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 and Prevention (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 for diagnosing and reporting the disease, and monitoring the organism.  However, there are no health standards for safe levels of exposure to Legionella.

Common infections caused by Legionella are Legionnaires’ Disease (LD), a severe pneumonia, or Pontiac Fever, a flu-like illness. Legionnaires’ Disease is most commonly linked to exposure to Legionella pneumophila. However other species (i.e., L. micdadeiL, longbeachae) can cause LD as well. L. pneumophila has many subgroups called serotypes. L. pneumophila serotypes 1, 3, 5, and 6 have been the causative agents of Legionnaires’ Disease. Co-infections with different species and/or serotypes have occurred.


Legionella are commonly found in aquatic environments and some species have been found in soil. The organisms are found in a wide range of environmental conditions and are relatively resistant to low pH, dissolved oxygen levels, and routine chlorination techniques for drinking water. Temperatures above 104° F promote rapid multiplication of the organism. The organisms are consistently found in the biofilm that forms in aquatic environments, cooling towers and potable water systems.


The health risk factors for Legionellosis are people who are immunocompromised by an underlying medical condition, those taking immunosuppressive drugs, heavy smokers, those who have chronic lung conditions, and the elderly. Several studies have documented cases of pediatric Legionellosis in neonates, premature infants, and children under 1 year of age. Exposure is through inhalation or aspiration of contaminated, aerosolized water. Based on current research and our existing knowledge, once a person has Legionnaires’ disease, getting it a second time is possible but rare.

While it was previously believed that Legionella was not contagious; there were 2 cases that occurred in Portugal that researchers suspect have 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).

Monitoring Guidelines

As a result of the Legionnaires’ Disease outbreak that occurred in the Bronx in August, 2015 both the State of New York and New York City Department of Health and Mental Hygiene have passed the first US regulations specifically for testing cooling towers (and drinking water systems in healthcare facilities) for Legionella. The regulations specify action levels for requiring cooling tower disinfection.  In addition the US Centers for Disease Control and Prevention (CDC) recommends routine monitoring for Legionella in all hospitals in the US that perform bone marrow and organ transplants. Routine monitoring in hospitals is required or recommended in NY, TX, MD, Los Angeles County and Allegheny County PA.  Canada has guidelines for monitoring healthcare facilities and certain provinces in Canada have regulations for monitoring government owned or leased buildings.  On June 26, 2015 The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) passed ASHRAE Standard 188 for the prevention of legionellosis in buildings. While this voluntary standard of practice doesn’t specifically address Legionella testing, it does offer basic recommendations for establishing a building water safety plan

Sampling and Analysis

Be sure to use sterile bottles containing a chlorine neutralizing agent. Since biofilms are the actual reservoirs for the bacteria, it is also recommended to take sterile swab samples of biofilm in areas where it is present. Samples should be shipped overnight to the lab on freezer packs.

Analyzing for Legionella in environmental samples is difficult and time consuming. Therefore analysis should be conducted by a CDC ELITE proficient lab using culture methods (ISO, CDC, or Legiolert). These culture tests have a turnaround time of 2 weeks and provide confirmed identification and enumeration of the organism.  Unfortunately the culture test cannot be rushed. Testing by Polymerase Chain Reaction (PCR) may be useful for providing fast, presumptive results when a case or outbreak is encountered. Testing by Next Generation Whole Genome Sequencing (WGS) will provide a genetic fingerprint that will identify a link between an environmental source isolate to a patient clinical isolate and test relatedness as well. The use of Pulsed Field Gel Electrophoresis (PFGE) or Sequence Based Typing (for L. pneumophila serotype 1) has been replaced by Whole genome Sequencing. WGS has the discriminatory power to determine unequivocally if the molecular fingerprints from pure culture isolates are related based comparing the similarity/difference between the 3.4 Million individual genomic units of the bacterium.