LAC DPH Health Advisory:

Locally Acquired Case of Dengue in Los Angeles County

October 15, 2025

Key Messages

Situation

A case of locally acquired dengue has been identified in a resident of the San Gabriel Valley. The individual has no history of domestic or international travel. They became symptomatic at the end of September and are currently recovering.

The Los Angeles County Department of Public Health (LAC DPH) is working with the local vector control district to respond to the situation. The vector control district has increased mosquito trapping and monitoring activities and is conducting additional mosquito abatement to reduce the risk of further spread in the neighborhood.

This is the first case of locally acquired dengue reported in LA County in 2025. In 2024, a total of 14 locally transmitted dengue cases were identified in the county.

The overall risk of sustained local dengue transmission in LA County remains low. However, this case highlights that infected mosquitoes are present locally and underscores the importance of early recognition, reporting, and prevention to limit further spread.

Travel-associated dengue cases continue to be identified in LA County and across the U.S. Globally, many countries are reporting higher than usual numbers of dengue cases, which increases the likelihood of importation. With mosquitoes capable of transmitting dengue established in LA County and frequent travel-associated infections, sporadic local transmission continues to be a concern.

In LA County, peak mosquito season is during the warmer months of July through November, when the risk of local dengue transmission is greatest. Travel-associated cases, however, can occur year-round.

LAC DPH encourages healthcare providers to suspect and test for mosquito-borne diseases, to promptly report suspected cases, and to routinely discuss prevention strategies with patients and travelers.

Actions Requested of Providers

Dengue Overview

Background

Dengue is an arbovirus infection caused by any of the four distinct but closely related dengue viruses (DENV-1, DENV-2, DENV-3, DENV-4). People can be infected with dengue more than once due to the four different dengue viruses. Dengue is endemic in many tropical and subtropical regions worldwide and global incidence of dengue in 2024 has been the highest level on record.

Dengue is primarily transmitted to humans by the bite of an infected Aedes mosquito. Local transmission of dengue in the United States has been limited, with sporadic cases reported in Florida, Hawaii, Texas, and more recently Arizona and California.

Clinical Presentation

Dengue can range from asymptomatic infection or mild illness to severe disease. The typical incubation period is 5-7 days.

Approximately one in four patients develop symptoms and most commonly have a mild to moderate, nonspecific, acute febrile illness. Typical presentation includes acute onset fever accompanied by a combination of the following: headache, retro-orbital pain, myalgia, bone pain, arthralgia, nausea and vomiting, rash, a positive tourniquet test, and leukopenia.

Approximately one in twenty symptomatic patients develop life-threatening disease with severe bleeding, shock, and/or respiratory distress. 

Warning signs of severe dengue usually appear as fever starts to decline, and include persistent vomiting, severe abdominal pain, fluid accumulation, mucosal bleeding, lethargy/restlessness, postural hypotension, liver enlargement, and progressive increase in hematocrit.

Persons at increased risk of severe dengue include those with a prior history of dengue infection, infants, older adults (≥ 65 years), pregnant women, and those with chronic medical conditions. A second infection with a different DENV virus from the first infection carries a higher risk of severe disease than the first, third or fourth infection, however any infection can lead to severe disease.

See CDC Clinical Features of Dengue for more details.

Laboratory Testing

All patients with suspected dengue should be tested with either one of these test combinations:

These tests should be ordered regardless of the time from symptom onset, although the test sensitivity of RT-PCR and NS1 antigen tests decrease after the first 7 days. A serum sample is preferred for dengue testing.

IgG detection in a single serum sample should not be used to diagnose acute dengue because it does not distinguish between current and previous infection.

Testing for West Nile virus, a mosquito-borne virus endemic in LA County, should be obtained at the same time as dengue tests. West Nile can have a similar clinical presentation. In addition, cross-reactivity among arboviral antibody tests can result in false-positive test results.

These tests are available at commercial laboratories. If access to commercial testing is not available, testing can be conducted at LAC DPH Public Health Laboratory after obtaining approval from LAC DPH.

See CDC Clinical Testing Guidance for Dengue for more details.

Management

No specific antiviral treatment for dengue is available. Treatment is supportive and requires careful volume management. Appropriate triage, management, and follow-up remain the most effective interventions to reduce dengue morbidity and mortality.

Patients with mild symptoms should be advised to stay well hydrated and avoid aspirin and other nonsteroidal anti-inflammatory drugs (such as ibuprofen) because of their anticoagulant properties. Fever should be controlled with acetaminophen and tepid sponge baths.

For at least one week, patients should avoid mosquitoes and use insect repellent to prevent additional spread.

Patients with severe disease require hospitalization. Intravenous fluid therapy is the mainstay of supportive care. Close observation and frequent monitoring in an intensive care unit may be required. Prophylactic platelet transfusions and steroids are not recommended.

See CDC Treatment Recommendations and Dengue Case Management Pocket Guide.

Prevention

Preventing mosquito bites is the best way to prevent dengue and other mosquito-borne diseases. While there is an FDA-approved vaccine, it is only approved for children 9-16 years old with previous dengue infection and living in endemic areas. See CDC Dengue Vaccine.

Aedes mosquitoes bite during the day and night and live indoors and outdoors. The best way to prevent mosquito bites is to use insect repellent, wear loose-fitting, long-sleeved shirts and pants, and take steps to control mosquitoes in and around the home. Information for the public, including print resources (in English and Spanish) are available on the CDC Preventing Mosquito Bites webpage.  

Patients can be referred to their local vector control district to report problems with mosquitoes and request assistance. Local vector control district by zip code can be found at www.socalmosquito.org .

See CDC Preventing Dengue.

Reporting

Additional Resources

CDC:

Dengue for healthcare providers

Clinical management pocket guide (see image)

Dengue general  

LAC DPH: Dengue--resources for the public and healthcare providers

CDPH: Dengue home page

This communication was sent by Dr. Sharon Balter, Director, Acute Communicable Disease Control Program, Los Angeles County Department of Public Health.