Legionella

Background

Small, fastidious Gram-negative bacillus
Uses proteins rather than carbohydrates as an energy source
- Requires media enriched with L-cysteine
L. pneumophila is an facultative intracellular parasite of a number of amoebae
Close relative of Coxiella burnetii
Also have a quiescent state that is "viable but not cultivable"

L. pneumophila serogroup 1: most common cause (70-80%)
L. micdadei: sometimes referred to as the "Pittsburgh pneumonia agent" or Tatlockia micdadei
L. pneumophila serogroups 2-15
L. dumoffi
L. longbeachae
L. bozemanii

Ubiquitous in water: lakes, streams, oceans
- Warm water has higher burden of bacteria
- Free-living water amoebae support intracellular growth of Legionella
No person-to-person transmission (one case report)
Most cases are sporadic, but there are also epidemics
- Half of cases are associated with travel
- Other risk factors include cell-mediated immunocompromise (e.g. anti-TNF-alpha)
- Also spas, hot tubs, potting mix
Most common strain is L. pneumophila Pontiac subtype serogroup 1 (90%)

Inhalation or microaspiration
Phagocytosed by alveolar macrophages
- Replicate intracellularly, then escape and are rephagocytosed by other macrophages
The population of bacteria increases in the lungs
- Infiltration by neutrophils etc causes pulmonary edema

Presents as a community-acquired or healthcare-associated pneumonia
May have a prodrome of hours to days that involves headaches, myalgias, asthenia, and anorexia, possibly fevers
- Diarrhea and abdominal pain
- Cough, sometimes chest pain, sometimes productive
Fevers with pulse-temperature dissociation, myalgias, nonproductive cough, diarrhea, confusion, hyponatremia, hypophosphatemia, and elevated liver enzymes
Can have leukopenia, thrombocytopenia, DIC, leukocytosis, pyuria

Rare, in immunocompromised patients or severe illness
- Except for pleural effusions, which are common
Can develop abscesses in brain, spleen, extrathoracic lymph nodes, and skeletal and myocardial myscles
Case reports of endocarditis

Incubation period 4 hours to 3 days
Self-limited, short febrile illness
- Fevers, myalgias, headaches, and asthenia
- Only diagnosed when there's an outbreak, since they're usually not sick enough to present to healthcare
- Outbreaks associated with industrial processes using sprayed water, recreational spas, decorative water fountains, and cooling towers
Can have cough, dyspnea, anorexia, arthralgia, and abdominal pain

Legionella urine antigen if the most widely-used test
- Most sensitive for L. pneumophila serogroup 1, less sensitive for others
Cultured best on buffered charcoal yeast extract (BCYE) agar is preferred
- Colistin
- Needs cysteine
Stained best with Warthin-Starry or Dieterle, but stains poorly

Test	Specimens	Sensitivity	Specificity	Notes
Culture	Spurum, other lower respiratory tract secretions, lung, pleural fluid, blood, extrapulmonary tissues, fluids	20-95%	100%	Positive several days after treatment, needs special media
Antigen	Urine	60-95%	>99%	Highest sensitivity for serogroup 1, Pontiac type; positive for days to months, even up to a year
Immunofluorescent microscopy	Same as culture	20-50%	99%	Highest specificity with monoclonal antibody
Antibody	Paired serum (acute and convalescent)	20-70%	95-99%	Highest specificity for serogroup 1
Molecular amplification	Sputum (ideally BAL), other lower respiratory tract secretions, urine	70-95%	90-95%	Not well standardized, good performance in reference labs

Treatments include: Levofloxacin 750 mg PO or IV once daily, moxifloxacin 400 mg PO or IV once daily, or azithromycin 500 mg PO or IV once daily
Alternative is doxycycline 100 mg p.o. twice daily
Usual duration 7-10 days
No need to treat Pontiac fever