An ecoregional classification for the state of Roraima, Brazil: the importance of landscape in malaria biology

Understanding the different background landscapes in which malaria transmission occurs is fundamental to understanding malaria epidemiology and to designing effective local malaria control programs. Geology, geomorphology, vegetation, climate, land use, and anopheline distribution were used as a basis for an ecological classification of the state of Roraima, Brazil, in the northern Amazon Basin, focused on the natural history of malaria and transmission. We used unsupervised maximum likelihood classification, principal components analysis, and weighted overlay with equal contribution analyses to fine-scale thematic maps that resulted in clustered regions. We used ecological niche modeling techniques to develop a fine-scale picture of malaria vector distributions in the state. Eight ecoregions were identified and malaria-related aspects are discussed based on this classification, including 5 types of dense tropical rain forest and 3 types of savannah. Ecoregions formed by dense tropical rain forest were named as montane (ecoregion I), submontane (II), plateau (III), lowland (IV), and alluvial (V). Ecoregions formed by savannah were divided into steppe (VI, campos de Roraima), savannah (VII, cerrado), and wetland (VIII, campinarana). Such ecoregional mappings are important tools in integrated malaria control programs that aim to identify specific characteristics of malaria transmission, classify transmission risk, and define priority areas and appropriate interventions. For some areas, extension of these approaches to still-finer resolutions will provide an improved picture of malaria transmission patterns.     

Biting indices,host-seeking activity and natural infection rates of anopheline species in Boa Vista,Roraima, Brazil from 1996 to 1998

The epidemiology of the transmission of malaria parasites varies ecologically. To observe some entomological aspects of the malaria transmission in an urban environment, a longitudinal survey of anopheline fauna was performed in Boa Vista, Roraima, Brazil. A total of 7,263 anophelines was collected in human bait at 13 de Setembro and Caran? districts: Anopheles albitarsis sensu lato (82.8%), An. darlingi (10.3%), An. braziliensis (5.5%), An. peryassui (0.9%) and An. nuneztovari (0.5%). Nightly 12 h collections showed that An. albitarsis was actively biting throughout the night with peak activities at sunset and at midnight. An. darlingi bit during all night and did not demonstrate a defined biting peak. Highest biting indices, entomological inoculation rates and malaria cases were observed seasonally during the rainy season (April-November). Hourly collections showed host seek activity for all mosquitoes peaked during the first hour after sunset. An. darlingi showed the highest plasmodial malaria infection rate followed by An. albitarsis, An. braziliensis and An. nuneztovari (8.5%, 4.6%, 3% and 2.6%, respectively). An. albitarsis was the most frequently collected anopheline, presented the highest biting index and it was the second most frequently collected infected species infected with malaria parasites. An. albitarsis and An. darlingi respectively, are the primary vectors of malaria throughout Boa Vista.     

The Genome of Anopheles darlingi,the main neotropical malaria vector

Anopheles darlingi is the principal neotropical malaria vector, responsible for more than a million cases of malaria per year on the American continent. Anopheles darlingi diverged from the African and Asian malaria vectors ∼100 million years ago (mya) and successfully adapted to the New World environment. Here we present an annotated reference A. darlingi genome, sequenced from a wild population of males and females collected in the Brazilian Amazon. A total of 10 481 predicted protein-coding genes were annotated, 72% of which have their closest counterpart in Anopheles gambiae and 21% have highest similarity with other mosquito species. In spite of a long period of divergent evolution, conserved gene synteny was observed between A. darlingi and A. gambiae. More than 10 million single nucleotide polymorphisms and short indels with potential use as genetic markers were identified. Transposable elements correspond to 2.3% of the A. darlingi genome. Genes associated with hematophagy, immunity and insecticide resistance, directly involved in vector–human and vector–parasite interactions, were identified and discussed. This study represents the first effort to sequence the genome of a neotropical malaria vector, and opens a new window through which we can contemplate the evolutionary history of anopheline mosquitoes. It also provides valuable information that may lead to novel strategies to reduce malaria transmission on the South American continent. The A. darlingi genome is accessible at www.labinfo.lncc.br/index.php/anopheles-darlingi.     

Parity and age composition for Anopheles darlingi root (Diptera: Culicidae) and Anopheles albitarsis Lynch-Arribálzaga (Diptera: Culicidae) of the northern Amazon Basin, Brazil.

Parity and age composition for Anopheles darlingi and Anopheles albitarsis in the northern Amazon Basin, Brazil, were investigated. Anopheline ovaries and ovarioles were examined in order to determine whether hourly and seasonal parity status for the vectors An. albitarsis and An. darlingi would vary in two different landscapes (forest and savanna/forest) where malaria is endemic in the northern Amazon Basin. A total of 1,199 anophelines (535 An. darlingi and 664 An. albitarsis) was dissected for parity status, ovariole dilatations, and follicular stages. The total number of nulliparous and parous females for both species varied by time of collection, locality, and season. During the rainy season for the first two h of collection, more nulliparous An. albitarsis and An. darlingi females were collected in the first hour (18:00-19:00), but during the second hour (19:00-20:00) more parous females of both species were captured. During the dry season in Copaíbas, more parous females of An. albitarsis were observed in the first hour while more nulliparous females were observed in the second hour. Nulliparous and parous females of both species for both hours were not significantly different at Road 19 in the dry season. This location was characterized by a forest malaria pattern of transmission with higher numbers of parous females and population stability in the dry season. In Copaíbas, the density and parity of An. darlingi increased during the rainy season, and it could be classified as an alluvial malaria pattern of transmission. For Copaíbas, control measures would be more successful if adopted at the transition from dry to rainy season. Further investigation on longitudinal spatio-temporal change in longevity and survival rates would help us to clarify differences in vector competence for An. darlingi and An. albitarsis and add to the understanding of differences regarding prevailing landscapes in malaria epidemiology in the northern Amazon Basin.     

Relations between Anopheles darlingi breeding habitats, rainfall, river level and malaria transmission rates in the rain forest of Suriname

Irregular peaks of P.falciparum malaria incidence were monitored in Suriname during 1982-86. An attempt is made to correlate these fluctuations of malaria rates with rainfall and river level in relation to the periodic availability of breeding habitats for the malaria vector Anopheles darlingi Root. Densities of adult female An.darlingi were recorded in the Upper-Marowijne region of Suriname, in a focus of malaria along the Marowijne River. Peaks in biting densities correlated well with periods of (i) high water level in the long rainy season, (ii) low water level in the long dry season, and (iii) abundant rainfall in the short rainy season. Critical levels of river-height and rainfall were defined which could explain most of the monthly fluctuations in malaria parasite incidence observed in this area.     

Man biting rate seasonal variation of malaria vectors in Roraima, Brazil

Malaria control has been directed towards regional actions where more detailed knowledge of local determinants of transmission is of primary importance. This is a short report on range distribution and biting indices for Anopheles darlingi and An. albitarsis during the dry and rainy season that follows river level variation in a savanna/alluvial forest malaria system area in the Northern Amazon Basin. Distribution range and adult biting indices were at their highest during the rainy season for both An. darlingi and An. albitarsis. During the rainy season the neighboring alluvial forest was extensively flooded. This coincided with highest rates in malaria transmission with case clustering near the river. As the river receded, anopheline distribution range and density decreased. This decrease in distribution and density corresponded to a malaria decrease in the near area. An exponential regression function was derived to permit estimations of An. darlingi distribution over specified distances. Anopheline spatio-temporal variations lead to uneven malaria case distribution and are of important implications for control strategies.     

Morphological and biochemical analyses of the salivary glands of the malaria vector, Anopheles darlingi.

Adult Anopheles darlingi salivary glands are paired organs located on either side of the esophagus. The male glands consist of a single small lobe. The female gland is composed of two lateral lobes, with distinct proximal and distal portions, and a medial lobe. The lobes are acinar structures, organized as a unicellular epithelium that surrounds a salivary canal. The general cellular architecture is similar among the lobes, with secretory material appearing as large masses that push the cellular structures to the periphery of the organ. Cells of the proximal-lateral lobes show asynchronous cycles of secretory activity and contain secretory masses with finely filamentous aspect. In the distal-lateral lobes, cells display synchronous cycles of activity, and have a dense secretory product with mottled pattern. Cells of the medial lobe have secretory masses uniformly stained and highly electrondense. Biochemical analysis of the adult female salivary glands revealed apyrase, alpha-glucosidase and lysozyme activities. Alpha-glucosidase and lysozyme activities are detected mostly in the proximal lobes while apyrase is mainly accumulated in the distal lobes. This differential distribution of the analyzed enzymes reflects a specialization of different regions for sugar and blood feeding. Thus, the morphological differences observed in the lobes correlate with functional ones.     

Molecular population genetics of the malaria vector Anopheles darlingi in Central and South America

To analyze the genetic relatedness and phylogeographic structure of Anopheles darlingi from 19 localities throughout Central and South America, we used a minimum spanning network, diversity measures, differentiation, neutrality tests, and mismatch distribution with mitochondrial cytochrome oxidase subunit I (COI) sequences. All the Central American haplotypes were separated by seven mutational steps from the South American haplotypes and the FST distance-based neighbor-joining tree showed a primary division between Central and South America, evidence for a putative gene pool division. More ancestral and diverse haplotypes were found in Amazonian and southern Brazil populations, suggesting that Central American populations may have originated in South America. The patterns of the mtDNA haplotype diversity and five of six tests for equilibrium implicate demographic expansion in the South American populations as the historical structure, but mismatch distribution depicts populations at mutation drift equilibrium (MDE). In South America, the departure from equilibrium was consistent with an expansion that occurred during the Pleistocene.     

Spatial expansion and population structure of the neotropical malaria vector, Anopheles darlingi (Diptera: Culicidae)

Extensive population structuring is known to occur in Anopheles darlingi , the primary malaria vector of the Neotropics. We analysed the phylogeographic structure of the species using the mitochondrial cytochrome oxidase I marker. Diversity is divided into six main population groups in South America: Colombia, central Amazonia, southern Brazil, south-eastern Brazil, and two groups in north-east Brazil. The ancestral distribution of the taxon is hypothesized to be central Amazonia, and there is evidence of expansion from this region during the late Pleistocene. The expansion was not a homogeneous front, however, with at least four subgroups being formed due to geographic barriers. As the species spread, populations became isolated from each other by the Amazon River and the coastal mountain ranges of south-eastern Brazil and the Andes. Analyses incorporating distances around these barriers suggest that the entire South American range of An. darlingi is at mutation–dispersal–drift equilibrium. Because the species is distributed throughout such a broad area, the limited dispersal across some landscape types promotes differentiation between otherwise proximate populations. Moreover, samples from the An. darlingi holotype location in Rio de Janeiro State are substantially derived from all other populations, implying that there may be additional genetic differences of epidemiological relevance. The results obtained contribute to our understanding of gene flow in this species and allow the formulation of human mosquito health protocols in light of the potential population differences in vector capacity or tolerance to control strategies.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 854–866.     

Geographic distribution,evolution, and disease importance of species within the Neotropical Anopheles albitarsis Group (Diptera,Culicidae)

The Anopheles albitarsis group of mosquitoes comprises eight recognized species and one mitochondrial lineage. Our knowledge of malaria vectorial importance and the distribution and evolution of these taxa is incomplete. We constructed ecological niche models (ENMs) for these taxa and used hypothesized phylogenetic relationships and ENMs to investigate environmental and ecological divergence associated with speciation events. Two major clades were identified, one north (Clade 1) and one south (Clade 2) of the Amazon River that likely is or was a barrier to mosquito movement. Clade 1 species occur more often in higher average temperature locations than Clade 2 species, and taxon splits within Clade 1 corresponded with a greater divergence of variables related to precipitation than was the case within Clade 2. Comparison of the ecological profiles of sympatric species and sister species support the idea that phylogenetic proximity is related to ecological similarity. Anopheles albitarsis I, An. janconnae, and An. marajoara ENMs had the highest percentage of their predicted suitable habitat overlapping distribution models of Plasmodium falciparum and P. vivax, and warrant additional studies of the transmission potential of these species. Phylogenetic proximity may be related to malaria vectorial importance within the Albitarsis Group.     

The distribution of two major malaria vectors, Anopheles gambiae and Anopheles arabiensis, in Nigeria.

The distribution of Anopheles gambiae and An. arabiensis across the ecological zones of Nigeria (arid savanna in the north gradually turns into humid forest in the south) was investigated. Results of the present study were compared to the distributions determined from samples of indoor-resting females reported by an earlier study over 20 years ago. Larvae were sampled in the rainy seasons of 1997 and 1999 from 24 localities, 10 of which were sampled in both years. Specimens were identified by the polymerase chain reaction method. Results showed that species composition changed significantly among the 10 localities in both years (chi2=13.62, P = 0.0002), but this change was significant in only four of the 10 localities. The identity of the prevalent (more abundant) species changed between 1997 and 1999 in only three of 10 localities. An. arabiensis was prevalent in several localities in the southern Guinea savanna, an area where it was virtually absent over 20 years ago. The data suggest that An. arabiensis has extend its range, although differences in sampling technique (larval sampling versus adult collection) can not be ruled out as a possible explanation.     

The prospect of eliminating malaria transmission in some regions of Brazil

This paper discusses malaria epidemiology and control in Brazil as well as the prospect of interrupting transmission in some areas of the country. The concepts of receptivity and vulnerability of an area to malaria transmission are analysed to predict where elimination might occur in a near future. Outside of the Amazon Region and in the oriental states of the Amazon, such as Tocantins, Maranh?o and Mato Grosso, it is likely that malaria transmission can be eliminated with the development and sustained use of a good surveillance system.     

Habitat suitability of Anopheles vector species and association with human malaria in the Atlantic Forest in south-eastern Brazil

Every year, autochthonous cases of Plasmodium vivax malaria occur in low-endemicity areas of Vale do Ribeira in the south-eastern part of the Atlantic Forest, state of S?o Paulo, where Anopheles cruzii and Anopheles bellator are considered the primary vectors. However, other species in the subgenus Nyssorhynchus of Anopheles (e.g., Anopheles marajoara) are abundant and may participate in the dynamics of malarial transmission in that region. The objectives of the present study were to assess the spatial distribution of An. cruzii, An. bellator and An. marajoara and to associate the presence of these species with malaria cases in the municipalities of the Vale do Ribeira. Potential habitat suitability modelling was applied to determine both the spatial distribution of An. cruzii, An. bellator and An. marajoara and to establish the density of each species. Poisson regression was utilized to associate malaria cases with estimated vector densities. As a result, An. cruzii was correlated with the forested slopes of the Serra do Mar, An. bellator with the coastal plain and An. marajoara with the deforested areas. Moreover, both An. marajoara and An. cruzii were positively associated with malaria cases. Considering that An. marajoara was demonstrated to be a primary vector of human Plasmodium in the rural areas of the state of Amapá, more attention should be given to the species in the deforested areas of the Atlantic Forest, where it might be a secondary vector.     

Biting patterns and seasonal densities of Anopheles mosquitoes in the Cayo District, Belize, Central America with emphasis on Anopheles darlingi.

The present study utilized an experimental hut to conduct human-baited landing collections for characterizing the all-night biting patterns and seasonal densities of adult Anopheles darlingi in the centrally located Cayo District of Belize, Central America. A total of 25 all-night collections (i.e., sunset to sunrise) were conducted from January 2002 to May 2003, capturing a total of 18,878 An. darlingi females. Anopheles darlingi exhibited a bimodal nightly biting pattern with one predominate peak occurring three h after sunset and a smaller peak occurring one h prior to sunrise. Biting females were collected throughout the night in higher densities indoors (9,611) than outside (9,267) the experimental hut (O:I=1.00:1.04). Seasonal adult collections show An. darlingi densities were highest during the transitional months between the end of the wet and beginning of the dry season (January) and the end of the dry season and beginning of the wet season (May). A total of 2,010 An. darlingi females was captured in 31 two-h, human-baited landing collections performed from January to October 2002. Anopheles darlingi monthly population densities were found to have no significant associations with high or low temperatures, precipitation, or river level. However, qualitative data examination indicates an inverse relationship between river level and An. darlingi adult collections suggesting a disturbance of larval habitats. All-night biting and seasonal distribution patterns for other anopheline species are also described. None of the adult specimens collected throughout the entire study tested positive for Plasmodium spp. infection using the VecTest rapid diagnostic kit.     

Anopheles and malaria transmission in Ambohimena, a village in the Occidental fringe of Madagascar Highlands

The anopheline vectors and malaria transmission were investigated in the Middle West of Madagascar, in the village of Ambohimena (at the altitude of 940 meters) during two years (August 1996 to July 1998). This village is located outside the vector control area, where yearly DDT house spraying campaigns have been conducted between 1993 and 1998. Collection of mosquitoes was mainly based on all night man billing collections (650 man-nights), pyrethrum spray catches (224 bedrooms) and direct collections in outdoor resting places (140 toilets, 61 pigsties, 33 holes, 19 sheds, 79 sisal hedges, 70 cart shelters). Blood fed anophelines allowed analysis of the origin of blood with an ELISA method. Presence of circum-sporozoite protein was assessed with another ELISA method. The total number of collected anophelines was 14,280. Two malaria vectors were identified: Anopheles funestus Giles, 1900 and An. arabiensis Patton, 1902. An. funestus was the most abundant mosquito, especially during the hot rainy season. Two peaks of abundance were observed (in December and April). Endophagic rate (for mosquitoes aggressive for man) of 35.3%, an endophilic rate (for resting mosquitoes) of 78.0% and an anthropophilic rate (for indoor resting mosquitoes) of 64.0% were calculated. The average parity rate was relatively low (61.2%). The Plasmodium falciparum immunological sporozoite rate was 0.20%. An. funestus presented a higher vectorial capacity during the first round of rice cultivation (January) than during the second round (April-May). An. arabiensis was mostly abundant in December and January at the beginning of the rainy season. This species was exophagic (endophagic rate = 27.5%) and zoophilic (anthropophilic rate = 7.8%). The sporozoitic index was determined as zero (number of examined mosquitoes = 871). In this village, An. arabiensis presented only marginal importance for malaria transmission. Malaria transmission occurred from December to April. Annual entomological inoculation rate, only due to An. funestus, was 8.96 during the first year, and 3.17 during the second year. In this area where transmission is moderately stable, we suggest an extension of vector imagocidal control activities up to the western fringes of the Highlands.     

Study of a population of Biomphalaria tenagophila (Orbigny, 1835) and of schistosomiasis transmission in "Alto da Boa Vista", Rio de Janeiro

The present study was performed using data from a Biomphalaria tenagophila population located in a watercress garden in the Alto da Boa Vista region representing an isolated focal point of schistosomiasis in the city of Rio de Janeiro. The density and age structure of this B. tenagophila population and its rate of infection by Schistosoma mansoni were studied for a period of 15 months. The snail population showed seasonal variation in density, with a decrease in number of individuals at the beginning of the rainy season. At the end of this season, the population consisted mainly of adults (92.8% in May 1985 and 82.8% in April 1986). The population growth curve was logistic and of sigmoidal configuration. Schistosoma mansoni cercariae were eliminated over a short period of time (March, April and May 1986). The release of cercariae of S. mansoni and of birds seems to depend on environmental temperature, which during certain months would show a daily variation of up to 13 degrees C, with the lower thermal limit approaching the limit value for sporocyte development.     

Impact of permethrin-treated bednets on malaria transmission by the Anopheles gambiae complex in The Gambia

Malaria vector mosquitoes belonging to the Anopheles gambiae complex were studied in four hamlets in The Gambia. All inhabitants were given bednets treated either with a placebo (milk) in two hamlets or with the pyrethroid insecticide permethrin (500 mg/m2) in two other hamlets. Malaria transmission occurred mainly during a few weeks of the rainy season, in September and October 1987. The indoor resting densities of mosquitoes in permethrin-treated hamlets were reduced, and we estimated over 90% reduction in biting on man by An. gambiae Giles sensu stricto in these hamlets. No mosquitoes were found under permethrin-treated bednets compared with eighty-one recovered from placebo-treated bednets. Mosquitoes exited more readily from rooms where permethrin-treated bednets were used than from rooms with placebo-treated nets. The annual mean probability that a child would receive an infective bite was estimated to be 0.09 in hamlets with insecticide-treated bednets, compared with 1.9 where placebo-treated bednets were used. Permethrin-treated bednets are therefore recommended as a means of effectively reducing the risk of exposure to malaria transmission, particularly in areas of low seasonal transmission.     

The importance of human FcgammaRI in mediating protection to malaria

The success of passive immunization suggests that antibody-based therapies will be effective at controlling malaria. We describe the development of fully human antibodies specific for Plasmodium falciparum by antibody repertoire cloning from phage display libraries generated from immune Gambian adults. Although these novel reagents bind with strong affinity to malaria parasites, it remains unclear if in vitro assays are predictive of functional immunity in humans, due to the lack of suitable animal models permissive for P. falciparum. A potentially useful solution described herein allows the antimalarial efficacy of human antibodies to be determined using rodent malaria parasites transgenic for P. falciparum antigens in mice also transgenic for human Fc-receptors. These human IgG1s cured animals of an otherwise lethal malaria infection, and protection was crucially dependent on human FcgammaRI. This important finding documents the capacity of FcgammaRI to mediate potent antimalaria immunity and supports the development of FcgammaRI-directed therapy for human malaria.     

Multimodal pyrethroid resistance in malaria vectors, Anopheles gambiae s.s., Anopheles arabiensis, and Anopheles funestus s.s. in western Kenya

Anopheles gambiae s.s., Anopheles arabiensis, and Anopheles funestus s.s. are the most important species for malaria transmission. Pyrethroid resistance of these vector mosquitoes is one of the main obstacles against effective vector control. The objective of the present study was to monitor the pyrethroid susceptibility in the 3 major malaria vectors in a highly malaria endemic area in western Kenya and to elucidate the mechanisms of pyrethroid resistance in these species. Gembe East and West, Mbita Division, and 4 main western islands in the Suba district of the Nyanza province in western Kenya were used as the study area. Larval and adult collection and bioassay were conducted, as well as the detection of point mutation in the voltage-gated sodium channel (1014L) by using direct DNA sequencing. A high level of pyrethroid resistance caused by the high frequency of point mutations (L1014S) was detected in An. gambiae s.s. In contrast, P450-related pyrethroid resistance seemed to be widespread in both An. arabiensis and An. funestus s.s. Not a single L1014S mutation was detected in these 2 species. A lack of cross-resistance between DDT and permethrin was also found in An. arabiensis and An. funestus s.s., while An. gambiae s.s. was resistant to both insecticides. It is noteworthy that the above species in the same area are found to be resistant to pyrethroids by their unique resistance mechanisms. Furthermore, it is interesting that 2 different resistance mechanisms have developed in the 2 sibling species in the same area individually. The cross resistance between permethrin and DDT in An. gambiae s.s. may be attributed to the high frequency of kdr mutation, which might be selected by the frequent exposure to ITNs. Similarly, the metabolic pyrethroid resistance in An. arabiensis and An. funestus s.s. is thought to develop without strong selection by DDT.