The dynamics of transmission and spatial distribution of malaria in riverside areas of Porto Velho, Rondônia, in the Amazon region of Brazil

Abstract

The study area in Rondônia was the site of extensive malaria epidemic outbreaks in the 19(th) and 20(th) centuries related to environmental impacts, with large immigration flows. The present work analyzes the transmission dynamics of malaria in these areas to propose measures for avoiding epidemic outbreaks due to the construction of two Hydroelectric Power Plants. A population based baseline demographic census and a malaria prevalence follow up were performed in two river side localities in the suburbs of Porto Velho city and in its rural vicinity. The quantification and nature of malaria parasites in clinical patients and asymptomatic parasite carriers were performed using microscopic and Real Time PCR methodologies. Anopheles densities and their seasonal variation were done by monthly captures for defining HBR (hourly biting rate) values.

Main results: (i) malaria among residents show the riverside profile, with population at risk represented by children and young adults; (ii) asymptomatic vivax and falciparum malaria parasite carriers correspond to around 15% of adults living in the area; (iii) vivax malaria relapses were responsible for 30% of clinical cases; (iv) malaria risk for the residents was evaluated as 20-25% for vivax and 5-7% for falciparum malaria; (v) anopheline densities shown outdoors HBR values 5 to 10 fold higher than indoors and reach 10.000 bites/person/year; (vi) very high incidence observed in one of the surveyed localities was explained by a micro epidemic outbreak affecting visitors and temporary residents. Temporary residents living in tents or shacks are accessible to outdoors transmission. Seasonal fishermen were the main group at risk in the study and were responsible for a 2.6 fold increase in the malaria incidence in the locality. This situation illustrates the danger of extensive epidemic outbreaks when thousands of workers and secondary immigrant population will arrive attracted by opportunities opened by the Hydroelectric Power Plants constructions.

Figure 1. Porto Velho City and a general view of the Madeira River valley.

Location of the Porto Velho Municipality, Porto Velho City and a general view of the Madeira River valley, showing the sites of the future construction of the Hydroelectric Power Plants in Santo Antônio and Jirau. Riverside areas of the Madeira River to the south of Porto Velho City, showing localities that will suffer environmental and demographic impacts with the construction of the Hydroelectric Power Plant in Santo Antônio.

Figure 2. Candelária map representation in 2005 and 2006.

Candelária household's representation (little squares). Sub area A contains households up to 100 m distance of mosquitoes' breeding sites (dark spots). Sub area B contains households more than 100 m distant of the breeding sites. Red colors represent houses with malaria cases (2005 and 2006).

Figure 3. Teotônio map representation in 2007.

Teotônio household's representation (little squares). R  =  Riverside area with households of “riberinhos” (amplified in the right upper angle); NR  =  Non riverside area with households of “non ribeirinhos”. Red colors represent houses with malaria cases (2007).

Figure 4. Vivax and falciparum malaria cases monthly distribution.

Monthly distribution (histograms) of vivax and falciparum malaria cases in Vila Candelária (A) and Cachoeira de Teotônio (B), in relation to variation of HRB (hour bitting rates), and rain precipitation in Vila Candelária (2005–2006) and Cachoeira do Teotônio (2007).

Figure 5. Malaria cases monthly distribution in Teotônio.

Monthly distribution of malaria cases in Teotônio among permanent residents (Resident) and temporary residents and visitors (Non resident). Pv  =  Plasmodium vivax; Pf  =  Plasmodium falciparum.