Legionnaires' disease, also known as legionellosis, is an infection of the upper respiratory tract. It is classified as an atypical pneumonia because it is caused by a disease agent different from the bacteria that are most commonly associated with pneumonia, (Streptococcus pneumoniae). Legionnaires' disease is caused by bacteria of the genus Legionella, most often Legionella pneumophila. Unlike many other emerging diseases, Legionnaires’ disease is not spread by insects or other animals; L. pneumophila is found naturally in fresh water, where it often lives within amoebae, and as a contaminant of air conditioning systems, hot tubs, whirlpool baths and spas, and hot water heaters. The disease is named for the 1976 convention of the American Legion in Philadelphia, the location of a major outbreak that led to the identification of L. pneumophila in 1977.

Pontiac fever is a milder, flulike infection that is also caused by Legionella bacteria. It is named for an outbreak that occurred in Pontiac, Michigan, in 1968, when several workers in the city's public health department came down with fever and flulike symptoms that did not include pneumonia. After L. pneumophila was identified as the cause of Legionnaires' disease in 1977, the Michigan health department reexamined the blood samples from the 1968 outbreak of Pontiac fever and found that L. pneumophila was the causative organism. An outbreak of Pontiac fever in New Zealand in 2007 was traced to contaminated potting soil, which was the first time an infectious Legionella species had been found in a substance other than water.

Cases of Legionnaries' disease, 2000–2015

The incidence of Legionnaires' disease worldwide is not known, as the World Health Organization (WHO) notes that many countries lack the resources to track the disease. In Europe, Australia, and Canada, researchers estimate that there are about 10 to 15 cases diagnosed per million population. In the United States, about 5,000 cases of Legionnaires' disease are reported annually to the Centers for Disease Control and Prevention (CDC); however, some researchers think that only about 10% of legionellosis cases are reported. It is thought that the disease accounts for 3%–15% of all cases of community-acquired pneumonia. Several outbreaks have occurred in the United States since 1976, including three outbreaks in the Bronx between December 2014 and September 2015; an outbreak in San Quentin Prison in the summer of 2015; and an outbreak in Flint, Michigan, between June 2015 and January 2016. Major outbreaks in Europe include one in the United Kingdom in April 1985; an outbreak in the Netherlands in March 1999; and the largest recorded outbreak to date, which occurred in Murcia, Spain, in July 2001, with 696 suspected cases and 449 confirmed.

Although outbreaks of Legionnaires' disease receive considerable media attention, 85% of cases are sporadic; that is, they occur in single, isolated individuals. In addition, some people who are infected with the bacterium are never diagnosed because they are either without symptoms or develop only mild, self-limited symptoms and do not seek treatment.

Legionnaires' disease is sometimes described as the pneumonic (lung-related) form of legionellosis. It resembles other types of pneumonia in that patients typically have fever, headaches, muscle aches, shortness of breath, and coughing. They may also complain of diarrhea, nausea, vomiting, and, in some cases, mental confusion or other neurological symptoms. Legionnaires' disease can be classified according to whether it is community acquired, hospital acquired (nosocomial), or travel acquired.

Pontiac fever is caused by the same bacterium that causes Legionnaires' disease but is a milder illness. Patients with Pontiac fever usually have a fever, headache, fatigue, and malaise (a general feeling of being unwell), and they may also have nausea and diarrhea, but they generally do not have difficult or painful breathing. Pontiac fever is sometimes called nonpneumonic legionellosis because the patient's lungs are not infected.

The prognosis of legionellosis is highly variable; some patients recover rapidly, while others recover only slowly in spite of therapy. Mortality is highest among older males and those who acquire the infection in a hospital. The death rate from nosocomial infections approaches 50%. According to WHO, the overall death rate from Legionnaires’ disease is 5%–10% but runs as high as 40%–80% in patients with compromised immune systems and those who are not treated. Progressive respiratory failure is the most common cause of death in persons with Legionnaires’ disease.

The prognosis of Pontiac fever is almost always excellent, as the infection is self-limited, and people can recover fully without treatment.

The primary role of public health officials is twofold: (1) to maintain surveillance of legionellosis and respond to group outbreaks as well as record sporadic cases and (2) to educate the general public as well as building managers and owners of public hot tubs about proper maintenance of water systems. These systems include water used primarily for drinking or washing in offices or apartments within large buildings as well as spas and other recreational water systems.

WHO states that proper maintenance of water systems must include regular inspection and necessary repair of water pipes, cooling towers, and similar devices; regular cleaning and disinfection of hot tubs, pools, and spas; flushing unused water taps in buildings on a weekly basis to prevent water from stagnating; and using either temperature levels or chemical means to limit the growth of Legionella bacteria. The organism's preferred temperature range lies between 25°C–45°C (77°F–113°F). Hot water should therefore be kept at 60°C (140°F) and cold water below 20°C (68°F).

Legionella bacteria and the amoebae that harbor them are found in freshwater bodies worldwide, which means that each country must take measures to inspect and maintain its own water systems. As there was no vaccine for legionellosis as of 2017, mass vaccination programs could not be undertaken.

Risk factors for Legionnaires’ disease include:

Race or ethnicity is not a risk factor for Legionnaires' disease.

The cause of both Legionnaires' disease and Pontiac fever is bacteria of the genus Legionella, usually L. pneumophila in the case of Legionnaires’ disease. The two species other than L. pneumophila that are associated with Pontiac fever are L. longbeachae and L. micdadei. Legionella bacteria are aerobic bacilli found in freshwater streams and lakes worldwide. They cannot be grown on standard laboratory media.

Legionella persist in nature because they can infect and replicate within freshwater amoebae. They prefer warm, moist environments and can live in biofilms that form on the inside of water pipes and water containers. People become infected when they breathe in aerosols of contaminated water formed by shower heads, plant misters, humidifiers, ornamental fountains, whirlpool baths, or water cooling towers for air conditioning (standard home air conditioning units do not contain water and are therefore safe). Nosocomial legionellosis usually results from contamination of the hospital's hot water supply or respiratory therapy equipment; travel-associated infections usually involve aircraft, cruise ships, or hotel water systems. As far as was known as of 2017, people cannot transmit legionellosis to one another.

Once breathed in, L. pneumophila replicates within the cells of the alveoli, the tiny air sacs in the lungs in which oxygen and carbon dioxide are exchanged. The bacteria are engulfed by a type of white blood cell called a macrophage, inside of which they multiply, kill the macrophage, and are released to infect other macrophages. The incubation period is two to ten days; it is shorter (usually only two to three days) for Pontiac fever.

The symptoms of Legionnaires’ disease include fever as high as 105°F; headache, chills, and shaking; coughing with sputum or phlegm; occasional hemoptysis (coughing up blood); and painful breathing (dyspnea). The patient may or may not have abdominal pain, confusion, altered mental status, lethargy, vomiting, or diarrhea.

Complications of Legionnaires' disease include dehydration, inflammation of the valves of the heart, respiratory insufficiency, kidney failure, rapid breakdown of skeletal muscle, multiple organ failure, bacterial invasion of the bloodstream, coma, and death.

Legionnaires' disease is most likely underdiagnosed even in countries with advanced healthcare systems because it is a relatively uncommon form of pneumonia, and many clinicians have never seen a case. Accurate diagnosis is based on a combination of the patient's history, changes in vital signs, and test results.

The doctor may suspect Legionnaires' disease based on the patient's history of recent hospital inpatient treatment or travel by airplane or cruise ship; fever above 102°F accompanied by headache and coughing up sputum; and rapid breathing but a slower-than-normal heart rate. The diagnosis must be confirmed by test results, however.

The patient is given a chest x-ray to determine the extent of infection in the lungs, although this type of imaging study cannot be used to distinguish Legionnaires’ disease from other types of pneumonia. The definitive diagnostic tests are a urine test for the presence a Legionella bacteria protein and culturing the bacteria from a sample of the patient's sputum. Page 251  |  Top of ArticleIt takes about three to five days for the bacteria to form visible colonies on the culture medium.

The patient is also given a blood test to evaluate liver and kidney function, electrolyte levels, and evidence of dehydration.

Delay in treatment significantly increases the risk of a fatal outcome in Legionnaires' disease; therapy is usually begun as soon as possible. The patient is admitted to the hospital and given antibiotics; oxygen therapy and rehydration may also be required. Initial antibiotic therapy is given intravenously, followed by a 10- to 14-day course of oral antibiotics after the patient begins to improve. A 21-day course is recommended for patients that have compromised immune systems, are diabetic, or have a severe case of Legionella pneumonia.

Pontiac fever needs no treatment other than rest and a mild pain reliever for headache and muscle aches.

The antibiotics most often used to treat Legionnaires' disease as of 2017 were levofloxacin, azithromycin, ciprofloxacin, doxycycline, moxifloxacin, tigecycline, clarithromycin, and rifampin. Because the bacteria multiply within living cells, effective antibiotics must have the ability to penetrate cell walls. It is thought that one reason the mortality rate of the initial outbreak in 1976 was so high (29 deaths out of 182 patients) is that the patients were treated with penicillin and cephalosporins, antibiotics that are relatively ineffective at penetrating cells.

As noted above, the prognosis of legionellosis is variable, depending on the patient's age and general health status, the source of the infection, and the speed of diagnosis and treatment.

Because there was no vaccine for legionellosis as of 2017, prevention of Legionnaires’ disease depends primarily on adequate inspection and maintenance of water systems in large buildings, hospitals, and airplanes and ships. Individuals can reduce their risk by not smoking, using alcohol only in moderation, and minimizing their use of public hot tubs or other recreational water features if they are unsure of the facility's safety standards.