Vector Infestation Research Articles

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.

Spatial Vulnerability to Dengue in a Brazilian Urban Area During a 7-Year Surveillance

This study considers the vulnerability of the urban area of the City of Belo Horizonte to dengue. A total number of 89,607 cases registered in the surveillance system from 1996 to 2002 were analyzed. Seven epidemic waves were identified during this period. Cases were grouped into 2,563 census areas, and three risk categories were proposed based on how many times each area reached a threshold established for each epidemic wave. The association between the risk categories and the socioeconomic, demographic and urban-infrastructure characteristics was evaluated. Analysis included Kruskal-Wallis test variance comparisons and multivariate regression using multinomial models. Incidence rates differed significantly among the three risk categories in most of the epidemic waves. The factors that best characterized the areas were low educational level (< or =4 years of schooling), low income of the head of the family (< or =2 minimum wages per household), household density, and proportion of children and elderly women. Information related to basic sanitation was not enough to discriminate levels of susceptibility to dengue, and study of population density and concentration of establishments considered vulnerable to vector infestation yielded questionable results. It is important to consider different levels of exposure of the population to explain the heterogeneous pattern of distribution of dengue cases in an urban setting. Understanding the dynamics of dengue fever is essential for surveillance purposes, to improve control measures and to avoid epidemics of this disease.

Evaluation of methoprene effect on Aedes aegypti (Diptera: Culicidae) development in laboratory conditions

Several Brazilian Aedes aegypti populations are resistant to the larvicidae temephos. Methoprene, that inhibits adult emergence, is one of the alternatives envisaged by the Brazilian Dengue Control Program (PNCD). However, at Brazil vector infestation rates are measured through larvae indexes and it has been claimed that methoprene use in the field could face operational problems. In order to define a standardized protocol, methoprene effect was evaluated in laboratory conditions after continuous exposure of larvae (Rockefeller strain) to a methoprene formulation available to the PNCD. Methoprene-derived mortality occurs mainly at the pupa stage and pupa development is inversely proportional to methoprene concentration. Number and viability of eggs laid by treated and control females are equivalent. A methoprene dose-dependent delay in the development was noted; however, strong correlations were found for total mortality or adult emergence inhibition if data obtained when all control mosquitoes have emerged are compared to data obtained when methoprene-treated groups finish development. The cumulative record of total methoprene-induced mortality at the time control adults emerge is proposed for routine evaluation of field populations. Mortality of all specimens, but not of larva, could account for adult emergence inhibition, confirming the inadequacy of larvae indexes to evaluate methoprene effect.

Infestação pelo Aedes aegypti e ocorrência da dengue em Belo Horizonte, Minas Gerais

To assess the association between the proportion of buildings positive for Aedes aegypti larvae measured by means of building infestation rate and dengue incidence rate. Autochthonous dengue cases were selected and building infestation rates assessed in the coverage areas of health districts in the city of Belo Horizonte, Brazil, from October 1997 to May 2001. After grouping building infestation rates according to their distribution in quartiles, mean dengue incidence rates (for the month following the building infestation assessment) were compared using ANOVA. A weak though statistically significant correlation was observed between monthly dengue incidence and building infestation rates for the health districts (r=0.21; p=0.02) and coverage areas (r=0.14; p=0.00) in the study period. After grouping building infestation rates in quartiles, coverage areas of building infestation between 0.46% and 1.32% (second quartile) showed a mean monthly dengue incidence rate twice as high as areas of building infestation lower or equal to 0.45% (first quartile). Areas of building infestation between 1.33% and 2.76% (third quartile) and equal to or higher than 2.77% had a mean monthly dengue incidence rate five and seven times, respectively, higher than those areas showing 0.45% or less. Despite the well-known limitations of building infestation rates for estimating vector infestation and prediction of dengue epidemics, the study results indicate that higher building infestation rates were associated to a higher risk of disease transmission in the health districts and coverage areas of Belo Horizonte.

Knowledge, attitudes, and practices concerning Chagas disease in schoolchildren from an endemic area in Peru

This study analyzes knowledge, attitudes, and practices concerning Chagas disease among 241 primary schoolchildren in "La Tinguiña", Ica, Peru (December 2000 - January 2001). Less than 1% of those interviewed knew that triatomines transmit Chagas disease, while nearly a quarter recognized the illness based on the appearance of "lumps" on the skin; 35.27% knew that vector infestation is controlled using insecticides; 26.56% recognized the adult stage of the vector, and 21.16% the nymphal instar; 14.11% knew triatomines or "kissing bugs" by the name "chirimacha"; 82.57% would accept an entomological survey, 66.80% would submit to a serological study, and 63.90% would participate in a triatomine search. The study shows that the population, despite having very limited knowledge on the disease and its vectors, shows interest in collaborating. Therefore, it is recommended that Chagas disease surveillance and control include educational programs and community participation.

Host Spatial Heterogeneity and the Spread of Vector-Borne Infection

We analyze how spatial heterogeneity in host density affects the advance of vector-borne disease. Infection requires vector infestation. The vector spreads only between hosts occupying the same neighborhood, and the number of hosts varies randomly among neighborhoods. Simulation of a spatially detailed model shows that increasing heterogeneity in host abundance reduces pathogen prevalence. Clumping of hosts can limit the advance of the vector, which inhibits the spread of infection indirectly. Clumping can also increase the chance that the pathogen and vector become physically separated during the initial phase of the epidemic process. The latter limitation on the pathogen's spread, in our simulations, is restricted to small interaction neighborhoods. A mean-field model, which does not maintain spatial correlations between sites, approximates simulation results when hosts are arrayed uniformly, but overestimates infection prevalence when hosts are aggregated. A pair approximation, which includes some of the simulation model's spatial correlations, better describes the vector infestation frequencies across host spatial dispersions.

Reemergence of dengue in Cuba: a 1997 epidemic in Santiago de Cuba.

After 15 years of absence, dengue reemerged in the municipality of Santiago de Cuba because of increasing migration to the area by people from disease-endemic regions, a high level of vector infestation, and the breakdown of eradication measures. The 1997 epidemic was detected early through an active surveillance system. Of 2,946 laboratory-confirmed cases, 205 were dengue hemorrhagic fever, and 12 were fatal. No deaths were reported in persons under 16 years of age. Now the epidemic is fully controlled.

Treatment of onchocerciasis.

Onchocerciasis ('river blindness') has for several centuries been the scourge of people living in certain areas of the world where the disease is endemic. The treatment available up to 10 years ago, diethylcarbamazine, had very severe secondary effects. The availability of ivermectin--a well tolerated and highly effective microfilaricidal drug--has completely changed this scenario. Ivermectin is now considered to be the drug of choice for the treatment of onchocerciasis. The prognosis for people with onchocerciasis has changed greatly. It is now possible to avoid the heavy infection loads seen previously, and patients, especially expatriates, may have their symptoms relieved by treatment. Ivermectin, used in mass treatment, may also improve the epidemiological situation, reducing the level of microfilariae in the skin of infected people and thus reducing the source for vector infestation. However, the treatment has to be repeated because the drug has no macrofilaricidal effect. Research today is focused on the finding of a drug able to destroy the adult worms that go on producing microfilariae for the length of their lives.

Dengue in Brazil--situation, transmission and control--a proposal for ecological control.

This article discusses dengue in terms of its conceptual and historical aspects, epidemiological and clinical/pathological nature, and evolution up to the present situation in Brazil. The author discusses the ecological relationship in both the production of dengue and its control. Comparison is made between traditional dengue-control programs and a proposed socially-controlled program of an ecological nature without the use of insecticides. Stress is placed on interdisciplinary technical and scientific activity, broadbased participation by communities in discussing methodological aspects involving them, and prospective evaluation comparing the communities selected for intervention and control communities with regard to clinical and subclinical dengue cases and vector infestation rates in relation to climatic, socio-economic, and behavioural factors.

Incidence of rice tungro virus disease and its vector in South Sulawesi, Indonesia

Abstract In South Sulawesi rice tungro virus (RTV) disease was found to be endemic in double‐cropped areas, where it was likely to be carried over to the next season in stubbles, ratoons or volunteer plants. The leaf hopper vector, Nephotettix virescens population started to increase 2–3 months after sowing both in wet and dry seasons, and it constituted more than 80% of total green leaf hopper recorded. It was found consistently that incidence of RTV increased with increase of the vector population. Late plantings were more vulnerable to vector infestation and tungro incidence in both wet‐ and dry‐season crops. Tungro epidemics were likely to occur more in the wet season than in dry season due to availability of large vector population.