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Legionella (Legionaires Disease)

What is Legionella?

Legionella is a Gram negative bacterium, including species that cause legionellosis. Legionellosis takes two distinct forms:

  • Legionnaires’ disease, also known as “Legion Fever”, is the more severe form of the infection and produces pneumonia.
  • Pontiac fever is caused by the same bacterium but produces a milder respiratory illness without pneumonia that resembles acute influenza.

Legionnaires’ disease acquired its name in July 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. Some people can be infected with the Legionella bacterium and have only mild symptoms or no illness at all.

Outbreaks of Legionnaires’ disease receive significant media attention. However, this disease usually occurs as single, isolated cases not associated with any recognized outbreak. When outbreaks do occur, they are usually in the summer and early autumn, though cases may occur at any time of year. The fatality rate of Legionnaires’ disease has ranged from 5% to 30% during various outbreaks. “The death rate for patients who develop Legionnaire’s disease while in the hospital is close to 50%, especially when antibiotics are started late”, according to the NIH and U.S. National Library of Medicine service’s MedlinePlus. Most infections occur in those who are middle-age or older.

Symptoms of the Disease

Patients with Legionnaires’ disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Some patients also have muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia), and occasionally diarrhea and vomiting. Laboratory tests may show that patients’ renal functions, liver functions and electrolytes are deranged, including hyponatremia. Chest X-rays often show pneumonia with bi-basal consolidation. It is difficult to distinguish Legionnaires’ disease from other types of pneumonia by symptoms or radiologic findings alone; other tests are required for diagnosis. The time between the patient’s exposure to the bacterium and the onset of illness for Legionnaires’ disease is 2 to 10 days

Persons with Pontiac fever experience fever and muscle aches without pneumonia. They generally recover in 2 to 5 days without treatment. The time between the patient’s exposure to the bacterium and the onset of illness for Pontiac fever is shorter, generally a few hours to 2 days.

Causes of the Disease

Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action breaks the surface of the water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye and small enough to be inhaled into the lungs.

This often occurs in poorly ventilated areas such as prisons where a condensating air conditioner can spread it throughout the entire room, infecting anyone not immune to the strain of bacteria. Potential sources of such contaminated water include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, evaporative coolers, hot water systems, showers, windshield washers, whirlpool spas, architectural fountains, room-air humidifiers, ice making machines, misting equipment, and similar disseminators that draw upon a public water supply.

The disease may also be transmitted from contaminated aerosols generated in hot tubs if the disinfection and maintenance program is not done rigorously. Freshwater ponds, creeks, and ornamental fountains are potential sources of Legionella. The disease is particularly associated with hotels, cruise ships and hospitals with old, poorly maintained pipework and cooling systems. A study published by the European Journal of Epidemiology points to automotive windshield washing systems as a source, recommending the addition of an antibacterial agent to the system’s reservoir.

Prevention of the Disease

Control of Legionella growth can occur through chemical or thermal methods. Copper-silver ionization is a chemical process that disperses and destroys biofilms and slimes that can harbor Legionella over the long term. Hyperchlorination with chlorine dioxide or monochloramine is a similarly dispersive alternative treatment. Ultraviolet light, thermal eradication, and ozone are short-term (nondispersive) treatments


A short-term chemical treatment, chlorine must be repeated every 3–5 weeks. Corrosion factors apply.

Copper-Silver Ionization

Industrial-size copper-silver ionization is approved by the U.S. Environmental Protection Agency for Legionella control and prevention on a case by case basis. As long as levels of copper and silver ions are sufficient to perform their disinfection function disinfection can occur as rapidly as one week.

Chlorine Dioxide

Chlorine dioxide is an approved potable water disinfectant with potentially serious health issues to women and children if ClO2 levels exceed the regulatory limit of (0.8 ppm). When applied properly, it can reduce Legionella levels in cold water easier than in hot water. It can take as long as 17 months for complete control of Legionella through an entire system. Monochloramine is an alternative.

Thermal Eradication

Thermal eradication (superheating to 140 °F (60 °C) and flushing) is a nonchemical treatment that typically must be repeated every 3–5 weeks.