Clinical Pearls & Morning Reports
Published February 28, 2018
Legionella is an important cause of both community-acquired and nosocomial pneumonia. Sporadic cases are more common than the widely publicized common-source outbreaks associated with contaminated water in hotels and office buildings. Read the latest NEJM Clinical Problem-Solving article here.
Q: What patient characteristics are common among those diagnosed with legionnaires’ disease?
A: Legionnaires’ disease shows a seasonal pattern, with peak activity in warm, rainy, and humid conditions. It affects primarily older patients, and it is more common among cigarette smokers, immunocompromised patients, and patients with chronic lung disease.
Q: Are there any distinguishing features of legionnaires’ disease?
A: Legionnaires’ disease cannot be prospectively differentiated from other types of pneumonia on clinical grounds, but it is associated with higher rates of hyponatremia and relative lymphopenia, as well as high fevers and prominent gastrointestinal symptoms, including nausea, diarrhea, and abdominal pain.
A: Legionella is an aerobic, faintly gram-negative bacillus that is difficult to detect by Gram’s staining. It does not grow on routine microbiologic mediums, and it requires buffered charcoal yeast extract agar for growth and isolation. Identification from clinical samples requires 3 to 5 days. Urinary antigen testing has become the first-line diagnostic test because it is inexpensive, provides rapid results, remains positive for days after administration of antibiotics, and has high specificity (estimated specificity, 99.1%). However, false negative urinary antigen test results are well recognized; the overall sensitivity has been estimated to be 74%, although it is higher with greater disease severity. Urinary antigen testing detects only infections caused by L. pneumophilia serogroup 1.
A: Advanced-generation macrolides (e.g., azithromycin) and respiratory fluoroquinolones (e.g., levofloxacin) are considered to be first-line therapy, and one of these should be included in the empirical treatment regimens of all cases of community-acquired pneumonia that are severe or that lead to hospitalization. Alternative agents (e.g., doxycycline) are also effective in mild to moderately severe cases. There are no definitive data from trials to guide therapy, but a retrospective analysis has suggested that fluoroquinolones are associated with a more rapid clinical response and fewer complications than macrolides. Generally, therapy should be continued for 10 days, or for 21 days if the patient is highly immunocompromised (e.g., if the patient had received a transplant). Person-to-person disease transmission is believed to be extremely rare, and isolation of hospital patients is unnecessary.