4.1. Legionella Risk Post Hurricanes
Legionella risk in captured rainwater or recycled water has been discussed in the previous studies [
2,
20,
31]. Yet not much is known of the impact of the hurricanes on rain cistern water quality and associated microbial infection risk. High occurrence of
Legionella spp. discovered in rain cistern water on St. Thomas, VI, post hurricane season promoted the investigation of infection risk of this ubiquitous pathogen. The QMRA results indicated that
Legionella risks post hurricanes were not significantly higher in comparison with two previous studies estimating the risks in captured rainwater (
Table 2). The median annual risk of the current study is slightly higher than the value reported by Ahmed et al. (2010) [
32] but is an order of magnitude lower than the risk estimated by Hamilton et al. (2017) [
33]. At first glance, the comparative results suggest that hurricanes do not appear to increase the
Legionella risk through shower water in the disaster-stricken region. However, discussions are warranted to better understand the contribution of the current study to the knowledge field and the limitation of the risk estimation.
The
Legionella concentrations in cistern water were estimated based on the fraction of
Legionella spp. identified among the total bacterial community by NextGen sequencing of 16S rRNA gene [
9]. The underlying assumption used in this study is that the fraction of
Legionella spp. among the total microbial community in cistern water is the same regardless of assay methods (genome-based versus culture-based). Therefore, combining the
%Leg with
Cbac (CFU/L) in cisterns could estimate the concentration of viable
Legionella spp. in the water. However, a direct comparison between the sequencing-based approach (#
Legionella spp. gene copies/# total gene copies) and the culture-based methods (#
Legionella spp. CFU/# total CFU) has not been performed. No relationship between HPC counts and
Legionella concentration or prevalence currently exists. Therefore, the assumption used here is an important limitation to the estimation of viable
Legionella spp. in the water. Past research in estimating the risk of
Legionella relied on PCR-based approaches to quantify
Legionella-specific genes, which assumed the genetic fragments of
Legionella equal to infectious
Legionella. Therefore, a PCR-based approach is also not a perfect solution to identify infectious
Legionella. Despite the limitations of 16S rRNA gene based NextGen sequencing, they are powerful data to understand the microbial composition in water. The HPC baseline data served to model a range of probabilistic values of
Legionella. High HPC counts can result from cistern stagnation, lack of disinfection, or inadequate temperatures, which can also lead to
Legionella growth [
6]. The HPC values, therefore, reflect the condition of the cisterns, which bounds the range of
Legionella. The HPC data were also used to correct for overestimation of infectious
Legionella by the genetic method [
2,
20,
34].
Significant progress has been made in recent years on improving the method for culture detection of
Legionella in drinking water and the water distribution network [
35,
36,
37]. The
Legionella monitoring methods by ISO 11731 and the U.S. CDC also provide a reliable framework for culturing
Legionella in drinking water [
38,
39]. However, these methods are still labor intensive and time consuming in comparison with genetic-based methods, which limit their implementation in a field study in the absence of a functional microbiology lab during the post-hurricane period. Nevertheless, culture-based methods should be considered whenever possible. Future studies should also carry out side-by-side investigations of culture-based vs. genetic-based
Legionella detection in drinking water under various conditions. The outcomes of these comparisons will improve our understanding on the limitations of genetic-based methods and develop credible correlations between the two different monitoring approaches to improve future risk estimations.
A comparison of the range of
Legionella concentrations from this study with those from other related studies is shown in
Table 2. Hamilton et al. (2016) reported
Legionella concentrations between 3 × 10
3 and 3.1 × 10
6 gene copies/L by qPCR based on 134 roof-top harvested rainwater samples collected in Southeast Queensland, Australia. Ahmed et al., (2014) [
2] reported
Legionella concentrations by qPCR to be between 1.6 × 10
4 and 1.0 × 10
5 copies/L, with a median concentration of 8.8 × 10
3 copies/L in 72 rainwater tank samples also from the same region in Australia. The simulated concentrations in this study ranged between 2.5 × 10
2 and 1.2 × 10
5 CFU/L, which are roughly one order of magnitude lower than qPCR results by Hamilton et al. (2016) [
20]. The high end-values in this study are similar to the report of Ahmed et al. (2014) [
2]. However, those previous studies reported concentrations as gene copies/L, whereas in this study the concentrations are CFU/L to represent viable counts. In drawing comparisons between the
Legionella concentrations, it should be noted that qPCR results may overestimate the viable
Legionella. Previous studies concluded that qPCR is useful for rapid detection and risk assessment, but often detect higher amounts than by culture methods, especially from water tanks which have been disinfected [
34,
35]. On the other hand, Hamilton et al., (2017) [
33] noted in a study on seasonality and RHRW premise plumbing pathogens that culture-based methods can underestimate concentrations due to the presence of viable but nonculturable (VBNC) cells. These distinctions are important in interpreting risk results as conservative or liberal estimates. Nevertheless, in the absence of a better method to estimate the infectious
Legionella, the genome-based approach in combination with the culture-based assessment of total HPC presents a useful method to estimate the viable
Legionella for risk quantification.
Comparison of the annual risks for combined warm and cold shower scenarios with those by Hamilton et al. (2016) [
20] and Ahmed et al. (2014) [
2] revealed a much wider range of estimated risks than those in the previous studies. This is due to a very large variability of the viable HPC detected in different cisterns. This variability could be attributed to the unevenness in cistern maintenance on the island. Alternative, the SMEWW 9215C method for HPC could also generate viable results because it uses a very small volume of water (0.1 mL) that could hit or miss of particle bound bacteria. The fractions of
Legionella detected also varied among cisterns. Some of the cisterns may be impaired by the hurricane-induced storms as noted in the study by Jiang et al. (2020) [
9]. During the sample collection effort, we covered as a broad of an area on the island as we could, but we did not have a pre-existing knowledge of income level of the households at the time of sample collection. Future study design should consider water quality assessment across different income levels. It should also be noted that both previous studies [
2,
20] equated qPCR genome copies with the viable CFU in the dose–response model, which may overestimate the risk of infectious
Legionella.Moreover, the uncertainty of the dose–response model is not only limited to ambiguity of the infectious Legionella concentration. Both this and previous studies adopted a dose–response model developed using guinea pigs rather than humans. Human infectivity requires clinical trials of exposing humans to Legionella, which are highly unlikely due to the ethical concerns. Research and data on Legionella infectivity in humans would be useful to further improve the risk assessment. Detailed epidemiology investigations of human exposure to contaminated water and health outcomes could be useful data to refit the human dose–response model.
The results of this study suggest investigations of HPC in cisterns can significantly further the understanding of the water quality and
Legionella risk since HPC can reflect the condition of cisterns. The HPC is relatively simple to perform, but an identical HPC method should be used for comparison across seasonal and spatial samples. Moreover, identification of
Legionella to the species level could improve our understanding of the pathogenic vs. non-pathogenic species in water. Additionally, our model does not account for additional
Legionella that may be growing in the plumbing and showerhead due to biofilm release or the presence of amoeba [
36]. There have also been reported differences in
Legionella concentrations between the cistern and the in-home faucet due to fluctuations in water age and in uncertain chlorine residuals from chlorinated cisterns, causing pipes to act as
Legionella reservoirs [
31]. The growth in the plumbing and showerhead is especially important in stagnant water when the home is abandoned during a time of disaster. However, this situation was not applicable to this study. All households sampled during this study were occupied during the hurricane season because evacuation from an isolated island far from the mainland was more challenging.
It is unclear whether the hurricanes exacerbated the Legionella risk due to the lack of historical data on the cistern water quality for the Virgin Islands. The impact of the hurricanes on the water safety in the Virgin Islands may be reflected through the lack of access to chlorine or other disinfection methods after the disaster struck. Regardless of the source of the Legionella, the outcomes of this risk analysis suggest the need of a routine water quality monitoring and maintenance program to reduce the risk of Legionella. Since rain cisterns are considered private property, a public education program should be put in place to promote the self-monitoring and routine cleaning of the cisterns.
4.2. Risk Perception and Risk Management
Although the majority of islanders perceive their water to be safe or somewhat safe for household uses based on the on-site surveys, the QMRA results indicate otherwise. The median annual risk values for both warm and cold showers exceed the EPA recommended threshold of 10−4 pppy (per person per year). The perceived water safety may be related to water use patterns because over 90% residents answered that they used bottled water for drinking. Washing water is considered “less risky”, and the aerosol transmission of pathogen through shower mist is not well known. We found that the perceived risk was divided by income levels; twice as many high-income participants deemed their water “safe”, whereas most low-income participants answered only “somewhat safe.” These low-income families may not have access to treatment methods such as chlorine, UV light, or filtration to disinfect their cistern water when electricity is compromised, which was the case during and after the hurricanes. They live in older, poorly maintained housing communities with aging water infrastructure and lack of economic resources to perform routine upkeep of the cisterns. They rely more on the cistern water that is free of charge, especially in times of crisis.
The large discrepancy between the risks estimated based on the QMRA and the perception of adequate water quality suggests that the prevalence of Legionella in cistern water and its risks are not always apparent. Public education and routine monitoring programs are necessary for public health protection. HPC monitoring could be a simple solution for reducing the risk of Legionella.
Temperatures in the range of 20 °C (68 °F) to 45 °C (113 °F) favor the growth of
Legionella. Therefore, finding
Legionella in the cisterns on the Virgin Islands, where the temperature is around 24 °C year-round, is not surprising.
Legionella can be inactivated when temperatures rise above 50 °C [
37]. Heating water to 60 °C is effective at reducing
Legionella in shower water. However, thermal inactivation is only effective on the heated portion of the water, while
Legionella may still be present in the cold-water portion mixed to achieve a desired final shower water temperature. In fact, our results showed the warm shower risk was higher than the cold-water risk. This is because, as shown in
Table 1, a hotter shower produces more aerosols per minute in the shower stall resulting in a higher concentration of aerosols within the shower stall. A shower temperature that is reasonably hot and much higher than room temperature causes a chimney effect in the shower stall, in which aerosols are carried upward with convective flow [
25]. A higher aerosolization rate therefore results in more aerosols inhaled for the duration of the shower. The increase in aerosolization between warm- and cold-water showers is up to a factor of 100 and is directly proportional to the increase in the dose for each shower event. Although the high temperature provided by a conventional water heater is adequate for heat inactivation of the bacterium, it is not enough to reduce the risk. The amount of hot water needed from the water heater to mix with ambient temperature water is low since the ambient water temperature in the Virgin Islands is relatively warm due to the warm, tropical climate. The amount of inactivation for a reduction in the concentration of
Legionella is small in comparison to the increase in aerosolization.
To better mitigate risk associated with RHRW household use, routine cleaning of cisterns and flushing of premise plumbing should be planned on a fixed schedule to reduce the opportunistic pathogens in shower water. In anticipation of an increase in Legionella prevalence in the rainwater by hurricane-induced storms, infrastructure damage (i.e., connection of underground cisterns with surface floodwater), and loss of power, stocking up on chlorine tablets before hurricane season and organizing quick transport and distribution of chlorine tablets immediately after the hurricanes to the disaster area could be helpful to reduce the waterborne and water-related illness. Other water treatment methods, such as UV and reverse osmosis membrane filtration, are less effective in the case of loss of power from electricity grids. Other types of fuel (no natural gas, gasoline is in significant shortage) are hard to access on the islands during and after the hurricanes.
Public education is an important tool to enhance the awareness of Legionella risks. Currently, there is no routine monitoring program, cistern water quality standard, nor uniformed recommendations for cistern management on the islands. Cisterns are considered private property; the governmental “interference” on the cistern water was not embraced by the local residents. There is a general mistrust of governmental agencies in advising of water quality and water use. Such mistrust and dissatisfaction among the public was reflected in our survey results, regardless of the household income levels. Public outreach programs using the research outcomes from objective QMRA could instill trust in the local residents about the governmental role in cistern water management. Development of transparent public policy with sufficient time for residents’ input and buy-ins are necessary to improve the relationship between the government and the citizens. This trust is critical to improve cistern water quality through monitoring, routine cleaning, and addressing technological treatment requirements. Cistern water quality management is above and beyond the hurricane seasons.