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
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
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

Diagnosis of Legionnaires` disease

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