BACKGROUND: Minnesota Department of Health (MDH) epidemiologists interview all Legionnaires’ disease (LD) cases/proxies about potential exposures; approximately 50 cases were reported annually in recent years. On 9/7/16, MDH noted 4 recent cases either lived or worked within a 1.7 mile area in the city of Hopkins, but no single common exposure was evident. Past surveillance data revealed no LD cases had reported living or working in Hopkins over the prior 4 years.
METHODS: On 9/9 MDH launched an investigation and alerted health care providers and the public. Outbreak cases met the CSTE case definition for confirmed LD, had symptom onset since 8/1, and lived, worked, or spent time in Hopkins within 10 days before onset. MDH and Hennepin County evaluated potential sources of aerosolized water. Samples from cases and from 14 cooling towers (CTs) within 1 mile of the geographic mean center of cases were tested by PCR, Legionella culture and serotyping, pulsed-field gel electrophoresis (PFGE), and whole genome sequencing (WGS).
RESULTS: A total of 24 cases (onset dates from 8/4 to 9/22) were identified. Fourteen (58%) lived in Hopkins, 7 (29%) worked in Hopkins, and 3 (13%) visited Hopkins. Median age was 59 years (range, 29-97), 17 (71%) were male, 18 (75%) were hospitalized, 6 (25%) required mechanical ventilation, and 1 (4%) died. All cases were laboratory-confirmed by urine antigen; 4 cases had Legionella pneumophila serogroup 1 (Lp1) isolated from respiratory specimens. All case isolates were indistinguishable by PFGE and WGS. The lack of a single common exposure location among cases suggested potential aerosolization from a CT. CTs are not regulated in Minnesota and there was no centralized information on CT locations. Of 14 CTs tested, one at an industrial plant in Hopkins yielded Lp1 isolates indistinguishable from case isolates by PFGE and WGS. The plant had two CTs with automated biocide delivery based on demand. The implicated CT operated during periods of elevated demand, and biocide was injected less consistently in that CT.
CONCLUSIONS: A community outbreak of LD was associated with an industrial CT using an automated biocide delivery system; low periodic demand for the CT may have contributed to intermittent biocide, stagnation, and Legionella amplification. CT identification was challenging. Molecular comparison of case and environmental Legionella isolates by PFGE and WGS was critical to identifying the source.