Adriana Troyo

The effects of human movement on the persistence of vector-borne diseases

With the recent resurgence of vector-borne diseases due to urbanization and development there is an urgent need to understand the dynamics of vector-borne diseases in rapidly changing urban environments. For example, many empirical studies have produced the disturbing finding that diseases continue to persist in modern city centers with zero or low rates of transmission. We develop spatial models of vector-borne disease dynamics on a network of patches to examine how the movement of humans in heterogeneous environments affects transmission. We show that the movement of humans between patches is sufficient to maintain disease persistence in patches with zero transmission. We construct two classes of models using different approaches: (i) Lagrangian models that mimic human commuting behavior and (ii) Eulerian models that mimic human migration. We determine the basic reproduction number R(0) for both modeling approaches. We show that for both approaches that if the disease-free equilibrium is stable (R(0)<1) then it is globally stable and if the disease-free equilibrium is unstable (R(0)>1) then there exists a unique positive (endemic) equilibrium that is globally stable among positive solutions. Finally, we prove in general that Lagrangian and Eulerian modeling approaches are not equivalent. The modeling approaches presented provide a framework to explore spatial vector-borne disease dynamics and control in heterogeneous environments. As an example, we consider two patches in which the disease dies out in both patches when there is no movement between them. Numerical simulations demonstrate that the disease becomes endemic in both patches when humans move between the two patches.

El Niño Southern Oscillation and vegetation dynamics as predictors of dengue fever cases in Costa Rica

Dengue fever (DF) and dengue hemorrhagic fever (DHF) are growing health concerns throughout Latin America and the Caribbean. This study focuses on Costa Rica, which experienced over 100 000 cases of DF/DHF from 2003 to 2007. We utilized data on sea-surface temperature anomalies related to the El Niño Southern Oscillation (ENSO) and two vegetation indices derived from the Moderate Resolution Imaging Spectrometer (MODIS) from the Terra satellite to model the influence of climate and vegetation dynamics on DF/DHF cases in Costa Rica. Cross-correlations were calculated to evaluate both positive and negative lag effects on the relationships between independent variables and DF/DHF cases. The model, which utilizes a sinusoid and non-linear least squares to fit case data, was able to explain 83% of the variance in weekly DF/DHF cases when independent variables were shifted backwards in time. When the independent variables were shifted forward in time, consistently with a forecasting approach, the model explained 64% of the variance. Importantly, when five ENSO and two vegetation indices were included, the model reproduced a major DF/DHF epidemic of 2005. The unexplained variance in the model may be due to herd immunity and vector control measures, although information regarding these aspects of the disease system are generally lacking. Our analysis suggests that the model may be used to predict DF/DHF outbreaks as early as 40 weeks in advance and may also provide valuable information on the magnitude of future epidemics. In its current form it may be used to inform national vector control programs and policies regarding control measures; it is the first climate-based dengue model developed for this country and is potentially scalable to the broader region of Latin America and the Caribbean where dramatic increases in DF/DHF incidence and spread have been observed.

First Report of the Isolation and Molecular Characterization of Rickettsia amblyommii and Rickettsia felis in Central America

During 2010, 15 adult ticks, identified as Amblyomma cajennense, were collected from horses in Cahuita and Turrialba districts, whereas 7 fleas, identified as Ctenocephalides felis, were collected from a dog in San Jose city, Costa Rica. In the laboratory, three A. cajennense specimens, two from Cahuita and one from Turrialba, were individually processed for rickettsial isolation in cell culture, as was a pool of seven fleas. Rickettsiae were successfully isolated and established in Vero cell culture from the three ticks and from a pool of seven fleas in C6/36 cell culture. The three tick isolates were genotypically identified as Rickettsia amblyommii, and the flea isolate was identified as Rickettsia felis through DNA sequencing of portions of the rickettsial genes gltA, ompA, and ompB of each isolate. In addition, other seven ticks were shown to contain rickettsial DNA. Polymerase chain reaction products of at least two of these ticks were sequenced and also showed to correspond to R. amblyommii. Overall, 66.7% (10/15) of the A. cajennense adult ticks were found to be infected with rickettsiae. This is the first report of a successful isolation in cell culture of R. amblyommii and R. felis from Central America.

Seasonal profiles of Aedes aegypti (Diptera: Culicidae) larval habitats in an urban area of Costa Rica with a history of mosquito control

ABSTRACT Dengue is the most important arboviral disease worldwide and the principal vector-borne disease in Costa Rica. Control of Aedes aegypti populations through source reduction is still considered the most effective way of prevention and control, although it has proven ineffective or unsustainable in many areas with a history of mosquito control. In this study, seasonal profiles and productivity of Aedes aegypti were analyzed in the city of Puntarenas, Costa Rica, where vector control has been practiced for more than ten years. Households contained more than 80% of larval habitats identified, although presence of habitats was more likely in other locations like lots and streets. In the wet season, habitats in the “other” category, like appliances, small manholes, and miscellaneous containers, were the most frequent habitats observed as well as the most common and productive habitats for Ae. aegypti. In the dry season, domestic animal drinking containers were very common, although concrete washtubs contained 79% of Ae. aegypti pupae collected. Individually, non-disposable habitats were as likely or more likely to contain mosquito larvae, and large containers were more likely to harbor mosquito larvae than the small ones only in the dry season. Considering various variables in the logistic regressions, predictors for Ae. aegypti in a habitat were habitat type (p<0.001), setting (p=0.043), and disposability (p=0.022) in the wet season and habitat capacity in the dry season (p=0.025). Overall, traditional Ae. aegypti larval indices and pupal indices in Puntarenas were high enough to allow viral transmission during the wet season. In spite of continued vector control, it has not been possible to reduce vector densities below threshold levels in Puntarenas, and the habitat profiles show that non-household locations, as well as non-disposable containers, should be targeted in addition to the standard control activities.

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.

A geographical sampling method for surveys of mosquito larvae in an urban area using high-resolution satellite imagery

ABSTRACT Entomological surveys in urban areas are often biased by selecting houses or locations with known high vector densities. A sampling strategy was developed for Puntarenas, Costa Rica, using high-resolution satellite imagery. Grids from the Advanced Spaceborne Thermal Emission and Reflection Radiometer and a QuickBird classified land cover map were used to determine the optimal final grid area for surveys. A random sample (10% of cells) was selected, and sample suitability was assessed by comparing the mean percentage of tree cover between sample and total cells. Sample cells were used to obtain entomological data from 581 locations: 26.3% of all locations positive for mosquito larvae were not households, they contained 29.5% of mosquito-positive habitats and 16% of Aedes aegypti pupae collected. Entomological indices for Ae. aegypti (pupae per person, Breteau index, container index, location index) were slightly lower when only household data were analyzed. High-resolution satellite imagery and geographical information systems appear useful for evaluating urban sites and randomly selecting locations for accurate entomological surveys.

Dengue in Costa Rica: the gap in local scientific research

El dengue es una enfermedad de gran importancia a escala mundial. Los esfuerzos para su control en la mayoria de las regiones del mundo no han logrado detener el aumento en su incidencia, y se fomenta la investigacion cientifica para contribuir a desarrollar nuevos enfoques de control basados en la evidencia cientifica o a mejorar los existentes. Enfermedad recientemente reemergente en Costa Rica, el dengue se ha convertido en un grave problema de salud en ese pais. A pesar de esta critica situacion, se dispone de pocas publicaciones cientificas sobre dengue en Costa Rica. En una busqueda realizada en la base de datos PubMed se encontraron solo 11 articulos, mientras que en varias bases de datos bibliograficas centradas en Costa Rica y otros paises de America Latina se hallaron 19 articulos mas. Ademas, se encontraron 10 tesis relacionadas con el dengue realizadas en universidades costarricenses. Por lo tanto, son pocos los articulos cientificos originales publicados sobre este tema en revistas cientificas arbitradas, especialmente en aspectos vinculados con la epidemiologia, la salud publica y la idoneidad y eficacia de las intervenciones en marcha. Esta brecha en las investigaciones puede deberse a diversos factores, como el contexto historico y politico, insuficientes recursos financieros y humanos, deficiencias en la colaboracion entre las instituciones y dificultades para disponer de los datos. Costa Rica puede aprender de la experiencia de otros paises de la Region de las Americas (como Cuba y Trinidad y Tobago), donde se ha investigado mucho mas sobre dengue. Las investigaciones de esos dos paises han aportado evidencias cruciales para el desarrollo de estrategias locales y generales dirigidas al control y la prevencion del dengue. En dependencia del contexto local, algunos metodos de control pueden ser mas eficaces que otros, por lo que las acciones basadas en la evidencia deben adaptarse para las diferentes regiones del mundo, paises y comunidades. La produccion cientifica en Costa Rica puede aumentar si se incrementa la colaboracion interinstitucional en investigaciones, se realizan mas entrenamientos, se eleva la capacidad investigativa y se estimula la publicacion oportuna de los resultados investigativos. Comenzar a acortar la brecha relacionada con las investigaciones sobre el dengue en los paises en desarrollo seria un paso importante hacia el control mundial de esta enfermedad.

Diversidad larval de mosquitos (Diptera: Culicidae) en contenedores artificiales procedentes de una comunidad urbana de San José, Costa Rica

A larval survey of mosquitoes was performed in artificial containers from indoor and outdoor enviroments in the neighborhood “La Carpio”, San José, Costa Rica. “La Carpio” is a community that suffered a dengue outbreak during the last months of the rainy season in 2002. During 2003, 582 and 369 houses were visited in the dry and rainy season, respectively, and the potencial mosquito breeding sites were searched for larvae. Of 1160 water containers identified, 152 (13.0%) were positive for at least one species of mosquito larvae and two mosquito species were found in 9 (0.78%) of the containers. In order to estimate the mosquito diversity, an infestation index by species (Ii) was calculated for each kind of container. Aedes aegypti, Culex quinquefasciatus, Cx. restuans, Cx. coronator, and Limatus durhamii were identified. Ae. aegypti was the most common species, and it showed the highest infestation index for all the containers (p < 0.05). In addition to Ae. aegypti, the presence of other species of Culicidae could mask the success of campaigns for prevention of dengue. This is also an important factor to consider in order to prevent the ocurrence of emerging vector borne diseases such as West Nile Virus Encephalitis.A larval survey of mosquitoes was performed in artificial containers from indoor and outdoor enviroments in the neighborhood “La Carpio”, San Jose, Costa Rica. “La Carpio” is a community that suffered a dengue outbreak during the last months of the rainy season in 2002. During 2003, 582 and 369 houses were visited in the dry and rainy season, respectively, and the potencial mosquito breeding sites were searched for larvae. Of 1160 water containers identified, 152 (13.0%) were positive for at least one species of mosquito larvae and two mosquito species were found in 9 (0.78%) of the containers. In order to estimate the mosquito diversity, an infestation index by species (Ii) was calculated for each kind of container. Aedes aegypti, Culex quinquefasciatus, Cx. restuans, Cx. coronator, and Limatus durhamii were identified. Ae. aegypti was the most common species, and it showed the highest infestation index for all the containers (p < 0.05). In addition to Ae. aegypti, the presence of other species of Culicidae could mask the success of campaigns for prevention of dengue. This is also an important factor to consider in order to prevent the ocurrence of emerging vector borne diseases such as West Nile Virus Encephalitis.

Rickettsia felis in Ctenocephalides felis from Guatemala and Costa Rica

Rickettsia felis is an emerging human pathogen associated primarily with the cat flea Ctenocephalides felis. In this study, we investigated the presence of Rickettsia felis in C. felis from Guatemala and Costa Rica. Ctenocephalides felis were collected directly from dogs and cats, and analyzed by polymerase chain reaction for Rickettsia-specific fragments of 17-kDa protein, OmpA, and citrate synthase genes. Rickettsia DNA was detected in 64% (55 of 86) and 58% (47 of 81) of flea pools in Guatemala and Costa Rica, respectively. Sequencing of gltA fragments identified R. felis genotype URRWXCal(2) in samples from both countries, and genotype Rf2125 in Costa Rica. This is the first report of R. felis in Guatemala and of genotype Rf2125 in Costa Rica. The extensive presence of this pathogen in countries of Central America stresses the need for increased awareness and diagnosis.

Factores asociados con la incidencia de dengue en Costa Rica

OBJETIVO: Determinar el grado de influencia de diferentes variables socioeconomicas, demograficas, geograficas y climaticas con respecto a la incidencia de dengue y dengue hemorragico (D/DH) en Costa Rica durante el periodo 1999-2007. METODOS: Se realizo un estudio epidemiologico de tipo correlacional, analizando la incidencia acumulada de D/DH desde 1999 a 2007 y su asociacion con diferentes variables en los 81 cantones del pais. La informacion se obtuvo de fuentes secundarias y las variables independientes usadas para el analisis se seleccionaron considerando su representatividad en cuanto a aspectos sociodemograficos, ambientales y de cobertura sanitaria que afectan a la epidemiologia de D/DH. Estas variables se organizaron en cuatro grupos de indicadores: demograficos, socioeconomicos, de vivienda y climaticos y geograficos. Los datos se analizaron por medio de regresiones de Poisson simples y multiples. RESULTADOS: Los cantones de Costa Rica con mayor incidencia de D/DH se localizaron principalmente cerca de las costas, coincidiendo con algunas de las variables estudiadas. La temperatura, la altitud y el indice de pobreza humana (IPH) fueron las variables mas relevantes para explicar la incidencia de D/DH, en tanto que la temperatura fue la mas significativa en los analisis multiples. CONCLUSIONES: Los analisis permitieron relacionar una mayor incidencia de D/DH con cantones de menor altitud, mayor temperatura y un IPH elevado. Esta informacion es relevante como un primer paso para estratificar prioridades y optimizar acciones de prevencion y control de esta enfermedad.