Unstable Hypoendemic Malaria in Rondonia (Western Amazon Region, Brazil): Epidemic Outbreaks and Work-Associated Incidence in an Agro-Industrial Rural Settlement

Abstract

A longitudinal study was conducted from January 1991 to January 1992 on the Urupa farm, a rural agro-industrial forestry settlement in Rondonia state (Western Amazon Region, Brazil) to define the parasitologic and clinical profile of malaria. Three cross-sectional, parasitologic, and clinical surveys were performed. In the intervals between surveys, malaria cases were monitored by twice a week medical visits to the farm and permanent local surveillance. The population of residents was approximately 170 and was characterized by high mobility. The slide positive rates found in the cross-sectional surveys were 0.5, 4.2 and 2.1, respectively, for the total population (Plasmodium vivax plus P. falciparum). Spleen rate values in children 2-9 years old were always less than 1%. However, this basically hypoendemic malaria situation was unstable, with occurrence of a typical epidemic outbreak at the end of the dry season. The total number of malaria cases recorded from January to December 1991 was 163, giving an annual parasite index of 970 per 1,000 inhabitants. However, sex and age distribution of cases showed rare incidence of malaria in infants and low incidence in children less than the age of 10. Male adults 16-40 years of age represented the main risk group. The observed clustering of cases allowed us to identify the place of work as a factor responsible for high incidence of malaria among adults. The general epidemiologic profile indicated that indoors transmission of malaria by the local Anopheles vector was low or absent.

Single-nucleotide polymorphism, linkage disequilibrium and geographic structure in the malaria parasite Plasmodium vivax: prospects for genome-wide association studies

“…All sites in Brazil are characterized by year-round but hypoendemic malaria transmission, with P. vivax prevalence rates typically below 1% [29,30]. The 249 samples from Brazil were collected in the rural settlement of Granada and the town of Plácido de Castro (50 km south of Granada), both in Acre State, and in the city of Porto Velho (500 km east of Granada), in Rondônia State.…”

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

“…The rapid flow rate of the water is inappropriate for in situ mosquitoes breeding, since larvae and eggs are removed. However, by the end of the rainy season, when rivers recover their original courses, large stable water collections are formed in areas previously inundated along their borders (Marques 1986, Camargo et al 1994. These stagnated water collections comprise excellent breeding sites for An.…”

“…In the longitudinal follow up, from 2001 to 2004, it was observed a period of high malaria incidence in the rainy season, peaking in April-May, and a season of lower incidence in the dry season (June to August) characteristic of flooding areas of the rural riverine profile model described for Amazonian localities (Alves et al 2002(Alves et al , 2005. However, other high incidence periods were irregularly observed in the dry season, as described in dry land areas of Rondônia (Camargo et al 1994(Camargo et al , 1996. This second period of higher incidence may persist up to the beginning of the following rainy season in November/ December.…”

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

“…A control group of 100 residents in the district of Triunfo, in the municipality of Candeias, 150 km from Porto Velho, was examined using the same protocol of clinical, parasitological, and PCR examinations. The Triunfo district is a typical agro-industrial settlement in the dry land areas of Rondônia where previous studies by our team have failed to identify asymptomatic malaria carriers by microscopy examination of blood thick smears (Camargo et al 1994). This control group was chosen because its API value was 340 in 2002, equivalent to the one we observed in Vila Candelária (see Results).…”

“…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.…”