Overview
Legionella pneumophila is a Gram-negative aerobic bacterium that colonises water tanks. It is the causative agent of Legionnaire’s disease, characterised by fever, malaise, and atypical pneumonia, named after an outbreak in a 1976 American Legion convention.
The classic exam presentation is a person returning from holiday or staying in a place with poorly maintained air conditioners.
Pathophysiology
Legionella pneumophila enters the body via inhalation and once in the lungs, is phagocytosed by alveolar macrophages. However, they release factors that allow them to survive and replicate, resulting in the macrophage being killed, releasing the bacteria, and allowing the cycle to repeat. Over time, bacteria, macrophages, and neutrophils accumulate in the alveoli.
Epidemiology
- In England and Wales in 2020, there were 295 confirmed cases of Legionnaire’s disease
- Men are affected more than women
Risk Factors
- Poorly maintained/stagnant water systems – such as air conditioning, recent plumbing, living near a cooling tower
- Smoking
Presentation
Features include:
- Acute flu-like symptoms (e.g. malaise, myalgia, headaches) and fever (~95%)
- Dry cough
- Relative bradycardia – poorly understood, but tends to be seen in Gram-negative and intracellular pathogens
- Features of pleural effusion (~1/3) – this occurs due to lung inflammation and infiltration of leukocytes
- Confusion and distress
Investigations
Overview
Some investigations include:
- Full blood count (FBC): may show lymphopenia:
- This may be due to T-cells migrating to the lungs during infection and their apoptosis.
- Urea and electrolytes (U&Es): may show hyponatraemia:
- This may be due to infection triggering a syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Liver function tests (LFTs): may show elevated transaminases:
- Legionella is the most common pneumonia to cause LFT derangements, and some studies have found Legionella bacteria in liver and spleen tissue
- Chest x-ray: may show mid-lower zone consolidation and pleural effusions in 1/3 of people
- Urinary antigen is the diagnostic test of choice:
- Legionella antigens are excreted in the urine and this has high sensitivity and specificity compared to cultures due to culturing requiring special media and expertise.
Differential Diagnoses
Mycoplasma pneumoniae
- Although both Legionella pneumophila infection and Mycoplasma pneumoniae infection can cause pneumonia and deranged LFTs, Mycoplasma pneumoniae is associated with erythema multiforme and autoimmune haemolytic anaemia and is not as strongly associated with lymphopenia and hyponatraemia.
Management
Overview
The antibiotic of choice is a macrolide, specifically clarithromycin or erythromycin.
Management
treat with erythromycin/clarithromycin
Prognosis
- Mortality rates can be over 10% in healthy people
