RESUMO
BACKGROUND: Comprehensive, longitudinal field studies that monitor both disease and vector populations for dengue viruses are urgently needed as a pre-requisite for developing locally adaptable prevention programs or to appropriately test and license new vaccines. METHODOLOGY AND PRINCIPAL FINDINGS: We report the results from such a study spanning 5 years in the Amazonian city of Iquitos, Peru where DENV infection was monitored serologically among approximately 2,400 members of a neighborhood-based cohort and through school-based absenteeism surveillance for active febrile illness among a subset of this cohort. At baseline, 80% of the study population had DENV antibodies, seroprevalence increased with age, and significant geographic variation was observed, with neighborhood-specific age-adjusted rates ranging from 67.1 to 89.9%. During the first 15 months, when DENV-1 and DENV-2 were co-circulating, population-based incidence rates ranged from 2-3 infections/100 person-years (p-years). The introduction of DENV-3 during the last half of 2001 was characterized by 3 distinct periods: amplification over at least 5-6 months, replacement of previously circulating serotypes, and epidemic transmission when incidence peaked at 89 infections/100 p-years. CONCLUSIONS/SIGNIFICANCE: Neighborhood-specific baseline seroprevalence rates were not predictive of geographic incidence patterns prior to the DENV-3 introduction, but were closely mirrored during the invasion of this serotype. Transmission varied geographically, with peak incidence occurring at different times among the 8 geographic zones in approximately 16 km(2) of the city. The lag from novel serotype introduction to epidemic transmission and knowledge of spatially explicit areas of elevated risk should be considered for more effective application of limited resources for dengue prevention.
Assuntos
Anticorpos Antivirais/sangue , Vírus da Dengue/imunologia , Dengue/epidemiologia , Dengue/transmissão , Absenteísmo , Adolescente , Adulto , Fatores Etários , Idoso , Idoso de 80 Anos ou mais , Criança , Pré-Escolar , Estudos de Coortes , Feminino , Febre de Causa Desconhecida/epidemiologia , Geografia , Humanos , Incidência , Estudos Longitudinais , Masculino , Pessoa de Meia-Idade , Peru/epidemiologia , Estudos Soroepidemiológicos , Adulto JovemRESUMO
Large-scale longitudinal cohort studies are necessary to characterize temporal and geographic variation in Aedes aegypti (L.) (Diptera: Culicidae) production patterns and to develop targeted dengue control strategies that will reduce disease. We carried out pupal/demographic surveys in a circuit of approximately 6,000 houses, 10 separate times, between January 1999 and August 2002 in the Amazonian city of Iquitos, Peru. We quantified the number of containers positive for Ae. aegypti larvae and/or pupae, containers holding pupae, and the absolute number of pupae by 4-mo sampling circuits and spatially by geographic area by using a geographic information system developed for the city. A total of 289,941 water-holding containers were characterized, of which 7.3% were positive for Ae. aegypti. Temporal and geographic variations were detected for all variables examined, and the relative importance of different container types for production of Ae. aegypti was calculated. Ae. aegypti larvae and pupae were detected in 64 types of containers. Consistent production patterns were observed for the lid status (lids: 32% wet containers, 2% pupal production), container location (outdoor: 43% wet containers, 85% pupal production), and method by which the container was filled with water (rain filled: 15% wet containers, 88.3% pupal production); these patterns were consistent temporally and geographically. We describe a new container category (nontraditional) that includes transient puddles, which were rare but capable of producing large numbers of pupae. Because of high variable pupal counts, four container categories (large tank, medium storage, miscellaneous, and nontraditional) should be targeted in addition to outdoor rain-filled containers that are not covered by a lid. The utility of targeted Ae. aegypti control is discussed, as well as the ability to achieve control objectives based on published but untested threshold values.
Assuntos
Aedes/crescimento & desenvolvimento , Animais , Demografia , Geografia , Humanos , Larva , Peru , Densidade Demográfica , Pupa , Fatores de Tempo , Saúde da População UrbanaRESUMO
The expense and ineffectiveness of drift-based insecticide aerosols to control dengue epidemics has led to suppression strategies based on eliminating larval breeding sites. The present work attempts to estimate transmission thresholds for dengue based on an easily-derived statistic, the standing crop of Aedes aegypti pupae per person in the environment. We have developed these thresholds for use in the assessment of risk of transmission and to provide targets for the actual degree of suppression required to prevent or eliminate transmission in source reduction programs. The notion of thresholds is based on 2 concepts: the mass action principal- the course of an epidemic is dependent on the rate of contact between susceptible hosts and infectious vectors, and threshold theory - the introduction of a few infectious individuals into a community of susceptible individuals will not give rise to an outbreak unless the density of the vectors exceeds a certain critical level. We use validated transmission models to estimate thresholds as a function of levels of pre-existing antibody levels in human populations, ambient air temperatures, and the size and frequency of viral introduction. Threshold levels were estimated to range between about 0.5 and 1.5 Ae. aegypti pupae per person for ambient air temperatures of 28 degrees C and initial seroprevalences ranging between 0 percent to 67 percent. Suprisingly, the size of the viral introduction used in these studies, ranging between 1 and 12 infectious individuals per year was not seen to significantly influence the magnitude of the threshold. From a control perspective, these results are not particularly encouraging. The ratio of Ae. aegypti pupae to human density has been observed in limited field studies to range between 0.3 and >60 in 25 sites in dengue-epidemic of dengue-susceptible areas in the Caribbean, Central America, and South East Asia. If, for purposes of illustration, we assume an initial seroprevalence of 33 percent, the degree of suppression required to essentially eliminate the possibility of summertime transmission in Puerto Rico, Honduras, and Bangkok, Thailand was estimated to range between 10 percent and 83 percent; however in Mexico and Trinidad, reductions of >90 percent would be required.(AU)
Assuntos
21003 , Feminino , Humanos , Aedes/crescimento & desenvolvimento , Simulação por Computador , Dengue/transmissão , Insetos Vetores/crescimento & desenvolvimento , Modelos Biológicos , Anticorpos Antivirais/sangue , Dengue/epidemiologia , Dengue/prevenção & controle , Vírus da Dengue/crescimento & desenvolvimento , Honduras/epidemiologia , Mordeduras e Picadas de Insetos/epidemiologia , Mordeduras e Picadas de Insetos/virologia , México/epidemiologia , Porto Rico , Pupa/crescimento & desenvolvimento , Medição de Risco , Processos Estocásticos , Temperatura , Tailândia/epidemiologia , Trinidad e Tobago/epidemiologiaRESUMO
We report the results of a country-wide pupal survey of Aedes aegypti (L.) in Trinidad designed to identify the most important Aedes aegypti-producing containers, importance being a function of a container's abundance and productivity. Numerically, the most common loci were outdoor drums, water storage tanks and buckets, laundry tubs, discarded tyres and small miscellaneous containers. The average number of foci per hectare was 287 (range 65 to 499). The average standing crop per container of Ae.aegypti pupae was 9.5 and ranged 12-fold, the most and least productive being the flower pot (> 30) and the small indoor vase (<3), respectively. Seven of the 11 types were responsible for < 10 percent of all Ae.aegypti pupae and the remaining 4 types, outdoor drums, tubs, buckets and small containers, accounted for > 90 percent. If targeted, source reduction programmes were directed by the importance of various container types, efforts designed to eliminate the ubiquitous small receptacle and tyres would reduce mosquito densities by 43 percent and the provision of an adequate water supply, precluding the need for water storage, could eliminate an additional 38 percent for a total of > 80 percent of Ae.aegypti in the country. The traditional Stegomyia indices used to document the density of Ae.aegypti and predict the threat of transmission - the house, container, and Breteau indices - were seen to have virtually no correspondence with the actual number of pupae per hectare or per person. We conclude that pupal survey is more appropriate for assessing risk and directing control operations (AU)