A 54-year-old woman develops fevers and cough on the final day of her vacation on a cruise ship in the Caribbean. She returns to her home in Florida, and over the following 3 days, she has continued fevers, productive cough, and new dyspnea on exertion. She presents to the ED and her vitals at that time are:
Her chest radiograph demonstrates multifocal patchy opacities and increased interstitial markings throughout all lung fields. Ceftriaxone and azithromycin are administered, along with 2 L of lactated ringers solution intravenously and her urinary Legionella antigen subsequently returns positive. She is admitted to the ICU on highflow nasal cannula at FiO2 0.7 and flow 50 L per minute.
Which of the following statements is most accurate regarding Legionella pneumonia?
A. Legionella pneumophila urine antigen is highly sensitive for both serogroups 1 and 2Correct Answer: B
Legionella infection of the lungs results from exposure to aerosolized Legionella species that reside in water sources. The most recognized sites of outbreaks have been cruise ships, hotels, resorts, and healthcare facilities where large water storage and heating systems are present. The most common causative agent is Legionella pneumophila serogroup 1, which is also the only species reliably detected by urinary antigen testing (answer A is incorrect). Serogroup 1 may account for nearly 85% of cases by some estimates, and urine-antigen testing is estimated to have a sensitivity of at least 70%. Legionella culture is technically challenging for a number of reasons: at least half of the patients with legionella have no sputum production, culture requires specific agar (buffered charcoal yeast extract), the bacteria can be slow growing (5+ days), and the sensitivity is highly dependent on the technical skill of the diagnostic laboratory. For this reason, Legionella culture is generally insensitive, with a sensitivity that is estimated to range from 20% to 80% (answers C and D are incorrect). The identification of Legionella on sputum culture is 100% specific for Legionella infection. Legionella is more prevalent among patients with severe CAP than in nonhospitalized patients with pneumonia and accounts for an even greater proportion of pneumonia in patients requiring ICU admission; no other atypical pathogen commonly causes severe CAP requiring ICU admission (answer E is incorrect).
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