Associations between meteorological variables, vector indices and dengue hospitalizations in Can Tho, Vietnam: a field survey
Authors: Toai Nguyen Phuong , Van Chinh Dang , Huy Nguyen Ngoc , Vittor Amy Y. 
Published: June 2019

Introduction

Dengue is a significant cause of morbidity and mortality in Can Tho, a province in the Mekong Delta in Vietnam. In this region, average temperatures have increased by 0.5°C since 1980, and river levels have risen. In a time-series analysis, we previously found that relative humidity was the most important meteorological predictor for dengue hospitalizations in Can Tho. To better understand proximate factors mediating this association, this study examines weather variables in relation to dengue hospitalization rates, vector indices, container productivity and larval elimination and mosquito avoidance behaviors.

Methods

Four hundred households were sampled bimonthly for one year in Can Tho. Vector indices of the immature forms of the dengue vector, Aedes aegypti, and the productivity of different types of household containers were determined. Dengue hospitalization rates were determined for the study period. Associations between these variables and mean temperature, relative humidity, precipitation, and the number of hours of sun were estimated using mixed effects Poisson regression analysis. Relative productivity of containers was determined by collecting Ae. aegypti pupae using a sweep method and adjusting by a calibration factor. Ae. aegypti larval density risk factors were determined using multivariate generalized estimating equations with a negative binomial distribution. To examine possible mechanisms mediating the relationship between climate, vectors and dengue, we also interviewed households about mosquito avoidance and larval elimination behaviors.

Results

The house-(HI), container-(CI), Breteau (BI), and pupal (PI) indices were associated with relative humidity (1-month lag, IRRHI=1.10 (95% CI 1.06, 1.13) per 1% increase), IRRCI=1.10 (95% CI 1.02, 1.19), IRRBI=1.17 (95% CI 1.14, 1.21), IRRPI=1.12 (95% CI 1.10, 1.14)). Vector indices were also associated with precipitation (1-month lag) and to a lesser degree, hours of sun and mean temperature. Ae. aegypti larval density was associated with not cleaning water storage containers (RR=2.50, 95% CI 1.59, 3.66), not having access to municipal waste pick-up (RR=3.15, 95% CI2.09, 4.75), disheveled clothes in the home (RR=1.85, 95% CI 1.24, 2.74) and season (RR[rainy season]=3.10, 95% CI 2.18-4.48). The most productive containers were water storage containers (relative pupal productivity 87%). Dengue hospitalization rates were associated with relative humidity (2-month lag, IRR=1.11 (95% CI 1.06, 1.17) per 1% increase). Only the PI (1-month lag) was significantly associated with dengue hospitalization rates (IRR 1.04, 95% CI 1.00, 1.07). Mosquito avoidance behaviors were more frequent in the dry season (92.5% vs. 86.0% of interviewees endorsed one or more forms of mosquito prevention, p<0.001). There was also less use of larval elimination strategies (39.2% vs. 50.5%, p<0.001) during the rainy versus the dry season.

Conclusion

Our study reveals a strong effect of relative humidity on vector indices and dengue hospitalization rates. This may be due to the mosquito’s vulnerability to desiccation, and the association warrants further study. Our findings also demonstrate, however, that during the rainy season when mosquito prevention is most needed, the use of fans, repellant coils and maintenance of water storage containers is actually reduced. Water storage containers were by far the most productive of pupae, and should be targeted in vector control activities.

Author summary

Climate plays an important role in the geographic distribution and burden of disease due to dengue, owing to the vector and virus’ sensitivity to temperature, humidity, and rainfall. In the Mekong Delta in Vietnam, where dengue poses a significant health burden, average temperatures have increased by 0.5°C since 1980. To better understand the influence of climate on dengue, this study examines its influence on dengue hospitalization rates, vector breeding behavior and human mosquito avoidance behaviors. We sampled 400 households every 2 months for one year for the presence of the dengue vector, Aedes aegypti, and the productivity of different types of household containers. Human mosquito avoidance behaviors, such as the use of fans, mosquito repellant, and larval elimination strategies were also recorded. The association between dengue hospitalizations, mean temperature, relative humidity, precipitation, and the number of hours of sun were established, and risk factors for the abundance of Ae. aegypti larvae were determined. We found that relative humidity is positively associated with the presence of Ae. aegypti immature forms, and that large jars used for water storage serve as the most important source of this vector. We also determined that people engage in mosquito avoidance/larval elimination strategies more frequently in the dry season versus the rainy season, despite increased vector breeding and dengue hospitalizations during the rainy season. This temporal disconnect between peak vector activity and dengue hospitalization rates vis-à-vis mosquito control strategies is a potential area for intervention.