Relationship between transmission intensity and incidence of dengue hemorrhagic fever in Thailand

Spatial distribution of arboviruses and its association with a social development index and the waste disposal in São Luís, state of Maranhão, Brazil, 2015 to 2019

OBJECTIVE

To detect spatial and spatiotemporal clusters of urban arboviruses and to investigate whether the social development index (SDI) and irregular waste disposal are related to the coefficient of urban arboviruses detection in São Luís, state of Maranhão, Brazil.

METHODS

The confirmed cases of Dengue, Zika and Chikungunya in São Luís, from 2015 to 2019, were georeferenced to the census tract of residence. The Bayesian Conditional Autoregressive regression model was used to identify the association between SDI and irregular waste disposal sites and the coefficient of urban arboviruses detection.

RESULTS

The spatial pattern of arboviruses pointed to the predominance of a low-incidence cluster, except 2016. For the years 2015, 2016, 2017, and 2019, an increase of one unit of waste disposal site increased the coefficient of arboviruses detection in 1.25, 1.09, 1.23, and 1.13 cases of arboviruses per 100 thousand inhabitants, respectively. The SDI was not associated with the coefficient of arboviruses detection.

CONCLUSION

In São Luís, spatiotemporal risk clusters for the occurrence of arboviruses and a positive association between the coefficient of arbovirus detection and sites of irregular waste disposal were identified.

Aedes aegypti and Aedes albopictus (Diptera: Culicidae) ecology, biology, behaviour, and implications on arbovirus transmission in Thailand: Review

Aedes aegypti and Aedes albopictus (Aedes) transmit highly pathogenic viruses such as dengue, chikungunya, yellow fever, and Zika which can cause life-threatening diseases in humans. They are the most important vectors of arboviruses in Thailand. Their vectorial capacity (VC) is highly complex mainly due to the interplay between biotic and abiotic factors that vary in time and space. A literature survey was conducted to collate and discuss recent research regarding the influence of Aedes vector biology, behaviour, and ecology on arbovirus transmission in Thailand. The survey followed guidelines of preferred reporting items of systematic reviews and meta-analyses (PRISMA). All fields, keyword search was conducted in the Web of Science database for the period of 2000-2021. The search yielded 821 records on Ae. aegypti and 293 records on Aedes albopictus, of which 77 were selected for discussion. Genomic studies showed that there is a high genetic variation in Aedes albopictus whereas Ae. aegypti generally shows low genetic variation. Along with genetically unstable arboviruses, the interaction between Aedes and arboviruses is largely regulated by genomic events such as genetic mutations and immune response protein factors. Temperature and precipitation are the major climatic events driving arbovirus transmission. Human exposure risk factors are mainly due to multiple feeding patterns, including endophagy by Aedes albopictus and zoophagic behaviour of Ae. aegypti as well as diverse human-associated breeding sites. Integration of the One Health approach in control interventions is a priority with a rigorous focus on Aedes-arbovirus surveillance as a complementary strategy.

Epidemiology and challenges of dengue surveillance in the WHO South-East Asia Region

Dengue poses a significant health and economic burden in the WHO South-East Asia Region. Approaches for control need to be aligned with current knowledge on the epidemiology of dengue in the region. Such knowledge will ensure improved targeting of interventions to reduce dengue incidence and its socioeconomic impact. This review was undertaken to describe the contemporary epidemiology of dengue and critically analyse the existing surveillance strategies in the region. Over recent decades, dengue incidence has continued to increase with geographical expansion. The region has now become hyper-endemic for multiple dengue virus serotypes/genotypes. Every epidemic cycle was associated with a change of predominant serotype/genotype and this was often associated with severe disease with intense transmission. Classical larval indices are widely used in vector surveillance and adult mosquito samplings are not implemented as a part of routine surveillance. Further, there is a lack of integration of entomological and disease surveillance systems, often leading to inaction or delays in dengue prevention and control. Disease surveillance does not capture all cases, resulting in under-reporting, and has thus failed to adequately represent the true burden of disease in the region. Possible solutions include incorporating adult mosquito sampling into routine vector surveillance, the establishment of laboratory-based sentinel surveillance, integrated vector and dengue disease surveillance and climate-based early warning systems using available technologies like mobile apps.

Abundance and Updated Distribution of Aedes aegypti (Diptera: Culicidae) in Cabo Verde Archipelago: A Neglected Threat to Public Health

BACKGROUND

Mosquito-borne viruses, such as Zika, dengue, yellow fever, and chikungunya, are important causes of human diseases nearly worldwide. The greatest health risk for arboviral disease outbreaks is the presence of the most competent and highly invasive domestic mosquito, Aedes aegypti. In Cabo Verde, two recent arbovirus outbreaks were reported, a dengue outbreak in 2009, followed by a Zika outbreak in 2015. This study is the first entomological survey for Ae. aegypti that includes all islands of Cabo Verde archipelago, in which we aim to evaluate the actual risk of vector-borne arboviruses as a continuous update of the geographical distribution of this species.

METHODS

In order to assess its current distribution and abundance, we undertook a mosquito larval survey in the nine inhabited islands of Cabo Verde from November 2018 to May 2019. Entomological larval survey indices were calculated, and the abundance analyzed. We collected and identified 4045 Ae. aegypti mosquitoes from 264 positive breeding sites in 22 municipalities and confirmed the presence of Ae. aegypti in every inhabited island. Results: Water drums were found to be the most prevalent containers (n = 3843; 62.9%), but puddles (n = 27; 0.4%) were the most productive habitats found. The overall average of the House, Container, and Breteau larval indices were 8.4%, 4.4%, and 10.9, respectively. However, 15 out of the 22 municipalities showed that the Breteau Index was above the epidemic risk threshold.

CONCLUSION

These results suggest that if no vector control measures are considered to be in place, the risk of new arboviral outbreaks in Cabo Verde is high. The vector control strategy adopted must include measures of public health directed to domestic water storage and management.

Control of Aedes aegypti Breeding: A Novel Intervention for Prevention and Control of Dengue in an Endemic Zone of Delhi, India

Background and objective

The study is based on hypothesis that whether continuous entomological surveillance of Ae. aegypti and simultaneous appropriate interventions in key containers during non-transmission (December–May) months would have any impact on breeding of Aedes and dengue cases during the following transmission months (June–November). The impact of the surveillance and intervention measures undertaken during non-transmission months were assessed by entomological indicators namely container index (CI), house index (HI), pupal index (PI) and breteau index (BI).

Methods

A total of 28 localities of West Zone of Delhi with persistent dengue endemicity were selected for the study. Out of these localities, 20 were included in study group while other 8 localities were in control group. IEC and various Aedes breeding control activities were carried out in study group in both non-transmission and transmission season whereas control group did not have any such interventions during non-transmission months as per guidelines of MCD. These activities were undertaken by a team of investigators from NIMR and SDMC, Delhi. In control group, investigators from NIMR carried out surveillance activity to monitor the breeding of Aedes mosquito in localities.

Results

Comparison of baseline data revealed that all indices in control and study group of localities were comparable and statistically non-significant (p>0.05). In both study and control groups, indices were calculated after pooling data on seasonal basis, i.e., transmission and non-transmission months for both years. The test of significance conducted on all the four indices, i.e., HI, PI, CI, and BI, revealed a significant difference (p<0.05) between the study group and control group during transmission and non-transmission months except in HI. Due to consistent intervention measures undertaken in non-transmission months in study group, reduction in CI, HI, BI and PI was observed 63%, 62%, 64% and 99% respectively during transmission months as compared to control group where increase of 59%, 102%, 73% and 71% respectively. As a result of reduction in larval indices, no dengue case (except one NS1) was observed in study group, whereas 38 dengue cases were observed in control group.

Conclusion

Through this pilot study, it is concluded that proper intervention in non-transmission season reduces vector density and subsequently dengue cases in transmission season.

A stage structured mosquito model incorporating effects of precipitation and daily temperature fluctuations

An outbreak of dengue fever in Guangdong province in 2014 was the most serious outbreak ever recorded in China. Given the known positive correlation between the abundance of mosquitoes and the number of dengue fever cases, a stage structured mosquito model was developed to investigate the cause of the large abundance of mosquitoes in 2014 and its implications for outbreaks of the disease. Data on the Breteau index (number of containers positive for larvae per 100 premises investigated), temperature and precipitation were used for model fitting. The egg laying rate, the development rate and the mortality rates of immatures and adults were obtained from the estimated parameters. Moreover, effects of daily fluctuations of temperature on these parameters were obtained and the effects of temperature and precipitation were analyzed by simulations. Our results indicated that the abundance of mosquitoes depended not only on the total annual precipitation but also on the distribution of the precipitation. The daily mean temperature had a nonlinear relationship with the abundance of mosquitoes, and large diurnal temperature differences can reduce the abundance of mosquitoes. In addition, effects of increasing precipitation and temperature were interdependent. Our findings suggest that the large abundance of mosquitoes in 2014 was mainly caused by the distribution of the precipitation. In the perspective of mosquito control, our results reveal that it is better to clear water early and spray insecticide between April and August in case of limited resources.

Reconstruction of disease transmission rates: Applications to measles, dengue, and influenza

Transmission rates are key in understanding the spread of infectious diseases. Using the framework of compartmental models, we introduce a simple method to reconstruct time series of transmission rates directly from incidence or disease-related mortality data. The reconstruction employs differential equations, which model the time evolution of infective stages and strains. Being sensitive to initial values, the method produces asymptotically correct solutions. The computations are fast, with time complexity being quadratic. We apply the reconstruction to data of measles (England and Wales, 1948-1967), dengue (Thailand, 1982-1999), and influenza (U.S., 1910-1927). The Measles example offers comparison with earlier work. Here we re-investigate reporting corrections, include and exclude demographic information. The dengue example deals with the failure of vector-control measures in reducing dengue hemorrhagic fever (DHF) in Thailand. Two competing mechanisms have been held responsible: strain interaction and demographic transitions. Our reconstruction reveals that both explanations are possible, showing that the increase in DHF cases is consistent with decreasing transmission rates resulting from reduced vector counts. The flu example focuses on the 1918/1919 pandemic, examining the transmission rate evolution for an invading strain. Our analysis indicates that the pandemic strain could have circulated in the population for many months before the pandemic was initiated by an event of highly increased transmission.

Utility of mosquito surveillance data for spatial prioritization of vector control against dengue viruses in three Brazilian cities

Background

Vector control remains the primary defense against dengue fever. Its success relies on the assumption that vector density is related to disease transmission. Two operational issues include the amount by which mosquito density should be reduced to minimize transmission and the spatio-temporal allotment of resources needed to reduce mosquito density in a cost-effective manner. Recently, a novel technology, MI-Dengue, was implemented city-wide in several Brazilian cities to provide real-time mosquito surveillance data for spatial prioritization of vector control resources. We sought to understand the role of city-wide mosquito density data in predicting disease incidence in order to provide guidance for prioritization of vector control work.

Methods

We used hierarchical Bayesian regression modeling to examine the role of city-wide vector surveillance data in predicting human cases of dengue fever in space and time. We used four years of weekly surveillance data from Vitoria city, Brazil, to identify the best model structure. We tested effects of vector density, lagged case data and spatial connectivity. We investigated the generality of the best model using an additional year of data from Vitoria and two years of data from other Brazilian cities: Governador Valadares and Sete Lagoas.

Results

We found that city-wide, neighborhood-level averages of household vector density were a poor predictor of dengue-fever cases in the absence of accounting for interactions with human cases. Effects of city-wide spatial patterns were stronger than within-neighborhood or nearest-neighborhood effects. Readily available proxies of spatial relationships between human cases, such as economic status, population density or between-neighborhood roadway distance, did not explain spatial patterns in cases better than unweighted global effects.

Conclusions

For spatial prioritization of vector controls, city-wide spatial effects should be given more weight than within-neighborhood or nearest-neighborhood connections, in order to minimize city-wide cases of dengue fever. More research is needed to determine which data could best inform city-wide connectivity. Once these data become available, MI-dengue may be even more effective if vector control is spatially prioritized by considering city-wide connectivity between cases together with information on the location of mosquito density and infected mosquitos.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0659-y) contains supplementary material, which is available to authorized users.

Revisiting Rayong: shifting seroprofiles of dengue in Thailand and their implications for transmission and control

Dengue virus has traditionally caused substantial morbidity and mortality among children less than 15 years of age in Southeast Asia. Over the last 2 decades, a significant increase in the mean age of cases has been reported, and a once pediatric disease now causes substantial burden among the adult population. An age-stratified serological study (n = 1,736) was conducted in 2010 among schoolchildren in the Mueang Rayong district of Thailand, where a similar study had been conducted in 1980/1981. Serotype-specific forces of infection (λ(t)) and basic reproductive numbers (R0) of dengue were estimated for the periods 1969-1980 and 1993-2010. Despite a significant increase in the age at exposure and a decrease in λ(t) from 0.038/year to 0.019/year, R0 changed only from 3.3 to 3.2. Significant heterogeneity was observed across subdistricts and schools, with R0 ranging between 1.7 and 6.8. These findings are consistent with the idea that the observed age shift might be a consequence of the demographic transition in Thailand. Changes in critical vaccination fractions, estimated by using R0, have not accompanied the increase in age at exposure. These results have implications for dengue control interventions because multiple countries in Southeast Asia are undergoing similar demographic transitions. It is likely that dengue will never again be a disease exclusively of children.

Simulations to compare efficacies of tetravalent dengue vaccines and mosquito vector control

SUMMARY

Infection with dengue, the most prevalent mosquito-borne virus, manifests as dengue fever (DF) or the more fatal dengue haemorrhagic fever (DHF). DHF occurs mainly when an individual who has acquired antibodies to one serotype is inoculated with another serotype. It was reported that mosquito control may have increased the incidence of DF and DHF due to age-dependency in manifesting these illnesses or an immunological mechanism. Tetravalent dengue vaccine is currently being tested in clinical trials. However, seroconversions to all four serotypes were achieved only after three doses. Therefore, vaccines may predispose vaccinees to the risk of developing DHF in future infections. This study employed an individual-based computer simulation, to emulate mosquito control and vaccination, incorporating seroconversion rates reported from actual clinical trials. It was found that mosquito control alone would have increased incidence of DF and DHF in areas of high mosquito density. A vaccination programme with very high coverage, even with a vaccine of suboptimal seroconversion rates, attenuated possible surges in the incidence of DF and DHF which would have been caused by insufficient reduction in mosquito abundance. DHF cases attributable to vaccine-derived enhancement were fewer than DHF cases prevented by a vaccine with considerably high (although not perfect) seroconversion rates. These predictions may justify vaccination programmes, at least in areas of high mosquito abundance. In such areas, mosquito control programmes should be conducted only after the vaccination programme with a high coverage has been initiated.

Secondary infection as a risk factor for dengue hemorrhagic fever/dengue shock syndrome: an historical perspective and role of antibody-dependent enhancement of infection

Today, dengue viruses are the most prevalent arthropod-borne viruses in the world. Since the 1960s, numerous reports have identified a second heterologous dengue virus (DENV) infection as a principal risk factor for severe dengue disease (dengue hemorrhagic fever/dengue shock syndrome, DHF/DSS). Modifiers of dengue disease response include the specific sequence of two DENV infections, the interval between infections, and contributions from the human host, such as age, ethnicity, chronic illnesses and genetic background. Antibody-dependent enhancement (ADE) of dengue virus infection has been proposed as the early mechanism underlying DHF/DSS. Dengue cross-reactive antibodies raised following a first dengue infection combine with a second infecting virus to form infectious immune complexes that enter Fc-receptor-bearing cells. This results in an increased number of infected cells and increased viral output per cell. At the late illness stage, high levels of cytokines, possibly the result of T cell elimination of infected cells, result in vascular permeability, leading to shock and death. This review is focused on the etiological role of secondary infections (SI) and mechanisms of ADE.

Quantifying the emergence of dengue in Hanoi, Vietnam: 1998-2009

BACKGROUND

An estimated 2.4 billion people live in areas at risk of dengue transmission, therefore the factors determining the establishment of endemic dengue in areas where transmission suitability is marginal is of considerable importance. Hanoi, Vietnam is such an area, and following a large dengue outbreak in 2009, we set out to determine if dengue is emerging in Hanoi.

METHODS AND PRINCIPAL FINDINGS

We undertook a temporal and spatial analysis of 25,983 dengue cases notified in Hanoi between 1998 and 2009. Age standardized incidence rates, standardized age of infection, and Standardized Morbidity Ratios (SMR) were calculated. A quasi-Poisson regression model was used to determine if dengue incidence was increasing over time. Wavelet analysis was used to explore the periodicity of dengue transmission and the association with climate variables. After excluding the two major outbreak years of 1998 and 2009 and correcting for changes in population age structure, we identified a significant annual increase in the incidence of dengue cases over the period 1999-2008 (incidence rate ratio  =  1.38, 95% confidence interval  =  1.20-1.58, p value  =  0.002). The age of notified dengue cases in Hanoi is high, with a median age of 23 years (mean 26.3 years). After adjusting for changes in population age structure, there was no statistically significant change in the median or mean age of dengue cases over the period studied. Districts in the central, highly urban, area of Hanoi have the highest incidence of dengue (SMR>3).

CONCLUSIONS

Hanoi is a low dengue transmission setting where dengue incidence has been increasing year on year since 1999. This trend needs to be confirmed with serological surveys, followed by studies to determine the underlying drivers of this emergence. Such studies can provide insights into the biological, demographic, and environmental changes associated with vulnerability to the establishment of endemic dengue.

Population density, water supply, and the risk of dengue fever in Vietnam: cohort study and spatial analysis

BACKGROUND

Aedes aegypti, the major vector of dengue viruses, often breeds in water storage containers used by households without tap water supply, and occurs in high numbers even in dense urban areas. We analysed the interaction between human population density and lack of tap water as a cause of dengue fever outbreaks with the aim of identifying geographic areas at highest risk.

METHODS AND FINDINGS

We conducted an individual-level cohort study in a population of 75,000 geo-referenced households in Vietnam over the course of two epidemics, on the basis of dengue hospital admissions (n = 3,013). We applied space-time scan statistics and mathematical models to confirm the findings. We identified a surprisingly narrow range of critical human population densities between around 3,000 to 7,000 people/km² prone to dengue outbreaks. In the study area, this population density was typical of villages and some peri-urban areas. Scan statistics showed that areas with a high population density or adequate water supply did not experience severe outbreaks. The risk of dengue was higher in rural than in urban areas, largely explained by lack of piped water supply, and in human population densities more often falling within the critical range. Mathematical modeling suggests that simple assumptions regarding area-level vector/host ratios may explain the occurrence of outbreaks.

CONCLUSIONS

Rural areas may contribute at least as much to the dissemination of dengue fever as cities. Improving water supply and vector control in areas with a human population density critical for dengue transmission could increase the efficiency of control efforts. Please see later in the article for the Editors' Summary.

The early clinical features of dengue in adults: challenges for early clinical diagnosis

BACKGROUND

The emergence of dengue throughout the tropical world is affecting an increasing proportion of adult cases. The clinical features of dengue in different age groups have not been well examined, especially in the context of early clinical diagnosis.

METHODOLOGY/PRINCIPAL FINDINGS

We structured a prospective study of adults (≥ 18 years of age) presenting with acute febrile illness within 72 hours from illness onset upon informed consent. Patients were followed up over a 3-4 week period to determine the clinical outcome. A total of 2,129 adults were enrolled in the study, of which 250 (11.7%) had dengue. Differences in the rates of dengue-associated symptoms resulted in high sensitivities when the WHO 1997 or 2009 classification schemes for probable dengue fever were applied to the cohort. However, when the cases were stratified into age groups, fewer older adults reported symptoms such as myalgia, arthralgia, retro-orbital pain and mucosal bleeding, resulting in reduced sensitivity of the WHO classification schemes. On the other hand, the risks of severe dengue and hospitalization were not diminished in older adults, indicating that this group of patients can benefit from early diagnosis, especially when an antiviral drug becomes available. Our data also suggests that older adults who present with fever and leukopenia should be tested for dengue, even in the absence of other symptoms.

CONCLUSION

Early clinical diagnosis based on previously defined symptoms that are associated with dengue, even when used in the schematics of both the WHO 1997 and 2009 classifications, is difficult in older adults.

Geographical gradient of mean age of dengue haemorrhagic fever patients in northern Thailand

Dengue haemorrhagic fever (DHF) is caused by dengue virus transmitted by Aedes mosquitoes; mean age of patients varies temporally and geographically. Variability in age of patients may be due to differences in transmission intensity or demographic structure. To compare these two hypotheses, the mean age of DHF patients from 90 districts in northern Thailand (1994-1996, 2002-2004) was regressed against (i) Aedes abundance or (ii) demographic variables (birthrate, average age) of the district. We also developed software to quantify direction and strength of geographical gradients of these variables. We found that, after adjusting for socioeconomics, climate, spatial autocorrelation, the mean age of patients was correlated only with Aedes abundance. The geographical gradient of mean age of patients originated from entomological, climate, and socioeconomic gradients. Vector abundance was a stronger determinant of mean age of patients than demographic variables, in northern Thailand.

Immunological and viral determinants of dengue severity in hospitalized adults in Ha Noi, Viet Nam

Background

The relationships between the infecting dengue serotype, primary and secondary infection, viremia and dengue severity remain unclear. This cross-sectional study examined these interactions in adult patients hospitalized with dengue in Ha Noi.

Methods and Findings

158 patients were enrolled between September 16 and November 11, 2008. Quantitative RT-PCR, serology and NS1 detection were used to confirm dengue infection, determine the serotype and plasma viral RNA concentration, and categorize infections as primary or secondary. 130 (82%) were laboratory confirmed. Serology was consistent with primary and secondary infection in 34% and 61%, respectively. The infecting serotype was DENV-1 in 42 (32%), DENV-2 in 39 (30%) and unknown in 49 (38%). Secondary infection was more common in DENV-2 infections (79%) compared to DENV-1 (36%, p<0.001). The proportion that developed dengue haemorrhagic fever (DHF) was 32% for secondary infection compared to 18% for primary infection (p = 0.14), and 26% for DENV-1 compared to 28% for DENV-2. The time until NS1 and plasma viral RNA were undetectable was shorter for DENV-2 compared to DENV-1 (p≤0.001) and plasma viral RNA concentration on day 5 was higher for DENV-1 (p = 0.03). Plasma viral RNA concentration was higher in secondary infection on day 5 of illness (p = 0.046). We didn't find an association between plasma viral RNA concentration and clinical severity.

Conclusion

Dengue is emerging as a major public health problem in Ha Noi. DENV-1 and DENV-2 were the prevalent serotypes with similar numbers and clinical presentation. Secondary infection may be more common amongst DENV-2 than DENV-1 infections because DENV-2 infections resulted in lower plasma viral RNA concentrations and viral RNA concentrations were higher in secondary infection. The drivers of dengue emergence in northern Viet Nam need to be elucidated and public health measures instituted.

Dengue is estimated to affect 50 million people each year and can occur as explosive outbreaks that overwhelm health systems. Despite significant advances the available knowledge is not sufficient to predict the outcome of individual infections or the occurrence of epidemics. Studies from low dengue transmission settings are lacking but offer the potential to better understand the contribution of age, primary versus secondary infection and serotype because there are likely to be more adult and primary infection patients and fewer serotypes circulating compared to high transmission settings. This is the first reported study of clinical dengue in Ha Noi, the largest urban area of Northern Viet Nam. Records kept by the Preventive Medicine Center indicate that <2500 clinical dengue cases attended government health care facilities in Ha Noi each year from 1999 until 2007. Patients in Ha Noi were older than in high transmission settings, the contribution of primary infection to overt and severe illness was greater and associations between serotype, plasma viral RNA concentration and overt and severe illness were distinct. The dengue situation in Ha Noi provides an opportunity to further examine the roles of serotype and prior immunity in dengue severity and epidemic emergence.

Effectiveness of peridomestic space spraying with insecticide on dengue transmission; systematic review

OBJECTIVE

To review the evidence on effectiveness of peridomestic space spraying of insecticides in reducing wild Aedes populations and interrupting dengue transmission.

METHODS

Comprehensive literature search of MEDLINE, EMBASE, LILACS, Web of Science, WHOLIS, MedCarib and CENTRAL, and a manual search of reference lists from identified studies. Duplicates were removed and abstracts assessed for selection. All field evaluations of peridomestic space spraying targeting wild adult Aedes vectors in dengue endemic countries were included. Data were extracted, and the methodological quality of the studies was assessed independently by two reviewers.

RESULTS

Fifteen studies met the inclusion criteria. Outcome measures were heterogeneous, foregoing the possibility of meta-analysis. Thirteen studies showed reductions in immature entomological indices that were not sustained for long periods. The remainder showed space spray interventions to be ineffective at reducing adult and/or immature entomological indices. Only one study measured human disease indicators, but its outcomes could not be directly attributed to space sprays alone.

CONCLUSION

Although peridomestic space spraying is commonly applied by national dengue control programmes, there are very few studies evaluating the effectiveness of this intervention. There is no clear evidence for recommending peridomestic space spraying as a single, effective control intervention. Thus, peridomestic space spraying is more likely best applied as part of an integrated vector management strategy. The effectiveness of this intervention should be measured in terms of impact on both adult and immature mosquito populations, as well as on disease transmission.

Spatial evaluation and modeling of Dengue seroprevalence and vector density in Rio de Janeiro, Brazil

BACKGROUND

Rio de Janeiro, Brazil, experienced a severe dengue fever epidemic in 2008. This was the worst epidemic ever, characterized by a sharp increase in case-fatality rate, mainly among younger individuals. A combination of factors, such as climate, mosquito abundance, buildup of the susceptible population, or viral evolution, could explain the severity of this epidemic. The main objective of this study is to model the spatial patterns of dengue seroprevalence in three neighborhoods with different socioeconomic profiles in Rio de Janeiro. As blood sampling coincided with the peak of dengue transmission, we were also able to identify recent dengue infections and visually relate them to Aedes aegypti spatial distribution abundance. We analyzed individual and spatial factors associated with seroprevalence using Generalized Additive Model (GAM).

METHODOLOGY/PRINCIPAL FINDINGS

Three neighborhoods were investigated: a central urban neighborhood, and two isolated areas characterized as a slum and a suburban area. Weekly mosquito collections started in September 2006 and continued until March 2008. In each study area, 40 adult traps and 40 egg traps were installed in a random sample of premises, and two infestation indexes calculated: mean adult density and mean egg density. Sera from individuals living in the three neighborhoods were collected before the 2008 epidemic (July through November 2007) and during the epidemic (February through April 2008). Sera were tested for DENV-reactive IgM, IgG, Nested RT-PCR, and Real Time RT-PCR. From the before-after epidemics paired data, we described seroprevalence, recent dengue infections (asymptomatic or not), and seroconversion. Recent dengue infection varied from 1.3% to 14.1% among study areas. The highest IgM seropositivity occurred in the slum, where mosquito abundance was the lowest, but household conditions were the best for promoting contact between hosts and vectors. By fitting spatial GAM we found dengue seroprevalence hotspots located at the entrances of the two isolated communities, which are commercial activity areas with high human movement. No association between recent dengue infection and household's high mosquito abundance was observed in this sample.

CONCLUSIONS/SIGNIFICANCE

This study contributes to better understanding the dynamics of dengue in Rio de Janeiro by assessing the relationship between dengue seroprevalence, recent dengue infection, and vector density. In conclusion, the variation in spatial seroprevalence patterns inside the neighborhoods, with significantly higher risk patches close to the areas with large human movement, suggests that humans may be responsible for virus inflow to small neighborhoods in Rio de Janeiro. Surveillance guidelines should be further discussed, considering these findings, particularly the spatial patterns for both human and mosquito populations.