The first recognized outbreak of Legionnaires’ disease occurred in the US at the American Legion Convention in Philadelphia during the summer of 1976. There were several hundred people who were stricken. Thirty four people died from the disease. As a result of the efforts of the US Centers for Disease Control (CDC), this was the first time the bacteria was cultured and identified. Earlier outbreaks of the disease went undiagnosed. Since that time, there have been many identified outbreaks in this country and abroad prompting professional organizations and health departments worldwide to implement guidelines and regulations for diagnosing and reporting the disease, monitoring the organism, and implementing remediation action levels. In 2015 as a result of an outbreak that affected over 100 people, both the New York State Department of Health and the New York City Department of Health and Mental Hygiene passed groundbreaking legislation for cooling towers and healthcare facilities.

At the same time the Bronx NY outbreaks were occurring, the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) passed a voluntary professional standard of practice entitled AHSRAE 188 Legionellosis: Risk Management for Building Water Systems. These two historic events brought much more recognition to Legionellosis as a seriously under reported set of diseases. It is important to emphasize that there are no health standards or regulations for safe exposure to Legionella because the infectious doses for both healthy and immunosuppressed people are unknown.

Transmission and Epidemiology

Ubiquitous in all aquatic environments, Legionella bacteria are found in groundwater, municipal water as well as fresh, brackish and marine surface waters. The bacteria enter our plumbing systems, whirlpool spas, and cooling towers via these water sources.  Unless control measures are conducted properly and routinely, the biofilm, scale, and corrosion that increases over time in these systems will protect the organism and allow it to multiply.

Contaminated aerosolized water from cooling towers, whirlpool baths, nebulizers, faucets and showerheads become aerosolized. When a susceptible host inhales the contaminated aerosol, legionellosis can occur. Aspiration of contaminated water or melted ice chips can also cause the disease. Legionella can cause a very severe form of pneumonia (Legionnaires’ disease) often accompanied by serious long term health effects, or it can cause the mild, transitory, flu-like illness called Pontiac Fever. Blood infections, organ infections, native and prosthetic heart valves, prosthetic hip and knee replacements and asymptomatic infections may occur.

Risk factors include age, gender (male), compromised immune systems, and pre-existing medical conditions such as chronic obstructive pulmonary disease, cancer, diabetes, kidney failure, asthma, the use of immunosuppressive drugs, and chemotherapy. Men over 50 years of age who are immunocompromised, heavy smokers and drinkers are at greatest risk.  However, there have been cases of the disease in healthy, younger people and women. Premature, immunocompromised, or ventilated neonates are also at risk from hospital acquired infection. Babies born in home birthing pools filled with tap water have also developed Legionnaires’ disease resulting in septicemia, organ failure, and death.

While it was previously believed that Legionella was not contagious; there were 2 cases that occurred in Portugal that researchers suspect had been transmitted from son to mother based on the temporal occurrence of the two illnesses and the identical molecular fingerprints of isolates obtained from the bacteria of the two patients (Correia et. al. 2016).

Legionellosis is routinely under reported. Most of the cases are caused by building potable water systems and frequently occur in hotels and healthcare facilities. Outbreaks affecting larger numbers of people are associated cooling towers. For a full list of water systems that have been associated with cases and outbreaks, please see this link:

While community-acquired outbreaks involving cooling towers and whirlpool spas receive the most media attention, studies indicate that building potable water sources account for most of the infections. This is particularly true in hospitals and nursing homes where there are large numbers of immunosuppressed or critically ill people. For these reasons, many state health departments have guidelines that recommend routine monitoring for Legionella in critical care hospitals and nursing home facilities. In August, 2014 the Veterans Administration (VA) developed a directive that requires all VA locations that have overnight care to implement monitoring of their potable water systems. There are also draft guidelines for the prevention of  legionellosis in building water systems that were created by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). 

Recently CDC released statistics indicating that the fatality rate of community acquired cases have a fatality rate of about 10% while healthcare acquired cases have a fatality rate of up to 25%.

Choosing Sampling Methods

Proper methods for collecting and analyzing samples are necessary to ensure defensible results that are based on the correct sample size.  Ensuring that a representative number of samples are taken is also critically important before interpreting results. Since the bacteria in drinking water are present in very low levels, a 1000 milliliter (ml) potable water sample is recommended by the US Public Health Centers for Disease Control (CDC). This sample size allows for the bacteria in the water to be concentrated, allowing for a more sensitive detection and quantitation limit. Many professional guidelines recommend semi-annual sampling for potable water sources.

In non-potable water sources such as cooling tower water, a 250 ml sample size is sufficient.  Professional guidelines suggest these sources be monitored quarterly.

Sampling should be conducted in a way that maximizes recovery of the organism and mimics the route of exposure. Legionella samples should be collected wherever water aerosolization may occur.

Sampling water alone, however, will likely miss the real source of the organism. The actual reservoir for the bacterium is the biofilm or slime that is always found in our plumbing systems, cooling towers, and whirlpool baths.

Biofilm (slime) consists of other bacteria, blue green algae, amoeba, and protozoans. Biofilm protects Legionella from direct exposure to ultra-violet (UV) light, desiccation, and the chemicals used to control its growth. Legionella bacteria are ingested by the protozoans and amoeba and will continue to multiply inside these organisms. The infection of these host organisms will cause them to die releasing huge numbers of Legionella bacteria into the surrounding environment. This is exactly what happens inside our bodies when our pulmonary macrophages become infected by Legionella.

Because biofilm protects the organism and enhances Legionella multiplication, incorporating swabs in your sampling protocol is very important. Swab sampling of biofilm found in cooling tower sumps, potable water faucets, showerheads, and whirlpool spa filters is necessary to obtain a true picture of the presence of Legionella. Very often, biofilm swab samples demonstrate the presence of Legionella undetected by water sampling alone.  Taking swab samples of drinking water and ice machines may reveal Legionella contamination due to stagnating water.

While collecting air samples for Legionella mimics the route of exposure, this sampling method is not recommended for non-research sampling projects. During air sampling, the bacteria likely are killed from the impaction of the bacterial cells on the collection media.

Legionella are unlikely to survive the exposure to sunlight and desiccation for long periods of time. Air transmission of Legionella from cooling towers is greater during periods of cloudy, high humidity days and after rainfall (Simmerling, 2017).

When taking samples an N95 or HEPA particulate respirator, face shield or splash googles, and waterproof gloves should be worn. When taking samples of hot water storage tanks, it is prudent to wear a face shield, a thermal resistant apron, and thermal, water resistant gloves. Take care not to generate any aerosols when collecting the samples.  Only sterile, appropriately preserved bottles and sterile, Dacron (not cotton) swabs obtained from your lab should be used for sampling.  Potable water bottles should be preserved with sodium thiosulfate to neutralize halogen biocides in the water sample.

These preserved bottles should also be used when collecting water from cooling towers that have been treated with chlorine or bromine compounds.

After collecting a water sample, be sure to leave an air space in the bottle.  Since Legionella require oxygen for their survival, an air space in the bottle will ensure that aerobic conditions are maintained during shipment to the lab.

Samples should be packed and shipped to minimize the multiplication of non-Legionella bacteria. Using an insulated cooler with freezer packs is recommended to avoid temperature extremes during shipping. Samples should be shipped overnight to the lab.