The U.S. territory of American Samoa has had one mosquito-borne outbreak after another — chikungunya in 2014, dengue type 3 in 2015, Zika in 2016, all carried by Aedes mosquitoes. In November 2016, with Zika still waning, a new one began: dengue virus type 2 (DENV-2).
The islands
American Samoa is five islands in the Pacific. Of its 55,519 residents in 2010, 95% lived on the largest, Tutuila — 76.8 square miles. Since 2016 the territory has run electronic surveillance for dengue, chikungunya and Zika: each week an automated search of health records at Lyndon B. Johnson Tropical Medical Center and three of the five regional health centers flags patients with fever-like illnesses.
How it spread
| When | What happened |
|---|---|
| November 2, 2016 | a fisherman from the Solomon Islands is seen in Sua County with fever, joint pain, rash and shortness of breath. He tests negative for Zika and chikungunya, positive for dengue — type 2. Two days later he leaves American Samoa; soon suspected cases turn up in neighboring counties |
| To March 2017 | up to four confirmed cases a month |
| March 2017 | cases climb, and American Samoa declares the outbreak |
| July 2017 | 75 confirmed cases in the month; then two months of decline |
| December 2017 | the peak: 120 cases |
| May 2018 | 96 cases, then a gradual fall |
| October 2018 | 6 cases; the last confirmed case fell ill on October 25 |
Testing
Suspected cases were tested on the island with a rapid test for the dengue NS1 protein and for IgM antibodies; positives, and anyone meeting the case definition, were sent for real-time RT-PCR at the Hawaii State Laboratories, which also typed the virus. People positive only on the rapid IgM test — 434 of them — were left out, because IgM can linger, can cross-react with antibodies from Zika, and had not been validated in American Samoa.
| November 2016 – October 2018 | |
|---|---|
| Patients tested | 3,240 |
| Laboratory-confirmed dengue | 1,081 (33.4%) — 19.5 per 1,000 people |
| Positive by RT-PCR | 941 of 3,122 — every one DENV-2; no Zika, chikungunya or other dengue type |
| Positive by rapid NS1 test | 421 of 2,151 |
Figure 1. Confirmed dengue cases by month of illness onset. CDC
Who got sick
| Female | 50.6% |
| Median age | 16 (from newborns to 87) |
| Travel off the islands in the 14 days before illness | only 8 people |
| Symptoms | fever 92.0%, muscle pain 63.6%, headache 48.6%, vomiting 28.5% |
| Hospitalized | 380 (35.2%) |
| Of 89 hospitalized patients whose records were reviewed | 33.7% dengue with warning signs, 25.8% severe dengue |
| Age | Cases per 1,000 |
|---|---|
| 0–9 | 16.7 |
| 10–19 | 38.1 — the highest |
| 20–29 | 17.7 |
| 30–39 | 11.1 |
| 40–49 | 10.6 — the lowest |
| 50–59 | 10.8 |
| 60–69 | 12.7 |
| 70 and older | 13.0 |
By county, incidence was highest in Ituau (29.5 per 1,000), next to the county that contains the capital, Pago Pago; elsewhere it ranged from 12.1 to 19.9.
Figure 2. Confirmed dengue cases per 1,000 residents, by county. CDC
One patient died: a 68-year-old man transferred from neighboring Samoa, who died within 24 hours of arriving. No death was reported from infection within American Samoa during this outbreak.
The response
The Zika response was still running, and it was turned on dengue:
- Mosquito breeding sites: the Environmental Health Division of the American Samoa Department of Public Health inspected private properties and businesses, citing those breaking the law on breeding-site removal. In August–September 2017, about 108 tons of solid waste and scrap metal were hauled from yards and public spaces.
- Spraying: indoor residual spraying in every public and private school, with inspections focused on the villages with the most cases.
- Messages: the territorial epidemiologist spoke on radio and television about seeking care for fevers and avoiding bites; billboards went up in busy places; repellent was handed out at community health clinics.
- Clinical care: training in dengue patient management was reinforced.
Why care matters
Dengue is the world's most common mosquito-borne viral disease — an estimated 58 million symptomatic infections and 13,000 deaths in 2013. About 75% of infections cause no illness, but 5% of patients go on to severe dengue, and 0.5% to 5.0% of hospitalized patients die — a rate that better, faster clinical care can reduce.
During the 2015 DENV-3 outbreak, about 900 suspected cases included four confirmed deaths. American Samoa and CDC then ran clinical dengue management training, and no further DENV-3 deaths were reported; nor has any death resulted from this outbreak in American Samoa. Similar training could help elsewhere.
Dengue is not new here: outbreaks go back to DENV-2 in 1972, and a 2010 survey found 96% of people had antibodies to dengue. The electronic surveillance set up after 2015 is what caught this outbreak's first case. Even so, the 1,081 confirmed cases probably understate its size, given who seeks care, what doctors look for, the limits of the tests, and the excluded IgM results.
What to do
Transmission lasted at least 24 months, which shows the need for vector control that is effective and sustainable. Meanwhile:
- Anyone in a dengue area with a sudden fever should seek medical care immediately, and clinicians should know how to test for and manage dengue.
- Use insect repellent, wear long sleeves and pants, and stay where doors and windows have screens.
Sources
Based on Cotter CJ, Tufa AJ, Johnson S, et al., "Outbreak of Dengue Virus Type 2 — American Samoa, November 2016–October 2018," MMWR Morbidity and Mortality Weekly Report volume 67, number 47, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's table gives nausea as 314 cases and 31.5%, which do not agree, so that figure is left out.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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