 four dengue virus serotypes. Phylogenetic trees showed their similarity to their prototypic strains and not to the prototypic strain of other three serotypes confirming their identity (Fig.4).
An outbreak of dengue fever occurred in Delhi after monsoon season in 2013. We have investigated the outbreak by conducting dengue virus specific RT-PCR on 378 suspected acute phase blood samples. Dengue viruses were detected in 71% of the samples confirming that the fever outbreak was caused by dengue viruses. The outbreak was mainly caused by DENV-2 serotype viruses. Dengue fever outbreaks are generally dominated by a particular serotype as reported from different parts of the world. DENV-2 dominated fever outbreaks were reported from Africa in 2007 and again in 201017. DENV-3 dominated outbreaks have been reported in 2013 in Solomon Islands18, in 2009-2010 from Nicaragua19 and in 2004 from Indonesia20. Dengue type 1 dominated outbreak was reported in 2001 from Myanmar21. A dengue 4 dominated outbreak was reported in 2009 in French Polynesia22.
Three dengue virus serotypes were detected in the present study. DENV-2 dominated the outbreak with DENV-1 and DENV-3 being detected in small number of samples. DENV-4 was not detected in our study. This is the first report of molecular detection of dengue viruses during 2013 dengue fever outbreak from Delhi, India. Chakravarti and colleagues23 had reported detection of all 4 serotypes of dengue during 2007-2009 in Delhi. Another recent study has reported a DENV-2 dominated outbreak in Orissa state of India in 2010-2011 in which 72% samples were detected positive by RT-PCR and IgM ELISA24. A DENV-2 dominated outbreak was earlier reported in the year 199625 in Delhi. All four serotypes were detected without any dominance by a particular serotype in 2003 outbreak in Delhi26 whereas DENV-3 was found to be the dominant serotype in the 2006 dengue outbreak27.The last dengue epidemic occurred in Delhi in 2010 in which 12.5% samples were detected positive for dengue viruses by RT-PCR and DENV-1 serotype was detected in 63% samples28. Our results show a change in serotype dominance from Dengue virus type 1 in 2010 to dengue virus type 2 in 2013 in Delhi.
Co-infection with more than one dengue virus serotypes was also detected (14%) in the present investigation. Bharaj et al had reported concurrent infection in 19% of the positive samples in 2006 in Delhi27. A very high percentage of co-infection by different serotypes (56.8%) was reported from Kerala, Southern state of India in 200829 . DENV-1 strains of American African genotype, DENV-2 strains of the Cosmopolitan genotype and DENV-3 strains of Genotype III were detected in the 2013 outbreak.. The genotypes reported in the present study are known to be circulating in India in recent years (Afreen, unpublished data,23,30,31). Alteration in circulating DENV-1, 2 and 3 genotypes was not observed in the year 2013. This may be due to complete dominance of these genotypes over any other genotype which may have been imported but failed to establish. Another factor could be a selective advantage of these genotypes in Indian hosts (both human and mosquito) or in Indian climate.
Molecular detection and serotyping of dengue viruses were carried out on acute phase blood samples that were collected during 2013 dengue fever outbreak from Delhi. Dengue viruses were detected in more than two third of the samples. Dengue virus serotype 2 was detected as the dominant serotype in the outbreak. Co-infection by different serotypes was also detected in the outbreak. Twenty nine outbreak strains (10 DENV-1, 12 DENV-2 and 7 DENV-3) were sequenced for partial envelope gene. Phylogenetic analysis grouped DENV-1 strains in the American African genotype, DENV-2 strains in the Cosmopolitan genotype and DENV-3 in Genotype III. The study will be helpful in the study of the epidemiology of dengue fever.
The authors have declared that no competing interests exist.
AcknowledgementsNazia Afreen is supported by Senior Research Fellowship of the University Grants Commission, Govern