Organism: Legionella pneumophila Flashcards
Give an overview of Legionnaires’ Disease
A new organism, Legionella pneumophila, was isolated from the lungs of some of the patients who died Subsequent investigation (serology on stored blood samples) revealed previous cases of infection with this organism, and it had had also caused outbreaks of febrile illness (“Pontiac Fever”) (Pneumophila = lung-loving)
Name the Legionella species
Family Legionellaceae 50 species, >70 serogroups
Legionella pneumophila causes 85% of human infections with the species
L. pneumophila serogroup 1 is responsible for most infections
Describe the epidemiology of L. pneumophila
Sporadic/endemic disease
Account for 2 – 9% of cases of community acquired pneumonia
Ubiquitous organisms
Natural habitats are aquatic bodies
Colonisation is enhanced by warm temperatures (25-42oC), stagnation, scale and sediment
Isolated from water, air-conditioning units, shower heads, piped water supplies & taps, in hotels, hospitals
Infection arises following inhalation of aerosols from contaminated water
What are the source of L. pneumophila?
Cooling towers Domestic water systems Air conditioning units Nebulisers Humidifiers Whirlpools or hot tubs Jacuzzis Showers Car washes Aerosol generators
Name the laboratory features of Legionella species?
L. pneumophila serogroup 1 is responsible for most infections
Non-capsulated Gram negative bacillus
Possesses flagellae
Strict aerobe
What are the Virulence Features of Legionella pneumophila and pathogenesis
Pili - adherence Outer membrane proteins - adherence Flagellae Enzymes: protease, phosphatase, lipase, DNAse, RNAse – cell damage LPS – endotoxic activity
L. pneumophila is an intracellular pathogen, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole – the mechanism by which this occurs is not yet known
Virulence proteins secreted into the vacuole enable survival and replication
Describe the clinical features of Legionnaires’ Disease.
Male: female ratio of 2:1, patients typically aged 50-60 years; uncommon <30 years
Pontiac fever: brief flu-like illness (ILI), self-limiting
Often under-recognised as a cause of community-acquired pneumonia
Predisposing factors to severe disease include
old age
cigarette smoking
immunosuppression
Incubation period is 2-10 days
Abrupt onset of fever, chills, dry cough, headache
May get multi-organ disease of GIT, CNS, Liver
Renal involvement results in proteinuria, haematuria & hyponatraemia (due to renal tubular acidosis)
Overall mortality of 15-20%
higher in patients with severely depressed cell-mediated immunity (transplant recipients) or if treatment is delayed
How is Legionella disease diagnosed?
Culture
Culture requires specific growth media
e.g., buffered charcoal yeast extract (BCYE) agar & antibiotics to which the organism is resistant to inhibit growth of other bacteria
If you suspect LD – you need to tell the microbiology laboratory
Sputum /tracheal aspirate/bronchoalveolar lavage
Colonies appear after 2 - 3 days incubation
Explain Urinary antigen detection as a method of legionella disease diagnosis
the most commonly used test
Sensitivity serogroup 1; 60-90%, does not detect other serogroups/species
Rapid test
Discuss Serology (Antibody detection)
not available routinely
Antibody is detectable about the 8th day of the illness
Diagnosis: 4-fold increase/rise in the level of antibodies produced between serum taken at acute stage of illness & repeated up to 2 weeks later (convalescent sample)
What is Immunofluorescent microscopy?
Difficult to see L. pneumophila by Gram stain
Specific immunofluorescence (antibodies) staining of legionella antigen in tracheal aspirate, bronchial lavage or lung biopsy
High specificity but lacks sensitivity
How is Legionella disease treated?
βlactam agents are not effective
Drugs with good intracellular penetration are required
Examples of agents used:
Respiratory fluoroquinolones, e.g., levofloxacin
Macrolides, e.g., azithromycin or clarithromycin
May combine with rifampicin in the seriously ill
What precautions are taken to prevent the occurrence of legionella disease?
Attention to plumbing:
Avoid areas of water stagnation - ‘dead legs’, unused outlets
Programme of flushing taps and showers, water tank cleaning and maintenance
Chlorination of water supply
Keep the hot water hot (>500C) and keep the cold water cold (<200C)
Danger of scalding with very hot water – thermostatic mixing valves in taps to avoid scalding
Emerging technologies e.g., copper-silver ionisation
