Snapshot A 60-year-old man is brought to the emergency department for confusion, cough, and diarrhea. He had recently attended a conference and stayed at an old hotel where other people reported similar symptoms. His past medical history includes chronic obstructive pulmonary disease and hypertension. He smokes 1 pack per day. On physical exam, his oxygen saturation is 92% on room air. There are fine crackles bilaterally on chest auscultation. A chest radiograph reveals diffuse patchy infiltrates. He is also found to have hyponatremia. A urine antigen test was sent for definitive diagnosis. Introduction Classification Legionella pneumophila an aerobic, facultative, intracellular gram - rod transmission aspiration of contaminated water/soil air conditioning hot tubs swimming pools Epidemiology demographics more common in the elderly risk factors cruise ships smoking immunosuppression hospitalization Pathogenesis organism is phagocytosed into alveolar macrophages, where they replicate bacterial spread occurs when they escape from macrophage vacuoles leading to host cell osmotic lysis Associated conditions Legionnaires disease (most common) lobar or insterstitial pneumonia gastrointestinal and central nervous system involvement more common in smokers and those with chronic lung diseases Pontiac fever mild and flu-like without significant pulmonary involvement Prognosis Pontiac fever is self-limited Presentation Symptoms Legionnaires disease cough with sputum production gastrointestinal symptoms nausea or vomiting water diarrhea central nervous system symptoms confusion encephalopathy Pontiac fever malaise headache malaise Physical exam fever Legionnaires disease altered mental status dyspnea relative bradycardia pneumonia Imaging Chest radiography indication all patients findings lobar consolidation or diffuse patchy infiltrates Studies Labs urine antigen test diagnostic does not Gram stain well, so it requires a silver stain sputum culture requires special media for culture charcoal yeast extract agar buffered with iron and cysteine hyponatremia transaminitis Making the diagnosis based on clinical presentation and laboratory studies Differential Mycoplasma pneumoniae distinguishing factor typically does not present with gastrointestinal or central nervous system involvement Treatment Medical antibiotics indication all patients drugs macrolides respiratory fluoroquinolones (levofloxacin and moxifloxacin) Complications Respiratory failure