Hot water plumbing in residences and office buildings have distinctive risk of Legionella pneumophila contamination

Abstract

Aim:

To observe how Legionella pneumophila, the causative agent for legionellosis, can transmit through the hot water plumbing of residences and office buildings.

Method and results:

Using qPCR, L. pneumophila and L. pneumophila Serogroup (Sg)1 were measured in hot water samples collected from 100 structures, consisting of 70 residences and 30 office buildings. The hot water samples collected from office buildings had a higher L. pneumophila detection frequency of 53% (16/30) than residences, with a 10³ GU/L (median) concentration. An office building’s age was not a statistically significant predictor of contamination, but its area (>100,000 sq. ft.) was, P =<0.001. Hot water samples collected at residences had a lower L. pneumophila detection frequency of 36% (25/70) than office buildings, with a 100 GU/L (median) concentration. A residence’s age was a significant predictor of contamination, P = 0.009, but not its area. The water’s secondary disinfectant type did not affect L. pneumophila detection frequency nor its concentration in residences, but the secondary disinfectant type did affect results in office buildings. Legionella pneumophila’s highest detection frequencies were in samples collected in March–August for office buildings and in June–November for residences.

Conclusion:

This study revealed that the built environment influences L. pneumophila transport and fate. Residential plumbing could be a potential “conduit” for L. pneumophila exposure from a source upstream of the hot water environment. Both old and newly built office buildings had an equal probability of L. pneumophila contamination. Legionella-related remediation efforts in office buildings (that contain commercial functions only) might not significantly improve a community’s public health.

1. Introduction

Legionella spp. is an environmental bacterium that causes the human respiratory disease, legionellosis. Legionellosis is contracted by breathing or aspirating soil or water droplets contaminated with Legionella. Legionellosis is associated with two clinical manifestations: Legionnaires’ disease (severe) and Pontiac fever (mild). Legionnaires’ disease signs and symptoms are similar to pneumonia (Mandell et al., 2007).

In the United States, between 2000 and 2017, the Legionnaires’ disease rate increased 5.5-fold from 0.42 to 2.29 per 100,000 persons (Barskey et al., 2020). In 2017, among the Legionnaires’ disease cases reported to the Centers for Disease Control and Prevention (CDC) National Notifiable Disease Surveillance System (NNDSS), the case fatality rate was 7% (Barskey et al., 2020). Most Legionnaires’ disease cases are reported in the Summer to Fall months. Lower case numbers are reported in the Winter to Spring (Barskey et al., 2022). In 2014 the estimated cost in the United States for Legionnaires’ disease treatment was 402 million dollars (Collier et al., 2021).

In 1976, the United States’ first recognized outbreak of Legionnaires’ Disease occurred at Philadelphia’s Bellevue-Stratford Grand Hotel (TIME, 1976). The bacteria, L. pneumophila had contaminated the water supply that the building used for its air conditioning cooling system, enabling exposure via aerosolization. Since the 1976 outbreak, many legionellosis cases and outbreaks have been caused by exposure to contaminated water aerosols in either a residence or office building setting (Garrison et al., 2016; Schumacher et al., 2020; Smith et al., 2019). In addition, many studies have detected L. pneumophila in hot water samples from residential and office buildings (Collins et al., 2017; Donohue et al., 2019a; Stout et al., 1992) (Buse et al., 2020; Donohue et al., 2019a, 2019b; Flannery et al., 2006; Moore et al., 2006; Pierre et al., 2019; Schwake et al., 2016).

In situations where there is a diagnosis of Legionnaires’ disease, a residence or office building’s hot water supply is often assumed to be the source of transmission (Garrison et al., 2016; Stout et al., 1992a; Wadowsky et al., 1982). The hot water environment provides a unique opportunity where a microbe can survive and multiply. The elevated temperature simultaneously increases a microbe’s growth rate and decreases the amount of chemical disinfectant (e.g., chlorine or chloramine) in the water (Brazeau and Edwards, 2013; Cullom et al., 2020). The primary reason why a disinfectant is added to drinking water is to prevent microbial growth. Research by Jacangelo et al. (2002) and States et al. (1989); shows that the disinfectant type and its amount influence the likelihood of L. pneumophila’s detection and growth. Since not all disinfectants have the same efficacy in preventing microbial growth, it is important to identify the factors (physical and chemical) that are associated with the risk of microbial contamination.

The objective of this study was to measure L. pneumophila contamination (detection frequency and concentration) in the hot water plumbing lines of three structure types: residences, apartments, and office buildings. A longitudinal sampling approach was utilized to determine if a structure had a sporadic (a single positive sample) or persistent (multiple positive samples) occurrence pattern. Additionally, a structure’s age and area (sq. ft.) were assessed to determine if they were indicators of contamination. In addition to characterizing L. pneumophila incidence in the three structure types, the hot water’s disinfectant residual was measured to identify the concentration at which efficacy was lost. Also, L. pneumophila environmental monthly detection rates by structure type were compared to CDC’s legionellosis monthly case reporting data to determine if there are shared patterns. We hypothesize that a building’s structural parameters could be risk factors for L. pneumophila contamination and the spread of legionellosis.

2. Materials and methods

Study Design.

From January 2011 to October 2019, hot water samples from 100 taps in offices and residences were tested for L. pneumophila contamination. The offices and residences were in 31 states across the United States. The structures in this study were geographically dispersed, had no known L. pneumophila contamination, and were not associated with any legionellosis investigation. Only one tap at each structure was sampled at three independent time points within a year. There was an average three-month gap between sampling events. A total of 296 water samples were collected at 66 single-family homes (195 samples), four apartments (12 samples), and 30 office buildings (89 samples). Three single-family homes and one office building only had two sampling events due to the participant moving away from the structure. All sampling locations received public water. Secondary disinfectant usage was determined through the location’s water quality report.

Following a standard operating procedure (SOP), participants collected water samples from a hot water tap at their structure. Briefly, the procedure required the participant to flush the line for 15 s, then collect the water in a 1-L high-density polypropylene (HDPP) bottle (Nalgene Inc., Rochester, NY). The 15-s water flush ensured that the water collected was from behind the hot-cold water interface in cases where a single faucet delivered both hot and cold water. Additionally, the 15-s flush also models how a person may collect a glass of water. The US Environmental Protection Agency’s (EPA) drinking water guidelines are based on drinking water exposure modeling. Once the water sample was collected, it was packed in an insulated box with ice packs and returned overnight to the EPA laboratory. Upon arrival at the EPA laboratory, the water samples were immediately processed.

Residual Testing.

Upon the sample arrival, the water’s total chlorine concentration was measured colorimetrically using the N, N-diethyl-p-phenylenediamine (DPD) method (Hach Company, Loveland, CO). The manufacturer’s instructions were followed. The color reaction was measured using a DR 3900 spectrophotometer (Hach Company).

Structure Characteristic:

The year the structure was built and its square footage were obtained from publicly available property information databases. Square footage was used as a surrogate for building complexity and as an inference for the presence of a complex plumbing system (e.g., dead ends, pipe bends, and underutilized taps). The structure age was determined by subtracting the year of the last sample collection from the year of building construction.

Definitions.

The following are the terms and definitions used in this paper. The word “occurrence” or “detection frequency” refers to any structure with at least one positive sample for L. pneumophila. The term “sporadic” indicates that only one of the three sampling events was positive for L. pneumophila or L. pneumophila Sg1. “Persistence” refers to the repeated detection of L. pneumophila or L. pneumophila Sg1 in two or more water samples from the same tap at a residence, apartment, or office building. The term “structure” refers to residences, apartments, and office buildings. A “residence” is defined as a structure (single-family homes or apartments) where activities related to home life (e.g., showering, sleeping, gardening, cooking) occur, be it a single-family home (n = 66) or a unit in an apartment complex (n = 4). The term “office building” (n = 30) was defined as a business place where office/professional or service transactions were performed as determined using the International Building Code (IBC) criteria (Council, 2000).

Legionella pneumophila and L. pneumophila Sg1 qPCR assays qPCR sample filtration.

One liter was vacuum filtered through a sterilized Whatman^® Nucleopore^™ Track-Etched Membrane, 47 mm, 0.4 μm polycarbonate membrane (Cytiva, Marlborough, MA). After filtration, the polycarbonate membrane filter was aseptically inserted into a sterile 2 mL O-ring screw cap microcentrifuge tube containing 0.30 ± 0.05 g of 0.1 mm sterile glass beads (BioSpec Products, Bartlesville, OK). The study generated 296 samples, not including method blanks, extraction blanks, and positive and negative controls. The samples were stored at −80 °C until their evaluation for L. pneumophila and L. pneumophila Sg1 presence.

qPCR DNA extraction.

Details of the DNA extraction from membrane filters were previously published (Donohue et al., 2019b). Briefly, each polycarbonate membrane was mini-bead beaten in a bead beater (BioSpec Products, Bartlesville, OK) with 500 μL of Tissue and Cell Lysis Solution (Lucigen Corporation, Middleton, WI). After a 5 min cool down on the ice, the bottom of the tube was punctured with an 18 G needle (Becton, Dickinson and Company, Franklin Lakes, NJ). Next, the bead beating tube was inserted into a sterile 1.5 mL microcentrifuge tube (Eppendorf, Enfield, CT). The lysate was collected into the 1.5 mL microcentrifuge tube body by centrifugation at 3500 rpm for 5 min. Next, 2 μL of Proteinase K (50 μg/μL) (Lucigen Corporation) was added and followed by incubation at 65 °C in a water bath for 15 min. Next, 2 μL of RNase A (5 μg/μL) (Lucigen Corporation) was added to the mixture and incubated at 37 °C for 30 min. Subsequently, 350 μL of MPC Protein Precipitation Reagent (Lucigen Corporation) was added to precipitate the cellular proteins. The resulting supernatant was transferred to a sterile microcentrifuge tube containing an equal volume of ice-cold isopropanol (~−4 °C). The samples were inverted manually up to 40 times and centrifuged at 10,000×g for 10 min. The supernatant was poured off, and the resulting DNA pellets were washed with 500 μL of ice cold (~−4 °C) 70% ethanol. Samples were centrifuged, and the ethanol removed by pipet. The DNA pellet was air-dried for 15 min to remove residual ethanol. Then the DNA pellets were re-suspended in 50 μL of nuclease-free sterile water and stored at −80 °C until analyzed.

Assays and Conditions for qPCR.

Two primer-probe sets were used to detect and quantify L. pneumophila. One primer-probe set targeted the L. pneumophila 16S rRNA gene (Lp16S), and the other set targeted the L. pneumophila Sg1 wzm gene (LpSg1) (Donohue et al., 2014; Merault et al., 2011). All DNA extracts were analyzed using the Lp16S which targets the 16S rRNA gene. Any DNA extract positive for L. pneumophila Lp16S was also analyzed for the presence of L. pneumophila Sg1 using the LpSg1 primer-probe set. All primer-probes and qPCR conditions were previously published (Donohue et al., 2014). The Lp16S assay linearity (R²), amplification efficiency (AE), and Limit of Quantification (LOQ) are R² = 0.999, AE = 0.96%, and LOQ = 100 GU/Rx. The LpSg1 assay linearity, amplification efficiency, and LOQ are R²= 0.995, AE = 0.96%, and LOQ = 100 GU/Rx. More assay details, qPCR conditions, and controls were previously published (Donohue et al., 2019b). In addition, specific information on the limit of detection, the limit of quantification, and sensitivity was previously published (Donohue et al., 2014).

Preparation of qPCR Standard/Positive Control. A previously published method was used to prepare the DNA standards for the qPCR method described below (Donohue et al., 2014).

qPCR Controls.

Controls were included on each plate to ensure the integrity of the method and confidence in the results. Genomic DNA extracted from L. pneumophila Sg1 ATCC 33125^™ (American Type Culture Collection; Manassas, VA) was used as a positive control for each assay. A serial dilution of seven concentrations ranging from 10⁶ to a theoretical one genomic copy was made from this DNA. Negative controls included three non-template controls (NTC), where sterile water was used in place of the DNA extract (template).

Method blanks were prepared at the time of the water filtration. One hundred milliliters of sterile molecular grade water (5 Prime, Gaithersburg, MD) were vacuum filtered as described above, for every 10 samples filtered. If a method blank was positive, the set of samples corresponding with it was considered compromised, and the data discarded. Inhibition of the qPCR reaction was monitored using external controls for the L. pneumophilia and L. pneumophilia Sg1 assays. The external inhibition control was prepared as follows: all unknown samples were spiked with 1 μL of an exogenous control of 10,000 target gene copies extracted from L. pneumophila ATCC 33152^™. A reaction was considered inhibited if the observed C_q value of the external or internal controls drifted ≥1.5 C_q units from the standard C_q value.

Interpretation of qPCR.

An extract was considered L. pneumophila positive if both replicate’s quantification cycle (Cq) values were <39. As stated earlier, an L. pneumophila positive sample was also tested for the presence of L. pneumophila Sg1. For the L. pneumophila Sg1 assay, both replicate Cq values needed to be < 39 to be considered positive. The 5 μL DNA aliquot added to each qPCR reaction was a 100 mL concentrate of the original volume collected. Therefore, each sample’s DNA extract was analyzed in duplicate (2 × 100 mL or 200 mL of the original volume was tested). Performing the qPCR reaction in duplicate improves the assay’s precision and verifies a positive detection.

Statistical Analysis.

The Cq values were transformed using a standard curve into genomic units (GU). The average genomic unit per replicate was calculated for each sample with a Cq < 39. For positive taps, an average genomic target value was calculated. To determine if there was a statistically significant difference between detection frequencies between the different building types, the Fisher Exact test in SigmaPlot 14.0 (Systat Software Inc, San Jose, CA) was applied. The Chi-square (X²), Fisher Exact test, and McNemar’s tests were applied to the seasonal data analysis using SigmaPlot 14 (Systat Software Inc). The concentration data were checked for normality using the Shapiro-Wilk test. Mann-Whitney U test was used to determine statistical significance for the occurrence and persistence concentrations. A p-value of 0.05 or lower was considered significant.

3. Results

3.1. Water detection frequencies and concentrations by structure type

Water samples from the hot water plumbing were collected at 70 residences generating 207 samples, and at 30 office buildings generating 89 samples. Thirty-six percent (25/70) of residences had one or more water samples positive for L. pneumophila. Legionella pneumophila was detected in 53% (16/30) of the office buildings (Table 1).

Table 1.

Legionella pneumophila and L. pneumophila Sg1 detection rate, concentration, and percent positive by site.

Detection & Quantification	L. pneumophila			L. pneumophila Sg1
	Residences	Office Buildings	P-Value	Residences	Office Buildings	P-Value
	n (%)	n (%)		n (%)	n (%)
	70	30		70	30
Detection Frequency
No. of sites with one or more positive samples	25 (26)	16 (53)	Fisher Exact test: P = 0.123	5 (7)	6 (20)	Fisher Exact test: P = 0.082
No. of sites with no positive samples	45 (64)	14 (47)		65 (9)	24 (80)
Concentration
Median concentration GU/L	3.9 × 102	5.1 × 103	Mann-Whitney U test: P = 0.007	3.1 × 102	1.0 × 105	Mann-Whitney U test: P = 0.177
Average concentration GU/L	7.5 × 104	2.0 × 105		4.4 × 105	2.7 × 105

Since this study used a longitudinal sampling design, the duration of L. pneumophila contamination was examined. Duration of contamination (periodicity) was divided into two categories: sporadic (one of three sampling events positive per structure) and persistent (two or more sampling events positive per structure). Of the 25 residences positive for L. pneumophila, 15 residences had only one positive sampling event (sporadic), and ten residences had multiple sampling events positive (persistent). Additionally, of the 25 residences positive for L. pneumophila, L. pneumophila Sg1 was only detected in 5 residences as a sporadic, single positive sample. Among the 16 L. pneumophila positive office buildings, five office buildings had only one positive sample. There were 11 office buildings that had persistent L. pneumophila detections. These 11 office buildings were also persistently positive for L. pneumophila Sg1.

Fig. 1 shows L. pneumophila concentration by structure type and occurrence pattern (sporadic or persistent). Legionella pneumophila concentrations were significantly lower in residences than in office buildings, with the median value of 3.9 × 10² genomic unit (GU)/L versus 5.1 × 10³ GU/L, respectively (U test; P = 0.007) (Table 1).

Fig. 1.

Legionella pneumophila (Lp) and L. pneumophila Sg1 GU/L concentration by residences (single-family homes + apartments) and office buildings and by sporadic/persistent detections.

Legionella pneumophila Sg1 did not persist in the residential hot water environment. However, there were some infrequent positive hot water samples where L. pneumophila Sg1 concentrations exceeded 10⁵ GU/L. Without evidence of L. pneumophila Sg1 persisting in hot water, these infrequent detections could have come from the cold-water line or outside the residential setting. In contrast, an office building’s water samples were repeatedly L. pneumophila Sg1 positive, with higher concentrations of 1.0 × 10⁵ GU/L than residences 3.1 × 10² GU/L, U test: P =<0.177 (Table 1). An office building’s hot water infrastructure is more likely to harbor L. pneumophila Sg1 than a residence’s hot water plumbing.

3.2. Structure’s age and square footage

Fig. 2 shows L. pneumophila detection frequency in hot water by a structure’s age and square footage (area). In this analysis, apartments were separated from single-family homes due to their larger size (10,000 to 100,000 sq. ft.) and their potential use of multiple hot water tanks or a hot water recirculating system. Legionella pneumophila detection frequency in newer single-family homes and office buildings was lower than in the older structures (Fig. 2A). Regardless of the type of structure (house or building), a structure older than 20 years had more persistent L. pneumophila detections than newer construction (<20 years old), Fisher Exact test: P = 0.009.

Fig. 2.

Legionella pneumophila occurrence and persistence percentage by A) year built and by B) area (square footage) for each structure type.

The size of a structure influenced L. pneumophila detection frequency (Fig. 2B). Legionella pneumophila detection frequency increased as the structure’s area (size) increased. For example, the 1000 sq ft. single-family homes had a 27% (9/33) L. pneumophila detection frequency. In comparison, the 4000 sq ft. Homes had a 67% (4/6) detection frequency (Fisher-Exact test: P = 0.348). Interestingly, regardless of the single-family home’s size, L. pneumophila was detected persistently in 15% of homes (indicating colonization), Fig. 2B.

The size of the office building’s affected both L. pneumophila detection frequency and occurrence pattern. Legionella pneumophila detection frequency of the 10,000 sq. ft. or greater office buildings was 64% (11/17), with a 91% (10/11) likelihood of being colonized (persistent detections). In contrast, the L. pneumophila detection frequency of the 5000 sq. ft. or less office buildings was 43% (3/7), with only a 33% (1/3) likelihood of persistence. Interestingly residences and office buildings whose area was <5000 sq. ft. Have similar detection frequencies, 33% (21/63) and 43% (3/7), Fisher Exact test: P = 0.712. Even these structures’ L. pneumophila persistence rates are similar at 13% (8/63) and 14% (1/7), Fisher Exact test: P = 1.0.

3.3. Role of secondary disinfectant by structure type

Table 2 shows L. pneumophila and L. pneumophila Sg1 positive samples by structure type (single-family homes, apartments, and office buildings) and the water’s secondary disinfectant (chlorine or chloramine). Office buildings had the highest L. pneumophila detection frequency of 61% (11/18) among the structures with chlorine treated water. Apartments had the highest L. pneumophila detection frequency of 67% (2/3) among the structure with chloramine-treated water. Interestedly, L. pneumophila Sg1 detection frequency was similar across the structure types, and secondary disinfectants.

Table 2.

Legionella pneumophila and L. pneumophila Sg1 detection frequency by structure and disinfectant type. The bolded text was the highest detection frequency.

Structure Type	No. of Total Sites	Species/Serogroup	Chlorine n (%)	Chloramine n (%)
Single-Family	N		47	19
Home	66	L. pneumophila positive	16 (34)	7 (37)
		L. pneumophila Sg1 positive	2 (4)	1 (5)
Apartment	n		1	3
	4	L. pneumophila positive	0	2 (67)
		L. pneumophila Sg1 positive	0	2 (67)
Office	n		18	12
Building	30	L. pneumophila positive	11 (61)	5 (42)
		L. pneumophila Sg1 positive	4 (22)	2 (17)

It is well established that a relationship exists between L. pneumophila concentration and secondary disinfectant residual concentration. Fig. 3 shows each structure type with positive L. pneumophila sample concentration by secondary disinfectant residual concentration. In the single-family home setting (Fig. 3A), L. pneumophila concentrations were statistically similar for chlorine and chloramine treated water. However, as expected, the total chlorine concentrations were statistically different. Legionella pneumophila median concentration (densities) for chlorine and chloramine were 3.6 × 10² GU/L and 4.5 × 10² GU/L respectively, U test: P = 0.710; chlorine and chloramine residual median concentrations were 0 mg Cl₂/L and 1.7 mg Cl₂/L respectively; U test: P = <0.001.

Fig. 3.

Shows Legionella pneumophila concentration by total chlorine concentration for chlorine-and chloramine-treated water. A.) residences B.) office buildings. The table under the scatterplot is the number of samples in a specific total chlorine range (e.g., 0–0.2 mg Cl₂/L, 0.2–1.0 mg Cl₂/L, 1–2 mg Cl₂/L etc.). The black dotted line at 0.2 mg Cl₂/L is a reference point for the minimum residual amount at entry point into distribution. The vertical bar is the total chlorine median-mean range for a particular structure type (e.g., the pink bar is the total chlorine median-mean range detected in single-family homes with chlorinated water, the purple bar represents the single-family homes with chloraminated water, and the green bar represents apartments with chloraminated water).

In the office building setting (Fig. 3B), L. pneumophila positive samples in chlorine and chloraminated treated water were statistically different relative to concentration and disinfectant residual concentrations. L. pneumophila median concentrations were 4.9 × 10⁴ GU/L (chlorine) and 4.3 × 10² GU/L (chloramine), respectively; U test: P = 0.006. The office building’s total chlorine median values were 0.0 mg Cl₂/L (chlorine) and 0.7 mg Cl₂/L (chloramine), respectively, U test; P = 0.009. In the office building setting, the disinfectant concentrations were lower than concentrations measured in single-family homes, especially the chloramine treated water, t-test (Welch’s); P = 0.027. The lower total chlorine concentrations suggest that other factors, such as water age, and biofilm, possibly affect the L. pneumophila population count and the disinfectant decay.

3.4. Seasonal rates

Fig. 4 shows L. pneumophila and L. pneumophila Sg1 detection rates by the month of sampling for residences and office buildings. Monthly differences were observed in the frequency of L. pneumophila and L. pneumophila Sg1 detections (Fig. 4). In residences, water samples collected in August and November had the highest L. pneumophila detection frequency, 27% (4/15) and 28% (7/25), respectively (Fig. 4A). The L. pneumophila Sg1’s highest detection frequency in residences was August, 13% (2/15) (Fig. 4C). In office buildings L. pneumophila highest detection frequency of 62% (5/8) were in samples collected in June (Fig. 4B). Legionella pneumophila Sg1’s highest detection frequency of 40% (2/5) was in May (Fig. 4D).

Fig. 4.

Radial graphs show L. pneumophila and L. pneumophila Sg1 detection frequency by month for residences and office buildings. A) L. pneumophila-residential, B) L. pneumophila-office buildings, C.) L. pneumophila Sg1-residential, and D) L. pneumophila Sg1-office buildings.

Fig. 4 also demonstrates that there are seasons of higher L. pneumophila and L. pneumophila Sg1 detection frequencies than at other times of the year. In residences, Summer-Fall months (June–November) were significantly greater for L. pneumophila detections frequency of 23% (29/125) than Winter-Spring months (December–May) 12% (9/82), X²: P = 0.042 or McNemar’s Test: P = <0.001. In office buildings, L. pneumophila’s highest detection frequency of 45% (21/47) was in the Spring-Summer months (March–August) compared to Fall-Winter months (September–Feb) 24% (10/42), Fisher Exact test: P = 0.046 or McNemar’s Test: P = 0.170 (Fig. 4B).

Fig. 5 shows each month’s positive samples’ mean (dark purple) and median (green) concentrations. Depending on the month, L. pneumophila concentrations span a wide range of values, as observed in June–November for residences and February–August for office buildings. The mean and median concentration range did not change (zero to less than a 0.5 log difference) in January–June for residences and September–February for office buildings. These results indicate that time of year and the structure characteristic have different risk profiles. A monthly concentration analysis for L. pneumophila Sg1 was not performed because the dataset (n = 17) was too small to support statistical analysis.

Fig. 5.

Legionella pneumophila mean and median concentrations by month A) residences and B) office buildings.

3.5. Legionella pneumophila monthly occurrence (environmental) and monthly legionellosis cases (disease) comparison by structure type

Radial graphs in Fig. 4 show that each structure type experiences a different L. pneumophila seasonal pattern. These patterns were reminiscent of previously published monthly seasonal distribution of Legionnaires’ disease cases (Barskey et al., 2022; Hicks et al., 2012). Fig. 6 compares the L. pneumophila monthly environmental occurrence pattern for residences (the orange line) to the CDC’s NNDSS monthly percentage of legionellosis cases (the purple bars) for residences (Barskey et al., 2020; Hicks et al., 2012; Neil and Berkelman, 2008).

Fig. 6.

Compares this study’s Legionella pneumophila monthly detection frequency (orange line) to the average percentage of legionellosis cases by month (purple bars). A) Legionella pneumophila residential monthly detection frequency compared to Hick et al. 2011 average percentage of legionellosis cases occurring annually by month and B.) compares L. pneumophila office buildings monthly detection frequency to CDC’s NORs 2009–2017 (month of first reported legionellosis case) average percentage of legionellosis cases by month. The density of the purple bar color indicates legionellosis case percentages: light purple is <7% of cases, median purple is 7–10% of cases, and dark purple is >10% of legionellosis cases.

The similarity between the residential L. pneumophila detection frequency and legionellosis case reporting trends was noticeable in Fig. 6A. It suggests that most legionellosis cases potentially result from exposure via the residential setting. However, the legionellosis case rate and environmental occurrence comparison have limitations. The data source for the legionellosis case rates was an imperfect match with those for the residential settings. The data source that Neil and Berkelman (2008), Hicks et al. (2012), and Barskey et al. (2020) used included legionellosis cases from both “sporadic-community acquired,” or travel-associated, and outbreak situations. Only 4% of their legionellosis cases were related to outbreaks, and 15–16% of cases were associated with travel exposure. Thus, it was fair to assume that the seasonal pattern reflected in the papers cited was a good representation of sporadic community-acquired legionellosis cases. Additionally, the years are not perfect matches. The reported legionellosis cases distribution was 2000–2009, and the environmental sampling timeframe was from 2011 to 2018. Therefore, the environmental divergence observed in April and October–November could be due to case pattern changes not observed in Hicks et al. (2012), 2000 to 2009 analysis. Nevertheless, despite the time frame differences, the overall patterns are comparable between environmental occurrence and clinical case reporting.

Additionally, we hypothesized that L. pneumophila occurrence trends in office buildings might have a relationship with outbreak-associated legionellosis cases. Legionellosis case data (first month) was extracted from the CDC’s National Outbreak Reporting System (NORS) database- from 2009 to 2017; 147 water-related legionellosis outbreaks were reported, generating 635 confirmed legionellosis cases. Using the first month of a NORS’s reported legionellosis case, an annual monthly percentage was calculated, Fig. 6B purple bars. The number of legionellosis cases associated with an outbreak was approximately 2–6 cases per outbreak. Approximately 18 water-related legionellosis outbreaks occur annually in the US. (CDC, 2020).

Fig. 6B compares this study’s office building monthly L. pneumophila detection frequency to the NORS monthly percentage of legionellosis cases (first month). The outbreak data and office building monthly detection frequency data do not appear to be associated (Fig. 6B). The office building and NORS monthly detection frequency comparison in Fig. 6B have limitations. This study focused on the hot water in office buildings, whereas the NORS dataset includes water-related outbreaks from water fountains, cooling towers, and a building’s potable water. The NORS dataset includes more water sources than just portable water from buildings.

4. Discussion

The legionellosis disease rate is increasing in the United States. Therefore, identifying at-risk locations where L. pneumophila control and prevention can be implemented is important. Identifying the structural characteristics (e.g., age, size, and disinfectant residual) that favor L. pneumophila contamination could help support the selection of strategies that could reduce contamination risk. Although the data set for this study is small, (66 single-family homes, 4 apartment complexes, and 30 office buildings), it includes structures that represent a variety of ages, sizes, and types at locations distributed across the continental United States. Thus, the data generated in this study can support hypothesis development, identify characteristics that favor the survival of L. pneumophila at locations where people live and work, and develop strategies to reduce colonization risk.

Legionella pneumophila in Residences.

When this project was initiated, the residential setting was hypothesized to be where most community-acquired legionellosis cases occur. Approximately 96% of reported legionellosis cases were considered community-acquired, involving situations where a single individual contracted the disease (Garrison et al., 2016). Unfortunately, these solitary cases rarely trigger a field investigation where the setting and source of L. pneumophila contamination are identified. Schumacher et al. (2020) provide a rare insight into community-acquired legionellosis cases. In their paper, a L. pneumophila Sg3 incident was initially thought to be connected to a Healthcare-Associated Infection (HAI). However, the field investigation revealed that the patients contracted Legionnaires Disease within their home from exposure to the bathroom shower or kitchen sink sprayer. A concentration of 0.05–2 CFU/mL was measured at these locations. Samples collected from all other potential exposure sites within the home and the associated healthcare centers were negative. Schumacher’s study suggested that other L. pneumophila serogroups can cause Legionnaires Disease. The likelihood of other L. pneumophila serogroups in the residential setting is supported by this study and studies by Byrne et al. (2018); Collins et al. (2017); Lim et al. (2003). These studies show a higher recovery of L. pneumophila serogroups Sg3, Sg4, and Sg6 versus Sg1 in residential potable water.

Residential hot water plumbing in single-family homes is a much simpler design when compared to that in large office buildings. The water flow in the single-family home is unidirectional, although backflow events may occasionally occur. A water heater is used to raise the water temperature, and pipes of various materials take the water to areas of demand. A residence’s smaller size contains less total pipe length, fewer risers, and dead ends than a multifunctional building complex. The homeowner’s most important water maintenance task is replacing the hot water heater every 6 to 12 years—a far lower maintenance burden than what is required by an apartment or office building manager. Yet, despite a single-family home’s design simplicity, older homes are prone to persistent L. pneumophila contamination.

Legionella pneumophila contamination risk in older residences suggests that other age-related risk factors such as biofilm development and pipe corrosion might be involved (Cullom et al., 2020; Lau and Ashbolt, 2009). Additionally, the age of the hot water heater was not asked of the participant. Therefore, a lack of routine home maintenance cannot be excluded as a contributor to risk. The Collins et al. (2017) shower study in the United Kingdom observed fewer Legionella positive samples in newer homes than in buildings 30 years old. They also reported that most L. pneumophila occurrences in houses were sporadic, with little evidence of plumbing colonization. Therefore, one can conclude that residences experienced transient pockets of contamination originating upstream of the hot-water system, such as a biofilm sloughing event. Both Cohn et al. (2015); Collins et al. (2017) observed circumstantial evidence that Legionella spp. Contamination can originate upstream of the hot-water supply.

The present study is one of the first to report on L. pneumophila residual concentrations within residential structures’ hot water environment. An important insight observed in the residential setting was that, regardless of disinfectant type (chlorine or chloramine), L. pneumophila concentration was statistically the same. This observation indicates that irrespective of the secondary disinfectant type, there is still the possibility for L. pneumophila exposure and the potential for disease transmission in a residential setting. This finding is rational when combined with other data such as legionellosis cases and outbreaks reported in areas where the water’s secondary disinfectant is chloramine (Holsinger et al., 2022; Rhoads et al., 2020; Tucker et al., 2018). More research is needed to explore further whether there is a relationship between residual disinfectant and L. pneumophila occurrence.

4.1. Legionella pneumophila in buildings

Buildings, especially those with many levels, are at risk for L. pneumophila contamination. This study adds to the body of evidence established by Buse et al. (2020); Donohue et al. (2019a); Flannery et al. (2006); (Garrison et al., 2016); Moore et al. (2006); Pierre et al. (2019), that report higher Legionella detections in buildings than single-family residences. For instance, Garrison et al.’s (2016) analysis of 25 legionellosis field investigations revealed that 93% (25/27) of outbreaks occurred in a “building setting,” and 60% (15/25) of these outbreaks were due to deficiencies observed in the building’s potable water supply network.

The present study indicates that older and newly built office buildings had an equal probability of L. pneumophila contamination. Several newspaper and journal articles by Stout et al. (2000), Kool et al. (1999), and Francois Watkins et al. (2017) linked a legionellosis case to new construction, specifically for healthcare buildings. These instances occurred when the building’s plumbing was being tested months earlier before occupancy, and the water was left to stagnate before use.

A building’s area (sq. ft.) appeared to influence L. pneumophila occurrence, especially L. pneumophila Sg1 occurrence. Buildings >100,000 sq. ft. (office buildings and apartments) had statistically significant higher L. pneumophila occurrence (X²: P = 0.016) and L. pneumophila Sg1 persistence (X²: P = <0.001) than offices < 100,000 sq. ft. This information is highly relevant for the United States, since there are approximately 32.6 million multifamily housing units and commercial buildings (US Census Bureau, 2012, 2020).

In studies by Flannery et al. (2006); Kool et al. (1999); Moore et al. (2006), chloramine was found to be an effective tool for removing L. pneumophila and Legionella spp. in buildings. However, recent research utilizing molecular techniques has shown that chloramine might only be suppressing L. pneumophila growth while promoting the growth of other Legionella spp. and opportunist pathogens (Donohue, 2021). Also, the Baron et al. (2015) case study on a chloramine treatment unit showed Legionella spp. growth during a shutdown period of service. As data continues to be published, it will be interesting to see how chloramine is evaluated as an overall disinfectant that protects against water-borne pathogens and microbial growth.

Over the last few decades, an office building’s water age (i.e., the time the water spends in the building’s plumbing before being used) has significantly increased. In addition, commercial buildings have become larger and multifunctional, requiring a more complex plumbing system. Increased water age was potentially an unintended consequence of the US 1992 Energy Act designed to identify ways to reduce energy consumption (EPACT92, 1992). Over the last twenty years, efforts to reduce energy consumption have accelerated. Smaller appliances and fixtures, low-flow and automatic faucets, and lower hot-water temperatures have impacted water use, potentially minimizing water flow and increasing water age. Research by Brazeau and Edwards (2013); Nguyen et al. (2012) demonstrate that 2–8 h water stagnation time could result in a 3–4 mg/L chlorine loss or 1.5–3.5 mg/L chloramine loss, dependent on the pipe material.

Water age can reduce water quality. Water quality degrades as the disinfectant residual decreases, potentially allowing for microbial colonization, biofilm development, and pipe corrosion products, e.g., rust, and iron oxides, to develop (Cullom et al., 2020; Rogers et al., 1994a, b). These physio-chemical and biological shifts in water quality could increase the survival and persistence of microbes and water-borne pathogens, such as Legionella, in the potable water supply (Buse et al., 2012; Cullom et al., 2020).

5. Conclusion

This study researched L. pneumophila occurrence and persistence in residences and offices to better understand the potential risk for microbial contamination and disease transmission associated with the building’s structural characteristics. Although office buildings may be more prone to L. pneumophila contamination, they also have a smaller potential for disease transmission because fewer activities take place in office buildings whereby the occupants are exposed to water aerosols. The exception is healthcare and residential care centers, hotels, and multi-unit apartment complexes. This study suggests that Legionella-realated plumbing system remediation efforts in office buildings (that contain commercial functions only) might not significantly improve a community’s public health.

Residential structures were not prone to L. pneumophila colonization, but they do have a much larger window for potential disease transmission. This was because many human activities are performed at higher frequencies and for longer durations in places where water aerosols are generated and inhaled by the occupants. This study revealed that residential plumbing could be a potential “conduit” for L. pneumophila exposure from a source upstream of the hot water environment. Thus, a residence may be the setting for a legionellosis case, even when it was not the source of the contamination.

Legionella pneumophila and other Legionella subspecies are environmental microbes that adapt and change in response to their environment. This study revealed that the built environment influences L. pneumophila transport and fate. More work is needed to evaluate if the trends and observations in this study are supported by future research with larger samples size and longitudinal analyses.