dengue viruses , single - stranded positive polarity ribonucleic acid ( rna ) viruses of the family flaviviridae , are the most common cause of arboviral disease in the world . dengue viruses have four serotypes , designated dengue types 1 - 4 ; and are transmitted mainly by bite of aedes aegypti mosquito and also by aedes albopictus . more than two - fifths of the world 's population ( 2.5 billion ) live in areas potentially at risk for dengue.(12 ) the global incidence of dengue fever ( df ) and dengue hemorrhagic fever ( dhf ) has increased dramatically in recent decades , and has turned this disease into a serious public health problem , especially in the tropical and sub - tropical countries.(345 ) dengue infection is endemic in many parts of india . in india , the first epidemic of clinical dengue - like illness was recorded in madras ( now chennai ) in 1780 . the first virologically proven epidemic of df in india occurred in 1963 - 1964 , outbreaks have been reported from different parts of the country at regular intervals.(6789101112 ) delhi , a city in north india , is endemic for dengue infection and has experienced eight outbreaks of dengue virus infection since 1967 ; with the last reported in 2006.(111213 ) we report the experience of all india institute of medical sciences ( aiims ) , a tertiary care hospital in delhi city , in the 2010 dengue outbreak . the objectives of the study were to know the incidence of laboratory confirmed dengue cases among the clinically suspected patients , to study the clinical profile of dengue - positive cases , and to co - relate the above with the prevalent serotype and environmental conditions . the study included clotted blood specimens received in virology laboratory , from clinically suspected cases of dengue infection , from 1st may 2010 to 31st december 2010 . serum was separated aseptically and was stored at -70c until further processing . for the identification of serotype , virus isolation followed by indirect fluorescent antibody assay ( ifa ) was done . as isolation could be done only in the acute phase samples for which proper cold chain was maintained throughout the transport , it was done in only 55 samples which were collected within 6 days of fever from the suspected dengue cases and were transported to the virology laboratory in ice . virus isolation was carried out in the c6/36 clone of aedes albopictus cell lines as described earlier.(11 ) ifa was performed using specific monoclonal antibodies to dengue virus types 1 - 4 ( provided by dr . gubler , then at cdc , atlanta , during the 1996 outbreak).(14 ) the serum samples from patients having  5 days of fever ( n = 4370 ) were tested for dengue - specific igm antibodies . -capture dengue igm enzyme - linked immunosorbent assay ( elisa ) kit was used ( supplied by the national institute of virology , pune ; under the national vector borne disease control program ) . manufacturer 's instructions were strictly followed for performing the test and interpreting the results . optical density ( o.d ) was measured at 450 nm using elisa reader ( biotek , winooski , united states ) . clinical and demographic details of the patients were obtained from the patients requisition forms , or from treating clinicians if required . the details were recorded in microsoft - excel sheets and those of dengue igm elisa - positive patients were analyzed to discern the pattern . for the identification of serotype , virus isolation followed by indirect fluorescent antibody assay ( ifa ) was done . as isolation could be done only in the acute phase samples for which proper cold chain was maintained throughout the transport , it was done in only 55 samples which were collected within 6 days of fever from the suspected dengue cases and were transported to the virology laboratory in ice . virus isolation was carried out in the c6/36 clone of aedes albopictus cell lines as described earlier.(11 ) ifa was performed using specific monoclonal antibodies to dengue virus types 1 - 4 ( provided by dr . the serum samples from patients having  5 days of fever ( n = 4370 ) were tested for dengue - specific igm antibodies . -capture dengue igm enzyme - linked immunosorbent assay ( elisa ) kit was used ( supplied by the national institute of virology , pune ; under the national vector borne disease control program ) . manufacturer 's instructions were strictly followed for performing the test and interpreting the results . optical density ( o.d ) was measured at 450 nm using elisa reader ( biotek , winooski , united states ) . clinical and demographic details of the patients were obtained from the patients requisition forms , or from treating clinicians if required . the details were recorded in microsoft - excel sheets and those of dengue igm elisa - positive patients were analyzed to discern the pattern . out of 55 specimens subjected to virus isolation , 33 ( 60.0% ) were positive for dengue-1 , while one specimen ( 1.8% ) was positive for dengue-2 . thus , the prevalent serotype ( 97.1% of the 34 culture - positive df cases ) was dengue virus ( denv ) serotype-1 . dengue igm antibody test was done on a total of 4,370 samples , of which 1,700 were positive for denv - specific igm antibodies ( 38.9% ) . in the 1,700 serologically confirmed dengue cases , male - to - female ratio was 1.6:1 . the largest number of positive samples ( 30.8% ) was from the age - group 21 - 30 years [ table 1 ] . the youngest patient was a 1-month - old male child and the oldest was an 87-year - old male patient . age - wise distribution of dengue - positive patients clinical profile and month - wise distribution of dengue - positive patients in 2010 out of 1,700 serologically confirmed dengue cases , 175 ( 10.3% ) were dhf patients , of which 158 ( 9.3% ) were dhf grade i / ii , and 17 ( 1.0% ) were dhf grade iii / iv ( dengue shock syndrome ) . thrombocytopenia ( platelet count < 100,000 cells per microliter of blood ) was seen in 393 ( 23.1% ) dengue igm - positive patients and 311 ( 18.3% ) patients presented with myalgia . the first dengue igm sero - positive case was in the last week of may , a 5-year - old male child from jharkhand - a state in east - central india . maximum number of specimens was received in september ( n = 1745 ; 39.9% of 4,370 ) , followed by october ( n = 1330 ; 30.4% of 4,370 ) . the maximum cases diagnosed were also in september ( n = 782 ; 46.0% of 1,700 ) , followed by october ( n = 448 ; 26.4% of 1,700 ) . percentage positives of the suspected cases peaked in september and august ( 44.8% and 44.2% , respectively ) . out of 55 specimens subjected to virus isolation , 33 ( 60.0% ) were positive for dengue-1 , while one specimen ( 1.8% ) was positive for dengue-2 . thus , the prevalent serotype ( 97.1% of the 34 culture - positive df cases ) was dengue virus ( denv ) serotype-1 . dengue igm antibody test was done on a total of 4,370 samples , of which 1,700 were positive for denv - specific igm antibodies ( 38.9% ) . in the 1,700 serologically the largest number of positive samples ( 30.8% ) was from the age - group 21 - 30 years [ table 1 ] . the youngest patient was a 1-month - old male child and the oldest was an 87-year - old male patient . clinical profile and month - wise distribution of dengue - positive patients in 2010 out of 1,700 serologically confirmed dengue cases , 175 ( 10.3% ) were dhf patients , of which 158 ( 9.3% ) were dhf grade i / ii , and 17 ( 1.0% ) were dhf grade iii / iv ( dengue shock syndrome ) . only one serologically confirmed dengue patient died ( 0.06% ) . thrombocytopenia ( platelet count < 100,000 cells per microliter of blood ) was seen in 393 ( 23.1% ) dengue igm - positive patients and 311 ( 18.3% ) patients presented with myalgia . the first dengue igm sero - positive case was in the last week of may , a 5-year - old male child from jharkhand - a state in east - central india . maximum number of specimens was received in september ( n = 1745 ; 39.9% of 4,370 ) , followed by october ( n = 1330 ; 30.4% of 4,370 ) . the maximum cases diagnosed were also in september ( n = 782 ; 46.0% of 1,700 ) , followed by october ( n = 448 ; 26.4% of 1,700 ) . percentage positives of the suspected cases peaked in september and august ( 44.8% and 44.2% , respectively ) . in the year 2010 , four years after the last epidemic of 2006 , india witnessed a massive wave of dengue fever , involving almost all states , except those in the extreme north and extreme north - east . the total number of serologically confirmed dengue cases from all over india in 2010 was 28,066 , the highest on records . total dengue igm - positive cases from delhi were 6,259 ( 22.8% of 28,066 ) , considerably higher than those reported in the previous years ( 548 , 1312 , and 1153 in 2007 , 2008 , and 2009 , respectively).(15 ) as per the epidemiological definition , as the number of dengue fever cases clearly exceeded all previous records , which was definitely greater than expected , it constituted an outbreak . over the last one and half decades , outbreaks and sporadic cases have been ascribed to different serotypes of dengue virus . the 1996 epidemic was mainly due to dengue-2 virus.(1112 ) in 2003 outbreak , all four denv serotypes were found to be co - circulating.(13 ) in 2005 , dengue-3 was identified as the predominant serotype in delhi.(16 ) in the year 2010 , the predominant serotype found was dengue-1 , which seemed to have replaced the previous circulating serotype . it is known that infection with one dengue serotype provides lifelong homologous immunity , but only transient cross - protection against other serotypes . the change in prevalent serotype is the major factor responsible for the enormous number of dengue cases witnessed , as the exposed population was by and large non - immune and thus susceptible to serotype-1 . maximum number of cases was in the 5 - 20 year age - group , while in 2003 , maximum number of positive cases was in 21 - 30 year age - group.(111213 ) in our study , maximum dengue cases ( 524 ; 30.8% of 1,700 ) were from the age group 21 - 30 years . the shift from pediatric / adolescent population to young adults getting affected reflects the presence of non - immune adult population falling prey to the circulating serotype of dengue virus . classical df is characterized by sudden onset of high grade fever , accompanied by headache , retro - orbital pain , myalgia , and thrombocytopenia . in our study , myalgia was seen in 18.3% and thrombocytopenia in 23.1% cases . thus , most of dengue cases were clinically indistinguishable from other febrile illnesses , and could be missed lacking the clinical suspicion and timely diagnosis . the risk of developing dhf / dss is greatest following an anamnestic dengue infection , particularly if the most recent infection was with dengue-2 virus.(1718 ) dengue-2 virus was the predominant serotype in 1996 outbreak of df / dhf / dss , and was responsible for the witnessed severity and mortality ( 10.8% ) . in 2003 , a mortality of 1.3%was reported from aiims ; and in 2006 , 1.5% mortality was reported from india.(11131920 ) in 2010 , total number of deaths was 110 all over india ( mortality 0.4% of 28,066 cases ) , with 8 from delhi ( mortality 0.13% of 6,259 cases).(15 ) at aiims , with only one death , the mortality was even lower ( 0.06% ) . and , this suggests that the present dengue outbreak was larger in scale , but lesser in severity . the role of environmental factors in infectious diseases is well - known . in most countries , dengue epidemics are reported to occur , during the warm , humid , and rainy seasons , which favor abundant mosquito growth and shorten the extrinsic incubation period as well.(212223 ) in our study , the largest proportion of serologically positive cases was recorded in the post - monsoon period , which is in agreement with previous studies.(2224 ) the reason for large number of df cases can also be related to the fact that in 2010 , delhi witnessed one of the wettest monsoons recorded . delhi obtained 997.2 millimeters ( mm ) of rainfall between july 1 and september 30 . the total rainfall in these three months in 2010 was not only notably higher than that of the previous 4 years ( 412.1 mm , 408.2 mm , 268.3 mm , and 468.2 mm in 2006 , 2007 , 2008 , and 2009 , respectively ) but also appreciably exceeded the 25-year average of 530.8 mm ( 1981 - 2005).(25 ) the prolonged and heavy rains led to a rise in mosquito growth , thus increasing transmission of mosquito - borne infections . the huge amount of infrastructure development which took place prior to commonwealth games ( october 2010 ) , may also have made growth environment more conducive for mosquitoes . another factor which would have contributed to the number of dengue cases could be an increase in the reporting of cases due to improvement in diagnostic settings . the event was of a large magnitude , but was comparatively less severe . a change in the predominant circulating serotype , unprecedented rains , enormous infrastructure development , and increased reporting due to improved diagnostic facilities all accounted for the observed multitude of people affected .