Temporal genetic variation in Aedes aegypti populations in Ho Chi Minh City (Vietnam)

Aedes aegypti, the main vector of dengue viruses in Asia, displays variation in population density over time. The larval habitats of this species being unevenly distributed and transient (depending on cycles of drought and flood), the forces generating temporal variation in gene frequencies in populations are studied. We sampled seven mosquito populations from Ho Chi Minh City (Vietnam) and its suburbs on five occasions between April 1999 and August 2000. We investigated genetic variation by studying isoenzyme and microsatellite polymorphism and susceptibility to a dengue 2 virus strain. Ae. aegypti populations collected during the dry season (January-April) showed genetic differentiation (F(ST) = 0.016, P < 10(-6) for isoenzymes) and showed more differentiated infection rates of the dengue 2 virus. The genetic structure of the population is less marked during the rainy season (F(ST) = 0.081, P < 10(-6)). Thus, environmental factors, such as rainfall and factors related to human activity, such as breeding site density and insecticide treatment, control the genetic structure of Ae. aegypti populations in the short term. The implications of studies of this kind for the design of future control programmes are discussed.     

Endemic dengue associated with the co-circulation of multiple viral lineages and localized density-dependent transmission

Dengue is one of the most important infectious diseases of humans and has spread throughout much of the tropical and subtropical world. Despite this widespread dispersal, the determinants of dengue transmission in endemic populations are not well understood, although essential for virus control. To address this issue we performed a phylogeographic analysis of 751 complete genome sequences of dengue 1 virus (DENV-1) sampled from both rural (Dong Thap) and urban (Ho Chi Minh City) populations in southern Viet Nam during the period 2003-2008. We show that DENV-1 in Viet Nam exhibits strong spatial clustering, with likely importation from Cambodia on multiple occasions. Notably, multiple lineages of DENV-1 co-circulated in Ho Chi Minh City. That these lineages emerged at approximately the same time and dispersed over similar spatial regions suggests that they are of broadly equivalent fitness. We also observed an important relationship between the density of the human host population and the dispersion rate of dengue, such that DENV-1 tends to move from urban to rural populations, and that densely populated regions within Ho Chi Minh City act as major transmission foci. Despite these fluid dynamics, the dispersion rates of DENV-1 are relatively low, particularly in Ho Chi Minh City where the virus moves less than an average of 20 km/year. These low rates suggest a major role for mosquito-mediated dispersal, such that DENV-1 does not need to move great distances to infect a new host when there are abundant susceptibles, and imply that control measures should be directed toward the most densely populated urban environments.     

Phylogeography of Recently Emerged DENV-2 in Southern Viet Nam

Revealing the dispersal of dengue viruses (DENV) in time and space is central to understanding their epidemiology. However, the processes that shape DENV transmission patterns at the scale of local populations are not well understood, particularly the impact of such factors as human population movement and urbanization. Herein, we investigated trends in the spatial dynamics of DENV-2 transmission in the highly endemic setting of southern Viet Nam. Through a phylogeographic analysis of 168 full-length DENV-2 genome sequences obtained from hospitalized dengue cases from 10 provinces in southern Viet Nam, we reveal substantial genetic diversity in both urban and rural areas, with multiple lineages identified in individual provinces within a single season, and indicative of frequent viral migration among communities. Focusing on the recently introduced Asian I genotype, we observed particularly high rates of viral exchange between adjacent geographic areas, and between Ho Chi Minh City, the primary urban center of this region, and populations across southern Viet Nam. Within Ho Chi Minh City, patterns of DENV movement appear consistent with a gravity model of virus dispersal, with viruses traveling across a gradient of population density. Overall, our analysis suggests that Ho Chi Minh City may act as a source population for the dispersal of DENV across southern Viet Nam, and provides further evidence that urban areas of Southeast Asia play a primary role in DENV transmission. However, these data also indicate that more rural areas are also capable of maintaining virus populations and hence fueling DENV evolution over multiple seasons.     

Suitability of containers from different sources as breeding sites of Aedes aegypti (L.) in a cemetery of Buenos Aires City,Argentina

Cemeteries are ideal urban areas to study the importance of different types of containers as breeding sites of Aedes aegypti (L.). In the present study, the suitability of plastic, glass, ceramic and metal containers was evaluated in four patches within a cemetery of Buenos Aires City, Argentina. Between October 1998 and May 2000, we found 215 breeding sites of Ae. aegypti out of 13,022 water-filled containers examined. In two patches containing microenvironments sheltered from the sun, the use of the different types of containers was proportional to the offer (correlation coefficient = 0.99, P < 0.05 in both cases). In the remaining patches, plastic and metal containers were the most and less frequent breeding sites, respectively (P < 0.001 in both cases). The number of immatures per breeding site (median = 4.5) did not show significant differences among the four types of containers examined (H3, 215 = 1.216, P = 0.749). Differences found in patches from a same cemetery suggest that different microenvironmental conditions affect the suitability of each type of container for Ae. aegypti breeding. Plastic containers appeared as key breeding sites that should be removed to reduce the Ae. aegypti population in the study area.     

Trends in colorectal cancer incidence in Ho Chi Minh City,Vietnam (1996–2015): Joinpoint regression and age–period–cohort analyses

BackgroundLittle is known about the trends in colorectal cancer (CRC) in Vietnam. We aimed to investigate the trends in epidemiology and anatomical subsites of CRC in Ho Chi Minh City, Vietnam.MethodsBased on the Ho Chi Minh City Cancer Registry data during 1996–2015, we calculated the average annual percent changes (AAPCs) of the age-standardized incidence rates (ASRs) by sex, age groups, and anatomical subsites, using joinpoint regressions analysis. We further performed age–period–cohort (APC) analysis using the United States National Cancer Institute’s web-based statistical tool to explore the underlying reason for the incidence trend.ResultsOver 20 years the overall ASR of CRC increased from 10.5 to 17.9 per 100,000, a 1.7-fold increase. CRC incidence elevated more rapidly in men (AAPC 4.7, 95%CI 2.2–7.3) than in women (AAPC 2.6, 95%CI 0.6–4.8). The highest and lowest increasing rates of ASRs were observed in the 50–64-year-old age group (AAPC 5.3, 95%CI 2.8–7.9) and < 50-year-old age group (AAPC 1.1, 95%CI –0.7 to 2.9), respectively. Regarding subsites, rectal cancer had the highest rate of increase (AAPC 3.3, 95%CI 1.0–5.7). Furthermore, the APC analysis indicated significant increases in CRC incidence in birth cohorts after 1975 in both genders.ConclusionsThe CRC incidence in Ho Chi Minh City increased, with the more prominent rates being among men and older populations, in rectal subsites, and in people born after 1975. The upward trend of CRC incidence in Ho Chi Minh City may be due to the adoption of a westernized lifestyle.     

Higher genetic variation estimated by microsatellites compared to isoenzyme markers in Aedes aegypti from Rio de Janeiro

Aedes aegypti populations from five districts in Rio de Janeiro were analyzed using five microsatellites and six isoenzyme markers, to assess the amount of variation and patterns of gene flow at local levels. Microsatellite loci were polymorphic enough to detect genetic differentiation of populations collected at small geographic scales (e.g. within a city). Ae. aegypti populations were highly differentiated as well in the city center as in the outskirt. Thus, dengue virus propagation by mosquitoes could be as efficient in the urban area as in the outskirt of Rio de Janeiro, the main entry point of dengue in Brazil.     

Geographic genetic variation in populations of the dengue virus vector Aedes aegypti

Isoenzyme variation was assessed in 79 mosquito samples of Aedes aegypti, and susceptibility to a dengue 2 virus strain was evaluated in 83 samples. Analysis of FST values, differentiation indexes, and geographic distances separating populations revealed that genetic differences between populations depended on the species' history of migration and colonization. Three major clusters were identified: (1). the sylvan form, Ae. ae. formosus, from West Africa and some islands in the Indian Ocean; (2). the domestic form, Ae. ae. aegypti, from Southeast Asia and South America; and (3). Ae. ae. aegypti populations from the South Pacific islands. Two groups were identified on the basis of susceptibility to the dengue virus: (1). populations with high infection rates, mostly the Ae. ae. aegypti form, and (2). mosquitoes with lower infection rates, specifically Ae. ae. formosus. Other evolutionary and epidemiological implications of the genetic variability of Ae. aegypti are also discussed.     

中国与越南褐飞虱和白背飞虱生物型研究

【目的】监测我国与越南褐飞虱Nilaparvata lugens St?l和白背飞虱Sogatella furcifera Horvath生物型,为抗虫育种工作提供指导。【方法】应用群体集团检测法和蜜露量检测法研究了中国广西、云南、河南、湖南、重庆、贵州和越南河内、河静、顺化、胡志明市和九龙江田间褐飞虱和白背飞虱的致害特性和生物型组成结构。【结果】我国主要稻区(除云南思茅外)和越南中北部的田间褐飞虱以Ⅱ型的比例多,对含Bph1、bph2基因的鉴别品种表现为致害;云南思茅的田间褐飞虱以Ⅱ+Ⅱ型的比例多,对含Bph1、bph2和bph4基因的鉴别品种表现为致害或强致害;越南胡志明市、九龙江的田间褐飞虱以Ⅱ+Ⅱ型的比例多,对含Bph1、bph2、Bph3、bph4基因的鉴别品种主要表现为致害或强致害。我国白背飞虱以Ⅰ型比例较多;越南顺化和河内以Ⅱ型比例多;所有监测点白背飞虱的致害特性总体表现为对含Wph1、Wbph2基因的鉴别品种的致害能力较强,对Wbph3的致害能力表现不一,对含Wph5基因的鉴别品种表现为中等致害。【结论】抗虫育种选择抗源时,不要选含Bph1、bph2基因的水稻品种作为褐飞虱抗源,不要选含基因Wbph1或Wbph2的水稻品种作为白背飞虱抗源。     

桂东南地区普通野生稻遗传多样性研究

利用25个微卫星位点对广西壮族自治区贺州、崇左、防城港3市8个居群301份普通野生稻材料的遗传多样性和遗传结构进行研究,结果表明桂东南地区普通野生稻遗传多样性丰富,平均等位基因数A=10.2400,有效等位基因数Ae=5.0221,平均期望杂合度He=0.7641,实际观察杂合度Ho=0.4840.根据固定指数(F=0.5653)计算出的异交率(t=0.2777)表明,普通野生稻的繁育系统是典型的混合繁育系统.对其遗传结构分析表明,总的遗传变异中有34.59%存在于居群间(Fst=0.3459).进一步研究发现大多数居群偏离了Hardy-Weinberg平衡且杂合体不足(Fis=0.2680,Fit=0.4817).最后根据各居群的遗传变异特点和遗传多样性比较,建议居群QT、YJ和TJ需要优先保护.     

微卫星DNA标记探讨镜鲤的种群结构与遗传变异

采用30个微卫星分子标记, 对5个镜鲤群体的观测杂合度(^H_o)、期望杂合度(^H_e)、多态信息含量(PIC)和有效等位基因数(^A_e)等进行了遗传检测, 根据基因频率计算遗传相似系数和Nei氏标准遗传距离, 以c2检验估计Hardy-Weinberg平衡, 以近交系数(FST)和基因流(Nm)分析群体的遗传分化。同时, 使用PHYLIP3.63软件绘制基于Nei氏标准遗传距离的UPGMA聚类图, 并进行bootstrap自举检验验证进化树的可靠性。在德国镜鲤选育系(Scattered Cyprinus carpio L.)和来自4个不同养殖场(松浦、东岗、奉城和辽中)的德国镜鲤群体中共检测到7 083个扩增片段, 长度在102 ~ 446 bp之间, 在群体内扩增出等位基因1~16个不等, 共计356个等位基因。结果表明: (1)5个群体检测的有效等位基因数在1.07~12.30个不等, 平均多态信息含量为0.74、0.74、0.69、0.75和0.75, 无偏期望杂合度的平均值为0.74、0.78、0.70、0.76和0.78, 说明这几个群体属于高度多态, 遗传多样性水平较高。(2)群体间相似系数在0.52以上, 相似性较高。聚类分析显示, 东岗、奉城和辽中3个养殖场的德国镜鲤群体聚类成一个分支, 而德国镜鲤选育系与松浦群体聚类成另一分支。聚类的先后与它们在地理分布上距离远近有一定的相关性。(3)在与功能基因相关的多个微卫星基因座位上, 扩增产物呈现不同程度的缺失现象, 这些无效等位基因的产生可能与结构基因在育种中受到人工选择的影响较大有关。     

Worldwide patterns of genetic differentiation imply multiple 'domestications' of Aedes aegypti, a major vector of human diseases

Understanding the processes by which species colonize and adapt to human habitats is particularly important in the case of disease-vectoring arthropods. The mosquito species Aedes aegypti, a major vector of dengue and yellow fever viruses, probably originated as a wild, zoophilic species in sub-Saharan Africa, where some populations still breed in tree holes in forested habitats. Many populations of the species, however, have evolved to thrive in human habitats and to bite humans. This includes some populations within Africa as well as almost all those outside Africa. It is not clear whether all domestic populations are genetically related and represent a single 'domestication' event, or whether association with human habitats has developed multiple times independently within the species. To test the hypotheses above, we screened 24 worldwide population samples of Ae. aegypti at 12 polymorphic microsatellite loci. We identified two distinct genetic clusters: one included all domestic populations outside of Africa and the other included both domestic and forest populations within Africa. This suggests that human association in Africa occurred independently from that in domestic populations across the rest of the world. Additionally, measures of genetic diversity support Ae. aegypti in Africa as the ancestral form of the species. Individuals from domestic populations outside Africa can reliably be assigned back to their population of origin, which will help determine the origins of new introductions of Ae. aegypti.     

Population structure and dispersal of the saltmarsh mosquito Aedes vigilax in Queensland, Australia

Population genetics of the mosquito Aedes vigilax (Skuse) (Diptera: Culicidae), a major vector of arboviruses (e.g. Barmah Forest, Ross River), were investigated to obtain an indirect estimate of mosquito dispersal characteristics in typical habitats of Aedes vigilax in south-east Queensland: on the off-shore islands of Moreton Bay and on the mainland where disjunct breeding populations of Ae. vigilax are distributed along intertidal marsh. Six allozyme loci were assessed for genetic differentiation between samples from 11 localities. Significant larval variation between some breeding sites was attributed to site-specific selection. Nonsignificant genetic differentiation was found among collections of adult mosquitoes caught in light traps throughout the study area (exceeding 60x27 km), indicating widespread dispersal. As distances of < or = 9 km over water did not appear to act as effective barriers to Ae. vigilax dispersal, localized control activities applied to Ae. vigilax breeding sites are unlikely to be effective against the vagile adult population. Therefore, the contiguous shires programme of broad acre control is endorsed to prevent the spread of arboviruses carried by Ae. vigilax     

Global genetic diversity of Aedes aegypti

Mosquitoes, especially Aedes aegypti, are becoming important models for studying invasion biology. We characterized genetic variation at 12 microsatellite loci in 79 populations of Ae. aegypti from 30 countries in six continents, and used them to infer historical and modern patterns of invasion. Our results support the two subspecies Ae. aegypti formosus and Ae. aegypti aegypti as genetically distinct units. Ae. aegypti aegypti populations outside Africa are derived from ancestral African populations and are monophyletic. The two subspecies co‐occur in both East Africa (Kenya) and West Africa (Senegal). In rural/forest settings (Rabai District of Kenya), the two subspecies remain genetically distinct, whereas in urban settings, they introgress freely. Populations outside Africa are highly genetically structured likely due to a combination of recent founder effects, discrete discontinuous habitats and low migration rates. Ancestral populations in sub‐Saharan Africa are less genetically structured, as are the populations in Asia. Introduction of Ae. aegypti to the New World coinciding with trans‐Atlantic shipping in the 16th to 18th centuries was followed by its introduction to Asia in the late 19th century from the New World or from now extinct populations in the Mediterranean Basin. Aedes mascarensis is a genetically distinct sister species to Ae. aegypti s.l. This study provides a reference database of genetic diversity that can be used to determine the likely origin of new introductions that occur regularly for this invasive species. The genetic uniqueness of many populations and regions has important implications for attempts to control Ae. aegypti, especially for the methods using genetic modification of populations.     

Spatiotemporal Structure of Molecular Evolution of H5N1 Highly Pathogenic Avian Influenza Viruses in Vietnam

Background

Vietnam is one of the countries most affected by outbreaks of H5N1 highly pathogenic avian influenza viruses. First identified in Vietnam in poultry in 2001 and in humans in 2004, the virus has since caused 111 cases and 56 deaths in humans. In 2003/2004 H5N1 outbreaks, nearly the entire poultry population of Vietnam was culled. Our earlier study (Wan et al., 2008, PLoS ONE, 3(10): e3462) demonstrated that there have been at least six independent H5N1 introductions into Vietnam and there were nine newly emerged reassortants from 2001 to 2007 in Vietnam. H5N1 viruses in Vietnam cluster distinctly around Hanoi and Ho Chi Minh City. However, the nature of the relationship between genetic divergence and geographic patterns is still unclear.

Methodology/Principal Findings

In this study, we hypothesized that genetic distances between H5N1 viruses in Vietnam are correlated with geographic distances, as the result of distinct population and environment patterns along Vietnam''s long north to south longitudinal extent. Based on this hypothesis, we combined spatial statistical methods with genetic analytic techniques and explicitly used geographic space to explore genetic evolution of H5N1 highly pathogenic avian influenza viruses at the sub-national scale in Vietnam. Our dataset consisted of 125 influenza viruses (with whole genome sets) isolated in Vietnam from 2003 to 2007. Our results document the significant effect of space and time on genetic evolution and the rise of two regional centers of genetic mixing by 2007. These findings give insight into processes underlying viral evolution and suggest that genetic differentiation is associated with the distance between concentrations of human and poultry populations around Hanoi and Ho Chi Minh City.

Conclusions/Significance

The results show that genetic evolution of H5N1 viruses in Vietnamese domestic poultry is highly correlated with the location and spread of those viruses in geographic space. This correlation varies by scale, time, and gene, though a classic isolation by distance pattern is observed. This study is the first to characterize the geographic structure of influenza viral evolution at the sub-national scale in Vietnam and can shed light on how H5N1 HPAIVs evolve in certain geographic settings.     

Distributions of Competing Container Mosquitoes Depend on Detritus Types, Nutrient Ratios, and Food Availability

Coexistence of competitors may result if resources are sufficiently abundant to render competition unimportant, or if species differ in resource requirements. Detritus type has been shown to affect interspecific competitive outcomes between Aedes albopictus (Skuse) and Aedes aegypti (L.) larvae under controlled conditions. We assessed the relationships among spatial distributions of detritus types, nutrients, and aquatic larvae of these species in nature. We collected mosquitoes, water, and detritus from artificial containers across 24 Florida cemeteries that varied in relative abundances of Ae. aegypti and Ae. albopictus.We measured nutrient content of fine particulate organic matter in water samples as total N, P, and C and ratios of these nutrients. We quantified food availability via a bioassay, raising individual Aedes larvae in the laboratory in standard volumes of field-collected, particulate-containing water from each cemetery. Quantities of detritus types collected in standard containers were significant predictors of nutrients and nutrient ratios. Nutrient abundances were significant predictors of relative abundance of Ae. aegypti, and of larval survival and development by both species in the bioassay. Survival and development of larvae reared in particulate-containing water from sites decreased with decreasing relative abundance of Ae. aegypti. These data suggest that N, P, and C availabilities are determined by detritus inputs to containers and that these nutrients in turn determine the feeding environment encountered by larvae, the intensity of interspecific competition among larvae, and subsequent relative abundances of species at sites. Detritus inputs, nutrients, and food availability thus seem to contribute to distributions of Ae. aegypti and Ae. albopictus in cemetery containers throughout Florida.     

Distribution and habitats of mosquito larvae in the Kingdom of Tonga

Abstract  Mosquitoes are a significant pest and human health issue in the Kingdom of Tonga. The occurrence of species and habitats used by mosquito larvae were investigated to determine the potential for control through larval habitat management. Forty-two sites, including 22 villages and 20 farm plantations on the six islands of Tongatapu, Pangaimotu, Vava'u, Pangaimotu (Vava'u group), 'Utungake and Nuku, were surveyed in April 2006. A total of eight mosquito species were collected: Aedes aegypti (Linnaeus), Ae. horrescens (Edwards), Ae. nocturnus (Theobold), Ae. tongae (Edwards), Culex albinervis (Edwards), Cx. annulirostris (Skuse), Cx. quinquefasciatus (Say) and Cx. sitiens (Wiedemann). Several species were widespread, particularly Ae. aegypti and Ae. nocturnus on the main island of Tongatapu, whereas Ae. aegypti dominated sites on islands of the Vava'u group. Comparative sampling of 17 village and 17 rural sites showed that larval habitat was more abundant in towns than in rural areas. Larvae were found in a wide range of habitats but were particularly abundant in artificial water bodies (e.g. disused concrete water tanks, 44-gallon drums and used car tyres). In rural sites, habitats were generally sparse except in rain-filled branch stems of giant taro plants. Mosquito populations in artificial habitats could be markedly reduced by seeding disused water tanks with aquatic predators already present in Tonga, using mesh-net covers over 44-gallon drums, and drilling holes in used car tyres.     

The first report of Aedes (Stegomyia) albopictus in Haiti

Aedes albopictus was found in six of the 10 departments of Haiti and in 14 of the 35 communes surveyed. The survey found the larvae of Ae. albopictus in 13 different types of containers. Used tires and tins were by far the most common breeding sites used by this mosquito species. At the breeding sites, Ae. albopictus was associated with other mosquito species, such as Aedes aegypti, Culex nigripalpus and Aedes mediovittatus. The highest proportion of association was with Ae. aegypti. This study represents the first report of Ae. albopictus in Haiti.     

Spatial distribution pattern of oviposition in the mosquito Aedes aegypti in relation to urbanization in Buenos Aires: southern fringe bionomics of an introduced vector

The distribution of Aedes aegypti (L) (Diptera: Culicidae) oviposition in Buenos Aires City is spatially heterogeneous. Oviposition activity was monitored for a year with a grid of 279 traps at 850-m intervals that were serviced weekly. Geostatistics were used for the spatial analysis and generalized linear regression to model oviposition as a function of demographic and environmental variables. The proportion of weeks infested and the total number of eggs showed spatial continuity and were higher in areas that had higher densities of houses and were closer to industrial sites; they were lower in areas with higher human populations or higher densities of flats. When all sites were considered, the spatial structure showed a strong trend, but after regression, the residuals presented lower spatial dependence. When only infested sites were considered, the oviposition variables were spatially autocorrelated and the regression residuals showed little or no spatial dependence. The spatial pattern of Ae. aegypti oviposition in a highly urbanized city such as Buenos Aires seems to be related to the urbanization gradient. These urban environments might present different resource availability or continuity between patches of resources.     

Distribution of dengue vectors in neighborhoods with different urbanization types of Manaus, state of Amazonas, Brazil

Aedes aegypti and Ae. albopictus are vectors of dengue viruses, which cause endemic disease in the city of Manaus, capital of the state of Amazonas, Brazil. More than 53 thousand cases have been registered in this city since the first epidemic in 1998. We evaluated the hypothesis that different ecological conditions result in different patterns of vector infestation in Manaus, by measuring the infestation level in four neighborhoods with different urbanization patterns, during the rainy (April), dry (August), and transitional (November) seasons. Ae. aegypti predominated throughout the study areas and sampling periods, representing 86% of all specimens collected in oviposition traps. High frequencies of houses positive for both species were observed in all studied sites, with Ae. aegypti present in more than 84% of the houses in all seasons. Ae. albopictus, on the other hand, showed more spatial and temporal variation in abundance. We found no association between infestation level and house traits. This study highlights the homogeneity of dengue vector distribution in Manaus.     

Seasonal pattern of abundance of Aedes aegypti (Diptera: Culicidae) in Buenos Aires City, Argentina

In Buenos Aires, the most crowded city of Argentina, there is a potential risk of dengue virus transmission by the mosquito Aedes aegypti during late summer. The temporal patterns of oviposition activity and abundance of breeding sites of this vector were studied in two cemeteries of the city. Between September 1998 and August 1999, we examined 142 ovitraps weekly and a total of 18,010 water-filled containers. Both study areas showed remarkable differences in the percentages of positive ovitraps (19% vs 8%) and breeding sites (18% vs 1%), but similar temporal abundance patterns. The percentage of breeding sites was higher in summer and autumn than in spring and winter, and the percentage of positive ovitraps was higher in summer than in the other three seasons. Immatures were recorded from the first week of October to the second week of July, and oviposition activity from the third week of October until the end of April. In both cemeteries and with both methodologies the highest infestation levels were registered in March (ovitraps: 41.8% and 20.6%, breeding sites: 39.2% and 3.4%). These highest abundances took place after several months with mean temperatures above 20 degrees C and accumulated rainfalls above 150 mm. A sharp decline in oviposition activity was observed when monthly mean temperature decreased to 16.5 degrees C, and no eggs were found below 14.8 degrees C. Seasonal fluctuation of Ae. aegypti abundances in mid-latitudes like Buenos Aires would allow reduction of the egg mosquito population through the elimination of containers during the coldest months, which are free of adults.