A local outbreak of dengue caused by an imported case in Dongguan China

Abstract

Background: Dengue, a mosquito-borne febrile viral disease, is found in tropical and sub-tropical regions around the world. Since the first occurrence of dengue was confirmed in Guangdong, China in 1978, dengue outbreaks have been reported sequentially in different provinces in South China transmitted by peridomestic Ae. albopictus mosquitoes, diplaying Ae. aegypti, a fully domestic vector that transmits dengue worldwide. Rapid and uncontrolled urbanization is a characteristic change in developing countries, which impacts greatly on vector habitat, human lifestyle and transmission dynamics on dengue epidemics. In September 2010, an outbreak of dengue was detected in Dongguan, a city in Guangdong province characterized by its fast urbanization. An investigation was initiated to identify the cause, to describe the epidemical characteristics of the outbreak, and to implement control measures to stop the outbreak. This is the first report of dengue outbreak in Dongguan, even though dengue cases were documented before in this city.

Methods: Epidemiological data were obtained from local Center of Disease Control and prevention (CDC). Laboratory tests such as real-time Reverse Transcription Polymerase Chain Reaction (RT-PCR), the virus cDNA sequencing, and Enzyme-Linked immunosorbent assay (ELISA) were employed to identify the virus infection and molecular phylogenetic analysis was performed with MEGA5. The febrile cases were reported every day by the fever surveillance system. Vector control measures including insecticidal fogging and elimination of habitats of Ae. albopictus were used to control the dengue outbreak.

Results: The epidemiological studies results showed that this dengue outbreak was initiated by an imported case from Southeast Asia. The outbreak was characterized by 31 cases reported with an attack rate of 50.63 out of a population of 100,000. Ae. albopictus was the only vector species responsible for the outbreak. The virus cDNA sequencing analysis showed that the virus responsible for the outbreak was Dengue Virus serotype-1 (DENV-1).

Conclusions: Several characterized points of urbanization contributed to this outbreak of dengue in Dongguan: the residents are highly concentrated; the residents' life habits helped to form the habitats of Ae. albopictus and contributed to the high Breteau Index; the self-constructed houses lacks of mosquito prevention facilities. This report has reaffirmed the importance of a surveillance system for infectious diseases control and aroused the awareness of an imported case causing the epidemic of an infectious disease in urbanized region.

Figure 1

Number of clinically diagnosed and laboratory confirmed dengue cases at different time points during the dengue outbreak. The outbreak of dengue occurred from July 22, 2010 to September 14, 2010, with a concentrated occurring between Aug 31 and September 14, 2010, which accounted for 80.6% (25/31) of all dengue cases (24 laboratory confirmed and 7 clinically diagnosed cases, and 31 in total)

Figure 2

Molecular Phylogenetic analysis of dengue virus isolates from this study compared with the other established dengue virus based on the alignment of E-protein gene. Phylogenetic analysis were conducted in MEGA5. The evolutionary history was inferred by using the Maximum Likelihood method based on the General Time Reversible model. The bootstrap consensus tree inferred from 1000 replicates is taken to represent the evolutionary history of the dengue virus isolates analyzed. Branches corresponding to partitions reproduced in less than 50% bootstrap replicates are collapsed. The percentage of replicate trees in which the associated dengue virus isolates clustered together in the bootstrap test (1000 replicates) are shown next to the branches. Initial tree(s) for the heuristic search were obtained automatically as follows. When the number of common sites was < 100 or less than one fourth of the total number of sites, the maximum parsimony method was used; otherwise BIONJ method with MCL distance matrix was used. The analysis involved 141 nucleotide sequences. There were a total of 1427 positions in the final dataset. The cDNA of DG2010, D10029-DG and D10030-DG in E-protein region shared 100% identity with each other. The molecular phylogeny analysis result indicated that the D10030-DG isolate has a nearest relationship with DENV1/D10167-GZ/2010 (GenBank: JN029814.1), DENV1/D10102-SZ/2010 (GenBank: JN029813.1), and DENV1/D10007-DG/2010(GenBank: JN029807) (Figure 2), which were isolated in Guangdong in 2010, and didn't cause local outbreak. This cluster is statistically supported (bootstrap value = 96%) and is within a clade contain the sequence DENV1/SG/(EHI)DED142808/2008, isolated in Singapore in 2008 also statistically supported (bootstrap value = 85%)

Figure 3

Surveillance of febrile cases in Qifeng clinic during the dengue outbreak. According to the report form Qifeng clinic near Xintang community, the number of the febrile cases peaked between September 12 and September 23, 2010. The febrile cases decreased apparently after September 23, 2010

Figure 4

The temporal dynamics of dengue cases and Aedes albopictus prevalence during the dengue outbreak in Dongguan. Since September 10, 2010, measures have been taken to control the mosquitoes, such as cleaning out ponds to minimize breeding places, and spraying insecticides to kill the mosquitoes. The Breteau Index (BI) dropped greatly from 68 on September 10, to 4 on September 15, and 2 on September 19. The BI has been stably maintained at 5 or less since September 19. The dengue case was not reported any more since September 14, 2010