Legionella Infections

Definition

Legionellosis refers to two clinical syndromes caused by Legionella bacteria: pneumonia and Pontiac fever. Legionella pneumonia (legionnaires disease) can be episodic or occur in outbreaks, and it can be acquired in the community or in health care settings. Pontiac fever is an acute, self-limited febrile illness caused by inhalation or aspiration of Legionella spp. Legionella extrapulmonary infections include meningitis, cellulitis, and skin and soft tissue abscesses. Endocarditis and wound infections also rarely occur.

The Pathogen

Legionellae are slender, noncapsulated, non-spore forming, obligatory aerobic, nutritionally fastidious, pleomorphic gram-negative bacilli (0.3-0.9 µm wide by 2-20 µm long) that belong to the Legionellaceae family ( E-Fig. 290-1A to 1D ). Legionella are facultative intracellular pathogens. L. pneumophila have a single polar flagellum. L. longbeachae differ in that they do not have a flagellum but have a capsule. Legionella spp. stain poorly on Gram stain. Warthin-Starry or Dieterle stains can demonstrate organisms in tissue. L micdadei and some other Legionella spp. may stain weakly acid-fast.

Legionella spp. have genomes that range in size from 2.37 to 4.88 Mb, but the core genome is comprised of only 1008 genes (6%). Recombination plays an important role in the organism’s evolution and likely accounts for the development of new virulent strains as well as strains adapted to surviving in man-made water systems.

Epidemiology

The Legionellaceae family has more than 60 species with more than 70 serogroups. L. pneumophila , which accounts for up to 90% of human infections, has more than 15 serogroups. Serogroup 1 is the most common cause of human infections, followed by serogroups 3, 4, and 6, although all groups can infect humans. L. longbeachae , which is more virulent than L. pneumophila , is the second most common species causing human disease, particularly in Australia and New Zealand. More than 25 Legionella spp. including: L. micdadei, L. bozemanae, L. feeleii, L. anisa, and L. dumoffii have been reported to cause human disease.

Legionella spp. are present in the environment worldwide. L. pneumophila and most Legionella spp. live in streams and lakes, and they thrive in man-made indoor and outdoor water systems, including drinking water, hot water pipes, fountains, air conditioning systems, and cooling towers. Almost all cases of legionnaires disease are due to exposure from Legionella contamination of man-made water sources. Legionella can live either freely or as intracellular parasites in a number of different protozoans including Acanthamoeba and Hartmannella species ( E-Fig. 290-1E ). Growth of Legionella is promoted by warm temperatures, stagnation, and the presence of sediment in water systems. Legionella can withstand temperatures of 32 to 154° F (0 to 68° C), and they form biofilms that help them resist chlorination. The reservoir of L. longbeachae is in soil, including potting soil and compost, in contrast to other Legionella , which are more commonly found in water.

Early Legionella outbreaks were difficult to solve and some persisted for months and even years. Since the eponymous outbreak at a Legionnaire’s convention in Philadelphia in 1976, owing to a contaminated air conditioning system, numerous large, waterborne outbreaks around the world have involved drinking water, air conditioning systems, and water cooling towers in hospitals, in long term care facilities, in hotels, in apartment buildings, and on ships.

The incidence of Legionella infections varies geographically, related to the concentrations of Legionella in water systems as well as local heat and humidity. L. pneumophila infection is more common in the northeastern United States and Mediterranean regions.

Humans are accidental hosts for Legionella . Infection is typically acquired from inhalation or aspiration of Legionella contaminated water, including water used for drinking and bathing, air conditioning systems, hot tubs, birthing pools, decorative fountains, swimming pools, and water cooling towers. Legionella have surpassed gastrointestinal pathogens as the most common cause of waterborne outbreaks.

Legionella are commonly present in very low concentrations in cold, disinfectant-treated, potable water; however, bacterial density is amplified by growth in biofilms and replication in warm water and in older water systems with low or no flow. Significant changes in water pressure or water flow can disrupt biofilms, thereby releasing large numbers of organisms into the water. Contaminated water can be inhaled, aspirated, or ingested, thereby resulting in sporadic and epidemic infections. The exact dose required for infection is not known, but high numbers of organisms (>1000) are likely necessary.

L. pneumophila infections are more common in summer ( E-Fig. 290-2 ), and most outbreaks occur between April and October. L. longbeachae , which is most commonly identified in soil and compost sources, usually causes disease in late spring and early summer in New Zealand and Australia, where it may account for 50% of Legionella cases.

Legionella can cause sporadic or epidemic, community-acquired pneumonia ( Chapter 85 ) and health care–associated pneumonia ( Chapters 85 and 261 ). Estimates of Legionella as a cause of community-acquired pneumonia requiring hospitalization range from 0.5 to 10% (average 2 to 5%). Legionella may account for up to 30% of nosocomial pneumonia. The reported incidence of legionnaires disease in the United States increased almost five- to six-fold from 1992 to 2018 (to approximately 2.29/100,000 persons), likely due to increased numbers of immunosuppressed individuals, the increased age of the population, increased testing, and greater awareness. Most U.S. cases are in the northeast, mid-Atlantic, and central regions. Of the 7500 annual cases, about 80% are in persons over age 50 years, about 60% of cases are in men, and the incidence rate is about 80% higher among Blacks compared with Whites.

An estimated 18,000 to 88,000 total cases may occur per year in the United States. Most cases are sporadic and occur in outpatients, whereas outbreaks account for only 5 to 10% of legionnaires cases in the United States. However, legionnaires disease is likely underdiagnosed and under-reported. For example, a switch from traditional serologic testing plus occasional culture to polymerase chain reaction testing in New Zealand increased the detection of legionella by 2.5 fold.

Activities that increase the risk of exposure to L. pneumophila spp. include overnight travel, recent plumbing work, disruptions of water supplies, older plumbing, use of whirlpools, hot springs, and proximity to cooling towers and decorative fountains. Travel–associated outbreaks are generally caused by contaminated potable water and cooling towers in hotels and contaminated hot tubs in ship–related outbreaks. Risk factors for infection with L. longbeachae include working with compost or potting soil, smoking, chronic obstructive lung disease, and lack of handwashing after working with soil. Risk factors for developing symptomatic legionnaires disease include male gender, smoking, advanced age, dialysis-dependent renal failure, chronic lung or cardiovascular disease, diabetes mellitus, hematologic malignancies, and immunosuppression, particularly decreased cell mediated immunity, and the use of biologic immunosuppressive agents.

Nosocomial infections occur when contaminated water is inhaled, aspirated, or inoculated into the respiratory tract, most commonly from use of contaminated tap water used in nebulizers, humidifiers, and nasogastric feeding. Health care–associated outbreaks typically account for more than half of the epidemics and more than 85% of the deaths due to Legionella . ^,

Pontiac fever has very high attack rates and more commonly occurs in outbreaks, in which 80 to 90% of exposed individuals become sick. Pontiac fever has been reported with L. pneumophila , L. micdadei , L. anisa , and other Legionella spp. High levels of L egionella contamination of potable water, showerheads, and cooling towers have been associated with outbreaks of Pontiac fever.

Pathobiology

Free-living Legionella can enter low metabolic states and remain viable but not cultivable, so they are both resistant to biocides and difficult to recover from the environment. They also form biofilms, which aid in their resistance to biocides and chlorination. The ability to grow in temperatures from 68 to 107.6° F (20 to 42° C) and tolerate pH ranges from 5 to 8.5 contributes to their resilience. Legionella cannot be cultured on ordinary media; buffered charcoal yeast extract agar is required to cultivate Legionella , which grow best at 35° C in humidified air. Colonies are generally visible at 3 to 5 days, but incubation up to 14 days may be necessary to identify unusual species. Given the difficulties growing Legionella in the laboratory, negative culture results do not exclude the diagnosis.

After inhalation or microaspiration of Legionella contaminated water, droplet nuclei, which are less than 5 µm in size, reach the alveoli. In the alveoli, bacteria attach to macrophages and epithelial cells using outer membrane porins, flagella, and pili. Human complement component C3 mediates phagocytosis in conjunction with a virulence factor, macrophage infectivity potentiator protein, which is expressed on the cell surface of L. pneumophila . Once phagocytized, L. pneumophila translocates hundreds of effector proteins into a specialized compartment, termed the Legionella -containing vacuole, through a Dot/Icm (delayed in organ trafficking/intracellular multiplication) type IV secretion system. Phagosome-lysosome fusion is inhibited by the effector proteins, which allow L. pneumophila to survive intracellularly in the Legionella containing vacuole. The vacuole is transformed into an organelle through recruitment of vesicles from the host’s endoplasmic reticulum. Intracellular bacterial replication causes the vacuole to expand, ultimately filling the cell. When the host cell’s nutrients are exhausted, the bacteria transition to a stationary growth phase and develop flagella. Caspase-1, which is triggered by the flagella, induces apoptosis and releases bacteria from the cell.

Legionella are able to resist complement and cationic peptides in the extracellular space, thereby avoiding free radicals. Bacteria in the extracellular space reenter more cells and trigger additional replication cycles, thereby amplifying the concentration of bacteria in the lung. Legionella uses a type II secretion system to secrete degradative enzymes and toxins. Infected macrophages release chemokines and cytokines that trigger a severe inflammatory response. A progressive, destructive pneumonia is caused by chemotactic substances, which attract polymorphonuclear leukocytes and monocytes, as well as the subsequent inflammatory response.

The predominant mechanism of immune control of Legionella is through Th1 cell mediated immunity. Infected alveolar macrophages incite the recruitment of neutrophils and stimulate natural killer cells to produce interferon gamma, which is key to eradicating Legionella from the lung. Innate immunity against Legionella involves toll-like receptors (TLR) 2, TLR 4, TLR5 and TLR89. Formation of inflammasomes leads to proteolytic activation of caspase-1. Cleavage of pro-interleukin – β (IL-Iβ) and release of IL-1β leads to an inflammatory form of cell death called pyroptosis. Pyroptosis is thought to be the main mechanism by which host cells control the growth of L. pneumophila . Individuals with TLR5 stop codon polymorphisms are more susceptible to legionnaires disease, whereas TLR4 polymorphisms are protective.

Pontiac fever is caused by inhaling Legionella contaminated aerosols. The aerosols often contain Legionella , endotoxins, and other microorganisms, so it is not clear if the disease is due to the Legionella bacteria alone, a polymicrobial aerosol, or endotoxins from Legionella or other organisms. The short incubation period of 4 to 6 hours in some patients supports a toxin-mediated illness, whereas the longer incubation of 35 hours in some patients suggests bacterial illness.

Very little is known about the pathogenesis, virulence, and cell biology of infections caused by Legionella spp. other than L. pneumophila . However, different species have different virulence factors and effector proteins. L. longbeachae lacks flagella and has a larger genome than L. pneumophila . These additional genetic sequences may increase L. longbeachae’s pathogenicity.

Gross pathology from post-mortem examination of patients who died from legionnaires disease demonstrates patchy or focal lesions in a third of cases, focal hemorrhage in one quarter of patients, and lobar pneumonia in 50% of patients. Microscopic examination demonstrates diffuse alveolar damage, bronchopneumonia and dense congestion with macrophages, neutrophils, and inflammatory debris ( E-Fig. 290-3A to 3D ). Blood vessel inflammation suggesting vasculitis may occasionally be present.

Clinical Manifestations