Review Question - QID 214446

This patient has Legionnaire disease as demonstrated by his lobar pneumonia (cough, fever, and lobar consolidation in Figure A), diarrhea, and hyponatremia. Legionella pneumophila causes the disease and grows on charcoal yeast agar with iron and cysteine.

Legionella pneumophila is an aerobic, intracellular, gram-negative rod. The bacteria can be visualized with a silver stain or cultured in charcoal yeast media with iron and cysteine. Transmission is through aspiration of contaminated water and/or soil; therefore, outbreaks often occur in poorly maintained cruise ships, water towers, or health care facilities. Affected patients most commonly present with Legionnaire disease, which involves lobar pneumonia, gastrointestinal (e.g., diarrhea) and CNS (e.g., confusion) symptoms, and hyponatremia. This condition is more common in smokers and those with chronic lung disease. Patients can also present with Pontiac fever, which is mild in comparison and involves flu-like symptoms without significant pulmonary involvement. Diagnosis is confirmed via urine antigen testing. Management is supportive.

Figure/Illustration A is a chest radiograph demonstrating extensive left lower lobe consolidation with obscuring of the left costophrenic angle (arrow) suggestive of lobar pneumonia.

Incorrect Answers:
Answer 1: Formation of caseating granuloma with central necrosis describes the pathogenesis of Mycobacterium tuberculosis. Although this patient is taking immunotherapy (i.e., infliximab) for his psoriasis, his symptoms are not suggestive of tuberculosis (which often presents with hemoptysis and night sweats). In addition, his chest radiograph demonstrates lobar pneumonia without any evidence of active tuberculosis (e.g., hilar lymphadenopathy or cavitary lesions).

Answer 3: High titers of cold agglutinin are seen in patients with Mycoplasma pneumonia, which is a common cause of atypical pneumonia. Patients often present with vague and mild symptoms of headache, nonproductive cough, and diffuse interstitial infiltrates on chest radiograph. This patient’s chest radiograph findings and symptoms (e.g., fever and productive cough) are more suggestive of Legionnaire disease.

Answer 4: Production of pyocyanin is a characteristic of Pseudomonas aeruginosa, which can result in Pseudomonas pneumonia. Although P. aeruginosa is an uncommon cause of community-acquired pneumonia, this patient is on immunomodulatory therapy, and it should be considered on the differential. However, this patient’s symptoms (i.e., diarrhea) and laboratory abnormalities (i.e., hyponatremia) are not usually seen in Pseudomonas pneumonia.

Answer 5: Sensitivity to optochin is a characteristic of Streptococcus pneumoniae, which is the most common cause of community-acquired pneumonia. Patients often present with rust-colored sputum, cough, fever, and evidence of lobar pneumonia on radiograph. However, gastrointestinal symptoms and hyponatremia are not seen.

Bullet Summary:
Legionella pneumophila, which grows on charcoal yeast media with iron and cysteine, can result in Legionnaire disease, which presents with severe pneumonia, gastrointestinal and CNS symptoms, and hyponatremia.