Dengue in U.S. Territories, 2010–2020: 30,903 Cases, Half of Them Under 20

Dengue is a flavivirus spread by infected Aedes mosquitoes, including Aedes aegypti and Aedes albopictus — one of the most common mosquito-borne viral infections in the world, and growing. In 2019 and 2020 the Americas reported about 5.5 million cases to the Pan American Health Organization, the most on record.

Every U.S. territory has reported local spread; their tropical climates suit Aedes mosquitoes well. Dengue is endemic — more than 10 cases in at least three of the previous 10 years — in American Samoa, Puerto Rico and the U.S. Virgin Islands (USVI). In Guam and the Commonwealth of the Northern Mariana Islands the risk is considered sporadic or uncertain. Yet the trends had not been well described. This CDC report looks at 2010–2020.

The report in brief. CDC / MMWR

How the data were gathered

Health departments report dengue to CDC through ArboNET, the national arboviral surveillance system built in 2000 to track West Nile virus. Dengue became nationally notifiable there in 2010, and cases are classed by the 2015 Council of State and Territorial Epidemiologists definition — confirmed by a virus, antigen or (where no other flavivirus circulates) antibody test, or probable on antibody alone where another flavivirus does. CDC's Dengue Branch Laboratory types the virus in a subset of specimens. Incidence uses 2020 census populations; the Northern Mariana Islands reported no local cases and are left out.

Puerto Rico's dengue record is long: a first report in 1899, sporadic reports until 1963, when the Puerto Rico Department of Health asked CDC to investigate a large outbreak, and again in 1969 during a DENV-2 outbreak — the year its Passive Dengue Surveillance System began. It still feeds ArboNET.

What was found

30,903 cases from four territories: 21,705 (70.2%) confirmed and 9,198 (29.8%) probable.

• About half (15,640; 50.6%) were under 20.
• 54.4% were male.
• 584 (about 2%) were severe dengue.
• 10,037 (32.4%) were hospitalized; 68 (0.2%) died.
• Only 28 (0.1%) followed travel to a country with dengue.

Dengue cases by territory and year of onset, 2010–2020. CDC / ArboNET

Yearly incidence per 1,000 population. CDC / ArboNET

Puerto Rico

Most cases came in the 2010 and 2013 outbreaks. 69.2% were confirmed, 54.6% were male, and over 99% were caught on the island. Incidence and hospitalization were highest among children aged 10–14 and 15–19. Of the 68 deaths, the largest number (10) were in people over 70, but six (8.8%) were under 20.

Puerto Rico: cases, incidence and hospitalization by age group, 2010–2020. CDC / ArboNET

Puerto Rico: deaths and mortality rate by age group, 2010–2020. CDC / ArboNET

American Samoa

Every case was confirmed and 50.3% were female. Cases and rates were highest under 20, with the highest incidence and hospitalization at ages 10–19.

American Samoa: cases, incidence and hospitalization by age group, 2010–2020. CDC / ArboNET

U.S. Virgin Islands

Most cases came in the 2012–2013 outbreak. 96.9% were confirmed, 53.3% were female, and all were local. After those aged 10–19, the most affected were 20–29 (15.0%); incidence was highest at ages 10–14.

U.S. Virgin Islands: cases and incidence by age group, 2010–2020. CDC / ArboNET

Guam

All 28 cases were confirmed and came in 2019 and 2020 — Guam's first locally acquired dengue since 1944, alongside wide spread in the Pacific — though it did not spread widely across the island, and nearly half were travel-associated. 60.7% were male; after ages 10–19, the most affected were 30–39 (21.4%). One reason for the difference may be the mosquito: Ae. albopictus is Guam's main dengue vector, Ae. aegypti the other territories'.

Which virus types

21,329 confirmed cases had their serotype — the type among the four dengue viruses, DENV-1 to DENV-4 — reported to or tested by CDC's Dengue Branch.

In Puerto Rico, DENV-1 and DENV-4 drove the 2010–2013 outbreaks. As chikungunya (2014) and Zika (2016) arrived, dengue fell from 2015 to 2019, all four types circulating at low levels — perhaps because Zika gave short-term cross-protection, as studies elsewhere in Latin America suggest. In 2019–2020 dengue rose slightly: every local case was DENV-1, with DENV-2 and DENV-3 arriving with travelers. While Central and South America had record dengue in 2019, none of the endemic territories had more than usual.

Low circulation of the other types may leave people more susceptible to them. As population immunity falls, large vaccine programs, training for clinicians and mosquito control guided by near real-time data matter more for preventing big outbreaks.

Puerto Rico: dengue cases by serotype and year, 2010–2020. CDC / ArboNET

The vaccine

Dengue has no specific antiviral, and controlling Ae. aegypti is hard: it breeds in hidden, hard-to-reach places, insecticide resistance is widespread, and new rules have retired some insecticides and slowed new ones. In June 2021 the Advisory Committee on Immunization Practices recommended Dengvaxia for children aged 9–16 with laboratory-confirmed previous dengue who live where dengue is endemic — the first dengue vaccine available in the United States for broad prevention. American Samoa, Puerto Rico and USVI all qualify.

Vaccinated children who have had dengue before get a lower risk of symptomatic disease, hospitalization and severe dengue — in the age group carrying the most disease. Clinicians there should know the eligibility rules and test for previous infection before vaccinating. Two more vaccines — Takeda's TAK-003 and the National Institutes of Health's TV003 — were in late-stage trials.

Limits

• The counts are too low. Not everyone who is ill seeks care, is tested or is reported. Studies in American Samoa found 16 DENV-3 cases in 2015 and 941 DENV-2 cases in an outbreak from November 2016 to October 2018, only partly reported to ArboNET.
• Severity is probably understated: many Puerto Rico reports lacked clinical outcomes.

To do better, the authors suggest reporting every case to ArboNET, more lab testing and serotyping (which public health labs can do), a standard case form, automated reporting, and training for clinicians and surveillance staff in the detailed case definition — in line with a Pan American Health Organization initiative, supported by CDC, to improve dengue reporting across the Americas.

Sources

Based on Ryff KR, Rivera A, Rodriguez DM, et al., "Epidemiologic Trends of Dengue in U.S. Territories, 2010–2020," MMWR Surveillance Summaries, volume 72, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report says both that American Samoa's cases were reported only in 2017–2018 and that it went without cases in 8 of the years, so neither is repeated here.