Legionnaires' disease outbreak investigation toolbox

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Diagnosis of Legionnaires' disease

An atypical pneumonia diagnosis of Legionnaires' disease is not obvious. Initially the patient will have non-specific symptoms including fever, malaise, loss of appetite and headache. Gastrointestinal symptoms are also prominent, with diarrhoea occurring in about 20 - 40% of cases. If left undiagnosed and untreated, the patient could go on to suffer respiratory- and even multi-organ failure [1]. A chest x-ray alone will be insufficient evidence for the observer to distinguish between legionellosis and other more usual causes of pneumonia [2], so specialised laboratory tests are required (see here for further discussion on the microbiological tests used). However, due to the specific nutrient requirements of Legionella, it will not grow on more usual standard laboratory media but requires Buffered-Charcoal Yeast Extract Agar, BCYA [3].

The gold standard test for Legionnaires' disease is to culture the organism using sputum or bronchial lavage from a suspected patient. However, as the organism can take some time to grow, it is not always the most efficient method in an ongoing outbreak setting. Legionella urinary antigen tests are significantly quicker and have a high sensitivity and specificity, showing a positive detection of >90% of cases in under 15 minutes of incubation [4]. The antigen can become present in the urine in as little as 1 to 3 days after symptomatic onset. The most significant problem with the urinary antigen test is that it is specific only to Legionella pneumophila serogroup 1, therefore a positive test indicates Legionnaires' disease, but a negative test cannot rule it out. However, as previously mentioned, because most outbreaks are the result of serogroup 1, it is the most widely-used test in an epidemiological investigation and has become commercially available [4].


Under-reporting and Under-diagnosis

The true incidence of Legionnaires' Disease in Europe is unknown, but it is believed to be a lot higher than the number of cases reported. There are several reasons why Legionnaires' disease is underreported and underdiagnosed [5]:
  • Because Legionnaires' disease is an uncommon form of pneumonia, with no particular clinical features that clearly distinguish it from other types of pneumonia, the identification of Legionnaires' disease is reliant upon clinicians including Legionnaires' disease in the differential diagnosis and requesting the appropriate investigations.
  • When a patient is diagnosed with pneumonia, treatment is generally started immediately. If the patient is treated with antibiotics that are effective against Legionella, the patient usually recovers, without further need to establish the cause of the pneumonia
  • A small proportion of the diagnostic methods for Legionnaires' disease lack sensitivity and may result in producing false negative results
  • The most commonly used method of diagnosis - urinary antigen detection, primarily detects Legionella pneumophila serogroup 1 infections. Infections due to other serogroups or other species, therefore, may not be detected by this method
  • Patients with a serious underlying disease involving immunosuppression are particularly at risk from Legionnaires' disease. If these patients die, death may be attributed to their serious condition, without diagnosing the Legionella infection
  • Milder cases of Legionnaires' disease may not be suspected and remain undiagnosed, since the disease is considered to be severe
  • Cases of travel-associated infection may be diagnosed in some countries but not forwarded to the national collaborator in the European surveillance scheme

Reference List

  1. STOUT J. E. & YU V.L. (1997) Legionellosis The New England Journal of Medicine 337, pp.682 - 687 http
  2. MULAZIMOGLU L. & YU V. L. (2001) Can Legionnaires' disease be diagnosed by clinical criteria? CHEST 120, pp.1049 - 1053 http pdf
  3. FIELDS B. S., BENSON R. F. & BESSER R. E. (2002) Legionella and Legionnaires' disease: 25 years of investigation Clinical Microbiology Reviews 15(3) pp.506 - 526 http
  4. DIEDEREN B. M. W. & PEETERS M. F. (2006) Evaluation of Rapid U Legionella Plus Test, a new immunochromatographic assay for detection of Legionella pneumophila serogroup 1 antigen in urine European Journal of Clinical Microbiology & Infectious Diseases 25, 733 - 735 http
  5. EWGLI (2005) European Guidelines for Control and Prevention of Travel Associated Legionnaires' Disease. Revised Edition. http pdf