Local Dengue Spread in Los Angeles County Led to 14 Cases in 2024

Locally acquired dengue transmission in Los Angeles County during late summer and fall 2024 was linked to 14 confirmed cases across 6 geographically distinct areas, according to an investigation published in Emerging Infectious Diseases.¹ The findings add to evidence that dengue can emerge in parts of the continental United States where Aedes mosquitoes are established, even though the disease is not considered endemic.

For clinicians, the report underscores a practical concern: several patients experienced delays in diagnosis, and some were not initially suspected of having dengue because they lacked travel history. The authors noted that delayed testing likely reduced the yield of polymerase chain reaction and nonstructural protein 1 (NS1) assays and complicated a timely public health response.

The investigation described a countywide public health and vector-control response after the first locally acquired case was recognized on August 30, 2024, in a patient without recent travel. Los Angeles County Department of Public Health (LACDPH) and partner vector control districts then conducted active case finding, household testing, door-to-door outreach, syndromic surveillance, and targeted mosquito abatement within 150-meter radii around case residences.¹

In total, 14 locally acquired cases were identified from August through November 2024. Median age was 54 years (range, 5-79 years); 57% of patients were female, and 43% required hospitalization.¹ No patients developed severe dengue or dengue hemorrhagic fever, and no deaths were reported.

The strongest evidence for sustained transmission occurred in 1 neighborhood, where 8 locally acquired cases were identified. Six of those cases occurred within a 1-mile radius over 7 weeks, which investigators considered consistent with an outbreak involving ongoing local transmission. Four of the 14 total cases were first identified through public health investigation rather than routine clinical diagnosis.

Diagnostic Delay, Clinical Presentation, and Field Response to Dengue Cases

Diagnostic delays were notable. Median time from symptom onset to seeking care was 2 days, but the median interval from symptom onset to specimen collection was 10 days, with a range of 4 to 34 days. The median time from specimen collection to reporting to LACDPH was 5 days. According to the authors, this timing often exceeded the preferred testing window for PCR and NS1 during the first week of illness, requiring plaque reduction neutralization testing for confirmation in several patients.

Clinical presentations were consistent with uncomplicated dengue. All patients had fever; 57% reported headache, and another 57% reported nausea or vomiting. Six patients in total were hospitalized, but none met the criteria for severe dengue. Laboratory data showed that all 14 patients underwent dengue IgM testing; 12 had positive results, 1 had equivocal results, and 1 had negative results. Nine patients had positive NS1, reverse transcription PCR, or both. Serotype information was available for 11 patients: 8 were DENV-3, 1 was DENV-1, and 2 were indeterminate between DENV-1 and DENV-3.¹

The field response was operationally intensive. Public health teams conducted 16 field operations and reached 866 households, 2 worksites, and 1 place of worship and school, completing 318 household surveys. A total of 224 persons were tested during response activities, but no additional locally acquired cases were identified through broad community testing beyond the confirmed case clusters. Syndromic surveillance from September through December identified no further locally acquired cases.^1

Vector surveillance found established Aedes aegypti populations near case households, whereas Aedes albopictus was detected in low abundance. No mosquito pools tested positive for dengue virus, although the investigators cautioned that negative mosquito testing does not exclude transmission because infection prevalence in mosquitoes may be very low.^1,2

How Does This Fit Into the Global Context of Dengue Transmission?

The report arrives as dengue activity has expanded globally, and sporadic local transmission has been documented previously in California, including Pasadena in 2023.³ In Los Angeles County, travel-associated dengue cases rose sharply to 222 in 2024, compared with 75 in 2023 and 35 in 2022, according to the investigators.¹ That increase likely heightened the opportunity for viral introduction into receptive mosquito populations.

The study did have several limitations, as surveillance largely depended on symptomatic case detection, so asymptomatic or mild infections were likely missed.¹ Participation in neighborhood outreach was incomplete, and many households either declined participation or could not be reached. Limited early clinical suspicion and inconsistent use of recommended dengue testing also may have delayed recognition of transmission chains.¹

Still, the findings have immediate clinical relevance in regions with established Aedes mosquitoes. The authors argued that clinicians should consider dengue in patients with compatible febrile illness even without travel history, particularly during mosquito season and in areas with known local transmission.¹ Earlier testing and reporting may improve both patient management and outbreak containment.

References

1. Vaughan AM, Park C, Ngo VP, Contreras ZA, Lee JJ, Danza P, et al. Investigation of and response to autochthonous dengue, Los Angeles County, California, USA, August-November 2024. Emerg Infect Dis. 2026;32(5). doi:10.3201/eid3205.251812

2. Otero LM, Medina J, Ruiz-Valcarcel J, Rivera R, Maldonado Y, Torres J, et al. Monitoring dengue virus in Aedes aegypti to improve dengue surveillance and control in Puerto Rico. Viruses. 2025;17:1539. doi:10.3390/v17121539

3. Feaster M, Patrick R, Oshiro M, Kuan M, Goh YY, Carmona M, et al. Notes from the field: first locally acquired dengue virus infections—Pasadena, California, October-December 2023. MMWR Morb Mortal Wkly Rep. 2024;73:955-956. doi:10.15585/mmwr.mm7342a4