Consecutive large dengue outbreaks in Taiwan in 2014–2015

Dear Editor, The World Health Organization (WHO) has recently raised a warning that many countries have dengue epidemics or outbreaks. Major dengue fever (DF) cases were reported in Southeast Asia.1 DF is a vector-borne disease caused by dengue virus that belongs to the genus Flavivirus in the family Flaviviridae. Dengue viruses are geographically distributed by two competent vectors, Aedes aegypti and Aedes albopictus. The Aedes aegyptimosquitoes are predominant in southern Taiwan and highly correlated with dengue epidemics or outbreaks.2 Previous reports have indicated that dengue epidemics or outbreaks have mainly occurred in the south of Taiwan, such as Tainan and Kaohsiung,3 which are neighboring cities. Dengue activity has mainly been found in Kaohsiung city.3 The co-circulation of two or even four dengue serotypes (DENV-1–4) has been reported in Kaohsiung city.4 A severe DF outbreak occurred in Taiwan in 2014. A total of 15 732 DENV laboratory confirmed cases were reported by the Centers for Disease Control, Taiwan (Taiwan CDC)4 (Supplementary Figure S1), including 15 492 indigenous and 240 imported cases (Supplementary Table S1). A total of 15 043 cases (96%) were reported in Kaohsiung city.4 We previously reported a correlation between the underground pipeline leaking gas explosion event, which was followed by continuous heavy rains, with the 2014 DF outbreak in Kaohsiung (Po0.0001; r= 0.87).5 The cavities created by the explosion might have led to an increase in stagnant waters suitable for breeding of the Aedes mosquitoes. We found that the Breteau index for measuring the density of mosquitoes increased from 10%–19% to 35%–49% from July to September in Kaohsiung. Serotyping using real-time reverse transcription polymerase chain reaction (real-time RT-PCR) for the NS5 gene and detection with type-specific primers and probes6 indicated DENV-1 as the predominant strain (Figure 1A; Supplementary Table S1). This 2014 DF outbreak was initiated by imported DF cases, and their viral envelope sequences were similar to the 2013 Indonesia isolates (Figure 1A; Supplementary Table S1). However, a consecutive larger DF outbreak occurred in Taiwan in 2015, and a total of 43 784 DF cases were reported by the Taiwan CDC4 (Supplementary Figure S1). Among them, 362 were imported cases (Supplementary Table S1). Most of the DF cases were distributed in Tainan (22 777; 52%) and Kaohsiung (19 784; 45%).4 Serotyping data indicated DENV-2 as the predominant strain in the 2015 DF outbreak (Figure 1A; Supplementary Table S1). DENV-2 infected cases were reported initially in May, and then the number surged to its highest peak in September but gradually decreased after October in Tainan city. In addition, the Taiwan CDC reported that DENV-2 was disseminated to the neighboring city, Kaohsiung, after July.4 We collaborated with the Kaohsiung Medical University Hospital (KMUH) and obtained 2000 DENV-positive serum samples in which the subjects were infected in 2015 as confirmed by the laboratory of Taiwan CDC.4 Written informed consent was obtained from each participant. All study protocols were approved by the institutional review board of the Kaohsiung Medical University. The serotype of 940 samples was determined using virus isolation and real-time RT-PCR.6,7 Most cases identified before July were DENV-1 infections. From August to September, 20% were DENV-1, and 80% were DENV-2. Samples collected after October were all DENV-2. Phylogenetic analysis (a neighbor-joining tree with p-distance inferred with 1000 bootstrap replicates in MEGA program version 5 for a 1485-nt fragment spanning the full gene of the envelope glycoprotein)8 proved that the DENV-2 isolated in Kaohsiung after August clustered with the 2015 outbreak-associated DENV-2 Tainan strain (bootstrap value was 100). However, it did not cluster with previous Taiwanese epidemic DENV-2 isolates (Figure 1A, left). Notably, the Taiwanese 2015 DENV-2 outbreak strains phylogenetically clustered with the 2014 China and 2015 Indonesia isolates. We suggest that this DF outbreak may have been caused by the imported DENV-2 infected cases from China or Indonesia. A similar phenomenon was found in the 2014 DENV-1 outbreak in Kaohsiung, which may have been caused by the imported cases infected with the 2013 Indonesia strain (Figure 1B, right) (Supplementary Table S1). We also