First isolation of dengue virus from the 2010 epidemic in Nepal

Abstract

Dengue is an emerging disease in Nepal and was first observed as an outbreak in nine lowland districts in 2006. In 2010, however, a large epidemic of dengue occurred with 4,529 suspected and 917 serologically-confirmed cases and five deaths reported in government hospitals in Nepal. The collection of demographic information was performed along with an entomological survey and clinical evaluation of the patients. A total of 280 serum samples were collected from suspected dengue patients. These samples were subjected to routine laboratory investigations and IgM-capture ELISA for dengue serological identification, and 160 acute serum samples were used for virus isolation, RT-PCR, sequencing and phylogenetic analysis. The results showed that affected patients were predominately adults, and that 10% of the cases were classified as dengue haemorrhagic fever/ dengue shock syndrome. The genetic characterization of dengue viruses isolated from patients in four major outbreak areas of Nepal suggests that the DENV-1 strain was responsible for the 2010 epidemic. Entomological studies identified Aedes aegypti in all epidemic areas. All viruses belonged to a monophyletic single clade which is phylogenetically close to Indian viruses. The dengue epidemic started in the lowlands and expanded to the highland areas. To our knowledge, this is the first dengue isolation and genetic characterization reported from Nepal.

Keywords: Dengue fever; Nepal; dengue 1 virus; epidemiology.

Fig. 1.

Dengue and Aedes aegypti confirmed districts in Nepal, 2010. (*) indicates Aedes aegypti positive districts and shaded areas indicate dengue positive districts. Numbers indicate the elevation (meter) of each district capital.

Fig. 2.

Epidemiological curve showing the distribution of dengue cases in 2010.

Fig. 3.

Phylogenetic tree of DENV-1 E region. Representative analysis of E region, 1,485 bps, 2,238 strains. Red and blue indicate Nepalese strains and Indian strains, respectively. The tree was constructed after 1,000 replicates of bootstrap analysis using neighbor joining method. Bootstrap values (%) greater than 50% are shown above branches. Labels of strains conform to the following format: (GenBank accession nos)_(Strain name)_(Country-region)_(Year of isolation). “NA” means that the information is not available.

Fig. 4.

Phylogenetic tree of DENV-1 E NS5-3’UTR spanning region. Maximum clade credibility (MCC) tree of NS5-3’UTR spanning region, 304 bps, of 579 strains with uncorrelated relaxed clock, GTR+G+I model.