Insecticide-Treated House Screens to Reduce Infestations of Dengue Vectors

The public health importance of the endophilic mosquito Aedes aegypti increased dramatically in the recent decade, because it is the vector of dengue, chikungunya, Zika and ­yellow­fever.­The­use­of­long-lasting­insecticidal­nets­(LLINs)­fixed­on­doors­and­win-dows, as insecticide-treated screening (ITS), is one innovative approach recently evaluated for Aedes ­control­in­South­Mexico.­From­2009­to­2014,­cluster-randomised­controlled­ trials were conducted in Acapulco and Merida. Intervention clusters received Aedes -proof houses (‘Casas a prueba de Aedes ’)­with­ITS­and­were­followed­up­during­2­years.­Overall,­ results­showed­significant­and­sustained­reductions­on­indoor­adult­vector­densities­in­the­treated­clusters­with­ITS­after­2­years:­ca.­50%­on­the­presence­(OR­≤­0.62,­P­<­0.05)­and­abundance­(IRR­≤­0.58,­P­<­0.05).­ITS­on­doors­and­windows­are­‘user-friendly’­tool,­with­high­levels­of­acceptance,­requiring­little­additional­work­or­behavioural­change­by­householders.­Factors­that­favoured­these­interventions­were­(a)­house­construction,­(b)­high­coverage­achieved­due­to­the­excellent­acceptance­by­the­community­and­(c)­col-laboration of the vector control services; and some operational complaints to and the installation process. ITS is a housing improvement that should be part of the current paradigms for urban vector-borne disease control.


Introduction
Dengue remains a priority for public health authorities across the globe. The viral disease is transmittedprimarilybythehuman-bitingmosquitoAedes aegypti, which also transmits other viruses, including yellow fever, chikungunya and Zika [1][2][3]. Although there are several vaccinecandidatesfordengue,chikungunyaandZika,butnoneishighlyeffective.Therefore, preventing or reducing Dengue (DEN) and the transmission of other Aedes-borne diseases depend entirely on control of the mosquito vectors or interruption of human-vector contact.
Control of A. aegypti is mostly performed in the context of public health and depends on structured programmes sustained by ministries of health. Routine vector control efforts, mostly targeting the outdoor allocation of Aedes vectors (i.e. source reduction, hand-applied larvicidestoartificialbreedingsitesandoutdoorultralowvolume(ULV)adulticiding),have to date achieved only a limited success in preventing diseases [4]. Improving urban Aedes control and achieving a measurable impact on DEN transmission require a reformulation of current strategies and a stronger focus on the adult mosquitoes that actually transmit the disease, both lowering vector abundance and preventing human-vector contact [5,6].
Females of A. aegypti mosquitoes have an endophilic behaviour (within buildings) and methods targeting these locations which have great potential for sustained impact. Methods currently accessible to reduce indoor adult vector abundance and prevent human-vector contact include essentially the control of adult vectors with insecticides (adulticides) applied by institutional programmes either as residual surface treatments or as space treatments; personalprotectiontoreduceexposuretobitingwiththeuseofchemicalproductssuchas domestic insecticides, repellents and long-lasting insecticidal nets (LLINs) deployed as curtains or bed nets and changes to human habitation or behaviour such as installing mosquito screening on doors, windows and other entry points.
We recently tested in Mexico an innovative intervention called Aedes aegypti-proof houses ('Casas a prueba de Aedes aegypti' in Spanish), involving insecticide-treated screening (ITS) withLLINspermanentlyfittedtowindowsanddoorsasaninnovativeapproachtoexclude A. aegypti from houses in dengue endemic areas. Here, we present the results of the evaluation oftheseITS-basedinterventionsonthedenguevectorsinSouthMexico.
LLINs are expected to reduce human-vector contact and reduce their life expectancy as a physical barrier, blocking mosquitoes, and as a chemical method, irritating/deterring or eventually killing mosquitoes [10,11].
Based on the successful control demonstrated against nocturnal endophilic Anopheles spp. vectorsandprotectiveefficacyofLLINs(intheformoftreatedbednets)inreducingmalaria transmission [8,12,13],theWHODengueScientificWorkingGroupof2006identifiedthe development/evaluation of LLINs as a primary global research stream [14]. However, LLINs were expected to be delivered in different approaches other than bed nets considering the diurnalactivitypatternsofAedes mosquitoes.
While ITCs can be easily introduced within DEN endemic areas, these studies showed, as found withbednets,thatITCsrequiredproperhandlingandusebylocalcommunitiestobeeffective. Coverage of the interventions based on ITCs typically falls dramatically over time [16,21,22], undoubtedlycompromisingefficacythroughoutthecommunity.Forexample,inIquitos,Peru, asociologicalstudyfoundthatproperuseofITCfallsdramaticallyovertime(45%inthesecond year of deploying) [21,22].Particularly,atthehouseholdlevel,theefficacyofITCsiscompromised when curtains remain open/tied back during daytime or when all house entry points cannot be protected [16][17][18][19][20]23].InGuatemala [19]andMexico [20], it was noted that families would remove or tie back the curtains to increase ventilation during the day, compromising the utility of the intervention as A. aegyptiisaday-bitingmosquito.Asolutiontothisistopermanentlyfix the LLIN to the doors and windows in the form of a screen rather than curtains [24,25].

House screening
Here, we use the term house screening to refer the use of insect screens in a house. An insect screenisbasicallyamesh(metalwire,fibreglassorothersyntheticfibres)stretchedinaframe (woodormetal)fixedontheopeningofahousesuchasadoororawindow.'Mosquito-proofing' of houses (with insect screens) is a form of environmental management based on changes to humanhabitationtoexcludevectorsandreduceman-vector-pathogencontactincludingmosquitoes [26,27].
Thefirstpublishedworkevaluatinghousescreeningasphysicalmethodtopreventmosquitoborne diseases was reported by Celli in Italy for the control of malaria among railroad workers and their families [28]. His study showed that screening porches and chimneys resulted in significantreductionsonmalariaincidence(4%withscreensvs.92%withouttheintervention) [29]. TheItalianexperienceledtowidespreadscreeningofhousesagainstmosquitoesinmalarious areas,notonlyinItalybutalsoaroundtheworld.Examplesofhousescreeningasamalaria control intervention include workers building the Panama Canal and rural homes in the SouthernUnitedStates [30].Nevertheless,thisprotectiveandefficaciousmethodwaslargely forgotten when the primary strategy of insecticidal control with DDT (dichloro-diphenyltrichloroethane) emerged [30].Modernstudiesonhousescreeninghaveprovensignificant reductions on malaria [31][32][33] and described to be widely accepted by communities [34].
TheintegrationofhousescreeningforthecontrolofdenguewasevaluatedinVietnaminthe 1990s.Nguyenetal. [35] and Igarashi [36] evaluated an intervention with permethrin nets covering all openings of houses (in addition to routine anti-Aedes health education and control measures)andreportedasignificantreduction(closeto100%)inthenumberofhousespositivefordenguevectors.Furthermore,indoorA. aegypti was undetectable levels for 7 months, while in the control group, infestation gradually increased during the epidemic season, and apositiveimpactinpreventingDENtransmissionduringtheepidemicseason(at6months after intervention) was observed. Therefore, screening doors and windows have been considered with the potential to limit transmission in the most recent systematic reviews [4,9].
Ingeneral,thestudiescompared10controland10interventionareasof100householdseach acrossbothcities.Routinevectorcontrolactivities-asimplementedbythelocalMinistry of Health-were performed in control clusters. Intervention clusters included insecticidetreated window and door screens (Acapulco and Merida) and targeted interventions in the productive water container types (in Acapulco only). As part of the national policy in response to dengue outbreaks and entomological risk indices [37], control and ITS intervention clusters could receive routine vector control activities (outdoor spraying with the organophosphates malathion or chlorpyrifos, indoor space spraying with the pyrethroid deltamethrin or the carbamates propoxur or bendiocarb and larviciding with the organophosphate temephos).
The indoor A. aegypti adult-based entomological indicators monitored for 2 years in both localities are shown in Figure 3. During the pre-intervention survey, similar infestation levelswerequantifiedinbothstudyarms.ITS protected houses against A. aegyptimosquitoes: significant reduction of indoor-resting adults by approximately50%onthepresence (OR≤ 0.62,P<0.05)andabundance (IRR≤0.58,P<0.05). The combination of ITS and interventions targeting productive container types was successful in continuing reducing the number of Aedes pupae and consequently of adult dengue vectors.
AsignificanteffectonindooradultAedes infestations was observed in houses protected with ITS but not in controls, despite the fact that Aedes populations at both sites were resistant to pyrethroids,andthisprotectionwassustainedbeyond24monthswhenITSwascombined with targeted treatment of productive breeding sites (as demonstrated in Acapulco, where the combinedinterventionmaintainedastatisticallysignificantprotectiveeffectonAedes adult andimmaturestagesuntiltheendofthestudy,approximately600daysafterITSinstallation).
MultiplefactorscouldexplainthelackofcompletesuppressionofA. aegypti indoors by ITS. First,thismethoddidnotaimtoreduceperidomesticbreedingorabundanceofmosquitoes. As intervention coverage on each block was not 100%, there is a possibility for mosquito breedingandhumanfeedingeveninthepresenceofscreens.InAcapulco,Mexico,combining ITS with peridomestic larval control on the most productive larval breeding habitats produced asynergisticeffectevidencedasareductionoftheabundanceofindoorfemalemosquitoes in screened houses [25].ThisfindingprovidesevidenceoftheimportanceofintegratingITS withmethodsfocusedonperidomesticcontrol.Anotherfactorexplainingindoorpresenceof A. aegypti in screened houses is the fact that maybe some adults did not contact and/or survive after the initial contact with the LLIS or probably not all contacted the screened surfaces on the day when collections were performed.
Anotherexplanationcouldbethelossofinsecticidalpowerandhighpyrethroidresistancein themosquitopopulationsofMerida.Exposuretosunlight,rainanddustimpactstheresidual  The physiological resistance in mosquitoes to the insecticide and the effect on the performance of a LLIN cannot be discounted. Pyrethroid resistance in A. aegypti populations of Mexico,includingMerida,hasincreasedduringthelastdecade [39][40][41][42]. Currently, ITS with LLIN will have to challenge insecticide resistance to pyrethroids because they are the only insecticide class recommended and available for LLIN [43].
Participants also appreciated a reduction in mosquito numbers. Though many were unaware that the net contained insecticide, they had noticed that mosquitoes and other pests died on contactwiththescreenandweresatisfiedwiththis.Theinsecticideinthescreenwasseenasbeneficialandacceptable,withfewreportsofsideeffectsorfearsaboutitsuse.Participantsreporteda reductioninfliesandcockroaches,and79.9%ofsatisfactionsurveyparticipantsreportedareductioninotherpests.Themajorityofparticipants(90.2%)hadthesameamountormorescreensin place now compared to the original amount installed, suggesting that very few were removed.

Conclusions
Intheabsenceofeffectivetreatmentorvaccinesandinthecontextofmultipleco-circulating viruses transmitted by Aedes mosquitoes, the development of preventive and long-lasting methods for Aedes control has become a top global health priority.
Thebenefitsofhousescreening,asaphysicalbarrier,relyonitsefficacytoexcludemosquitoes and eventually protect against mosquito bites, which is epidemiologically relevant if most transmissionoccursindoors.Fromanenvironmentalhealthperspective,residentialpremises (houseandperidomicile)offerimportanthabitatsforsupportingpopulationsofA. aegypti as they emerge from productive breeding sites, move in and out houses in search of food (human blood), refuge and mating and oviposit at the suitable breeding sites to complete their life cycle. Aedes is an anthropophilic, endophilic and endophagic species, and the house is the epidemiologicallymostsignificantpointofvector-humancontactforarbovirustransmission.
The adaptation of long-lasting insecticide nets permanently fitted as mosquito screens on windows and doors has advantages over other approaches (such as bed nets and curtains) because these interventions are in place permanently and require little additional work or behavioural change by householders.
OurstudiesinMexicodemonstratethatLLISdeployedasITSactsasabarrierandsignificantly restrainstheentranceofmosquitoestohousesforatleast2yearspostdeployment.Concisely,a houseprotectedwithITSondoorsandwindowshasatleast50%lesschancesofhavingA. aegypti femalesincomparisonwithanon-protectedhouse.ITSconfirmedasustainedprotectiveeffect on indoor female mosquitoes, the most epidemiologically important target for vector control, even in the presence of high levels of resistance to pyrethroids in the local mosquito population.
The present studies provide valuable and unique information on the use of house screening within cities endemic for mosquito-borne diseases, and at the time of writing, are unique in supportingthefeasibilityandpotentialbenefitofthismethodforthesimultaneousprevention and control of dengue, chikungunya and Zika transmission. The positive results from trialsusinghousescreening/fullscreeningofwindows/doorssuggestthatexcludingthevector A. aegypti from the home may prove to be an innovative approach in terms of environmental management (changes to human habitation), if it is proven ultimately to reduce transmission of the pathogens to humans. This simple classic method of vector control should be considered and encouraged by the National Ministries of Health.
As most human-mosquito contacts occur indoors, the observed reductions in abundance and prevalenceofinfestationinourstudiesmaybesignificantenoughtoimpactvirustransmissioninameasurableway.Assessingtheepidemiologicalimpactofexistingandnewparadigms on Aedes-borne disease transmission remains a top research and public health priority. ParticularlyforITS,ourentomologicalstudiesprovideevidenceforthedevelopmentofrobust RCT(RandomizedControlledTrials)evaluatingtheshort-andlong-termprotectiveefficacy ofthismethod.OfparticularinterestistheevaluationofthepotentialforITStobeincluded within an IVM (Integrated Vector Management) scheme that also includes other modes of vectorcontrolthattargetperidomesticbreedingoradultabundance(e.g.larviciding,ULV).
TheauthoritiesofMexicoareconsideringhowtoexpandAedes aegypti-proof housing to as many homes as possible, probably as a targeted intervention for high-risk areas (hot spots) of endemic localities. Given that the same mosquito transmits dengue, chikungunya and Zika, the local governmentofYucatanisplanningtoscalethisinterventiontoprotectthemostvulnerable,particularly pregnant women and their families. Therefore, and in accordance with the local MoH, our team is currently also developing a pilot study evaluating an integrated intervention on pregnant women, providing them with information and a kit of tools including repellents and Aedes aegypti-proof houses to protect them against mosquito biting and disease transmission.

Author details
Pablo