The aim of this study was to determine the frequency and clinical features of dengue infection in Medellin city. A longitudinal study in three schools and a cross-sectional study in other seven schools was performed. An active surveillance for febrile episode (≤7 days of evolution) was established and blood samples were drawn in acute and convalescence state of each participant. Furthermore, in the cross-sectional study one blood sample was drawn annually for three years (2010, 2011, 2012) from student participants. Mosquito indexes were determined in 94 schoolchildren´s houses and they were mapped using the software ArcGIS 10.1. A total of 2,379 schoolchildren were enrolled in 2010 and for 2011 only 1,840 continue included in the study. During the two years follow up, 264 schoolchildren had acute febrile illness and 23 (8.7%) dengue cases were confirmed. The four dengue serotypes were found with a major frequency of DEN-1, 41.7% and DEN-2, 33.3%. All cases were mild dengue cases. The most common symptoms of dengue cases compared with undifferentiated fever were asthenia (96% vs. 87%), anorexia (78% vs. 57%), rhinorrhea (65.2% vs. 58%), abdominal pain (56.5% vs. 47.8%), arthralgia (43% vs. 33%), low back pain (30% vs. 27%) and positive tourniquet test (13% vs. 3%) but there was no statistically significant difference. In the cross-sectional study, dengue incidence was 3% in 2010; 1.2% in 2011 and 3.2% in 2012. The seroprevalence of IgG antibodies was 48% in 2010; 57.9% in 2011 and 56.3% in 2012. Increased trend was observed in seroprevalence, increasing age and lower socioeconomic status p<0,001. Ratio of asymptomatic/symptomatic infections in 2010 and 2011 was 8.1:1 and 10.3:1 respectively. Aedes aegypti¿s larvae were found in 28.7% of the household. The low risk entomological indexes were the most predominant (>80%) in the hole study area, however most of the dengue cases were found in these areas with low entomological indexes. These results show an elevated dengue transmission in children, a higher risk in lower socioeconomic status and a high risk with the increase in age. All signs and symptoms are similar to other undifferentiated fever illness. No correlation was observed between the entomological indexes and the incidence of dengue cases.
