A clogged pipe, a heat map, and seventeen cases of Legionnaires disease

On 11 and 12 July 2022, Napa County Public Health received reports of
three positive urinary antigen tests for Legionella pneumophila
serogroup 1. By 21 July there were six confirmed cases, against a
baseline of one or two per year. The final count was **17 cases — 14
confirmed, three suspected** — through 15 August.

Legionnaires disease is **acquired by inhaling water droplets from human-made
building water systems**. Somewhere in downtown Napa, something was making
aerosol.

Drawing the circle

The patients' residences were plotted on a point density heat map, and a
high-risk zone defined as everything within a 1.0-mile (1.6 km) radius of
the centre of the resulting cluster.

That one step is what made the investigation tractable. Cooling towers,
fountains and misters are everywhere in a town centre; without a boundary,
sampling is unbounded. **All seven of the environmental samples that came back
positive were collected inside the zone** — of 11 sampled sites, six
positive for L. pneumophila and one for both that and Legionella anisa.

What the inspections found

A lack of maintenance at most cooling towers. Many had **low or no
detectable chlorine** at sampling, from **no routine biocide application,
improper distribution, or other system problems.**

And at facility A, the specific failure:

**A clog in the pipe leading to the chemical feed system impeded the
controller's ability to detect water flow, resulting in low or no injection
of biocide into the tower.**

The controller was not broken. It could not see water moving, so it did not
dose. **Maintenance records show the clog was detected in early July — at
approximately the same time as many of the case exposures — and was resolved
in early August.**

Which of the seven was it

Seven positive sites is a list of suspects, not an answer. Two laboratory
methods narrowed it to one:

• Sequence-based typing builds an allelic profile from seven loci;
  each unique profile is a sequence type (ST)
• Whole genome sequencing with SNP analysis measures relatedness — **fewer
  differences means more closely related**

**The environmental samples yielded ST1, ST10, ST35 and ST296. The clinical
isolate was ST35. The only environmental sample that yielded ST35 came from
facility A's cooling tower** — and no SNP differences at all separated the
two, while the other environmental isolates were genetically distant.

One clinical specimen was PCR-positive but culture-negative, so no isolate
could be grown from it. Nested sequence-based typing, a culture-independent
variant, was run on the specimen directly — and also found ST35. That is
the methodological point the report makes: when no isolate is recovered, the
case is not lost to the analysis.

The response

An outbreak alert asked local providers to test hospitalised patients with
community-acquired pneumonia, or those failing outpatient treatment. CDPH
notified other health departments; a press release asked people with
symptoms to seek care; Napa County put up a public webpage.

Facilities testing positive were told to begin remediation immediately.
**One that did not respond to oral and written communication received a legal
order to shut down its cooling tower until remediation was complete.**
Facilities outside the zone were told about the outbreak and about
maintenance practice.

The last case was detected on 15 August, by which time most facilities had
started or finished remediation.

Source: Centers for Disease Control and Prevention, MMWR.