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Odorless Emanators Can Passively Kill Disease-Bearing Mosquitoes in Endemic Areas Interview with:

Gregor J. Devine, Ph.D Mosquito Control LaboratoryQIMR Berghofer Medical Research Institute Brisbane, Queensland, Australia

Dr. Devine

Gregor J. Devine, Ph.D
Mosquito Control LaboratoryQIMR Berghofer Medical Research Institute
Brisbane, Queensland, Australia What is the background for this study?

Scale of the problem: Dengue, Zika and chikungunya are all transmitted by the same mosquito species.  That mosquito, Aedes aegypti, is superbly adapted to the human, urban environment – it lays its eggs and develops in the standing water that collects in the myriad containers associated with modern living (plastic bottles, food packaging, buckets, planters, crumpled tarpaulins etc.). Unusually they rely almost entirely on human blood for their nutritional requirements and they subsequently bite multiple times during each egg laying cycle. That reliance on human blood means that they are usually found resting indoors, a behaviour that also offers them some protection from weather extremes and predators. Once infected, and having incubated the virus until it is transmissible, a mosquito that survives for just a couple of weeks can infect many humans within the same and neighbouring households.

In poorer tropical urban environments with dense human populations, unscreened houses, no air-conditioning, and innumerable rain-filled containers to develop in, Aedes aegypti proliferates and so do those diseases, causing ca 400M annual infections of dengue alone by some estimates. The economic impact of the dengue, which normally causes a high fever, muscle and joint pains and nausea, is pronounced; especially in poor households with few savings and no welfare system. Every year, about 500,000 of those dengue cases develop into severe dengue, or dengue haemorrhagic fever (typified by plasma leakage, severe bleeding and organ impairment). There are about 25,000 deaths annually.

mosquito-Aedes aegypti-feeding-human.jpgThe Zika pandemic of 2015-2016 resulted in 1000s of babies born with microcephaly and damage to their brains and eyes. For 1000s of other children, the impacts of Zika on their cognitive development did not manifest in their first, formative years.  Chikungunya is endemic in Asia and Africa but between 2010 and 2014, outbreaks and epidemics spread across the Indian Ocean, the Caribbean, the Americas and the Pacific Islands. It causes severe, often debilitating joint pain in infected patients. Those affected also suffer from headaches, fever, severe muscle pain and conjunctivitis. Joint pain can persist in subacute or chronic form for several months or even years, particularly in older patients. The ubiquity of the mosquito Aedes aegypti across the tropics and sub tropics ensures that further epidemics of Zika and chikungunya will occur, outside their usual ranges. It’s simply impossible to predict when that will occur.

Few options for disease control: These viral diseases are of tremendous concern, because there are no effective vaccines or therapeutants available for their management. All we have are insecticides and mosquito control. However, control of Aedes aegypti is a slow, patchy process that struggles to have an impact on disease outbreaks. The preference of Aedes aegypti for the indoors means that the best way of killing potentially viraemic mosquitoes is to apply insecticides inside houses but the logistics of that, especially in big urban cities, is challenging. Teams move slowly though an area, house-by-house, many residents are not at home, and dogs and suspicious homeowners need to be managed or avoided. The sprays themselves have an odour and may leave visible residues that are not always acceptable. Moreover, the number of Aedes aegypti populations that have evolved resistance to the insecticides used in many public health programs

A solution? Volatile pyrethroids, exhibiting both lethal and behavioural effects on mosquitoes are available in formulations that release insecticides passively to the air, at room temperature. These may be suitable for deployment in houses with the aim of creating “bite-free” spaces. By removing the need for conventional application methods (insecticide spraying, indoors) these devices might be rapidly deployed with minimum disruption to households. This control method comes under the “spatial repellent” paradigm, currently listed by the World Health Organisation’s Vector Control Advisory Group (VCAG) as a promising novel intervention that requires a better evidence base. Can you describe how the emanators work? 

Response: Metofluthrin is widely registered as an insecticide across the globe and is available in a number of consumer products. It has never been evaluated in a public health context. The treated devices that we use are hung, one per room in a household. Insecticide volatises from the device without the need for heat or power. Metofluthrin is a member of the pyrethroid class of insecticides. Pyrethroids act upon the nervous system of insects to induce paralysis and death. Metofluthrin kills mosquitoes in the same way. At the tiny doses delivered by our treated devices, an additional sublethal impact on the nervous system appears to interfere with their host seeking behaviours. Affected mosquitoes stop flying, stop biting and spend more time resting. Subsequently, they may desiccate or starve or, if they stay near the emanator, they may die because of continued insecticide exposure. What types of insects would be effective against?

Response: Metofluthrin is effective against a range of dipterans (e.g. true flies and mosquitoes). In a public health context, its passive diffusion is suited to protecting humans indoors from bites that can transmit disease. What are the main findings?

Response: This is the first large-scale, randomized control trial to evaluate the entomological impacts of volatile pyrethroids in an urban environment. We used devices suitable for rapid deployment, treated with the chemical metofluthrin. These devices were hung, one per room, in 100 households in a town in the Yucatan, Mexico. Results were compared with 100 non-treated households.

The trial demonstrated a significant impact on Aedes aegypti densities (decreases of 60%) and, importantly, biting behaviour indoors (decreases of 90%).

These effects occurred despite the ubiquitous presence of pyrethroid- resistance alleles associated with conventional insecticide resistance.

In household surveys, the community perceived the new intervention favourably. They commonly observed that the emanators were “odorless”, “unobtrusive”, “environmentally friendly”, “easy to install” and “safe for the family and pets”. Almost all household heads thought that the intervention was suitable for wider use in the community. What should readers take away from your report?

Response: There are no effective vaccines or therapeutants for the control of most mosquito-borne diseases. All we have are insecticides, but these are challenged by coverage and resistance. Quick and highly effective interventions are desperately needed and these metofluthrin devices may provide additional tools for the fight.

In 2020, COVID-19 emerged as a formidable challenge to dengue surveillance and control operations. The diversion of resources, disruption of transport networks and social distancing restrictions, as well as pressure on the public health and hospital system, have played havoc with dengue outbreak responses despite their continuing presence. Many countries are now suffering dengue epidemics within the wider COVID-19 pandemic and are struggling to cope. More than ever, we need fast, reliable tools for control. The Zika pandemic (2015-2016) and the re-emergence of Yellow Fever in Brazil (2016-2018) remind us that the next public health emergency may be mosquito-borne. What recommendations do you have for future research as a result of this work?

Response: These trials will continue in 2021. This time, we will put the devices in the hands of the householders to determine whether this approach is suitable for community-led outbreak control. In large urban cities, community involvement will be essential if control tools are to be deployed with sufficient speed and coverage to affect transmission over large areas. The next step following that proof will be epidemiological trials that aim to demonstrate a reduction in disease transmission. Is there anything else you would like to add?

 Response: This trial was funded by a USAID award to Gregor Devine, QIMR Berghofer. Prof Pablo Manrique’s team at the Campus de Ciencias, Biológicas y Agropecuarias, Universidad Autónoma de Yucatán deployed the emanators and conducted the assessments.

Gregor Devine spent many years working in vector control and ecology in Peru (2004-2009) and Tanzania (2010-2011) and led the dengue vector control team in North Queensland from 2011-2013.

Citation: Devine GJ, Vazquez-Prokopec GM, Bibiano-Marín W, Pavia-Ruz N, Che-Mendoza A, Medina-Barreiro A, et al. (2021) The entomological impact of passive metofluthrin emanators against indoor Aedes aegypti: A randomized field trial. PLoS Negl Trop Dis 15(1): e0009036. doi:10.1371/journal.pntd.0009036



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Last Updated on February 4, 2021 by Marie Benz MD FAAD