Legionellosis refers to the two clinical syndromes caused by bacteria of the genus Legionella. Pontiac fever is an acute, febrile, self-limited illness that has been serologically linked to Legionella species, whereas Legionnaires′ disease is the designation for pneumonia caused by these species. Legionnaires′ disease was first recognized in 1976, when an outbreak of pneumonia took place at a Philadelphia hotel during an American Legion convention.
The family Legionellaceae comprises more than 50 species with more than 70 serogroups. The species L. pneumophila causes 80–90% of human infections and includes at least 16 serogroups; serogroups 1, 4, and 6 are most commonly implicated in human infections. To date, 18 species other than L. pneumophila have been associated with human infections, among which L. micdadei (Pittsburgh pneumonia agent), L. bozemanii, L. dumoffii, and L. longbeachae are the most common. Members of the Legionellaceae are aerobic gram-negative bacilli that do not grow on routine microbiologic media. Buffered charcoal yeast extract (BCYE) agar is the medium used to grow Legionella.
The natural habitats for L. pneumophila are aquatic bodies, including lakes and streams. L. longbeachae has been isolated from natural soil and commercial potting soil. Legionellae can survive under a wide range of environmental conditions; for example, the organisms can live for years in refrigerated water samples. Natural bodies of water contain only small numbers of legionellae. However, once the organisms enter human-constructed aquatic reservoirs (such as drinking-water systems), they can grow and proliferate. Factors known to enhance colonization by and amplification of legionellae include warm temperatures (25°–42°C) and scale and sediment. L. pneumophila can form microcolonies within biofilms; its eradication from drinking-water systems requires disinfectants that can penetrate the biofilm. The presence of symbiotic microorganisms, including algae, amebas, ciliated protozoa, and other water-dwelling bacteria, promotes the growth of legionellae. The organisms can invade and multiply within free-living protozoa. Rainfall and humidity have been identified as environmental risk factors.
Sporadic community-acquired Legionnaires′ disease has been linked to colonization of residential, hotel, and industrial water supplies. Drinking-water systems in hospitals and extended-care facilities have been linked to health care–associated Legionnaires′ disease.
Cooling towers and evaporative condensers have been overestimated as sources of Legionella. Early investigations that implicated cooling towers antedated the discovery that the organism could also exist in drinking water. In many outbreaks attributed to cooling towers, cases of Legionnaires′ disease continued to occur despite disinfection of the cooling towers; drinking water was the actual source. Koch's postulates have never been fulfilled for cooling tower–associated outbreaks as they have been for hospital-acquired Legionnaires′ disease. Nevertheless, cooling towers have occasionally been identified in community-acquired outbreaks, including an outbreak in Murcia, Spain, in which several hundred suspected cases of Legionnaires′ disease occurred over a 3-week period. As mentioned above, L. longbeachae infections have been linked to potting soil, but the mode of transmission remains to be clarified.
Multiple modes of transmission of Legionella to humans exist, including aerosolization, aspiration, and direct instillation into the lungs during respiratory tract manipulations. Aspiration is now known to be the predominant mode of transmission, but it is unclear whether Legionella enters the lungs via oropharyngeal colonization or directly via the drinking of contaminated water. Oropharyngeal colonization has been demonstrated in patients undergoing transplantation. Nasogastric tubes have been linked to hospital-acquired Legionnaires′ disease; microaspiration of contaminated water was the hypothesized mode of transmission. Surgery with general anesthesia is a known risk factor that is consistent with aspiration. Especially compelling is the reported 30% incidence of postoperative Legionnaires′ disease among patients undergoing head and neck surgery at a hospital with a contaminated water supply; aspiration is a recognized sequela in such cases. Studies of patients with hospital-acquired Legionnaires′ disease have shown that these individuals underwent endotracheal intubation significantly more often and for a significantly longer duration than patients with hospital-acquired pneumonia of other etiologies.
Aerosolization of Legionella by devices filled with tap water, including whirlpools, nebulizers, and humidifiers, has been implicated. An ultrasonic mist machine in the produce section of a grocery store was the source in a community outbreak. Pontiac fever has been linked to Legionella-containing aerosols from water-using machinery, a cooling tower, air-conditioners, and whirlpools.
The incidence of Legionnaires′ disease depends on the degree of contamination of the aquatic reservoir, the immune status of the persons exposed to water from that reservoir, the intensity of exposure, and the availability of specialized laboratory tests on which the correct diagnosis can be based. Numerous prospective studies have ranked Legionella among the top four microbial causes of community-acquired pneumonia, accounting for 2–9% of cases. (Streptococcus pneumoniae, Haemophilus influenzae, and Chlamydophila pneumoniae are usually ranked first, second, and third, respectively.) On the basis of a multihospital study of community-acquired pneumonia in Ohio, the Centers for Disease Control and Prevention (CDC) estimated that as many as 18,000 cases of sporadic community-acquired Legionnaires′ disease occur annually in the United States and that only 3% of these cases are correctly diagnosed. Legionella is responsible for 10–50% of cases of nosocomial pneumonia when a hospital's water system is colonized with the organisms. The incidence of hospital-acquired Legionnaires′ disease depends on the degree of contamination of drinking water as defined by the rate of positivity of distal water sites (not as defined quantitatively by the number of colony-forming units per milliliter).
Risk factors for Legionnaires′ disease include cigarette smoking; chronic lung disease; advanced age; prior hospitalization, with discharge within 10 days before onset of pneumonia symptoms; and immunosuppression. Immunosuppressive conditions that predispose to Legionnaires′ disease include transplantation, HIV infection, and treatment with glucocorticoids or tumor necrosis factor α antagonists. However, in a large prospective study of community-acquired pneumonia, 28% of patients with Legionnaires′ disease did not have these classic risk factors. Surgery is a prominent predisposing ...