Characteristics of a Dengue Outbreak in a Remote Pacific Island Chain – Republic of the Marshall Islands, 2011–2012

Dengue is a potentially fatal acute febrile illness caused by four mosquito-transmitted dengue viruses (DENV-1–4). Although
dengue outbreaks regularly occur in many regions of the Pacific, little is known about dengue in the Republic of the
Marshall Islands (RMI). To better understand dengue in RMI, we investigated an explosive outbreak that began in October
2011. Suspected cases were reported to the Ministry of Health, serum specimens were tested with a dengue rapid
diagnostic test (RDT), and confirmatory testing was performed using RT-PCR and IgM ELISA. Laboratory-positive cases were
defined by detection of DENV nonstructural protein 1 by RDT, DENV nucleic acid by RT-PCR, or anti-DENV IgM antibody by
RDT or ELISA. Secondary infection was defined by detection of anti-DENV IgG antibody by ELISA in a laboratory-positive
acute specimen. During the four months of the outbreak, 1,603 suspected dengue cases (3% of the RMI population) were
reported. Of 867 (54%) laboratory-positive cases, 209 (24%) had dengue with warning signs, six (0.7%) had severe dengue,
and none died. Dengue incidence was highest in residents of Majuro and individuals aged 10–29 years, and ,95% of
dengue cases were experiencing secondary infection. Only DENV-4 was detected by RT-PCR, which phylogenetic analysis
demonstrated was most closely related to a virus previously identified in Southeast Asia. Cases of vertical DENV
transmission, and DENV/Salmonella Typhi and DENV/Mycobacterium leprae co-infection were identified. Entomological
surveys implicated water storage containers and discarded tires as the most important development sites for Aedes aegypti
and Ae. albopictus, respectively. Although this is the first documented dengue outbreak in RMI, the age groups of cases and
high prevalence of secondary infection demonstrate prior DENV circulation. Dengue surveillance should continue to be
strengthened in RMI and throughout the Pacific to identify and rapidly respond to future outbreaks.