A new study examines how effective Ovitraps are in controlling mosquitoes (and ultimately disease prevention) in Texan communities.
Aedes aegypti. Photo credit: Muhammad Mahdi Karim, https://commons.wikimedia.org/wiki/File:Aedes_aegypti.jpg
Aedes aegypti is a terrible mosquito. It thrives in warm, urban environments in subtropical and tropical regions around the globe. Ä. Egyptians are incredibly competent vectors for several viruses, including dengue, chikungunya, zika, and yellow fever. The disease burden and spread of these vectors is expected to increase due to climate change. Controlling these mosquito populations in and around households is of paramount importance to disease prevention, both now and in the future.
Control of Ae. Egyptians has included both chemical (insecticidal) and mechanical approaches. Insecticides have been used on larval habitats, screens and in fog campaigns. While mosquitoes are effective against malaria mosquitoes (anopheles Genus), Ä. Egyptians feeds during the day and does not make this control possible. In addition, control campaigns have focused on the removal of containers that are important breeding grounds (often artificial water containers). While some control measures are effective, they may not always remain in effect for long periods of time (e.g. insecticide resistance) or they may not work in all situations.
Ovitraps have long been used to catch and count pregnant (pregnant) female mosquitoes. Ovitraps are designed to attract egg-laden females by providing ideal egg-laying conditions – usually a black bucket filled with grass-soaked water. The mosquitoes penetrate the trap, but are prevented from reaching the water by a fine-meshed sieve. Sticky surfaces trap the female mosquitoes and allow important disease vectors in an area to be enumerated and identified. Although they have been used for mosquito research for decades, more recently scientists have tested them as a control tool to help reduce mosquito populations and disease.
A recent study looked at whether Ovitraps was used as a means of reducing Ä. Egyptians Populations. Although a previous study showed them to be an efficient control option in Puerto Rico, it is unknown how accepted and effective they are in other settings. This current study was conducted in the Lower Rio Grand Valley in South Texas. Ä. Egyptians is abundant and local transmission of Dengue, Chikungunya and Zika viruses has been documented.
The authors recruited households from low- and middle-income communities in the region. During the intervention periods, households had used improved autocidal Gravid Ovitraps inside and outside of each household. Mosquitoes were collected and identified weekly. All communities received the intervention, but the timing of the intervention was randomly assigned – so there were control communities at each point in the study.
Every week, scientists collected and identified mosquitoes at all monitoring and intervention centers. They were interested in how the use of Ovitraps could change the weekly frequency of female mosquitoes. They hypothesized that the interventions (Ovitraps) either 1) have immediate effects and last for the entire duration of the intervention, or 2) have temporary effects and do not last for the duration of the intervention. They used a special set of statistical models called generalized linear mixed models and generalized additive mixed models to account for the independence of measurements over time and space. This just means that they took into account the fact that measurements taken close together – either a few weeks apart or geographically close – are more similar than measurements taken further apart.
At all control locations, scientists recorded 2,929 women in 2017 and 4,1117 in 2018. By comparing intervention households with households without intervention, they showed that Ovitraps had a significant influence Ä. Egyptians Population, with a two-week delay. The middle-income households and indoor traps had smaller populations than low-income households and outdoor traps. The use of Ovitraps in 2018 resulted in a 77% reduction in female Ae. aegypti – very impressive!
Participation was not compulsory and of course there were households that did not participate. To take into account the different Ovitrap efforts across the area, they calculated a “trap coverage” that they should include in their statistical models. For example, more traps within 200 m (the approximate flight distance of mosquitoes in that area) could result in a greater decrease in mosquito population.
The scientists found that covering traps was very important to the success of the intervention. Interestingly, the coverage increases from none to a few cases (1 / house) – the abundance Ä. Egyptians actually increased. However, once the coverage was higher (2 / house) than the frequency fell below the densities found in a trap-free area. There is much evidence that mosquito larvae compete for resources as larvae. It is possible that by removing a little (but not a lot) that competition will decrease and allow for larger adult mosquito populations. This interesting result underscores the importance of testing interventions in the field and finding optimal control protocols.
The results of this study offer a positive control option Ä. Egyptians Population in urban areas. Of course, more research is needed to understand the impact of changes in population on disease in these communities, but it provides significant and substantial changes for vector populations relatively quickly. Interestingly, the scientists found that trap cover was a key factor in how effective this intervention was. They highlight the importance of engaging local communities and participating in vector control efforts.