An explosive outbreak of community-acquired pneumonia occurred in July of 1976. The outbreak was among attendees of the American Legion Convention at a hotel in Philadelphia, PA (1). Six months later, the causative agent was isolated from the lung tissue of Legionnaires’ cases by scientists at the Centers for Disease Control and Prevention (CDC), Atlanta, GA (2). The microorganism, an aerobic gram-negative bacterium, was named Legionella pneumophila. The pneumonia became known as Legionnaires’ disease because the outbreaks occurred in attendees at the American Legion Convention. The first reported epidemic of healthcare-associated Legionella pneumonia was identified retrospectively. It occurred in July 1965 at St. Elizabeth’s Hospital, a psychiatric institution in Washington, DC (3). In this outbreak, 81 patients were afflicted, with an attack rate of 1.4%. It was not until 1980 that hospital water distribution systems were first implicated as the source for healthcare-associated Legionnaires’ disease. Tobin isolated Legionella from showerheads in the hospital room of a patient with healthcare-associated Legionnaires’ disease (4). Legionella was subsequently isolated from potable water distribution systems of numerous hospitals experiencing outbreaks of Legionnaires’ disease (5, 6, 7, 8 and 9).

The Legionellaceae family has been characterized as one monophyletic family belonging to the gamma subdivision of the class Proteobacteria (10). Although a single genus and species (L. pneumophila) was originally proposed for the family Legionellaceae (11), the Legionellaceae family now contains >50 species and >70 serogroups in the genus Legionella (12, 13 and 14). Approximately half of these Legionella species have been implicated in human disease (15). Among the species, L. pneumophila is responsible for 90% of infections (Table 36-1) (15, 16 and 17). These microorganisms are facultative intracellular gram-negative bacteria found in natural and man-made water systems. They are saprophytic water bacteria that can be intracellular parasites of protozoa (in water) and macrophages and epithelial cells in humans (18). Most cases of legionellosis are caused by L. pneumophila serogroups 1, 4, and 6 (13,17,19).

Other species implicated in human infection include L. micdadei (the Pittsburgh pneumonia agent), L. bozemanii, L. dumoffii, L. tucsonensis, L. cincinnatiensis, L. feeleii, L. longbeachae, and L. oakridgensis (15,20). L. longbeachae is responsible for approximately 30% of Legionnaires’ disease in Australia and New Zealand (15). Most patients with nonpneumophila Legionella species infections have been severely immunocompromised because of corticosteroid therapy, organ transplantation, or malignancy (21,22,23,24).

Legionella species are small (0.3-0.9 µm in width and ˜2 µm in length), faintly staining gram-negative rods with polar flagella (except L. oakridgensis) (25). They generally appear as small coccobacilli in infected tissue or secretions, whereas long filamentous forms (up to 20 µm in length) can be seen when they are grown in culture media. Legionellaceae are obligately aerobic slow-growing nonfermentative bacteria. They are distinguished from other saccharolytic bacteria by their requirement for L-cysteine and iron salts for primary isolation on solid media and by their unique cellular fatty acids and ubiquinones. Differences among species have been assessed by phenotypic (26) and chemotaxonomic tests. Phenotypic tests include composition of lipopolysaccharides, electrophoretic protein profiles, monoclonal antibodies, fatty acid composition, and cellular carbohydrates. Genotypic tests include random amplified polymorphic DNA profiles, heteroduplex analysis of 5S ribosomal RNA (rRNA) gene sequences, and computer-assisted matching of transfer DNA-intergenic length polymorphism patterns, and sequence-based typing (27, 28 and 29).

The microorganism can be visualized, with some difficulty, with Gram stains of clinical specimens taken from normally sterile sites (e.g., pleural fluid). Both the Gram and Gimenez stains can be used for clinical specimens, whereas silver impregnation stains, including the Dieterle and Warthin-Starry stains, can be used for paraffin-fixed tissue sections. L. micdadei (Pittsburgh pneumonia agent) can stain weakly acid-fast in tissue with Kinyoun and Fite stains and on smears with a modified acid-fast stain in tissue or sputum specimens. These microorganisms are nutritionally fastidious and do not grow on standard bacteriologic media, which explains why the microorganism was so difficult to isolate in the original American Legion outbreak.

Legionnaires’ disease can be acquired by the inhalation of aerosols containing Legionella or by aspiration of water or respiratory secretions containing Legionella (12). Other possible modes of transmission include direct inhalation or hematogenous dissemination from other foci of infection (30). Pneumonia is the presenting clinical syndrome in almost all cases of healthcare-associated legionellosis (31). Although rare, extrapulmonary Legionella infection has been documented (30,32, 33 and 34).

Cigarette smokers, patients with chronic pulmonary disease, and alcoholics are at increased risk for Legionnaires’ disease. For such individuals, mucociliary clearance is impaired and aspiration is common. As a barrier to entry, mucociliary clearance can be overcome by adherence of the microorganism to respiratory epithelial cells. After aspiration or inhalation, Legionella attaches to respiratory epithelial cells. Legionellae possesses pili that are known to mediate adherence to epithelial cells (15). A gene has been identified that demonstrates homology to the type IV pilin genes in other bacteria. Legionella has also been detected in oropharyngeal secretions of transplant patients (35), and symbiosis has been shown in vitro between oropharyngeal flora and Legionella (36).

Legionella is an intracellular pathogen both in humans and in aquatic environments (15,37,38). It has been suggested that the ability of L. pneumophila to replicate in protozoa is closely linked to its ability to replicate in human macrophages (15). Legionella survives and multiplies as parasites of single-celled protozoa in freshwater and moist soil (39). Virulence may be increased by replication in amoebae. In humans, cell-mediated immunity plays the central role in host defense against L. pneumophila as it does against other intracellular pathogens. Legionella replicates within mononuclear phagocytes, primarily monocytes, and alveolar macrophages (40). Phagocytosis occurs through a process mediated by complement component C3 and outer membrane proteins such as the macrophage infectivity potentiator (Mip) protein. The uptake of L. pneumophila is considered a virulence-directed process that is a consequence of properties of the organism (15). The macrophage readily phagocytoses Legionella, a process that is more avid in the presence of specific opsonizing antibody. Once inside the cell, the microorganism evades phagosome-lysosome fusion, converts to a replicative form that is acid tolerant, and multiplies until the cell ruptures (38). Liberated bacteria are phagocytosed by newly recruited cells, and the cycle of ingestion, multiplication, and liberation with cell lysis begins anew.

Although the resident alveolar macrophage normally degrades most microorganisms, Legionella is able to subvert this host defense. L. pneumophila evades destruction by inhibiting phagosome-lysosome fusion (38). Genes responsible for this survival mechanism have been identified as components of the Dot/Icm secretion system, which is required for intracellular replication and establishing the Legionella-containing vacuole (41). Intracellular growth and formation of a replication vacuole requires the products of >26 L. pneumophila dot/icm genes (42). This Dot/Icm type IV secretion system is used by Legionella to inject effector proteins into host cells to modulate host organelle function (12,23).

Intracellular multiplication of Legionella within human monocytes also depends on the availability of iron (45). The lymphokine interferon-γ (IFN-γ) stimulates human
alveolar macrophages and monocytes to resist Legionella infection by upregulating reactive oxygen production and downregulating cellular iron content. An analysis of Legionnaires’ disease patients showed that they produced less IFN-γ than did non-Legionnaires’ disease patients. Impaired IFN-γ response may increase susceptibility to the disease (43). Other cytokines and hemopoietic growth factors, such as interleukin-10 (IL-10) and granulocyte-macrophage colony-stimulating factor, have not been shown to enhance anti-Legionella activity (44). Significant rises in the T Helper-1 cytokines IFN-γ and IL-12 were detected in the serum of patients with Legionnaires’ disease, supporting the importance of cellular immunity in this disease (45). Neutrophils are less important, and neutropenic patients are not at undue risk for Legionnaires’ disease. Nevertheless, L. pneumophila is susceptible to oxygen-dependent microbicidal systems in vitro. Neutrophils inhibit Legionella growth but lack the capacity to kill L. pneumophila. Lysis of infected macrophages by lymphokine-activated killer cells or natural killer cells may also be an important cell-mediated immune function for eliminating intracellular Legionella. It appears that Legionella is resistant to the direct bactericidal functions of neutrophils, but a requirement for neutrophils in the induction of IFN-γ by natural killer cells has been demonstrated (46).

Humoral immunity plays a secondary role in host defense against Legionella infection. Patients with Legionnaires’ disease have measurable type-specific antibodies (immunoglobulin M and immunoglobulin G) within several weeks of infection. Antibodies do not promote complement-mediated killing nor inhibit intracellular proliferation (14,47). Moreover, immunized animals and patients develop a specific antibody response with subsequent resistance to Legionella challenge.

A number of factors have been postulated to contribute to the virulence of L. pneumophila: type I and type II secretion systems, a pore-forming toxin, type IV pili, flagella, a Legionella toxin, a 24-kd protein called Mip, a zinc metalloprotease, and proteases including enzymes that scavenge reduced-oxygenated metabolites (15).

Strains of L. pneumophila differ in virulence. L. pneumophila serogroup 1 is known to cause most cases of Legionnaires’ disease (17). Although multiple strains of L. pneumophila serogroup 1 may colonize water distribution systems, only a few strains are likely to cause disease in patients exposed to the water (48). Monoclonal antibody subtyping of strains of L. pneumophila serogroup 1 has shown that a surface epitope recognized by one particular monoclonal antibody (MAB-2/MAb3/1) may be associated with virulence. The immunodominant part of this virulence-associated epitope has been identified as the 8-0-acetyl group of the 0-specific polysaccharide chain of the lipopolysaccharides (49,50). A correlation between virulence-associated MAB-2/MAb3/1 epitope and charge density of the Legionella envelope may be the factor that discriminates highly virulent from less virulent strains (51).

Legionella species other than L. pneumophila appear to be less virulent and occur almost exclusively among immunocompromised hosts. They also respond more readily to antibiotic therapy (20,52).

Legionella is now the single most common cause of outbreaks involving drinking water (53). Most legionellosis outbreaks associated with drinking water occurred in healthcare facilities, and nursing homes (54). Heffelfinger et al. (55) reported that 25% of 152 hospitals surveyed had reported cases or outbreaks of healthcare-associated Legionnaires’ disease from 1989 to 1998. Although legionellosis is a reportable disease in many countries including the United States, the extent of this infection is still uncertain. Underestimates are likely due to cases that are overlooked because of the persistent lack of availability and utilization of the specialized laboratory tests needed to make the diagnosis (16,56,57). The CDC has reported significant increases of legionellosis in the United States by analyzing data submitted to the National Notifiable Disease Surveillance System. There was an increase of 1300 cases in 2002 to over 2000 cases yearly through 2005 (58). The minimum number of Legionella cases annually is estimated at 18,000 and approximately 25% are healthcare associated (16,59). CDC also reviewed Legionella case report data from 2005 to 2007 submitted to the Legionnaires’ Disease Supplemental Surveillance System and found that acute care hospitals accounted for 88% of the cases, with long-term care and rehabilitation facilities accounting for 12% of reported cases. The study documented that healthcare-associated Legionnaires’ disease continues to have a high case-fatality rate (34%). In a study of reported cases of Legionnaires’ disease in western Pennsylvania, Squier et al. also found a high mortality rate for healthcare-associated cases (38-53%), which is significantly higher than the 20% rate identified for community-acquired cases (16,59). Consequently, Legionnaires’ disease should be considered in the differential diagnosis for all pneumonia cases with prior acute care facility exposure, particularly the elderly, smokers, immunosuppressed patients, and those with chronic lung disease (60, 61 and 62).

More extensive use of Legionella diagnostic testing has revealed that many patients with Legionnaires’ do not fall into these typical risk groups. Squier et al. (59) found that 22% of the reported cases did not have any of the typical risk factors. This trend was also identified in a large study in the Netherlands (63). These studies further emphasize the need for clinicians to include Legionella in the differential diagnosis of healthcare-associated pneumonia. The variable infection rates among individuals reflect a dependence on multiple variables. These include a contaminated potable water system with Legionella, exposure of the host to the contaminated water, susceptibility of the patient exposed, and recognition of the disease by the physician.

Since 1986, legionellosis has also been monitored in Europe. Reports show Legionella species to be a common cause of pneumonia, with L. pneumophila being the most predominant (23).

Situations labeled as sporadic cases of Legionella may represent a chance discovery of the disease occurring at a low endemic period. Likewise, situations labeled as epidemic may represent a cyclical peak at a healthcare facility with endemic but previously undiscovered cases.

Cases surface because of a combination of circumstances: improved diagnostic methods, clinical suspicion of Legionnaires’ disease by an individual physician, or
isolation of the microorganism from open lung biopsy or postmortem lung culture (21).

Routine testing for Legionnaires’ disease at autopsy identified eight cases of healthcare-associated Legionella at a regional transplant center in the southwestern United States (64). The occurrence of three cases in early 1996 led to a retrospective review, which suggested that transmission had occurred for >17 years. An additional 14 cases were identified for a total of 25 culture-confirmed cases of Legionnaires’ disease. Thus, situations labeled as sporadic or nonepidemic may merely represent chance discovery of disease occurring at a low endemic level. Likewise, situations labeled as “epidemic” may merely represent a cyclical peak at a hospital with endemic but previously undiscovered disease.

Consistently identified risk factors for Legionnaires’ disease include advanced age, males, smoking, alcohol abuse, chronic pulmonary disease, and immunosuppression (malignancy, corticosteroid use). Males are affected at two to three times the rate of women; this may be related to cigarette smoking or underlying medical conditions (e.g., chronic obstructive pulmonary disease). Attributable mortality for Legionnaires’ disease is approximately 20%; however, the likelihood of death from Legionella infection increases in patients who are younger than 1 year, elderly, or male, with healthcare-associated infection, renal disease, predisposing underlying conditions such as malignancy, or immunosuppression, or delayed administration of appropriate antimicrobial therapy (23,56,64). Mortality can be as high as 40% for healthcare-associated cases (16). When Jespersen et al. (65) compared mortality rates between community-acquired and hospital-acquired legionellosis, they found case-fatality rate to be three times higher in the hospital-acquired group.

Healthcare-associated infections due to Legionella occur most frequently in immunosuppressed hosts. The patients at highest risk are organ transplant and hematopoetic stem cell transplant recipients (66). During an outbreak in an acute care hospital, 55% (5/9) of all patients undergoing kidney transplantation developed Legionnaires’ disease over a 5-month period (67). Healthcare-associated Legionella infection has been reported in renal (67,68), heart (64,69, 70 and 71), and bone marrow transplant recipients (64,71,72). Corticosteroids are an important independent risk factor. Neoplastic disease, diabetes, and renal failure are often cited as risk factors. The broader use of diagnostic testing may result in more patients being identified without these classic risk factors. A retrospective review of over 400 cases of Legionnaires’ disease in the Pittsburgh area showed that 25% of reported cases did not have the classic risk factors (73).

There is an association of Legionnaires’ disease with surgery. In the past, up to 40% of cases reported in the literature occurred in surgical patients (74). More recently, Legionella is mostly related to solid organ transplantation and to a lesser degree to head and neck surgery. Healthcare-associated Legionella infection increased with the use of general anesthesia and endotracheal intubation (64,75,76).

Surprisingly, neutropenic or leukemic hosts appear to have an attack rate no higher than that of the general population. The exception is patients with hairy cell leukemia (77,78). Likewise, the risk of Legionella infection in the HIV-infected patient appears to be no greater than other highrisk populations, with reports of <1% to 4% (52). However, these patients are prone to extrapulmonary manifestations, bacteremia, and lung abscesses.

Increasing use of diagnostic tests for Legionella has led to new risk groups of patients being discovered as susceptible victims for Legionnaires’ disease. They include immunocompromised children in pediatric hospitals colonized with Legionella and elderly patients residing in longterm care facilities and rehabilitation centers colonized by Legionella.

In a CDC survey of reported pediatric legionellosis cases, 72% were healthcare-associated; the source was related to exposure to tap water (79). A review of published reports by Yu and Lee showed that an outbreak involving 11 neonates and another 2 related to “water-birth” delivery were all related to exposed to contaminated water. However, the percentage of tap water site positivity was not reported (80).

Healthcare-associated cases have been reported in immunosuppressed children (71,81,82) and children with underlying pulmonary disease (30,80,83). In three hospitals in which epidemiologic investigations were conducted (81,83,84), a link to the hospital water supply was made.

Pneumonia in long-term care facilities has increased in recent years as the population of this group increases. However, it often is unclear if the cases should be considered community-acquired or healthcare-associated pneumonia. Increased reports of Legionnaires’ disease has occurred in assisted-living and long-term care facilities (59). Legionellosis is not a diagnosis typically sought out in this setting. Implementing Legionella prevention guidelines in western Pennsylvania raised the index of suspicion, and as a result, the proportion of cases of healthcare-associated Legionnaires’ disease diagnosed in long-term care facilities went up from 4% to 65% (59). One investigation in Canada identified Legionella in the potable water supply as the source for two outbreaks in nursing homes (85). Aspiration was presumed to be the mode of transmission. In one outbreak, eating pureed food was a significant risk factor for Legionella, consistent with aspiration originating from a swallowing disorder (85). In another prospective study, L. pneumophila serogroup 1 was isolated from a newly constructed long-term care facility (86). Six cases of Legionnaires’ disease were diagnosed over 2 years. DNA subtyping established that the patient isolates were identical to the environmental isolates from the water supply.

In a 10-year report of nursing home-acquired pneumonia by Polverino et al. (87), 150 cases were analyzed. L. pneumophila was found in 5% of cases; etiology was reported in only 57 cases. The authors reported inadequate treatment and lack of aspiration assessment.

The CDC and the European Working Group for Legionella Infection have surveyed travel-associated Legionnaires’ disease in the United States and Europe. They report an 85% increase from 2005 to 2008. Thus, a careful travel history is important to avoid the assumption that these cases might be related to a healthcare facility (12).