Seasonal Malaria Transmission and Variation of Anopheline Density in Two Distinct Endemic Areas in Brazilian Amazônia

Abstract

Studies on seasonal anopheline fauna variation were performed in two distinct settlements in the State of Rondônia, Brazil: one at the Madeira River banks (Portuchuelo) with stable native Amazonian population; the other at an inland lumber-extracting farm (Urupá) in dry land, in which adults are mostly migrants. During a 6-yr period (1994-2000), 8,638 adult anophelines were collected: 2,684 in Urupá and 5,954 in Portuchuelo. Anopheles darlingi represented >95% of total mosquitoes caught. Dissection of 4,424 A. darlingi females yielded a very low sporozoite infection index below 0.1%. Oocysts were found in both localities in approximately 0.1% of dissected mosquitoes. Determination of the hour biting rates disclosed seasonal variations in both localities. However, in Portuchuelo, mosquito density peaked at the acme of the rainy season, whereas at Urupá it peaked in the dry season. The increase in mosquito density and incidence of malaria cases were coincident. The high mosquito densities observed in the riverine settlement of Portochuelo sector B, which permits evaluation in > 10,000 mosquitoes' bites/person/year, could explain, in spite of the low mosquito's infection index, the previously described development of natural immunity in the local population that is not observed in the dry land agroindustrial settlement of Urupá.

Urban malaria in the Brazilian Western Amazon Region I: high prevalence of asymptomatic carriers in an urban riverside district is associated with a high level of clinical malaria

“…However, it was necessary to identify the source of malaria parasites responsible for the high transmission observed. In communities of rural riverside areas the incidence of new malaria's cases is quite low in the dry season and increases sharply at the end of the rainy season, after the increase of the Anopheles density (Alves 2003, Gil et al 2003. It had been suggested that the source of infection responsible for the malaria outburst were parasites present in asymptomatic carriers.…”

“…A first model, found in dry land agroindustrial settlements and urban communities in peripheral areas is characterized by an hypo-endemic profile with prevalence of P. vivax over P. falciparum infections and the presence of epidemic outbreaks starting at the end of the rainy season and extending throughout the dry season (Camargo et al 1994, Camargo 1998, Gil et al 2003. A second model was seen in isolated stable native populations in rural riverside areas characterized by hypo-and meso-endemic profiles, with outbreaks following the peak of the rainy season (Camargo 1998, Alves et al 2002.…”

“…A second model was seen in isolated stable native populations in rural riverside areas characterized by hypo-and meso-endemic profiles, with outbreaks following the peak of the rainy season (Camargo 1998, Alves et al 2002. In the first model, asymptomatic malaria infections are rare, or absent as previously ob-served by Prata et al (1988) and the population group at risk, similar to that of "frontier malaria", consists of male adults who due to their professional activities are more exposed to being bitten during dusk, when the Anopheles darlingi vector is more active (Gil et al 2003). In the rural riverside situation, the risk group consists of children and young adolescents following a profile similar to that observed in meso and hiperendemic areas of Africa (Trape et al 1993, Alves et al 2002, Alves 2003, Gil et al 2003.…”

“…In the first model, asymptomatic malaria infections are rare, or absent as previously ob-served by Prata et al (1988) and the population group at risk, similar to that of "frontier malaria", consists of male adults who due to their professional activities are more exposed to being bitten during dusk, when the Anopheles darlingi vector is more active (Gil et al 2003). In the rural riverside situation, the risk group consists of children and young adolescents following a profile similar to that observed in meso and hiperendemic areas of Africa (Trape et al 1993, Alves et al 2002, Alves 2003, Gil et al 2003. In parallel, with the higher malaria transmission rates and sedentary behavior of the people living in riverside areas, the prevalence of asymptomatic infections of P. vivax and P. falciparum malaria was observed in 20 to 65% of the local populations (Camargo 1998, Alves et al 2002, Alves 2003.…”

Urban and suburban malaria in Rondônia (Brazilian Western Amazon) II: perennial transmissions with high anopheline densities are associated with human environmental changes

“…This second period of higher incidence may persist up to the beginning of the following rainy season in November/ December. Therefore malaria is found year round, in contrast with seasonal variation in typical dry land areas and typical riverine rural areas (Gil et al 2003). In 2001 an active search and treatment of clinical malaria cases started in Vila Candelária.…”