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Temperature and the Oceanic Niño Index (ONI) may predict the seasonal prevalence of Aedes vectors with an estimated lead time of 6 months, allowing policy makers to implement effective infection control measures against dengue fever and other Aedes-borne diseases. These findings were published in Lancet Planetary Health.1
Using climate and weather data captured from 2010 to 2018, researchers conducted a 2-stage hierarchical analysis that assessed patterns of Aedes vector activity in 10 administrative subunits of Kalutara, Sri Lanka. The researchers sought to determine whether associations between meteorologic variables and 3 commonly used vector indices could be used to predict the prevalence of vector activity. The 3 vector indices assessed were the premise index, defined as the percentage of houses and land units infested with immature Aedes (larvae and pupae); the container index, defined as the percentage of water-holding containers infested with immature Aedes; and the Breteau index, defined as the number of positive water-holding containers per 100 residences. The researchers also obtained ONI data from the National Oceanic Atmospheric Administration Centre for Weather and Climate Prediction. Values of 0.5 or more and -0.5 and below on the ONI were representative of El Niño and La Niña events, respectively.
During the study period, 3 El Niño events occurred. Analysis of these events showed that cumulative monthly rainfall of 200 mm or more was associated with higher vector indices in the same month, indicating increased vector activity. For cumulative monthly rainfall measurements of 1000 mm, the estimated maximum relative risk (RR) for an increase in vector index values were 1.55 (95% CI, 1.12-2.14) and 1.48 (95% CI, 1.03-2.12) for the premise and Breteau indices, respectively.
Further analysis showed that higher vector index values suggestive of increased vector activity were associated with temperatures above 31.5 °C (88.7 °F). For a temperature of 34.0 °C (93.2 °F) with a 2-month lead time, statistically significant associations were observed for the premise (RR, 1.35; 95% CI, 1.07-1.49) and Breteau (RR, 1.26; 95% CI, 1.07-1.49) indices, but not for the container index (RR, 1.14; 95% CI, 0.99-1.31).
Statistically significant associations were noted between the ONI and all 3 vector indices. For a 6-month lead time, the maximum RR observed at an ONI value of 2 for the premise, Breteau, and container indices were 1.54 (95% CI, 1.25-1.90), 1.53 (95% CI, 1.25-1.88), and 1.29 (95% CI, 1.11-1.51), respectively.
This study was limited by the inability to assess environmental conditions related to La Niña events as El Niño conditions were both more prominent and intense during the study period.
According to the researchers, “[these findings] are relevant for understanding the consequences of climatic variability on vector activity and broadly on all Aedes-borne diseases, including chikungunya and Zika [virus], in Sri Lanka and other tropical countries.”
We spoke with lead study author Prasad Liyanage, PhD, and fellow author Yesim Tozan, PhD, to obtain further insight in these findings and how they might help public health authorities design and implement effective vector control measures. Dr Liyanage is a Medical Officer at Sri Lanka Ministry of Health and Researcher at Umeå University; and Dr Tozan is Assistant Professor of Global Health at New York University.
Dengue transmission rates in Sri Lanka were linked to monsoonal rainfalls, with this study finding that 3 indices (rainfall, temperature, and Oceanic Niño Index) were each related to mosquito activity. How might public health officials and infectious disease experts potentially collaborate to implement effective mosquito control interventions?
Dr Tozan: Dengue fever is a complex disease; its transmission dynamics in a locality are influenced by a myriad of factors, including climatic and environmental conditions, socioeconomic context, and public health interventions. Therefore, dengue control requires a multifaceted approach, such as bringing together experts from a range of disciplines and sectors as well as communities. Infectious disease experts, including epidemiologists and entomologists, can contribute by periodically predicting and communicating the risk for dengue fever over space and time. Climate information, when combined with entomologic and epidemiologic surveillance data, which includes temporal and spatial distribution of dengue cases and the circulating serotypes of dengue viruses, can provide reliable risk estimates across time and space. Continuous vector surveillance can provide insight into the most productive mosquito breeding sites for public health authorities to plan and launch targeted and community-based source reduction interventions. This collaborative approach can refine vector-control activities at the granular level of particular mosquito breeding places and provide adequate lead time to plan and implement effective and efficient mosquito-control interventions in resource-constrained settings.
Given the estimated 6 month lead time following El Niño weather events prior to the increased prevalence of Aedes mosquitoes, what early warnings and interventions could be effectively communicated to local populations?
Dr Liyanage: Community participation is essential for achieving effective mosquito control. The ability to promote a sustainable change in [behavior] to eliminate a specific mosquito breeding place depends on the extent to which public health authorities can create a collaborative environment. Such a community-based approach is not an easy task and requires advanced planning. For instance, if water-holding containers are the most productive breeding sites for mosquitoes in the locality, health education and communication activities would need to be designed to [mobilize] and engage the community to achieve the specific [behavioral] objective of covering water-storage containers with, ideally, freely available and appropriate materials. An advocacy campaign should also be conducted for policy makers to provide a continuous water supply to the locality. When triggered by early warnings from the climate, this 2-way process would be an effective intervention to reduce the burden of dengue fever. Other productive breeding places — such as roof gutters, tires, and covering items — need a similar but targeted approach.
Can any trends in data from a previous study that assessed the influence of El Niño events on mosquito populations in southern California be extrapolated to guide research in the United States?2
Dr Tozan: El Niño and La Niña events (El Niño Southern Oscillation Cycles) have differential effects on weather (rainfall and temperature) in different regions of the world, thus on the abundance of various types of mosquitoes. Our study proposed a unique methodological framework which can quantify the lagged effects of climate variables, including El Niño and La Niña events, on mosquito abundance in any setting. If the necessary data are available, the framework we proposed can easily be applied to predict mosquito abundance based on climatic factors in California and elsewhere in the United States.
References
1. Liyanage P, Tozan Y, Overgaard HJ, Tissera HA, Rocklov J. Effect of El Nino-southern oscillation and and local weather on Aedes vector activity from 2010 to 2018 in Kalutara district Sri Lanka: a two-stage hierarchical analysis. Lancet Planet Health. Published online July 6, 2022. doi:10.1016/S2542-5196(22)00143-7.
2. Heft DE, Walton WE. Effects of the El Nino-southern oscillation (ENSO) cycle on mosquito populations in southern California. J Vector Ecol. June 2008;33(1):17-29. doi:10.3376/1081-1710(2008)33[17:eotens]2.0.co;2.
