The mosquito index hit its highest ever, and nobody had heard

West Nile virus is **the leading domestically acquired arboviral disease in the
United States**, spread by Culex mosquitoes. **There is no vaccine and no
specific treatment** — care is supportive — and the **case fatality rate among
people with neuroinvasive disease is 10%.**

Maricopa County, Arizona has had it since 2003 (12 cases). Its largest
previous outbreak was 2004, with 355 cases.

2021

**The largest documented focal West Nile outbreak in a single county in the
United States** — more than four times the county's previous record, during a
pandemic that had already stretched the hospitals taking those 1,014
admissions.

The warning, and what it was worth

On 2 September 2021, vector control notified the health departments that
the **vector index — a measure of infected Culex mosquitoes — was
substantially elevated.**

By that date at least 100 human cases had already been reported. Within two
weeks the index reached its highest ever recorded level (53.61), with **a
tenfold increase in human cases** alongside it.

The surveillance worked: the mosquitoes were measured, the number was
extraordinary, and the response began. Vector control ran daily remediation
against the elevated index and resident complaints — **large numbers of
outdoor mosquitoes from unknown sources, and unmaintained swimming pools** as
breeding sites. The health department increased **messaging, education events
and media outreach.**

And then the finding that undoes it

**Despite outreach efforts to communities and health care partners,
clinicians and patients reported a lack of awareness of the WNV outbreak.**

The largest county outbreak in the country's history, with a public
communications push behind it, and the doctors seeing the patients did not know
it was happening. A patient with fever and confusion is tested for what the
clinician is thinking about.

Hence the two recommendations, which are both about knowledge rather than
mosquitoes:

• **Clinicians should consider West Nile testing in serum and cerebrospinal
  fluid** for a clinically compatible illness — both fluids, because
  neuroinvasive disease is the severe form and the CSF is where it shows
• Public health agencies should continually review messaging to broaden
  awareness

The other route

Transmission is also possible through blood transfusion. Since 2005 the
FDA has recommended **nucleic acid testing of minipools of donations, with an
automatic switch to testing individual donations once one comes back
positive** — a design that keeps routine screening cheap and becomes precise
exactly when it needs to.

**The frequency and location of outbreaks vary annually and are challenging to
predict**, which is why human and mosquito surveillance together is what
the report ends on.

Source: Centers for Disease Control and Prevention, MMWR.