Approach
Legionnaires' disease must be verified by laboratory diagnosis, whether by urine antigen test, polymerase chain reaction (PCR) testing of a respiratory specimen, serology, or culture on special media. Clinical features may be suggestive, but are not sensitive or specific for Legionella.[36] Therefore, all patients with pneumonia (both community-acquired and healthcare-associated) should be tested for Legionella.[37] Legionella pneumonia is an atypical pneumonia because Legionella will not grow on routine culture media in the microbiology laboratory, not because of unique signs or symptoms.
History
Legionnaires’ disease presents with pneumonia, which often requires hospitalization and can be fatal in approximately 10% of cases.[10] Typical symptoms include cough (with or without sputum production), shortness of breath, fever, myalgia, and pleuritic chest pain. Headache, nausea, vomiting, diarrhea, and abdominal pain may also be present.
A history of recent water exposure (e.g., hot tub, recreational spa, recent plumbing work at home) should increase the suspicion of Legionella infection, but is not essential. Symptoms typically start between 2 and 10 days after exposure.[10]
The index of suspicion should be particularly high during known outbreaks. However, in outbreaks, <5% of people exposed to the source develop Legionnaires’ disease. Current and ex-smokers, people with underlying lung disease, and people with immunosuppression are more likely than healthy people to develop disease after exposure.[10]
Pontiac fever is milder than Legionnaire’s disease and presents with fever, myalgia, and headache. Arthralgia, cough, anorexia, and abdominal pain may also occur. Clinical suspicion for Pontiac fever is usually based on the presence of a current known outbreak or cluster of cases with similar symptoms. Symptoms occur within 72 hours of exposure to an infected water source. It can affect healthy people as well as those with underlying illnesses. Up to 95% of people exposed during outbreaks of Pontiac fever will develop symptoms of disease. Nearly all patients make a full recovery without antibiotics or hospitalization. Due to the self-limiting nature of Pontiac fever, testing is not usually performed.[10]
Examination
Legionnaires’ disease may present with typical signs of pneumonia, such as tachypnea, fever, hypoxia, crackles or ronchi on auscultation, and chest wall tenderness. Two classic signs of Legionnaires' disease are fever that does not coincide with tachycardia and a prominent headache. Pontiac fever has nonspecific physical exam findings, such as fever and tachycardia.
Tests
A patient with suspected pneumonia should have a chest x-ray or abdominal CT scan, a sputum Gram stain and culture, a urine antigen test for Legionella pneumophila serogroup 1, and blood cultures (2 sets). A baseline complete blood count and complete metabolic profile should also be requested.
Although the urinary antigen enzyme immunoassay test is rapid and has high specificity (99%), a negative urinary antigen does not exclude the diagnosis of Legionella infection, because it primarily only identifies serogroup 1 (which is the most common serogroup, accounting for 80% to 90% of cases).[10] The test has a pooled sensitivity of 74%.[38] Despite higher sensitivities for detecting Lpneumophila serogroup 1, the assay’s ability to detect alternative species and serogroups is lower.
As a result, if the urine antigen test is negative but there remains a high suspicion for Legionella infection, for example there has been possible exposure to contaminated water (e.g., recent hotel stay, cruise, campsite, or other accommodation stay) in the previous 2 weeks, other tests may be performed. A throat swab or sputum sample may be obtained for polymerase chain reaction (PCR; though the test is not FDA-approved in the US), a lower respiratory specimen (sputum, bronchial fluid, or pleural fluid) may be placed onto atypical media for culture growth, or Legionella serology may be obtained.[39][40] These tests must be specifically requested of the microbiology laboratory, as they are not routinely performed. Culture and isolation of Legionella from clinical specimens constitutes the gold standard for diagnosis but takes 3 days or more to yield results, limiting its use in initial diagnosis.[4] PCR has the advantage of producing rapid results within hours and, unlike urine antigen testing, can detect all species and serogroups of Legionella. It is highly sensitive and specific. However, its use may be limited by the fact that a significant proportion of patients with Legionnaires’ disease do not produce sputum. Induced sputum and bronchoscopic lavage may increase diagnostic yield in this situation. Throat swabs can also be sent for PCR but the sensitivity is low.[41] Serology is rarely used now in clinical practice; its primary use is for retrospective diagnosis in epidemiologic investigations. Legionella infections where the diagnosis is made using only serological methods no longer meet the case definitions for the purposes of public health action or surveillance in the UK.[39]
For all confirmed cases of Legionnaires’ disease (positive urinary antigen test or PCR), a lower respiratory sample should be sent immediately for culture (if not already done).[39] Isolation of the infecting strain allows epidemiologic typing to be done, providing valuable data for the control and prevention of further cases.[4]
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