37 Legionella

Question 1

Describe the morphology of Legionella.

Answer 1

• Slender, pleomorphic, G(-) rods short coccobacilli, obligate aerobes and require L-cysteine and iron for primary isolation, nonfermenter, derive energy from metabolism of amino acids.

Question 2

Describe the pathogenesis of legionella.

Answer 2

• Being facultative intracellular parasites, they need to infect and replicate in macrophages for pathogenesis. They bind to CR3 complement receptors on mononuclear phagocytes, get inside, but are not killed because they inhibit phagolysosome fusion. Chemokines and cytokines released by infected macrophages stimulate a severe inflammatory response, characteristic of infections with Legionella.

Question 3

What is the natural reservoir for Legionella?

Answer 3

• Natural bodies of water such as lakes and streams, as wells as air conditioning cooling towers, and condensers and in water systems (showers, hot tubs).

Question 4

What are the clinical manifestations of Legionella?

Answer 4

• Asymptmatic (common), Pontiac fever, Legionnaires disease.

Question 5

Describe Pontiac fever.

Answer 5

• Influenza-like. Symptoms (fever, chills, myalgia, malaise and headache, without clinical evidence of pneumonia) develop over 12 hour period, persists for 2 to 5 days, and resolve spontaneously without antibiotic treatment.

Question 6

Describe Legionnaires disease.

Answer 6

• More severe than Pontiac fever that can lead to death in untreated. Incubation of 2-10 days, signs of acute illness appear abruptly (fever, chills, a dry, nonproductive cough, headache, multi organ disease). Pneumonia w/ multilobular consolidation, evident on X-ray and inflammation and microabscesses in lung tissue.

Question 7

What is the best method for laboratory diagnosis?

Answer 7

• Nucleic acid amplification (NAA)

Question 8

What is the treatment for Legionnaires disease?

Answer 8

• Macrolides or fluoroquinolones