A 60-year-old man with a history of hypertension and chronic obstructive pulmonary disease (COPD) presents to the Emergency Department complaining of fever and a productive cough. The symptoms started yesterday and have been progressive. He now also has vomiting, diarrhea, headache, and muscle and joint pain. His current medications include inhaled albuterol and ipratropium, hydrochlorothiazide, and lisinopril. He has a 30 pack-year smoking history and drinks alcohol moderately. He recently returned from a business trip, where he stayed in several hotels and ate all of his meals within these hotels. On examination, his temperature is 39.2°C with a heart rate of 110 beats per minute and a respiratory rate of 26 breaths per minute. Pulmonary examination reveals dullness to percussion in the left lower lobe with rales and bronchial breath sounds on auscultation. His laboratory values are shown below.
A urine dipstick shows 1+ protein and 1+ blood.
Which of the following is the most likely causative organism?
A. Streptococcus pneumoniaeLegionella pneumophila. This patient has CAP with features of Legionnaires’ disease caused by L. pneumophila, which is the most common Legionella species to cause disease in humans. It is a common cause of CAP and HAP. Although Legionnaires’ disease can occur in any age group, it is more common in older patients who have a history of smoking or underlying lung disease. This patient likely acquired this infection from recent travel; outbreaks typically involve a water source such as hotel drinking water. It presents as pneumonia with fever, productive cough, and dyspnea and may also have symptoms of headache, myalgias/arthralgias, nausea/vomiting, and diarrhea. Laboratory values frequently show hyponatremia, renal and/or hepatic dysfunction, and hematuria/proteinuria. Diagnosis can be made with a urinary antigen test and culture on buffered charcoal yeast extract agar. Treatment is with a macrolide or fluoroquinolone antibiotic. This is a high-yield question for the shelf examination; when an elderly patient presents with pneumonia, hyponatremia, and diarrhea, suspect Legionella. (Of note, Legionella can also cause a mild, self-limited pulmonary condition called Pontiac fever, though this is less common than Legionnaires’ disease.)
Almost all of the answer choices are reasonable choices for CAP; however, the constellation of findings argues for Legionella over the other organisms. It is helpful to know the organisms associated with unique clinical syndromes (e.g., Legionnaires’ disease) and with unique circumstances (e.g., Nocardia in an immunosuppressed patient). (A) S. pneumoniae is the most common cause of pneumonia worldwide. (C) Klebsiella is common in alcoholics and nursing home patients; look for the buzzwords “currant jelly sputum” and “mucoid colonies.” (D) Pseudomonas is a common water-loving nosocomial pathogen that is often multidrug resistant (MDR). It is also very common in cystic fibrosis patients. (E) Mycoplasma and other “atypical” pneumonias are common in young, healthy patients. H. influenzae and Moraxella catarrhalis are common causes of pneumonia in COPD patients; however, the constellation of findings in this patient argues for Legionella over H. influenzae. Stenotrophomonas maltophilia is an opportunistic MDR organism that can cause HAP and VAP. Peptostreptococcus anaerobius is an anaerobic organism that is often found in cases of aspiration pneumonia.