Organism:  Legionella pneumophila, Legionella micdadei (Pittsburgh pneumonia agent)

The Mode of Transmission:  aspiration, aerosolization, or instillation of water contaminated with Legionella into the respiratory tract.

SX:

• Legionnaires's disease is pneumonia caused by Legionella species.  Fever, cough, blood streaked sputumi in about 10% of pts, chest pain, pleuritic or nonpleuritic, in about 10% of pts, watery diarrhea, N&V, abd. pain are seen in 25-50% of cases;  headache & mental status changes are the most common neurologic Sx.
• Pontiac fever is an acute, self-limiting, flu-like illness caused by Legionella species with 24-48 hours' incubation period & high attack rate.  Pneumm onia does not occur.  The predominant Sx are malaise, myalgias, fever, chills, & headache.  Complete recovery is the rule even without any specific therapy.  It only requires symptomatic therapy;  no antibiotic therapy is required.

DX:

• Chest x-ray initially shows a unilateral alveolar infiltrate, which can progress to other lobes with consolidation. Diffuse interstitial infiltrates were seen in 25% of those affected in 1976 outbreak.  Radiologic improvement lags behind clnical improvement.  
• Lab. findings:  moderate leukocytosis, hyponatremia, small gram-negative rods in Gram stain of sputum.
• Isolation of the organism by culture from respiratory secretion is the definitive method of diagnosis.
• Direct fluorescent antibody stain of the specimen is rapid & highly specific.  Legionella antibody titers were often used in the past prior to the advent of culture, but they require both acute & convalescent serum.  A single titer of 1:!28 is presumptive evidence of disease, and a fourfold rise in titer is diagnosic.  The sensitivity of serology is about 40%.
• Detection of Legionella Pneumophila-soluble antigens in urine is a rapid diagnostic test that is highly sensitive.  Antigen in urine is detectable 3 days after the onset of clnical disease, even if specifc antibiotic therapy is given, and it persists for several weeks.

RX:

• Erythromycin 500 mg qid PO or 1 gm q6h IV daily for 10-14 days, upto 3 weeks for immunosuppressed pts.
• Combination of macrolide (erythromycin, azithromycin, clarithromycin) or quinolone (as Ciprofloxacin) with rifampin as initial Rx in confirmed cases of Legionnaires' disease.

Ref:
Current Therapy in Adult Medicine 4th Ed, 1997 - Jerome Kassierer & Harry Greene II