| dc.description.abstract |
Dengue infection has been an important flaviviral infection in Sri Lanka and is considered as an emerging and re-emerging disease causing epidemics from time to time. During the last 20 years hyperendemic transmission has been established over a geographically expanding area. Transmission of dengue virus is known to be influenced by overlapping and reinforcing factors related to human host, virus and mosquito vector. Present study is on the effect of silent transmission, clustering of dengue cases and the role of Aedes vectors on transmission of dengue in Gampaha and Kurunegala Districts of Sri Lanka. The present study was carried out using the cluster investigation method. Index clusters were selected from hospital based dengue cases. Control clusters were selected from low dengue transmission areas. Cluster enrolees recruited were residing within 100 m radius from the index case and blood samples and demographic data were collected. At each household, adult and larval mosquito surveillance was also carried out. Volunteer samples were tested for recent dengue infection by IgM EL1SA. A selected set of sera were tested for dengue Ag by Ag detection ELISA, and for dengue viral RNA by RT-PCR and Semi-Nested PCR. ffA I assay was done as a confirmatory test. Of the 629 volunteers recruited from 30 index clusters, 90 (14.3%) were positive for recent dengue infection while none of the 310 volunteers of 15 control clusters were positive. Of the 90, asymptomatic dengue infection was observed in 49 (54.4%) indicating symptomatic to asymptomatic rate of infection was 1:1.2 in the study locations. Previous dengue infected volunteers were more vulnerable to infection (r2=2.19, p<0.00!) and age was the most important factor determining asymptomatic infections (r=0.672, pO.OOl). Irrespective of the gender, dengue exposed population over 40 years of age were more likely to be asymptomatic (x2==21.87, pO.OOl). Of the 33 paired sera tested by HAI assay, dengue virus infection was confirmed in 4 volunteers, of them 3 were asymptomatic confirming the presence of silent transmission in the study population. Level of IgM antibody was less among asymptomatic volunteers compared to symptomatic volunteers and hospitalized patients (F=16.05, pO.OOl). Study confirmed the clustering of dengue cases around index cases (t=5.06, pO.OOl). Dengue infections were clustered centrally close to the index patient's house (F=5.894, pO.OOl) with a significant trend towards household risk of infection. There was a significant correlation between the presence of dengue infection with breteau index (+0.372) and Aedes albopictus was found in 70% of the localities. Of the 40 adult Aedes mosquito pools (3 Ae. Aegypti and 37^1e. albopctus) collected from index cluster localities, only one pool (Ae. albopictus) was positive for dengue viral RNA and it was found to be of dengue serotype 2 (DEN-2). Study confirms the presence of asymptomatic dengue infections, clustering of dengue cases and the probable role of Ae. albopictus on dengue transmission leading to silent spread of the disease. |
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