Legionnaires’ Disease: Things To Know

In July 1976, an infectious disease caught the attention of the American public when a series of deaths occurred among members of the American Legion (AL) who had joined a convention in the state of Philadelphia. Among the members who attended the convention, 182 people acquired a pulmonary disease of which 29 of them died within few days (Madigan 2006). The detection of the bacteria that caused the disease has not been so easy among microbiologists at that time and a lot of “incorrect” diagnoses and speculations were presented to the public. The Centers for Disease Control and Prevention (CDC) finally identified the previously unknown bacillus bacterium named Legionella pneumophila few months after the July 1976 event (Microsoft Student Encarta 2008). The bacterium was given the genus name “Legionella” to commemorate those (American Legion members) who have lost their lives because of the L. pneumophila infection. The disease caused by the bacterium has also been called Legionnares’ Disease or Legionellosis. Scientists have already identified 44 species of Legionella but not all of them cause disease (Madigan 2006).

Characteristics and Mode of Transmission of Legionella pneumophila

Biologists have been successful isolating Legionella species in natural waters like lakes and rivers. Moreover, the bacteria can grow in the warm water of air-conditioning cooling towers just like the structure found in the top of the building (Bellevue-Stratford Hotel) where the meeting of the American Legion occurred. Aerosols from that air-conditioning structure traveled down to the meeting place where it entered the nasal passages of the AL members. Interestingly, those who have just passed in front of the building also acquired the disease; perhaps they also inhaled aerosols coming from the air-conditioning structure at the top of the hotel building. Because the bacterium is airborne, hospitals, hotels, urban business districts and other establishments are taking extra care to avoid the spread of Legionnaires disease. Transmission of the bacterium between persons is not yet observed (Nester 2007; Madigan 2006).

L. pneumophila has been detected on water lines in hospitals that are usually warm. Most hospitals therefore lower the temperature of their water to 43-55 degrees Celsius because L. pneumophila loves warm water. The bacterium exhibits resistance to low level of chlorine allowing them to survive in water tanks with low amount of chlorine for long periods of time. The reason behind the resistance of L. pneumophila is its close interrelationship with waterborne amoeba. When they are engulfed by the amoeba, they resist the digestive enzymes of the amoeba and they begin to multiply. The amoeba therefore acts as a barrier against heat and chlorine for the bacterium (Nester 2007; Madigan 2006).

Symptoms of Legionnaires’ Disease

The disease is characterized by “headache, chest pain, lung congestion, and high fever (Microsoft Student Encarta 2008) of 40.5 degrees Celsius.” These are the general symptoms of pneumonia. Individuals aged 50 and above with low immunity are at higher risk of contracting the disease and dying. Survival rates are highest for non-smokers and non-alcoholics.

L. pneumophila is also the causative agent for Pontiac fever, which is another form of Legionellosis. The symptoms for the disease are muscular pains, fever, and most often cough. The two forms of Legionellosis may both occur during outbreaks (Nester 2007).

Diagnosis of Legionnaires’ disease

Culturing samples of L. pneumophila on a selective charcoal-yeast extract is considered to be the best diagnostic method. Doctors also examine respiratory specimens using fluorescent antibody methods and a DNA probe test (Madigan 2006).

Treatment of Legionnares’ Disease

The antibiotic erythromycin is proven effective against legionellosis. Other macrolide antibiotics like azithromycin are also prescribed by doctors (Madigan 2006; Nester 2007).


"Legionnaires’ Disease." Microsoft® Student 2008 [DVD]. Redmond, WA: Microsoft Corporation, 2007.

Madigan, Michael. 2006. Brock Biology of Microorganisms. Upper Saddle River, N.J.: Prentice Hall/Pearson Education.

Nester, Eugene. 2007. Microbiology: a human perspective. New York: McGraw-Hill Higher Education