Characteristics of a dengue outbreak in a remote pacific island chain--Republic of The Marshall Islands, 2011-2012

Abstract

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.

Figure 1. Epidemic curve of suspected dengue cases by date of illness onset in the Republic of the Marshall Islands, October 2011–February 2012.

Laboratory case definition and date of illness onset were compiled as indicated. Only one date per week is shown on the x-axis.

Figure 2. Atoll of residence of suspected dengue cases identified in the Republic of the Marshall Islands, October 2011–February 2012.

Laboratory case definition and atoll of residence were compiled as indicated. Asterisks indicate atolls with laboratory-positive cases. Ebeye island is located on Kwajalein atoll.

Figure 3. Age distribution of laboratory-positive dengue cases in the Republic of the Marshall Islands, October 2011–February 2012.

A: Age distribution by sex and rate of laboratory-positive cases; B: Primary and secondary DENV infections by age group from a representative sample of all laboratory-positive cases; error bars indicate standard error.

Figure 4. Bayesian maximum clade credibility phylogenetic tree of DENV-4 detected during the 2011–2012 dengue outbreak in the Republic of the Marshall Islands.

N = 40 E gene sequences (1,485 basepairs). Internal nodes supported by posterior probability. The dot indicates the DENV-4 isolated from the Republic of the Marshall Islands in 2011 (GenBank accession number JX891655).

Figure 5. Aedes species composition in juvenile mosquito samples collected from water-filled containers on Majuro atoll, Republic of the Marshall Islands, 3–8 November, 2011.

Examples of habitat types in each container class are provided in Table 2.