uted to various factors including the presence of inhibitors that could interfere with sample processing and thereby limit the recovery of Legionella isolates in environmental samples. The elevated level of iron detected in cool- ing tower E could have played a role in con- suming the oxidizing biocide residual, pro- moting growth, and interfering possibly with methods of detecting Legionella species dur- ing routine sampling and analysis. Legionella spp. are iron-dependent, and the growth of the organism is promoted with high levels of iron (Vittal et al., 2021). In addition to being a nutritional requirement for Legionella , excessive iron (due to corrosion or other factors) can remove free chlorine (Martin et al., 2020). This iron interference might have resulted in erroneous Legionella monitoring data and thus reports of compliance to the New York State cooling tower registry. In 2019, NYSDOH sta£ directed the facility to clean and disinfect the tower per Cooling Tower Subpart 4-1, Appendix 4-A (NYSDOH, 2023). Cooling tower E underwent increased monitoring, surveillance, and remediation for approximately 5 months (August 2019–Janu- ary 2020). The facility was also instructed to collect weekly samples for heterotrophic plate count (HPC) using their current dip slide method and to conduct weekly testing for Legionella during the time the site was being investigated. Monitoring the changes in such parameters as often as possible is essen- tial for ensuring treatment and remediations are working correctly and that public health is no longer at risk. The results of the wind direction analysis were inconclusive, as there was significant variability in the percentage of time that optimal wind direction was achieved during the estimated exposure periods for the four genetically similar 2019 cases (Figure 2). Three out of four of these LD cases reported activity that could have included walking outdoors. Interview data related to time-spe- cific activity was not made available. Under- standing of the transmission and persistence of LD in communities would benefit from more extensive travel history data from LD cases and continued evaluation of the influ- ence of wind direction on drift travel from cooling towers. This type of analysis should be expanded to account for the influence of wind speed and seasonal patterns on cooling tower drift travel.
TABLE 6
Hourly Optimal Wind Direction Frequency for 2019 Legionnaires’ Disease Cases With Whole Genome Sequencing Links
Case
Date Range
% Optimal Wind Direction
1 2 3 4
7/25/2019 to 8/7/2019 8/3/2019 to 8/16/2019 8/4/2019 to 8/17/2019 9/7/2019 to 9/20/2019
21.15 15.06 19.55
9.29
Note. The optimal wind direction percentage was calculated based on the percentage of hours with optimal wind direction during the estimated exposure period of 12 days prior to reported symptom onset. Models were based on data collected from the nearest New York State Mesonet weather station. Frequencies of optimal wind direction were calculated using Python 3.10, and figures were generated using Microsoft Excel.
Legionella species. Further, two out of three samples retaken from the cooling towers of the industrial heating plant were negative for any Legionella species. Cooling tower E was posi- tive for Lp5 (30 CFU/ml) in 2021 (Table 5), even though the tower had been disinfected by the owner. In July 2021, three samples were initially collected from the private hot tub visited by a 2021 LD patient case. A hot tub water sample was positive for Lp1 (5 CFU/ml; Table 5). A swab of the hot tub fil- ter was positive for Legionella species (3,450 CFU/ml; Table 5). Lp1 isolates recovered from the hot tub and clinical specimens from the LD patient case who visited the hot tub were also found to be genetically similar (3–7 single nucleotide polymorphisms [SNPs]) to the previously sequenced isolates from clinical samples and samples collected from cooling tower E in 2019 via whole genome sequencing (Figure 1). Two samples collected after remediation in August 2021 had nonde- tectable levels for Legionella. Wind Direction Analysis Wind direction analysis revealed that dur- ing the estimated exposure periods for case 1, case 2, case 3, and case 4 in 2019, the percentage of time that wind direction was optimal for potential exposure to aerosols from the drift of cooling tower E ranged from 9.29% to 21.15% (Table 6). Discussion Due to the practical diculties in collecting and successfully culturing sputum samples, it is often impossible to positively match an
environmental source of Legionella with LD patient cases. Sputum was collected from 14 of 25 potential cases, and Lp1 was successfully cultured from 8 out of 14 cases. A genetic link was observed between sputum samples from 5 of 8 of the positive sputum samples, indicat- ing that the outbreak cases were most likely exposed to a similar environmental source. Further investigation showed that Lp1 iso- lates recovered from four of the cases identi- fied in 2019 were genetically linked to isolates recovered from cooling tower E, suggesting that the cooling tower was the likely source of exposure for the outbreak. Additional genetic similarities between isolates recovered from clinical samples and hot tub samples in 2021 and cooling tower E further support the link between the cooling tower and the outbreak. Legionella species were not cultured in any of the LD case residences. Although Lp1 was cultured from the trickling filter discharge at the wastewater treatment plant investigated, there was a lack of treatment plant-associated colony isolates that could be matched to the clinical isolates. Therefore, the treatment plant was determined to not be a significant point of exposure. A shortcoming of this investigation is that only 5 of the 17 identified cases associated with this outbreak could be traced via whole genome sequencing to any of the environ- mental sources tested. Due to the diculty in obtaining and culturing clinical samples, it is uncommon to match clinical and envi- ronmental samples during routine investiga- tions, so these findings are significant. The lack of genetic matches could also be attrib-
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