Two dengue cases in Arizona, and 4,299 mosquito pools that said nothing

On 7 November 2022, dengue virus — **not endemic in the continental United
States** — was identified in a Maricopa County, Arizona resident. Patient A had
been admitted to hospital on 19 October with a dengue-like illness, seven days
after travelling to Mexicali, Mexico.

That alone is a travel-associated case. The question was whether the virus had
arrived with them.

The plan existed before the case did

A joint locally acquired mosquito-borne disease response plan already
existed between the county health department, the county environmental
services department and the state. **An incident command office was activated
on 10 November** — three days after identification.

Four things started at once:

1. Prospective investigation of provider and laboratory reports, plus
   suspected arboviral visits queried from the **National Syndromic
   Surveillance Program's BioSense platform**
2. Retrospective review of confirmed, probable and suspected dengue cases
   from 1 July onward
3. An alert to healthcare providers about possible local transmission
4. Advice to test for and report suspected dengue

None of it found anything. No local acquisition in 13 suspected arboviral
visits in BioSense, 10 reviews of closed cases, or 10 new investigations.

And neither did the mosquitoes

4,299 additional mosquito pools collected across the county between 18
September and 19 November — including every pool within five miles of patient
A's residence — were tested for dengue by RT-PCR.

All negative.

Two complete surveillance systems, one looking at people and one at
mosquitoes, both silent.

What did find the second case

After discussions with **CDC's Dengue Branch and Florida's public health
department** about best practice, teams canvassed **every residence within
0.09 miles (150 m)** of patient A's home during 17–19 November — to
interview residents, take specimens and assess properties for mosquito
breeding.

CDC confirmed DENV-3 by plaque reduction neutralisation testing. **The
person reported no travel in the two weeks before symptom onset** — a locally
acquired infection. **A household member had dengue-like symptoms but declined
testing.** Both have recovered.

Aedes aegypti mosquitoes and breeding sites were found at that residence,
and the mosquitoes trapped there tested negative — the same answer the
other 4,299 pools gave.

What it took, and what it says

One case found by testing four and a half thousand mosquito pools: none. One
case found by knocking on 241 doors and drawing blood from the 53 people who
agreed: one. Fewer than a third of households consented to anything at all.

The outbreak — two autochthonous infections — ended on 4 January 2023,
after more than 45 days without a further locally acquired case.

The county's response is shaped by what worked: **more mosquito traps around
patient residences**, public prevention education, and **a countywide provider
education campaign** to encourage testing irrespective of travel history —
because a first locally acquired case is, by definition, in someone who has not
travelled.

Source: Centers for Disease Control and Prevention, MMWR.