Dengue is emerging as a major public health problem in India. Since the first epidemic in Kolkata during 1963–64 many places in India have been experiencing dengue infection [6
]. One of the largest outbreaks in North India occurred in Delhi and adjoining areas in the year 1996. The 1996 epidemic was mainly due to dengue-2 virus [2
]. Following this in the post epidemic period, 1997, dengue-1 virus activity was seen in Delhi [7
]. Thereafter, in the year 2003 another outbreak occurred in Delhi and all four dengue virus serotypes were found to be co-circulating [4
]. However, dengue-3 was reported to predominate in certain parts of North India in 2003[8
In the following year (2004 and 2005) though no outbreak occurred in Delhi, definitely higher number of cases than usual were referred to our laboratory for testing. The seasonality of transmission of dengue with increased activity in the post monsoon season was seen in the present study; in accordance with the reported patterns of dengue transmission [9
]. Even in the post-epidemic period (2004& 2005) increased dengue virus activity was seen in post monsoon period September to November with peak in the second and third week of October. Similar observation was seen in the year 1997 following 1996 epidemic [7
]. These findings indicate that during epidemic and non-epidemic years dengue infections are mostly seen in post monsoon season hence preventive measures should be in full swing at the very onset of the monsoon.
Age wise distribution of the seropositive cases in all 3 years shows that statistically significant number of cases were in older age group (>10 yrs) as compared to the younger age group (≤ 10 yrs) (p value = <0.001). This observation is quite in accordance with our previous reported study [4
] and with other studies from Delhi [10
]. However many studies from South India [11
] found children more susceptible to infection than the adults.
The predominant dengue virus serotype seen in the year 1997, following dengue-2 epidemic in 1996 was dengue-1[7
]. In this study in the year 2004, dengue-1 was found to be circulating following 2003 outbreak, which involved all four serotypes. In the year 2005, however, in majority of cases dengue-3 was identified. dengue-2 and dengue-4 were not identified in 2005 indicating that dengue-3 seems to have replaced dengue-2 and 4 to establish itself as the predominant strain in Delhi.
Over the past two decades, dengue-3 has caused unexpected epidemics of DHF in Srilanka, East Africa and Latin America [12
]. Emergence of dengue-3 has also been reported in 2003 as well as 2004 from certain parts in North India [8
The present study reports the emergence of dengue-3 as the predominant serotype in Delhi in the year 2005. Further studies regarding the molecular characterization of these dengue-3 viruses are underway. Epidemiology of dengue infection in Delhi is rapidly changing face, with frequency of outbreaks increasing, even as dengue establishes itself as endemic disease. The need of the hour is to characterize the circulating serotypes of dengue virus in our community and understand the evolutionary processes influencing the dengue virus, as this is expected to impact on vaccination strategies for future.