Determination of entomological indices to assess dengue transmission, predict dengue outbreaks, and evaluate vector control interventions

Background. Dengue fever is the most rapidly spreading mosquito-borne viral disease globally causing about 390 million infections and 22,000 deaths annually. There is no effective vaccine or antiviral treatment. The major mosquito vector is Aedes aegypti. There are four dengue virus (DENV) serotypes and infection of one serotype generally gives lifelong protective immunity to that serotype. Prevention depends on mosquito control. Outbreaks occur periodically. Early warning is not yet effective enough to initiate control interventions in time. There is an urgent need to develop cost-effective and practical tools that can reliably measure dengue transmission and predict outbreaks. Objectives. The aim of the project was to contribute to the development of early warning systems for dengue epidemics by evaluating entomological and immunological indices that accurately and reliably represent transmission. The specific objectives were to 1) assess the accuracy of proposed indices in identifying households with or without confirmed dengue-infected individuals; 2) identify risk factors of dengue transmission; 3) assess the relationship between indices and dengue incidence longitudinally; and 4) assess their effectiveness in detecting the impact of a vector control intervention. Methods. The project was done in northeastern Thailand during 2016-2020 and included 1) a case-control study (CCS) and 2) a cluster randomized control trial (RCT). Fever patients at local hospitals were recruited into the CCS. Differences in dengue risk factors and indices between case (DENV+) and control (DENV-) households were assessed. The RCT assessed differences in dengue, entomological indices, prevalence of mosquito saliva antibodies in human blood between the intervention arm (pyriproxyfen, PPF, an insect growth regulator applied to breeding sites) and the control arm (no interventions). In both studies, entomological and socioeconomics, disease surveillance and human blood samples were collected. Molecular methods were used to detect DENV and other mosquito-transmitted viruses, e.g., Zika (ZIKV) and chikungunya (CHIKV) in human blood and in mosquitoes. Human exposure to mosquito bites was assessed by ELISA. Insecticide susceptibility was assessed in mosquitoes. Local climate-dengue spatial relationships in Khon Kaen province were assessed during 2006-2016. Results. A total of 166 dengue cases and 189 controls were included in the CCS with complete matched entomological and household information. Most mosquitoes collected were A. aegypti. The presence of DENV-infected Aedes in patient houses was associated with 4 times higher odds of dengue infection than in control houses. In contrast, immune response to Aedes bites was higher in control than in case patients. Non-infected Aedes abundance and presence of mosquito immature stages were unexpectedly higher in control than in case houses. Children and adolescents had higher odds of dengue infection than older patients. DENV-4 was the most abundant serotype. ZIKV and CHIKV were detected in about 4% and 5% of the patients, respectively. Co-infections of DENV and ZIKV occurred in patients. The CHIKV ECSA (East/Central/South Africa) strain was predominant. Close to 6% of individual mosquitoes were DENV-positive. ZIKV and CHIKV were detected in 17% and 3% of mosquito pools, respectively. The RCT showed no differences in entomological indices between the PPF-arm and the control arm. However, a strong positive relationship was found between human immune response to Aedes bites and adult Aedes abundance in households. PPF inhibition on mosquito emergence never reached 100% and declined quickly to only 10% sixty days after treatment. Additional findings indicate that vectors were resistant to PPF and to most insecticides used for Aedes control. The RCT coincided with an interepidemic period with exceptionally few fever and dengue cases. The climate-dengue results show that the risk of dengue increased by 5.5% for each degree Celsius increase in maximum temperature. Conclusions. DENV-infected adult Aedes mosquitoes is a better indicator of dengue risk than the traditional immature entomological indices. Unexpected entomological and immunological findings shows that vector-dengue relationships are very complex. These findings can be explained by transmission occurring away from the home or before mosquito collections were done or that the observed immune responses reflect human vector exposure during weeks before collections. Few dengue cases during the RCT prevented assessment of associations between entomological indices and dengue incidence. A rapid decline in PPF effectiveness may explain the lack of differences between the study arms. However, hidden and untreated breeding sites could also explain the observed results. The project advanced the scientific understanding of entomological and immunological indices and climate relationships useful for dengue early warning systems.

Dengue patients and households were involved in fever surveillance to understand the links between vectors at their houses and dengue, thus improving their knowledge, vigilance, and vector control to reduce disease burden. Nurses and hospital staff communicated with government vector control staff providing increased interest in dengue control. Government officials engaged in capacity building, early vector control, disease monitoring, and public communication. The project contributed to dengue epidemic early warning systems by assessing entomological and immunological indices in the field. Despite complicated biological relationships, we showed that the presence of DENV-infected Aedes vectors was more important for dengue disease than other indices. Current international early warning systems use weekly weather, Aedes mosquito and dengue case data to predict outbreaks. Our longterm spatiotemporal results show that temperature was significantly associated with higher dengue incidence.

Dengue fever is the most rapidly spreading arboviral disease in the world causing an estimated 390 million infections and 22,000 deaths annually in at least 100 countries. There is currently no cure and no licensed vaccine available. Disease prevention is done by mosquito vector control. Outbreaks occur periodically in most endemic countries, but outbreak detection is usually not accurate or timely enough to initiate control interventions in time. There remain critical gaps in knowledge and an urgent need to develop cost-effective and practical tools that can reliably measure dengue transmission and predict outbreaks. Entomological indices have been used for almost a century to evaluate mosquito infestation. However, currently no entomological index is accurate and universal enough to estimate dengue transmission or predict outbreaks. This project aims to contribute to the development of early warning systems for dengue epidemics by evaluating entomological and immunological indices that accurately represent dengue transmission and evaluate the impact of vector control interventions. Specifically, the project will assess the relationship between proposed indices and dengue incidence using vector density, vector infectivity, and human exposure to mosquitoes; and to assess the validity of the these indices in evaluating the impact of vector control interventions.