How your AC Can Kill You - Legionnaires Disease

Danger lurks in the recesses of your air conditioning unit. A bacteria that attacks the respiratory system can grow in your cooling towers and can affect those who are exposed to the contaminated mist and vapor that comes from them.

Legionella pneumophila is the causative agent of legionnaires disease, a specific type of pneumonia that is exclusively caused by this bacteria. This bacteria was first isolated in 1976 when several veterans attending the American Legion convention in Philadelphia became sick and showed symptoms of pneumonia. Since then, America, Europe, and Australia report 10 - 15 cases per 1 million each year.

Distinguishing properties of Legionella

Medical experts were once puzzled by the appearance of the Legionnaires’ disease as they didn’t know where the causative agent came from. However, later on, they found out that the legionellae bacteria are natural inhabitants of freshwater environments. Furthermore, they also found out that they are resistant to regular water treatment techniques as their symbiotic relationship with free-living amoeba protects them from disinfectants. Due to these facts, researchers realized that internal and external factors such as water stagnation, changes in water pressure, inadequate levels of disinfectants, pH fluctuations, water temperature fluctuations, presence of scale and sediment, biofilm, changes in municipal water quality, water main breaks, and construction can contribute to the legionella problem.

When the legionellae bacteria are stained with the Gram Staining Method they appear pink, meaning they are Gram-negative. Further research shows that they do, in fact, have thin cell walls like the typical Gram-negative bacteria and are a rightful member of this group. When attempting to demonstrate the colonization of the lungs by the legionellae bacterium through biopsy, the usual hematoxylin and eosin stain wouldn’t stain the bacteria. Special staining methods such as the Dieterle silver impregnation stain is capable of staining the lung-biopsy tissue and the offending microorganism.

During the first outbreak in 1976, medical personnel tried to grow the bacteria on the culture media which were available during that time. They realized that this bacteria required specific nutrients as the bacteria did not grow on any of the culture media that they used. Later on, they found out that this bacteria needs high levels of iron and cysteine to grow which led to the creation of culture media which are enriched with iron and cysteine.

Disease progression

The portal of entry is always through the mouth and nose since the legionellae bacteria are often transmitted from the reservoir to the susceptible host through mists, vapors, and droplets. The bacteria are naturally present in freshwater sources which may enter cooling towers and tap water reservoirs. The mist generated by the air conditioning units that are connected to the bacteria-containing cooling towers may harbor the bacteria which could enter the nose of anyone who is unfortunate enough to enter the air-conditioned room. Tap water may also liberate the bacteria through the tiny droplets generated from turning on the faucet or shower. These droplets could then enter the respiratory system of anyone who is unfortunate enough to be near the faucet or shower. The bacteria often attacks the lungs but if left untreated may spread systemically and cause bacteremia. The bacteria’s major virulence factor, or its primary disease-causing component, is its lipopolysaccharide or endotoxin.

Risk factors

For unknown reasons, males are more susceptible to acquiring legionnaires’ disease. Smoking and alcohol consumption increase one’s risk significantly. Those who are immunocompromised like AIDS and cancer patients and those who are taking immunosuppressants like transplant patients are at a greater risk of acquiring Legionella pneumonia. Even if the bacteria is spread through droplets and colonizes the lungs, cases of person to person transmission have yet to be reported.

Symptoms and clinical findings

Legionnaires disease would often present with a mild influenza-like illness at first but if left untreated can progress to severe pneumonia. Once the symptoms of pneumonia would appear, other symptoms like mental confusion, non-bloody diarrhea, proteinuria, and microscopic hematuria would soon follow.

As with any other type of pneumonia, coughing is a prominent symptom of legionnaires disease. However, there’s not a lot of sputum produced. When sputum is expectorated, they are often nonpurulent. One laboratory test that can differentiate Legionnaires’ disease from the other types of pneumonia, besides microbiological tests, is the laboratory test for serum sodium concentration. Legionnaires’ disease would often present with hyponatremia or less than normal serum sodium levels in the person affected.

Not all cases of Legionella infection would lead to atypical pneumonia. Sometimes, infection with the legionellae bacteria would lead to a flu-like illness called Pontiac fever.

Laboratory Diagnosis

More often than not, doctors wouldn’t immediately suspect that a patient has legionnaires’ disease. The doctor would most likely request microbiological analysis as a doctor would for any other type of atypical pneumonia. The patient’s blood, sputum, or both are collected, Gram stained, and are used for bacterial culture.

The Gram stains of the sputum would reveal that there are a lot of neutrophils but, interestingly, there wouldn’t be any bacteria present. Microbiological cultures using agars that are commonly used for sputum cultures such as sheep blood agar, chocolate agar, and MacCongkey agar would yield no growth. This will give the microbiologist some hint that other more specialized agar such as the buffered charcoal-yeast agar or BCYE with cysteine is needed. A BCYE agar with no cysteine is also cultured for differentiation purposes. If growth is observed in the BCYE agar with cysteine only, the microbiologist would begin to suspect that the bacterial isolate is legionellae. Further biochemical tests would confirm the identity of the bacterial isolates.

Serological methods, or laboratory tests that use serum, can be done to diagnose legionella infection. A patient’s antibody titer is determined through serological methods such as indirect immunofluorescence assay. If a person is infected with Legionella then their antibodies against this bacteria would increase.

A rapid test for legionella has been developed, this test detects pneumophila antigens in the patient’s urine. If the pneumophila antigens are present then the person is infected with the legionellae bacteria.


Antibiotics are the only treatment option for legionella infection. Drugs such as azithromycin or erythromycin are preferred. Fluoroquinolones like levofloxacin and trovafloxacin are great alternatives. Legionellae bacteria can produce lactamase enzymes that would render penicillins and cephalosporins ineffective.


For a person to lessen his or her likelihood of acquiring Legionnaires’ disease he or she can reduce cigarette and alcohol consumption. However, the best way to tackle this problem would be to target the source of the infection.

Water Treatment

Water distribution systems need to be disinfected by methods such as ultraviolet light sterilization, ozonation, instantaneous steam heating, hyper chlorination, copper-silver ionization, or a combination of two or more of these methods. Chemical and physical control methods like maintaining hot water temperatures, ensuring disinfectant levels are high enough at the point of use, and ensuring water pH levels are optimal; all of these prohibits the Legionellae bacteria from growing.

Adequate Disinfection

As mentioned previously, legionella can grow inside cooling towers, water tanks, etc. There are several water-handling processes in certain buildings that expose the water to improper heating, faulty storage, and unintentional or intentional disinfectant filtering that may inadvertently encourage the growth of the legionellae bacteria. If through testing, it is found that a building’s tap water is inadequately disinfected at the point of use but there are no problems with the source then supplemental disinfectants are recommended at the point of use. Supplemental disinfectants include chlorine, mono-chloramine, chlorine dioxide, ultraviolet light, and ozone.

Getting the Right Temperature

The optimum temperature to prohibit the Legionella bacteria from growing is less than 77oF and greater than 108oF. In other words, cold water should be cold and hot water should be hot. Legionella may still affect people who live in warmer climates. Although tap water in these places tends to reach high temperatures, there are areas in the water pipes that are cold and could encourage the legionella bacteria to grow. For people who live in cold climates, it is essential that the water is heated to the maximum but within anti-scald regulations. However, there are cases where the maximum temperature allowable by a certain state may be too low to have the power to inhibit the growth of the Legionella bacteria. The best way to tackle this problem would be to apply engineering control wherein the water would be heated at a temperature beyond the anti-scald regulations and then cooled within the regulations when it reaches the point of use. This way, the risk of legionella infection can be reduced.

Prevent Stagnation within the Pipes

For any bacteria to create a biofilm, they need to adhere to the surface first and then create their film. In an environment where water flows continuously, the legionella bacteria may find it difficult to stick to the insides of a water pipe or reservoir. Thus, it is recommended that buildings are checked for any signs of water stagnation within the water distribution system.

Operate and Maintain Equipment

Organic debris and corrosion that may come from out-dated equipment or through faulty operation could enter your water system. These may provide safety pockets for the legionella bacteria to grow. Aside from this, these could also provide nutrients for them.

Monitor Other External Factors

Water main breaks, construction operations, changes in municipal water quality, these are other factors that building owners should pay close attention to. All of these have the potential to contaminate and introduce the legionella bacteria into the water system.

Wrap Up

Free-living environmental bacteria can become pathogenic when introduced into the human body in a specific way. The Legionella bacteria is one bacteria that has this property. It is a natural bacteria of freshwater but can enter our water system and be introduced to our respiratory system through droplets. Once it reaches our respiratory system, it can colonize it and cause a specific type of atypical pneumonia, Legionnaires’ disease.

There are many things that make a person more susceptible to Legionella. Certain behavior like frequent smoking and heavy drinking can make you more susceptible. Certain conditions that can compromise your immune system and the intake of immunosuppressants can make a person susceptible to Legionella as well.

When a person is infected with Legionella they would exhibit symptoms similar to Influenza that would then progress to atypical pneumonia. Certain properties of Legionella infection can complicate Laboratory diagnosis. The person affected would produce fewer sputum compared to other respiratory infections. Aside from this, the sputum would not yield any bacteria when stained. Finally, the bacteria won’t grow in any media except on BCYE that is enriched with cysteine.

Treatment for Legionella infection is through drug medication. Preventive measures can be focused on the person itself through lifestyle changes. However, the best preventive measures are those addressed to the water system. By inhibiting the growth of the Legionella bacteria in the water system through chemical and physical means, those who would receive this water are at a significantly lower risk of acquiring the disease.


  1. Review of Medical Microbiology & Immunology, Tenth Edition, Warren Levinson, MD, PhD





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