Presumed unconstrained dispersal of Aedes aegypti in the city of Rio de Janeiro, Brazil Dispersão de Aedes aegypti em local presumidamente sem barreira limitante

OBJECTIVE: To evaluate dispersal of Aedes aegypti females in an area with no container manipulation and no geographic barriers to constrain mosquito ﬂ ight. METHODS: A mark-release-recapture experiment was conducted in December 2006, in the dengue endemic urban district of Olaria in Rio de Janeiro, Southeastern Brazil, where there is no evident obstacle to the dispersal of Ae. aegypti females. Mosquito traps were installed in 192 houses (96 Adultraps and 96 MosquiTRAPs). RESULTS: A total of 725 dust-marked gravid females were released and recapture rate was 6.3%. Ae. aegypti females traveled a mean distance of 288.12 m and their maximum displacement was 690 m; 50% and 90% of females ﬂ ew up to 350 m and 500.2 m, respectively. CONCLUSIONS: Dispersal of Ae. aegypti females in Olaria was higher than in areas with physical and geographical barriers. There was no evidence of a preferred direction during mosquito ﬂ ight, which was considered random or uniform from the release point.

In the city of Rio de Janeiro, Brazil, the mosquito Aedes aegypti is essentially associated with human density, and is especially abundant in urbanized and densely populated areas. 1 Ae.aegypti breeds almost exclusively in artifi cial large containers located in the peridomestic area, such as water tanks and metal drums 15 and seldom invade the fringe of urban forests, generally not laying eggs or fl ying through distances exceeding 100 m from houses. 12,14spersal may have several implications on mosquito population genetics, spreading of gene pools, such as insecticide resistance and virus susceptibility, and determination of control strategies by public health authorities.The dispersal ability of Ae. aegypti females have already been studied. 20However, several previous dispersal estimates might be biased.Common sources of bias in dispersal estimates seem to be site manipulation prior to mosquito release or recaptures performed in a limited area, such a small village or a single block, frequently close to the release point. 9,10,13,14,17,21,22For example, Honório et al 10 performed a mark-release-recapture experiment in a highly endemic area of dengue with female mosquitoes with amputated proboscis to avoid ethical issues and they also eliminated or removed most small containers from the study area.These interventions may have infl uenced the dispersal pattern of Ae. aegypti females, which displaced at least 800 m from their release point.Maciel-de-Freitas et al 13 evaluated dispersal of Ae. aegypti females in two areas

INTRODUCTION
with distinct landscape and physical barriers, such as sea and extensive highways, and observed differential displacement from the release site in both areas.
The objective of the present study was to evaluate dispersal of Ae. aegypti females in an area with no container manipulation and no geographic barriers to constrain mosquito fl ight.

METHODS
The residential district of Olaria (22°50'45''S; 43°15'39''W), located in the city of Rio de Janeiro (Southeastern Brazil), was selected to release dustmarked Ae. aegypti females.It has an estimated population of 62,509 inhabitants in an area of 369 hectare, nearly 169 inh./ha.Olaria is a lowland district with extensive paved streets, moderate traffi c and a busy rail station.Houses generally have 2-4 rooms and large peridomestic area, with usually no more than 4-5 persons per house.There is a regular service of garbage collection and water supply.Olaria is in one of the most important dengue endemic zone in the city, which registered 49,266 dengue cases from 1986 to 2001.According to the Health Department of the City of Rio de Janeiro, during the 2001 dengue epidemic, more than 8% (2,165/26,535 cases) of the city's total cases were reported there; in 2007 there were 208 dengue cases.In July 2006, fi ve months prior to experiment start, Olaria had a house index of 8.8.
Released Ae. aegypti females were derived from a laboratory colony that is renewed at least twice a year with eggs collected in ovitraps installed in Rio de Janeiro.Larvae were fed with fi sh food (Tetramin) and reared according to Consoli & Lourenço-de-Oliveira. 2 After emergence, females were separated from males and kept together at 25 ± 3°C and 65 ± 3% relative humidity and provided with sucrose solution up to a day before the fi rst blood feeding.
Two blood meals were offered on 3-4 days after emergence in an artifi cial membrane feeder apparatus. 19e expected all released females were gravid when released three days after blood meals.A total of 725 Ae. aegypti females with 6-7 days of emergence were marked with fl uorescent dust (Day-Glo Color Corp., Cleveland, OH) and released outdoor in the morning (between 08:00 AM and 09:00 AM) of December 2 nd , 2006, approximately one hour after dust marking.
To capture released Ae. aegypti females a total of 192 traps (96 Adultraps 8 and 96 MosquiTRAPs 6 ) were installed in Olaria.Briefl y, both traps were originally designed to capture gravid Ae. aegypti females.Adultrap is a black plastic jar with an external plastic screen using only water as attractant. 8MosquiTRAP has a glued car inside a black plastic jar, using a commercial mixture as attractant. 6Adultrap and MosquiTRAP were installed in the peridomestic area of adjacent houses and were left in the same place until the end of the experiment, covering an area of 3.14 km 2 .Recaptures were carried out for 18 days.All collected mosquitoes were brought to the laboratory to be identifi ed and checked for the presence of fl uorescent mark with an UV light.
As for dispersal, the 192 houses where mosquito traps were installed were geo-referenced using a Global Position System (GPS; Garmin eTrex personal navigator) to calculate distance between releasing and capture points.The fl ight behavior of Ae. aegypti females was summarized by a set of dispersal measures: mean distance traveled (MDT), maximum distance traveled (MAX), and fl ight ranges of 50% (FR 50 ) and 90% (FR 90 ) of the population. 11,16Frequency distributions of the marked mosquitoes that had traveled <100 m and >200 m from the release point were also evaluated.
We also used circular statistics to help understanding fl ight dynamics of Ae. aegypti females and most importantly, if they had an isotropic direction or a uniform distribution from the release point.For that, we calculated the angle formed between the release point and collection sites, forming a circular histogram, which shows the distribution of fl ight angles (to the southnorth axis), with the North direction set as 0°.Flight direction tendencies were visually interpreted based on the histogram shape.The Rayleigh test of uniformity was used to calculate the probability of the null hypothesis that the data have a uniform distribution. 23ircular histogram and data analysis were performed using Oriana program, v. 2.02. a Mark-release-recapture (MRR) experiment protocols were submitted and approved by Fundação Oswaldo Cruz (Fiocruz) Research Ethics Committee (CEP/Fiocruz protocol no.11591-2005).

RESULTS
A total of 46 (6.34%) dust-marked females were recaptured, 17 (36.9%) of them collected in Adultraps and 29 (63.1%) in MosquiTRAPs (Table 1).Dust-marked Ae. aegypti mosquitoes were recaptured up to nine days after release; the majority of them were recaptured up to four days (Table 1).
Ae. aegypti females had an average dispersal of 288.1 m from the release point.However, the maximum distance traveled observed was 690 m, corresponding to a single female that fl ew to the southeast direction.Additionally, 90% of released females fl ew up to 500.2 m and 69.0%displaced more than 200 m from the release point (Table 2).
Ae. aegypti fl ight showed an isotropic behavior from the release point, showing no evidence of a preferred direction (z = 0.242; p = 0.787).In fact, a circular histogram constructed with the angles formed by release and recapture points emphasizes the uniform distribution of fl ight direction (Figure).

DISCUSSION
Recapture rate of Ae. aegypti females were in accordance with other similar studies performed in Rio de Janeiro, even when recapture effort was confi ned to a smaller area. 13As there is no evident physical or geographical obstacle in Olaria, mosquito emigration was expected to be less controlled than in other studies in areas with physical barriers. 9,13,17,21,22spersal of Ae. aegypti females in Olaria was higher than that seen in two other districts of the city of Rio de Janeiro and in other areas with physical and geographical barriers. 9,13,17,21,22In Favela do Amorim, an urban area with high human density and container availability, Ae. aegypti females had an MDT of around 50 m and a MAX of 151 m.In Tubiacanga, a suburban area with a human density three times lower than Favela do Amorim, MDT was approximately 80 m with a maximum displacement of 363 m. 13 Since mosquito fl ight seems to be associated to the heterogeneity distribution of blood sources 7 and oviposition sites, 5 low availability or container elimination during dry season or control campaigns may increase mosquito dispersal rates because females would have to fl y longer distances to lay their eggs. 3,4,18Therefore, the elimination of small containers during the dry season by Honório et al 10 before Ae.aegypti releasing may have stimulated mosquito dispersal.These authors evaluated dispersal by collecting rubidium chloride (RbCl)-marked eggs with ovitraps set along varied distance from the release point.Several positive ovitraps were found at 800 m from the release site, the most distant collection point, suggesting that the displacement of Ae. aegypti females was beyond the most distant collection point. 10esides, Honório et al 10 amputated proboscis of released gravid Ae. aegypti females which may have infl uenced the observed dispersal range.
Dispersal is an important parameter in disease transmission dynamics and vector control.In theory, a disease vector able to displace long distances may spread pathogens to large areas.According to our results, an effective and effi cient control is required that can include container elimination and insecticide spraying in a radius of at least 500 m from a reported dengue case.
Bars refl ect the number of mosquitoes that fl ew in that specifi c direction.

.
Circular histogram of the distribution of fl ight angles of dust-marked Ae. aegypti females released in Olaria and recaptured with Adultraps and MosquiTRAPs.Rio de Janeiro,

Table 2 .
Dispersal measures of dust-marked Ae. aegypti females released in Olaria and compared with similar experiments performed in residential areas worldwide.Rio de Janeiro, Southeastern Brazil, 2006.
50: fl ight range of 50% of the population FR 90 : fl ight range of 90% of the population MAX: maximum distance traveled * Dispersal data from Favela do Amorim and Tubiacanga are presented as dry season/wet season estimates.