California's First Locally Acquired Dengue: Pasadena, 2023

Dengue is rising worldwide. It's caused by four dengue viruses (DENV-1 to 4) and spread by Aedes mosquitoes — and invasive Aedes species are now found in California. Its most common symptom is fever, often with headache, pain behind the eyes, muscle, joint or bone pain, nausea, vomiting and rash.

From 2013 to 2022, Pasadena had six dengue cases, all acquired while traveling. Then, on October 2, 2023, the Pasadena Public Health Department (PPHD) received a lab report of raised dengue antibodies in a patient — person A — who had not traveled. Person A had fallen ill in mid-September and needed hospital care.

CDC

Mosquito control, fast

The same day, PPHD alerted the San Gabriel Valley Mosquito and Vector Control District (SGVMVCD). From October 2 to 9, following California Department of Public Health guidance, the district trapped and tested mosquitoes within 250 m (0.16 miles) of person A's home.

The district also went door to door with mosquito-prevention advice and ran two rounds of truck-mounted adult and larval mosquito treatments. Mosquito pools tested negative for arboviruses.

Finding more infections

PPHD looked for an earlier traveler who might have brought the virus, and for other infections. From October 10 to November 14, it tried to reach 175 households near person A's:

Person A's DENV-1 infection was confirmed by RT-PCR on October 18. On December 5, the 24 households with any travel or symptoms were followed up, and six adults gave blood samples. One — person B — also had DENV-1, with no travel, no symptoms, and no known contact with travelers or sick people.

What it means

This was California's first confirmed symptomatic case of locally acquired dengue, with evidence of a second, symptomless infection nearby. Moving on mosquitoes and investigation before lab confirmation was key to cutting the risk quickly, and established partnerships between health departments and vector control agencies make that possible.

(California's recommended trapping radius was 250 m in 2023; in 2024 it became 150 m (0.09 miles).)

Sources

Based on Feaster M, Patrick R, Oshiro M, et al., "Notes from the Field: First Locally Acquired Dengue Virus Infections — Pasadena, California, October–December 2023," MMWR volume 73, number 42, Centers for Disease Control and Prevention; a work of the United States government in the public domain.