Notes from the Field: First Locally Acquired Dengue Virus Infections — Pasadena, California, October–December 2023

Investigations and Outcomes

Vector Risk Mitigation

On October 2, PPHD alerted SGVMVCD that person A had a suspected case of arboviral disease. During October 2–9, SGVMVCD trapped, counted, and tested mosquitoes within 0.16 miles (250 m) of person A’s home, per guidance from the California Department of Public Health* (4). Aedes mosquito counts were eight times as high as in other routine surveillance areas in the San Gabriel Valley for the same period (80 versus an average of 10.5 adult Aedes mosquitoes per trap) (5). During the same period, SGVMVCD conducted a door-to-door mosquito prevention educational campaign and two rounds of truck-mounted adulticide and larvicide treatments in the San Gabriel Valley. Mosquito trap counts 1 week after baseline confirmed a 62% reduction in the mean number of trapped adult mosquitoes (80 per trap pretreatment versus 30 posttreatment). Reverse transcription–polymerase chain reaction (RT-PCR) testing on mosquito pools were negative for arboviruses.

Public Health Investigations

PPHD activated community risk mitigation measures and conducted an epidemiologic investigation. This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.^†

Interviews with person A confirmed no recent travel history. The objectives of the investigation were to identify a potential index case preceding person A with relevant travel, identify potential additional secondary infections, and conduct enhanced case investigation in the surrounding neighborhood. During October 10–November 14, PPHD attempted to contact 175 households; 14 (8.0%) declined, and 31 (17.7%) could not be reached. Among the 130 (74.3%) households with a completed interview, 15 (12%) had a member with recent travel to regions where dengue is endemic, eight (6%) had a member with recent symptoms consistent with arboviral illness, and one (1%) had a member with both. Testing by RT-PCR confirmed person A to have DENV-1 infection on October 18. On December 5, follow-up interviews of the 24 households with at least one person with travel or symptoms were conducted, and six consenting adults provided serum samples. From these samples, one additional person (person B) was confirmed as having DENV-1 infection by RT-PCR. Person B had no recent travel history, no symptoms consistent with dengue, and no reported contact with recent travelers or symptomatic persons.

Preliminary Conclusions and Actions

This first confirmed symptomatic case of locally acquired dengue in California was identified in October 2023, with subsequent evidence of a second asymptomatic person with infection. Rapid deployment of vector control resources and epidemiologic investigations before laboratory confirmation were vital to timely mitigation of arboviral risk. Established, active partnership among public health and vector control agencies is important for rapid reduction of mosquito populations when local transmission of arboviral disease is suspected.