Legionella PneumoniaLegionella pneumophilia is a surprisingly common cause of community acquired pneumonia, one which is often not specifically diagnosed. Although a common cause of severe pneumonia requiring ICU admission (after Streptococcus pneumoniae and Chlamydia pneumoniae), it is in fact a much rarer cause of "walking" pneumonias.1 It is important to recognize as along with pneumococcus, it is the community acquired pneumonia most likely to take a healthy adult and give life threatening disease. The disease was first described at the infamous Legionnaires convention outbreak in Philidelphia. Interestingly, it is rare in Europe. Legionella pneumophilia causes 2 distinct syndromes; Legionnaires disease and Pontiac fever.2 Legionnaires has a 2-10 day incubation period during which patients develop fever, headache, malaise and myalgias followed by pneumonia. Many patients have prominent symptoms of chest pain and 25-50% have watery diarrhea. Nausea, vomiting, abdominal pain, renal and CNS manifestations are also described. Pontiac fever is an acute self-limited flu like illness without pneumonia. Typically the incubation period is only 24-48 hours and recovery occurs in a week. Although we tend to associate Legionella pneumonia with outbreaks, in fact more than 80% of the cases are sporadic. Regardless, public health does follow up interviews in Nova Scotia if culture or antibody testing is positive. The organism is found in water and proliferates under favourable conditions; generally a suitable temperature, sediment to adhere to, and symbiotic microorganisms for growth. This is ironic considering the bacterium is fastidious and difficult to culture in the lab. Different strains appear to have varying virulence; some release numerous toxins but the toxins are not necessarily linked to virulence. The lungs main defence against Legionella is related to mucociliary clearance of the bacterium. Therefore conditions which hinder this clearance render a patient susceptible i.e. cigarette smoking, COPD, alcoholism. The immunocompromised, including transplant recipients, HIV patients and those on steroids, are also at risk. Clinically, there is little to differentiate Legionella from other atypical organisms.3 Up to 50% of patients have diarrhea or GI symptoms, and there is a higher proportion of patients with headache. Laboratory tests are similar to other pneumonias except for a propensity towards a low serum sodium (Na<130). The Gram stain of sputum or fluid reveals lots of PMNs but few or no organisms (They are small faint pleomorphic gram negative rods - easily obscured by normal sputum flora). Chest X-ray is usually abnormal by the third day, typically with a segmental infiltrate. These often become multilobar and widespread involvement is common, even after the initiation of appropriate antibiotic treatment. Radiographic clearance takes 1-4 months. Specific testing for Legionella pneumophilia can be done in a special culture medium or by direct fluorescent antibody testing (DFA) of sputum or urine. DFA is less sensitive than culture. For DFA, a large number of organisms need to be present; therefore a positive result is more likely with a multilobar infiltrate on CXR. A four-fold rise in acute and convalescent IgG titres is also diagnostic, although the lag time to results makes this impractical as a clinically useful test. Treatment requires antibiotics with high intracellular concentrations. As Legionella typically causes severe disease often with GI involvement, parenteral treatment is recommended. The organism is sensitive to macrolides, quinolones and rifampin. Azithromycin 1gm followed by 500mg q24hr has replaced erythromycin as first line treatment - erythromycin IV can cause vein irritation and requires large fluid volumes to infuse. IV ciprofloxin or levofloxin are alternatives. For severe disease most texts recommend a macrolide IV in combination with po rifampin. Treatment for 2 weeks is recommended for mild disease and 3 weeks for severe disease or for immunocompromised patients. To date, antibiotic resistance has not been a problem in organisms causing atypical pneumonia.4 Inhalation of aerosolized water contaminated with Legionella pneumophilia is the method of acquiring this infection. During outbreaks attempts are made to identify the common water source which has caused the disease. Superheating the water (to 70° C) and flushing the source is necessary to eradicate Legionella. The bacteria is chlorine tolerant so hyperchlorination is not as effective a strategy for decontaminating water sources. As this is a common cause of severe community acquired pneumonias and our first line agents for these pneumonias are macrolides and quinolones, routine testing for Legionella seems unnecessary. Perhaps in severe cases, especially patients with multilobar infiltrates and markers of atypical disease - GI symptoms, normal WBC or low s-Na, it may be prudent to test for Legionella. When a patient is critically ill, a specific diagnosis is often helpful - it guides treatment plans and knowledge of the natural history of the disease can aid in predicting the course and outcome that might be expected in a particular patient. Thanks to Dr. Scott Rappard, Respirologist at St. Martha's Regional Hospital in Antigonish Nova Scotia for reviewing the draft copy of this article. References:
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