Pathophysiology
1) usual cause is infection with Legionella pneumophilia 2) transmission is via inhalation of bacteria in aerosols or aspiration from oropharynx
Signs and Symptoms
1) clinical severity is variable – febrile illness of limited duration to acute purulent pneumonia 2) non-productive cough 3) high fever and chills 4) malaise 5) headache 6) hypoxia 7) abdominal pain 8) mental confusion 9) shock 10) death
Characteristic Test Findings
Chest radiograph – 1) multilobar consolidation Laboratory – 2) hyponatremia 3) hypophosphatemia 4) thrombocytopenia 5) increased AST/ALT
Histology/Gross Pathology
1) grows within human alveolar macrophages 2) microabscesses in lung parenchyma 3) L. pneumophilia is fastidious, aerobic, gram-negative bacillus 4) difficult to culture (requires charcoal yeast extract with silver stain)
Associated Condtions
increased incidence with – 1) chronic steroid use 2) transplant patients 3) chemotherapy 4) diabetes mellitus 5) smoking, emphysema, chronic bronchitis 6) advanced age 7) alcoholism 8) malignancy 9) surgery
Biochemistry
absolute growth requirement is presence of L-cysteine
Inheritance/Epidemiology
1) L. pneumophilia flourishes in warm, moist environments (shower stalls, air conditioning units 2) infects debilitate or immunocompromised patients
Treatment
erythromycin, quinolones, tetracycline
Tips for USMLE
1) not contagious 2) grows in air-conditioning units 3) look for an older man who smokes and drinks and who develops a pneumonia after surgery.
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