Legionnaire's Disease (Legionellosis)

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Legionnaire's Disease (Legionellosis)


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Legionellosis refers to a disease caused by a type of bacteria called Legionella. Most commonly, the responsible organism is Legionella pneumophila.

The bacteria are normal residents of freshwater creeks, ponds, and lakes. They can also be present in the water supply inside buildings, where they have entered the air via tiny water droplets from ventilation or water ducts.

There are two forms of Legionellosis. The first is a more severe pneumonia that is known as Legionnaire's disease. The second includes a milder type of pneumonia and is called Pontiac fever.

Disease History, Characteristics, and Transmission

Legionellosis was first apparent in July 1976. At that time, an outbreak of pneumonia occurred during an American Legion convention being held at the Bellevue-Stratford Hotel in Philadelphia, Pennsylvania. Ultimately, 221 veterans were sickened during the outbreak. Thirty-four of these people eventually died of the infection, which was later dubbed Legionnaire's disease.

The disease outbreak caused national alarm, since it was feared to be the start of an epidemic of Swine Flu, which was at the time affecting Asia. However, an investigation conducted by the United States Centers for Disease Control and Prevention (CDC) determined that the Philadelphia outbreak was due to a newly discovered bacterium, which was eventually named L. pneumophila.

The outbreak was traced to bacteria growing in the hotel's cooling tower. Later, investigators showed that the bacterium is capable of growth as a surface-adherent structure called a biofilm. It is likely that bits of the biofilm broke off and were sucked into the hotel's ventilation system, where the bacteria were inhaled. Other outbreaks have been traced to biofilms growing on showerheads and in contaminated drinking water.

Legionellosis is an example of an opportunistic infection—an infection that is caused in some people by a bacterium that normally does not cause harm. For example, studies have determined that 5 to 10% of Americans contain Legionella antibodies even though they have not developed Legionellosis. However, in people whose immune systems are less capable of fighting off an infection, the bacteria can cause disease. Pneumonia due to Legionella comprises 2 to 15% of all pneumonia cases inU.S. hospitals, according to the CDC.

The majority of Legionellosis—over 90% of cases— is caused by L. pneumophila. L. micdadei can also cause legionellosis, especially in people who are immunocompromised.


BIOFILM: Biofilms are populations of microorganisms that form following the adhesion of bacteria, algae, yeast, or fungi to a surface. These surface growths can be found in natural settings such as on rocks in streams, and in infections such as can occur on catheters. Microorganisms can colonize living and inert natural and synthetic surfaces.

OPPORTUNISTIC INFECTION: An opportunistic infection is so named because it occurs in people whose immune systems are diminished or are not functioning normally; such infections are opportunistic insofar as the infectious agents take advantage of their hosts’ compromised immune systems and invade to cause disease.


The Coordinating Center for Infectious Diseases/Division of Bacterial and Mycotic Diseases states that:

  • Each year, between 8,000 and 18,000 people are hospitalized with Legionnaires' disease in the U.S. However, many infections are not diagnosed or reported, so this number may be higher. More illness is usually found in the summer and early fall, but it can happen any time of year.
  • Legionnaires' disease can be very serious and can cause death in 5% to 30% of cases. Most cases can be treated successfully with antibiotics (drugs that kill bacteria in the body), and healthy people usually recover from infection.
  • People most at risk of getting sick from the bacteria are older people (usually 65 years of age or older), as well as people who are smokers, or those who have a chronic lung disease (like emphysema).
  • People who have weak immune systems from diseases like cancer, diabetes, or kidney failure are also more likely to get sick from Legionella bacteria. People who take drugs to suppress (weaken) the immune system (like after a transplant operation or chemotherapy) are also at higher risk.

SOURCE: Coordinating Center for Infectious Diseases/Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention (CDC)

Approximately 10,000 to 40,000 Americans acquire Legionnaires’ disease every year, and 8,000 to 18,000 require hospitalization. The people who are the most likely to become ill are over age 50. The risk is greater for those with diminished immune system function due to illness, diabetes, cigarette smoking, and who are taking immunosuppressing drugs. Legionnaires’ disease can occur in children, but is not normally considered a disease of childhood. Children who are at risk are those who are on a respirator to assist with breathing, and those whose immune systems are impaired due to recent surgery or drug treatment. Curiously, those infected with the human immunodeficiency virus or who have developed acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome) do not appear to be at higher risk than others, although when contracting the disease, their symptoms are often more severe.

Legionnaire's disease is caused by inhaling Legionella suspended in minute water droplets, or by aspirating Legionella bacteria, which occurs when particles bearing the bacteria escape the gag reflex and fall directly into the respiratory tract. The bacteria can be naturally found in bodies of fresh water and whirlpool spas (the source of the first outbreak of Pontiac fever), where they can be dispersed into the air by the action of wind and waves. As well, the bacteria growing within biofilms in stagnant water at the intake of air conditioning cooling towers, humidifiers, faucets, shower heads, and even the water misters in supermarket produce departments can slough off and be carried on water droplets. Person-to-person transmission has not been demonstrated.

When inhaled or aspirated, Legionella bacteria enter the lungs. Normally, as bacteria enter the lungs they are engulfed and dissolved by cells called alveolar macrophages. However, Legionella are able to grow and divide inside the macrophages. Eventually, the infected macrophages burst, releasing the bacteria, which infect other macrophages and continuing the cycle of infection.

The symptoms of legionellosis develop 2 to 10 days after inhalation of the bacteria. At first, the symptoms include a feeling of tiredness, headaches, fever, chills, aching muscle, and a loss of appetite. A fever of up to 104°F (40°C) can develop. A dry and hacking cough also develops; it can change to a cough that involves the release of bloody mucus. The pneumonia affects breathing in about 50% of people and can cause chest pain in about 30% of those who get the infection. Some people develop a decreased heart rate, which can be dangerous when combined with the decreased breathing capability of the lungs.

In addition to pneumonia, legionellosis can involve other areas of the body. Other, less common complications include diarrhea, nausea with vomiting, abdominal pain, kidney failure and impaired urine production (which allows the build up of toxic by-products of body processes), and diminished mental capacity.

Pontiac fever is a milder form of legionellosis, which does not involve the lower respiratory tract. The symptoms, which are flulike and which typically appear within two days of exposure to the bacteria, include fever, headache, muscle aches, and fatigue. The infection passes within a few days and often, persons do not seek medical treatment.


The Centers for Disease Control and Prevention (CDC) states that “a person diagnosed with Legionnaires' disease in the workplace is not a threat to others who share office space or other areas with him or her. However, if you thought that there your workplace was the source of the person's illness, contact your local health department.”

SOURCE: Centers for Disease Control and Prevention (CDC)

Scope and Distribution

Legionellosis can occur almost anywhere in the world. A 2003 survey conducted by the 36-country European Working Group for Legionella Infections found the disease in 34 of the member nations. As one example, scientists investigating the May 1980 eruption of Mt. St. Helens became ill, likely with L. pneumophila found in ponds on the hillside.

Treatment and Prevention

Cases of legionellosis that occur as part of an outbreak are usually diagnosed more quickly than isolated cases. Diagnosis is complicated by the fact that the early symptoms and appearance of the chest in an x-ray are similar to other types of bacterial or viral pneumonia. Prompt diagnosis and treatment results in a better prognosis for persons with legionellosis. Death occurs about 5% of the time for previously healthy individuals and almost 25% of the time for people who were already ill or whose immune system was impaired when they contracted the disease. In severe cases that require mechanical assistance for breathing and kidney function, the death rate can be over 65%.

Legionellosis can be diagnosed by detecting anti-bodies to L. pneumophila produced by the immune system. A number of tests use the antibodies to detect the bacteria. For example, the antibodies can be linked to a fluorescent probe, and when samples are treated with the fluorescent antibody, L. pneumophila will appear as bright objects upon microscopic examination. Other tests can detect the presence of protein components of the bacteria in the urine, or the presence of the bacterial genetic material in urine and other body fluid.

Legionellosis is treated with antibiotics. As the bacteria reproduce inside host cells, the antibiotics must be capable of penetrating into the host cells. Typically, levofloxacin or azithromycin are used. Prompt antibiotic therapy leads to a complete recovery in the majority of cases.

Legionellosis is prevented by keeping ductwork, pipes, cooling towers, showerheads, and other potential breeding spots clean and free of stagnant water. In reality, this sort of vigilance can be difficult to maintain unless a mandated and inspection schedule is imposed and documentation required.

As of 2007, there is no vaccine for legionellosis.

Impacts and Issues

In the aftermath of the Philadelphia outbreak, regulations governing the cleaning and monitoring of air conditioning systems in public places were changed to minimize the development of L. pneumophila.

Legionellosis has the most impact in places where people gather and which are ventilated or have shower facilities; examples include indoor recreation centers, pools, spas, hotels, and hospitals. The latter is especially important since ill people are even more susceptible to the infection. Construction workers can also be at increased risk, since the bacteria may be dispersed into the air during excavation of the site.

In contrast to diseases such as bacterial meningitis and AIDS, there is no indication that poorer regions of the world are any more at risk than the more wealthy developed world. Indeed, the association of legionellosis with facilities such as hospitals and hotels has made the disease more of a problem in developed countries.

See AlsoOpportunistic Infection; Water-borne Disease.



Betsy, Tom and James Keogh. Microbiology Demystified. New York: McGraw-Hill Professional, 2005.

McCoy, William F. Preventing Legionellosis. London: IWA Publishing, 2006.


www.Legionella.org <http://www.legionella.org/> (accessed March 6, 2007).

Brian Hoyle

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