A 34-year-old man presents to the emergency department with a non-productive cough and intermittent fever for the past 12 days. He also reports nausea, vomiting, and diarrhea. He has been treated with amoxicillin-clavulanate for 7 days, but his condition has not improved. He has no significant past medical history. He works as an air-conditioning technician. He does not smoke cigarettes and drinks 2 beers per week. Vital signs reveal a low-grade fever and mild tachypnea. On physical examination, there is decreased tactile fremitus, dullness to percussion, and crackles in the left lower lung field. A chest X-ray reveals patchy left lower lobe infiltrates. Lab results reveal hyponatremia and mildly elevated hepatic tranaminases.
Which of the following organisms is the most likely cause of this patient’s symptoms?
The correct answer is:
B) Legionella pneumophila
Legionella pneumophila typically occurs after exposure to contaminated water or soil and may be spread by human-made water systems like cooling towers, hot tubs, and plumbing systems. Legionnaires’ disease is clinically and radiographically similar to other forms of pneumonia and most commonly reveals patchy unilobar infiltrates on chest x-ray. Features suggestive of Legionaires’s disease include gastrointestinal symptoms, hyponatremia, elevated transaminases, and failure to respond to beta-lactam monotherapy.
Answerchoice A: Klebsiella pneumoniae, is incorrect. Klebsiella pneumoniae often causes a more acute presentation with a productive cough (currant jelly and sticky sputum) and may be associated with more severe and localized findings, such as cavitary lesions on imaging in the upper lung lobes.
Answer choice C: Mycobacterium tuberculosis, is incorrect. Mycobacterium tuberculosis typically presents with a chronic cough, night sweats, and weight loss, and is less likely to cause a rapid onset of symptoms.
Answerchoice D: Pseudomonas aeruginosa, is incorrect. Pseudomonas aeruginosa typically affects immunocompromised patients, those with chronic lung disease such as cystic fibrosis, or those who have been on a ventilator.
Answer choice E: Streptococcus pneumoniae, is incorrect. Streptococcus pneumoniae is a common cause of community-acquired pneumonia should respond to beta-lactam therapy such as amoxicillin-clavulanate.
Key Learning Point
Legionnaires’ disease, caused by Legionella pneumophila, is clinically and radiographically similar to other forms of pneumonia and most commonly reveals patchy unilobar infiltrates on chest x-ray. Features suggestive of Legionaires’s disease include gastrointestinal symptoms, hyponatremia, elevated transaminases, and failure to respond to beta-lactam monotherapy.
