Legionella

Etiology

Legionellaceae are aerobic, non–spore-forming, nonencapsulated, gram-negative bacilli that stain poorly with Gram stain when performed on smears from clinical specimens. Stained smears of Legionella pneumophila taken from colonial growth resemble Pseudomonas. Unlike other Legionella species, Legionella micdadei stains acid fast. Although 58 species of the genus have now been identified, the majority (90%) of clinical infections are caused by L. pneumophila, and most of the remainder are caused by L. micdadei, Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae.

The organisms are fastidious and require l -cysteine, ferric ion, and α-keto acids for growth. Colonies develop within 3-5 days on buffered charcoal yeast extract agar, which may contain selected antibiotics to inhibit overgrowth by other microorganisms; Legionella rarely grows on routine laboratory media.

Epidemiology

The environmental reservoir of Legionella in nature is fresh water (lakes, streams, thermally polluted waters, potable water), and invasive pneumonia (legionnaires disease) is related to exposure to potable water or to aerosols containing the bacteria. Growth of Legionella occurs more readily in warm water, and exposure to warm-water sources is an important risk factor for disease. Legionella organisms are facultative intracellular parasites that grow inside protozoa present in biofilms, consisting of organic and inorganic material found in plumbing and water storage tanks and various other bacterial species. Epidemic and sporadic cases of community-acquired legionnaires disease can be attributed to potable water in the local environment of the patient. Risk factors for acquisition of sporadic community-acquired pneumonia include exposure to cooling towers, nonmunicipal water supply, residential plumbing repairs, and lower water heater temperatures, which facilitate growth of bacteria or lead to release of a bolus of biofilm containing Legionella into potable water. The mode of transmission may be by inhalation of aerosols or by microaspiration. Outbreaks of legionnaires disease have been associated with protozoa in the implicated water source; replication within these eukaryotic cells presumably amplifies and maintains Legionella within the potable-water distribution system or in cooling towers. Outbreaks of community-acquired pneumonia and some nosocomial outbreaks have been linked to common sources, including potable hot-water heaters, evaporative condensers, cooling towers, whirlpool baths, water births, humidifiers, and nebulizers. Travel-associated legionnaires disease and Pontiac fever are increasingly recognized in major outbreaks. Although person-to-person transmission has been reported, if it does occur, it is extremely rare.

Hospital-acquired infections are most often linked to potable water. Exposure may occur through 3 general mechanisms: (1) inhalation of contaminated water vapor through artificial ventilation; (2) aspiration of ingested microorganisms, including those in gastric feedings that are mixed with contaminated tap water; and (3) inhalation of aerosols from showers, sinks, and fountains. Extrapulmonary legionellosis may occur through topical application of contaminated tap water into surgical or traumatic wounds. In contrast to legionnaires disease, Pontiac fever outbreaks have occurred through exposure to aerosols from whirlpool baths and ventilation systems.

The incidence of legionellosis in the United States increased from 1,100 cases in 2000 to >6000 cases in 2015, for a national incidence rate of 1.9 per 100,000 population based on reporting to the Centers for Disease Control and Prevention (CDC) through the National Notifiable Disease Surveillance System. Because this is a passive reporting system, these are likely underestimates of the incidence of disease. An active laboratory-based and population-based surveillance system for tracking Legionella infections was recently launched by CDC, which will help to better assess its true incidence and epidemiology. (For up-to-date information, see https://www.cdc.gov/legionella/ .)

Legionellosis demonstrates geographic differences, and the vast majority of cases are classified as legionnaires disease (99.5%), with a small fraction as Pontiac fever (0.5%). Legionella infections are reported most frequently in fall and summer, and recent studies show an association with total monthly rainfall and humidity. Approximately 0.5–5.0% of those exposed to a common source develop pneumonia, whereas the attack rate in Pontiac fever outbreaks is very high (85–100%). Although Legionella is associated with 0.5–10% of pneumonia cases in adults, it is a rare cause of pneumonia in children, accounting for <1% of cases; however, infrequent testing for Legionella might underestimate its prevalence. Acquisition of antibodies to L. pneumophila in healthy children occurs progressively over time, although these antibodies presumably reflect subclinical infection or mild respiratory disease or antibodies that cross-react with other bacterial species. Community-acquired legionnaires disease in children is increasingly reported (1.7% of reported cases), and most cases occur in children 15-19 yr old, followed by infants. The incidence in infants is reported to be 0.11 per 100,000. Legionnaires disease is particularly severe in neonates. The epidemiology of hospital-acquired legionnaires disease in children is derived almost exclusively from case reports, so the true incidence of this entity is unknown.