.      Ileus

3.      Toxic megacolon

In this patient, fulminant CDI is not evident, but exact severity cannot be classified without the absolute WBC count and serum creatinine.

7. Management

7.1 Initial CDI Episode

1.      Discontinue the inciting antibiotic when clinically feasible

2.      Maintain appropriate fluid and electrolyte replacement

3.      Preferred treatment:

o    Fidaxomicin 200 mg orally twice daily for 10 days

4.      Acceptable alternative:

o    Vancomycin 125 mg orally four times daily for 10 days

5.      Fidaxomicin is preferred because it provides similar initial cure with a lower risk of recurrence, although access and cost may influence treatment choice

7.2 Fulminant CDI

1.      Vancomycin 500 mg orally or enterally every 6 hours

2.      Add IV metronidazole 500 mg every 8 hours

3.      If ileus is present, add rectal vancomycin 500 mg every 6 hours

4.      Manage with a multidisciplinary team, with early surgical involvement when there is:

o    Toxic megacolon

o    Perforation

o    Shock

o    Progressive organ failure

o    Clinical deterioration

7.3 Recurrent CDI

1.      First recurrence

o    Fidaxomicin, or

o    A tapered and pulsed oral vancomycin regimen

2.      Second recurrence or subsequent episodes

o    Fidaxomicin or an appropriate vancomycin regimen

o    Fecal microbiota-based therapy should be offered after treatment of a second recurrence

3.      FDA-approved microbiota-based products include:

o    Fecal microbiota, live-jslm (Rebyota)

o    Fecal microbiota spores, live-brpk (Vowst)

4.      These products are used to prevent recurrence after completion of antibacterial treatment for recurrent CDI, not to treat active acute CDI

5.      Microbiota-restoration therapy may be considered earlier in selected patients at particularly high risk of further recurrence

7.4 Infection Control

1.      Use contact precautions with gloves and gown in healthcare settings

2.      C. difficile spores persist in the environment and require appropriate sporicidal environmental cleaning

3.      Alcohol-based hand rub remains appropriate for routine healthcare hand hygiene unless hands are visibly soiled

4.      Soap-and-water handwashing is particularly emphasized:

o    When hands are visibly soiled

o    After restroom use

o    During CDI outbreaks

5.      Soap and water physically remove spores, whereas alcohol-based products have poor activity against C. difficile spores

8. Complications

1.      Dehydration and electrolyte abnormalities

2.      Severe colitis

3.      Toxic megacolon

4.      Colonic perforation

5.      Sepsis and shock

6.      Recurrent CDI

9. Key Clinical Insight

Recent fluoroquinolone exposure followed by new-onset watery diarrhea, abdominal cramping, leukocytosis, and a positive stool NAAT for toxigenic Clostridioides difficile is highly consistent with Clostridioides difficile infection. A positive PCR detects toxin genes and must be interpreted together with compatible clinical symptoms.

References

1.      Fischer M, Vaughn BP, Peery AF, Kelly CR. AGA Clinical Practice Update on Management of Clostridioides difficile Infection in Adults: Expert Review. Clin Gastroenterol Hepatol. 2026;24(9):2359-2369. doi:10.1016/j.cgh.2026.05.007.

2.      Johnson S, Lavergne V, Skinner AM, et al. Clinical Practice Guideline by the Infectious Diseases Society of America and Society for Healthcare Epidemiology of America: 2021 Focused Update Guidelines on Management of Clostridioides difficile Infection in Adults. Clin Infect Dis. 2021;73(5):e1029-e1044. doi:10.1093/cid/ciab549.

3.      Centers for Disease Control and Prevention. Clinical Guidance for C. diff Infection Prevention in Acute Care Facilities. Updated May 13, 2026.