on 28-Nov-2022 (Mon)

Predominant symptoms include fever, cough, and shortness of breath [1-10]. Symptoms typically arise 2 to 10 days after exposure to contaminated water or soil. Fever and fatigue often precede the onset of cough. Rales and/or other signs of consolidation can be present on physical examination. Sputum may be difficult to obtain.

Pneumonia caused by Legionella is clinically and radiographically similar to other forms of pneumonia.

Radiographic findings are varied and nonspecific; however, the most common findings are patchy unilobar infiltrates, which can progress to consolidations [ 11-13].

Although no clinical features reliably distinguish Legionnaires' disease from other types of pneumonia, certain features may raise the index of suspicion [1-6,10,14-17]. These include:

● Gastrointestinal symptoms such as nausea, vomiting, and diarrhea

● Hyponatremia

● Elevated hepatic transaminases

● C-reactive protein levels >100 mg/L

● Failure to respond to treatment for pneumonia with beta-lactam monotherapy

Mortality, even in tertiary care centers, ranges from about 1 to 10 percent [4,20,21]. Mortality after admission to ICU may be over 30 percent with some patients requiring extracorporeal membrane oxygenation [22]

Clinical features of Legionnaires' disease do not appear to vary with the infecting species or serotypes. In a case series evaluating over 100 patients with Legionnaires' disease, signs and symptoms of Legionella longbeachae pneumonia were largely similar to those of Legionella pneumophila pneumonia [4].

Patient risk factors include older age, smoking, and chronic lung, cardiovascular, or renal disease [23-26]. Immunocompromise, particularly impaired cell-mediated immunity, increases both the likelihood of infection and poorer outcome [27-29].

Failure to respond to beta-lactam monotherapy in a patient with pneumonia should also raise suspicion for Legionella infection. Concern may also be raised when the course of a viral infection follows an unexpected course.

Although certain clinical features (eg, altered mental status, gastrointestinal symptoms, and hyponatremia) might raise suspicion for Legionella infection, no individual symptom or sign or combination is pathognomic. Laboratory testing is necessary for diagnosis.

Because early diagnosis and administration of appropriate antimicrobial therapy is associated with improved outcomes in patients with Legionnaires' disease [31,32], we take an inclusive approach to diagnosis and generally test the following patients:

● All patients with moderate to severe CAP or patients with CAP who require hospitalization

● Any patient with CAP or nosocomial pneumonia who has a known or possible exposure to Legionella (eg, during an outbreak)

● Immunocompromised patients (who are at higher risk for Legionella infection and severe disease)

In addition, IDSA/ATS and BTS guidelines recommend testing for patients with nonsevere CAP who have specific risk factors or epidemiologic exposures [33,35-38].

When testing for Legionella in patients with pneumonia, we prefer to use PCR on a lower respiratory tract sample (eg, sputum or bronchoalveolar lavage specimen) because PCR has high diagnostic accuracy and detects all Legionella species and serogroups.

If PCR is not available or if sputum cannot be obtained, urine antigen testing is an acceptable alternative, especially in regions such as the United States where the prevalence of L. pneumophila serogroup 1 is high. The main advantages of the urinary antigen test are its rapid turnaround time and high specificity.

The major limitation of PCR is the inability to obtain adequate sputum samples from patients with Legionnaires' disease. For some patients, inducing sputum for testing may enhance diagnostic yield [44]

The sensitivity of urine antigen tests ranges from approximately 70 to 80 percent and the specificity approaches 100 percent in patients with Legionnaires' disease caused by L. pneumophila serotype 1 [39,46,47].

Legionella antigens can be detected in urine as early as one day after symptom onset and persist for days to weeks. The turnaround time for the assay is a few hours. Lateral flow devices are simple can be used in most clinical laboratories [48].

The major disadvantage of the assay is that it only detects L. pneumophila serotype 1. While L. pneumophila serotype 1 causes over 80 percent of reported cases of Legionnaires' disease in most regions of the world, L. longbeachae is common in some regions, such as Australia and New Zealand, which limits the utility of the assay [49]. Other urinary antigen systems that detect a broader range of species are under are under investigation [50]

Culture — Culture on special media is considered the gold standard for diagnosis of Legionella infections. Culture can be performed on nearly any sample type and results are typically obtained in approximately three to five days.

For Legionnaires' disease, the sensitivity of sputum cultures is variable with a reported range of <10 to 80 percent [39,46,51]. The specificity of culture nears 100 percent as colonization with Legionella species does not occur.