US dengue hit a record 3,798 cases in 2024, and 97% arrived with travellers

The United States recorded 3,798 dengue cases in 2024, a 359% increase above the 2010–2023 annual average of 828 cases, according to a surveillance summary published in MMWR [s1]. Over that earlier fourteen-year baseline, annual counts had ranged from 202 to 2,055 [s1]. The 2024 figure is not an incremental rise on a trend; it is close to double the previous highest year.

What the national picture shows

Cases are reported to ArboNET, the national arboviral surveillance system covering the 50 US states and the District of Columbia [s1]. Of the 3,798 cases, 97.2% were associated with travel outside the reporting jurisdiction during the two weeks preceding symptom onset, and the remaining 2.8% were locally acquired [s1].

The seasonal and demographic detail is specific. Cases peaked during July–September, which accounted for 41.6% of the annual total, and the highest share by age group — 21.8% — occurred among people aged 50–59 [s1]. Hispanic or Latino persons accounted for 57.5% of all cases [s1].

Among travel-associated cases, acquisition occurred primarily in the Caribbean including Puerto Rico and the US Virgin Islands (34.1%), North America meaning Mexico and the United States (24.3%), and Central America (15.6%) [s1].

Severity is the part that makes this more than a counting exercise. Among all patients, 36.1% required hospitalisation, 2.8% of cases were severe, and six patients — 0.2% — died [s1]. Among the 1,204 patients with a known dengue virus serotype, DENV-3 was the most commonly reported at 54.8% of the four serotypes [s1].

A hospitalisation rate above a third is high for a disease often described as self-limiting, and it almost certainly reflects who gets tested rather than who gets infected. Mild dengue in a returning traveller frequently never reaches a laboratory. Surveillance counts confirmed cases, and confirmed cases skew sick.

The 2.8%

Locally acquired dengue in the continental United States remains rare — 2.8% of a record year [s1] — but it is the number with implications for domestic vector control, and one 2024 cluster has now been described in detail.

During August–November 2024, 14 locally acquired dengue cases were identified in Los Angeles County, California [s2]. Epidemiologic evidence indicated cases occurred in several neighbourhoods, suggesting short transmission chains following introductions from returning travellers, though in one neighbourhood the evidence supported ongoing transmission for up to seven weeks [s2].

Median patient age was 54 years, with a range of 5 to 79; eight patients (57%) were female and six (43%) male; and six (43%) required hospitalisation [s2]. The investigation also documented delays: median time from symptom onset to specimen collection for dengue testing was nine days, ranging up to 34 [s2].

That nine-day median is the operational finding. Dengue viraemia is short, and a mosquito control response triggered nine days after symptom onset is responding to a transmission window that has largely closed. The authors frame local transmission in Los Angeles County as highlighting the emerging threat of mosquitoborne disease transmission in non-endemic areas and the need for rapid and coordinated public health and vector control responses to interrupt transmission [s2].

Travel surveillance as an early signal

A third analysis, published in February, argues that returning-traveller data can be used prospectively rather than only as a tally. It analysed 10,530 travel-associated dengue cases among US travellers reported to ArboNET during January 2010–April 2024, involving travel to 128 countries, and used negative binomial and Poisson models to develop country-specific thresholds at the 75th, 80th and 90th percentiles for identifying elevated risk [s3].

Applying a threshold of more than 10 cases in a three-month period improved specificity, and the resulting dual-criteria method identified high-risk periods including sustained transmission in countries with limited official reporting, such as Cuba in 2022–2023 [s3]. Threshold comparisons revealed a tradeoff between early detection and overclassification, while real-time and retrospective assessments produced consistent high-risk signals [s3].

The authors note the method's dependence on reporting speed: timeliness might be reduced if reporting delays increase beyond those observed [s3]. Their conclusion is that travel surveillance should be integrated into global dengue monitoring and preparedness [s3].

What the numbers do not settle

None of these papers establishes why 2024 was a record. A surveillance summary describes what was reported; it cannot separate a genuine rise in infection from increased travel volume, increased clinician awareness, or improved testing. The 359% figure is a comparison against a fourteen-year average [s1], and averages across periods with changing diagnostic practice are a blunt baseline.

Nor do the data support inferences about a reader's individual risk. The demographic pattern — the 50–59 age concentration, the 57.5% Hispanic or Latino share [s1] — largely reflects who travels where, not differential susceptibility, and the summary does not report population denominators that would allow rates to be calculated.

What the three papers jointly establish is narrower and firmer: a record year driven overwhelmingly by importation [s1], a documented instance of onward local transmission sustained for weeks after importation [s2], and a demonstrated method for reading traveller data as a leading indicator of where transmission is intense [s3]. The MMWR authors' own framing is that the findings underscore the need for enhanced prevention strategies, clinical awareness, and tailored public health messaging for travellers to areas where dengue is endemic [s1].

Sources

• Increase in Travel-Associated and Locally Acquired Dengue Cases — United States, 2024, MMWR, 14 May 2026
• Investigation of and Response to Autochthonous Dengue, Los Angeles County, California, USA, August–November 2024, Emerging Infectious Diseases, published online 6 April 2026
• Using Routine Surveillance Data to Assess Dengue Virus Transmission Risk in Travelers Returning to the United States, Emerging Infectious Diseases, February 2026