Preventing, Testing & Mitigating Legionella Risk in Building Water Systems

Feb. 1, 2021

Mitigating COVID-19’s impact on building water systems through a smart CWMP

About the author:

Michelle DeRosa is marketing coordinator for Phigenics LLC. Jonathan Kargas is product manager for Phigenics LLC. They can be reached at [email protected] or 844.850.4087.


The spread of COVID-19 has caused the closure and recommissioning of buildings and increased the adaptive reuse of space in the healthcare, hospitality and travel industries.  All of these factors have contributed to an increased risk of disease and injury caused by legionella and other waterborne pathogens in building water systems. Stagnant water conditions can pose a dangerous threat in facilities with COVID-19 patients and the communities at large with the risk of legionellosis. The healthcare industry should be aware of the impact waterborne pathogens like legionella can have on COVID-19 patients, especially individuals 50 years and older and the immunocompromised. 

The most effective way to control hazards and prevent legionella is through an industry standard-aligned Comprehensive Water Management Program (CWMP) empowered by facility water management teams. Proactive water management teams need tools to identify and address problems in a timely manner as part of their CWMP. One effective tool includes legionella testing as part of their validation strategy. Direct testing for legionella helps building owners and facility managers validate that their CWMP, when implemented as designed, is effectively controlling the hazard. Testing helps identify potential issues and address water safety and risk management challenges that may emerge from the extended closure of a building. Faster, more accurate, and more comprehensive testing is critical to ensure the proper implementation of a CWMP, leading to increased safety for all individuals who enter commercial buildings.

What is Legionella?

Legionella is a waterborne pathogen that can cause severe pneumonia called Legionnaires’ disease. It is a heterotrophic, aerobic, gram-negative, rod shaped bacteria that can be found naturally in freshwater environments and soil. Legionella requires a host to survive and proliferate. In building water systems, legionella infect host cells, such as common waterborne amoebae. Once inside the host, legionella will rapidly grow until overwhelming the cell, causing lysis (cell bursting), releasing large numbers of bacteria into the water system. Once the bacteria are in the system they can cause disease in humans after inhalation or  aspiration of contaminated water.

The U.S. Center for Disease Control and Prevention (CDC) stated in 2018, in the U.S., there were approximately 10,000 cases of Legionnaires’ disease reported by health departments (Legionnaires Disease History 2018). The CDC has stated there are no known safe levels of legionella in building water systems.

What You Can Do to Prevent Legionella

Legionella bacteria in building water systems and outbreaks of diseases associated with legionellosis are preventable. There are several steps building owners, facilities management staff and plumbing professionals can take to mitigate the risk of contamination. 

First, an industry standard-aligned CWMP should be developed, supported by a water management team (WMT). Forming a cross-functional WMT with authority and accountability is the most important part of developing a CWMP. WMTs are responsible for the successful development and implementation of the program and ensuring the program is effective. Second, verification and validation procedures should be established to confirm the program is being implemented as designed (verification) and confirm the program is effective when implemented as designed (validation). Third, molecular screening can be incorporated as a part of the comprehensive validation strategy, to provide accurate information about the facility and confirm control methods within one day after sampling. Implementing a CWMP and using molecular screening as part of the validation strategy can aid in safely reopening buildings and facilities after a period of dormancy, much like what is occuring during the COVID-19 pandemic.

How to Test for Legionella

There are a few different ways of testing for legionella. Traditional culture methods, based on the industry standard method ISO 11731, take a minimum of 10 to 14 days for results. In addition, the ISO method requires samples to be shipped to a laboratory.  The shipping process can result in changes to the water sample, which can introduce uncertainty with the results. An in-field sampler is available that shortens this time frame considerably with culture results available five days from the time of the sample.  

Molecular screening is one of the fastest testing methods available for detecting legionella in building water samples. This is made possible by using a molecular biology technique called Polymerase Chain Reaction (PCR). PCR detects the genetic material of the target organism, in this case, legionella DNA. Legionella PCR detects but cannot differentiate between live, recently disinfected, and viable but non-culturable (VBNC) legionella DNA. VBNC are cells which have gone into a protective state due to environmental stress and can no longer form colonies on solid media. The PCR diagnostic provides tremendous value as a negative screen. Negative PCR results correlate 99.8% with negative culture results. Therefore, when a sample is PCR negative, there is no technical need to culture that particular sample.

ASHRAE Guideline 12-2020, published in March of 2020, is a document that helps facility managers implement water management programs. In section C3, culture, in-field inoculation and molecular methods are described as appropriate test strategies for use in legionella detection.

The best application for the PCR diagnostic is as a negative screen. In this strategy, negative results are used to eliminate areas of concern, and positive results are used to indicate that further investigation is warranted. Further investigation could involve follow up cultures of positive locations to confirm viability. This allows facilities to target and allocate resources for areas of highest concern, optimizing testing budgets and manpower. The CDC has implemented a negative legionella screening strategy in certain non-outbreak situations or in research projects (Llewellyn et al., 2017).

PCR technology can help water management teams strategically increase the number of testing locations and effectively manage legionella positive detections. Traditional culture methods and PCR can be used in a complementary strategy to optimize testing and manpower resources. Water management teams should leverage the technology available through molecular methods and use the data to apply more focused, time-consuming culture methods only to positive areas within building water systems. 

Individuals in the water treatment industry, including those in facilities management and plumbing professions, rely on the detection of legionella DNA for fast, preliminary results to indicate whether or not building water systems are well-managed. Molecular screening using PCR can deliver rapid results that confirm the absence of legionella within building water systems or identify locations that need further attention. 

Putting the PCR Strategy Into Practice

A large hospitality and gaming corporation’s WMT implemented a strategy using PCR into their recommissioning plan to safely and efficiently open their establishments across the country. Once state officials cleared the hospitality and gaming properties to reopen after a prolonged shutdown, the WMT determined that their first step was to get the water moving in all of their property’s building water systems. 

They systematically flushed the water systems, ensuring that there was enough water moving to completely turnover the system. The WMT identified areas to test for legionella, choosing to test at each processing step (how water is processed, for example: heating, cooled, filtered, conditioned, stored, etc.) identified in their program. They used a high volume PCR screening strategy that provided results the next day. 

Because of the 99.6% negative predictive value of the PCR screen, the locations with negative legionella PCR results were cleared for use the day after sampling, Any locations with positive legionella PCR results were tested using an in-field culture testing method. Results for the in-field test were reported in four days, and while waiting for these results, the WMT continued to flush and move water through their building, specifically targeting the PCR positive locations. Once culture results were obtained, the culture-negative locations were cleared for use, which accounted for all but one location that came back culture positive. Further flushing and disinfection of the fixture was done, followed by additional PCR testing. After remediation, the PCR result came back negative, which indicated that there was no legionella DNA detected and no subsequent culture test was needed.

This recommissioning strategy provided the WMT with data that was used to make the defensible decision to reopen the properties. If the WMT decided to use traditional culture methods their decisions could have been delayed 10 days or more depending on results. Using molecular screening allowed the WMT to test more locations, more quickly, gathering important data while saving time and resources.

But Make it Comprehensive

It is important to remember that a WMP’s testing and validation strategy must be comprehensive to be defensible. Using molecular screening and traditional culture methods are essential to providing data for defensible decisions. A comprehensive validation strategy will ensure that WMTs can safely and efficiently operate their building water systems and that their CWMP is effective. 

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