Legionnaires' Disease and Pontiac Fever (Legionellosis)

X-ray image of lungs of a patient with Legionnaires'; disease

• Legionellosis is an infection that is caused by a bacterium.
• The bacterium thrives in droplets of water and moist conditions usually associated with water systems. The bacteria reproduce in high numbers in warm, stagnant water (95 F - 115 F)
• The bacterium can infest an entire building.
• The illness takes two distinct forms: Legionnaires' disease and Pontiac fever.
• Legionnaires' disease is the more severe form and can be fatal.
• Pontiac fever is the far milder form of the illness.
• Symptoms of Legionnaires' disease include fever, chills, and a cough.
• At its worst, Legionnaires' disease can cause severe pneumonia and respiratory failure.
• Although antibiotics are an effective medical treatment, the most useful approach is prevention with the maintenance of water systems.
• Outbreaks are usually recognized in the summer and early fall, but cases may occur all year long.

Legionella organisms can be found in many types of water systems. However, the bacteria reproduce in high numbers in warm, stagnant water (95 F-115 F), such as that which is found in certain plumbing systems and hot-water tanks, cooling towers, and evaporative condensers of large air-conditioning systems, and hot tubs. Cases of legionellosis have been identified throughout the United States and in several other countries. The disease likely occurs worldwide.

The bacterium thrives in the mist aerosolized from air-conditioning ducts. Thus, it can infest an entire building or airplane. Factors that are now known to enhance the growth of Legionella bacteria in human-made water environments include the following:

• Water temperatures of 77 F-107.6 F
• Stagnation of the water
• Scale and sediment in the water
• Certain free-living amoebae organisms in water are capable of supporting intracellular growth of Legionellae

Legionellosis is an infection that is caused by a Legionella species called Legionella pneumophila. The disease has two distinct forms:

• Legionnaires' disease is the more severe form of infection, which may involve pneumonia. The onset of this form of the disease is usually two to 10 days after infection but can occur up to 16 days later. Legionnaires' disease acquired its name in 1976 after an outbreak of pneumonia occurred among people attending the American Legion convention at the Bellevue-Stratford Hotel in Philadelphia. Later, the bacterium causing the illness was named Legionella pneumophila.
• Pontiac fever is a milder illness that develops from hours to two days after initial infection and resolves spontaneously. Sometimes, it may not even cause symptoms.

Outbreaks of legionellosis have occurred after people have inhaled aerosols that come from a water source (for example, air-conditioning cooling towers, hot tubs, or showers) contaminated with Legionella bacteria. People may be exposed to these aerosols in homes, workplaces, hospitals, or public places. Person-to-person infection does not occur with legionellosis, and there is no evidence of people becoming infected from vehicle air conditioners or household window air-conditioning units.

Cases of Legionnaires' disease have increased throughout the United States, but this most likely represents increased detection by better diagnostic tests. Some would suggest that this increase may be due to global warming, though this seems unlikely.

Hospital-acquired (nosocomial) infections have occurred and are usually due to contamination of the water supply. Two well-reported cases occurred in the 1980s in Los Angeles, one at the Wadsworth VA hospital and the other at the City of Hope. Guidelines have now been established for routine environmental testing for Legionnaires' in several states in the United States and by the Veterans Affairs health care system.

With regard to travel-associated infection, this usually occurs in hotels from contaminated water, most notably at the Legionnaires' convention in Philadelphia. This disease has also been associated with travel on cruise ships.

Some data suggest an increased incidence of Legionnaires' in the elderly in nursing homes linked to eating puréed food, probably also due to increased aspiration in this population.

People of any age can develop Legionnaires' disease, but the illness most often affects middle-aged and older people, particularly those who smoke cigarettes or have chronic lung disease, as these individuals have a greater likelihood of developing any respiratory illness.

People at an increased risk for Legionnaires' disease also include people whose immune systems are suppressed by diseases and conditions such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those who take medications that suppress the immune system are also at risk including those chronically on steroids.

Pontiac fever most commonly occurs in healthy people.

Patients with Legionnaires' disease usually develop a fever, chills, and a cough. The cough may either be dry or produce sputum. Some patients with Legionnaires' disease also have muscle aches, headache, tiredness, loss of appetite, and occasionally diarrhea.

Legionnaires' disease can cause severe pneumonia, seriously affect breathing, and even lead to respiratory failure and adult respiratory distress syndrome (ARDS). In some cases, the heart rate is slower than expected for the degree of fever.

• There are no specific symptoms that directly identify Legionnaires' pneumonia. Legionnaires' pneumonia presents in a manner similar to Chlamydia pneumonia and Mycoplasma pneumonia, so-called atypical pneumonias (previously referred to as "walking pneumonia"). These are referred to as atypical because the associated symptoms and signs are unlike typical pneumonia (as characterized by Streptococcus pneumonia), which involves spiking high fevers, sudden onset, cough, and purulent sputum and often chest pain and a localized infiltrate on chest X-ray.

People with Pontiac fever experience a self-limiting influenza-like illness with fever, chills, headache, and muscle aches but, by definition, do not have pneumonia. Sickened individuals generally recover in two to five days without treatment.

Laboratory tests may include a modest increase in white blood cells, mild abnormalities in liver function studies, low sodium in the blood, and even some decreased function of the kidneys. Nevertheless, these features can also be seen with a variety of different types of pneumonia. Chest X-rays often demonstrate abnormal densities (areas of the lungs that on the X-ray film appear whiter). However, it is difficult to distinguish Legionnaires' disease from other types of pneumonia by symptoms and chest X-ray alone. Additional specific tests are required for diagnosis.

These other tests are specialized and are not routinely performed on people with fever or pneumonia. Several types of tests are available. The most useful test uses a urine sample to detect identifiable proteins of the Legionnaires' bacterium (Legionella antigens). This test has the disadvantage of only testing for Legionella pneumophila serogroup 1 (this is the organism responsible for most of the cases). Additionally, detecting the bacteria in a culture of the sputum may be helpful. Lastly, tests that compare antibody levels of Legionella in two blood samples obtained three to six weeks apart showing a fourfold rise in the antibodies in the blood against the bacterium can be confirmatory after the disease is gone.

Because these tests complement each other, a positive result from each test when Legionnaires' disease is suspected increases the probability of confirming the diagnosis. However, because none of the laboratory tests is 100% sensitive, the diagnosis of legionellosis is not excluded even if one or more of the tests is negative. Of the available tests, the most specific is culture isolation of the bacterium from secretions from the respiratory tract.

Legionnaires' pneumonia is not unusual. In fact, it represents over 4% of all community-acquired pneumonias. An additional unknown number of people are infected with the Legionella bacterium but have only mild symptoms or no symptoms at all (so-called Pontiac fever).

Outbreaks of Legionnaires' disease have received the most media attention. A disease outbreak was associated with a party at the Playboy mansion in Los Angeles where at least four individuals contracted Legionnaires' disease. However, the disease most often occurs as single, isolated cases not associated with any identified outbreak. Outbreaks are usually recognized in the summer and early fall, but cases may occur all year long. More recently, outbreaks have been detected in gym participants in Florida and in a police station in New York.

Since the bacterium of Legionnaires' disease was identified in 1976, numerous hospital-acquired outbreaks of the disease have also been reported. These outbreaks have enabled health officials to study epidemics of legionellosis. A recent CDC evaluation reported that one in five Legionnaires' disease cases are linked to health care facilities.

There are three major classes of antibiotics that are effective in treating a Legionella infection. These include

• the fluoroquinolones such as levofloxacin (Levaquin), moxifloxacin (Avelox),
• the macrolides such as erythromycin, azithromyocin (Zithromax), and clarithromycin (Biaxin), and
• the tetracyclines including doxycycline (Vibramycin). A new class of antibiotics (glycylcyclines) is also effective.

The choice of antibiotic is often dependent on the patient's clinical state, tolerance to the medication, and a health care professional's degree of certainty as to the diagnosis.

• Zithromax and Levaquin are particularly effective because of decreased gastrointestinal irritation, higher potency, better penetration into tissue, and once-daily dosing.

In severe cases of Legionnaires' disease that seem more resistant to a single antibiotic, a second drug called rifampin (Rifadin) may be added.

Pontiac fever requires no specific treatment.

Prevention is aimed at improving the design and maintenance of cooling towers and plumbing systems. The goal is to limit the growth and aerosolization of Legionella organisms. Hyperchlorination tends to be ineffective at disinfecting the water supply. Copper-silver ionization, superheating, and monochloramine disinfection are newer methods of controlling and eliminating the growth of this bacteria.

During outbreaks, health department investigators seek to identify the source of disease transmission and recommend appropriate prevention and control measures, such as decontamination of the water source.

Detailed guidelines for the prevention of Legionnaires' disease both in hospitals with no identified cases ("primary prevention") and in hospitals with reported cases ("secondary prevention") have been published and are available in the U.S. from the Centers for Disease Control and Prevention.