Legionellosis - Diagnosis

Diagnosis

People of any age may suffer from Legionnaires' disease, but the illness most often affects middle-age and older persons, particularly those who smoke cigarettes or have chronic lung disease. Immunocompromised patients are also at elevated risk. Pontiac fever most commonly occurs in persons who are otherwise healthy.

The most useful diagnostic tests detect the bacteria in sputum, find Legionella antigens in urine samples (due to renal fibrosis), or the comparison of Legionella antibody levels in two blood samples taken 3 to 6 weeks apart. A urine antigen test which is simple, quick, and very reliable will only detect Legionella pneumophila serogroup 1, which accounts for 70% of disease cause by L. pneumophila. This test was developed by Richard Kohler and described in the Journal of Infectious disease in 1982 while Dr. Kohler was a junior faculty member at the Indiana University School of Medicine. In addition the urine antigen test will not identify the specific subtypes; so it cannot be used to match the patient with the environmental source of infection. Legionella can be isolated on a CYE agar as well.

Legionella stains poorly with gram stain, stains positive with silver, and is cultured on charcoal yeast extract with iron and cysteine.

There is a significant under-reporting problem with Legionellosis. Even in countries with effective health services and readily available diagnostic testing, about 90% of cases of Legionnaires’ disease are missed. This is partly due to Legionnaire’s disease being a relatively rare form of pneumonia, which many clinicians will not have encountered before and therefore may mis-diagnose. A further issue is that patients with Legionellosis can present with a wide range of symptoms some of which (such as diarrhea) may distract clinicians from making a correct diagnosis.