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Malaria on the Amazonian Frontier: Transmission Dynamics, Risk Factors, Spatial Distribution, and Prospects for Control

Little follow-up data on malaria transmission in communities originating from frontier settlements in Amazonia are available. Here we describe a cohort study in a frontier settlement in Acre, Brazil, where 509 subjects contributed 489.7 person-years of follow-up. The association between malaria morbidity during the follow-up and individual, household, and spatial covariates was explored with mixed-effects logistic regression models and spatial analysis. Incidence rates for Plasmodium vivax and Plasmodium falciparum malaria were 30.0/100 and 16.3/100 person-years at risk, respectively. Malaria morbidity was strongly associated with land clearing and farming, and decreased after five years of residence in the area, suggesting that clinical immunity develops among subjects exposed to low malaria endemicity. Significant spatial clustering of malaria was observed in the areas of most recent occupation, indicating that the continuous influx of nonimmune settlers to forest-fringe areas perpetuates the cycle of environmental change and colonization that favors malaria transmission in rural Amazonia.

Received March 14, 2008. Accepted for publication July 7, 2008.

Acknowledgments: The authors thank the inhabitants of Ramal do Granada for their participation in the study; Sebastião Bocalom Rodrigues, Damaris de Oliveira, and Nésio M. Carvalho (Municipal Government of Acrelândia), Raimundo A. Costa and the malaria control teams in Granada and Acrelandia, for their logistic support, Adamílson Luís de Souza and Carlos E. Cavasini for help in fieldwork, Francisco das Chagas O. Luz (Ministry of Health, Brasília, Brazil) for reviewing all malaria slides, Marília Sá Carvalho and Oswaldo Gonçalves Cruz (Program of Scientific Computation, Oswaldo Cruz Foundation) for their helpful advice in the statistical analysis, Cassiano P. Nunes, for artwork, and Tatiana Havryliuk for a critical reading of the manuscript.

Financial support: CNPq (470067/2004-7) and FAPESP (03/09719-6 and 05/51988-0) to M.U.F.; scholarships from FAPESP (M.dS.N. and C.J.), and CNPq (N.S.dS. and M.U.F.) are also acknowledged.

^* Address correspondence to Mônica da Silva-Nunes, Department of Parasitology, Institute of Biomedical Sciences, University of São Paulo, Av. Prof. Lineu Prestes 1374, São Paulo, São Paulo, Brazil, 05508-900. E-mail: msnunes1{at}yahoo.com.br

Authors’ addresses: Mônica da Silva-Nunes, Natal S. da Silva, Camila Juncansen, and Marcelo U. Ferreira, Department of Parasitology, Institute of Biomedical Sciences, University of São Paulo, Av. Prof. Lineu Prestes 1374, São Paulo, São Paulo, Brazil 05508-900, E-mails: msnunes1{at}yahoo.com.br, natalss{at}gmail.com, camilacj{at}gmail.com, and muferrei{at}usp.br. Cláudia T. Codeço, Program of Scientific Computation, Oswaldo Cruz Foundation, Av. Brasil 4365, Rio de Janeiro, Rio de Janeiro, Brazil 21045-900, E-mail: codeco{at}fiocruz.br. Rosely S. Malafronte, Laboratory of Protozoology, Institute of Tropical Medicine of São Paulo, Av. Dr. Enéas de Carvalho Aguiar 470, Cerqueira César, São Paulo, Sã o Paulo, Brazil 05403-000, E-mail: rmalafronte{at}usp.br. Pascoal T. Muniz, Department of Health Sciences, Federal University of Acre, BR 364 km 4, Rio Branco, Acre, Brazil 69915-000, E-mail: pascoal{at}ufac.br.

Reprint requests: Mônica da Silva-Nunes or Marcelo U. Ferreira, Department of Parasitology, Institute of Biomedical Sciences, University of São Paulo, Av. Prof. Lineu Prestes 1374, 05508-900 São Paulo (SP), Brazil, E-mails: msnunes1{at}yahoo.com.br and muferrei{at}usp.br.

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