Individual risk factors for Plasmodium vivax infection in the residual malaria transmission focus of Oaxaca, Mexico

Objective. To identify individual risk factors for malariainfection of inhabitants in the residual transmission focus onthe Pacific coast of Oaxaca, Mexico. Materials and Methods.A population-based, matched case-control study wasconducted from January 2002 to July 2003 comparing thefrequency of exposure to individual risk factors in subjectspresenting clinical malaria and uninfected controls. A malariacase was defined as an individual living in the study areapresenting malaria symptoms and a Plasmodium vivax-positivethick blood smear; controls were individuals negative to P.vivax parasites and antibodies of the same gender and with± five years as the case. A standardized questionnaire wasused to record information about the individual risk factorsassociated with malaria episodes in cases and two controlsfor each case. Results. In a multiple conditional logisticregression model analysis of data from 119 cases and 238controls, 18 out of 99 variables were significantly associated(p< 0.05) with increased risk of malaria, including: being bornin another locality (RM 3.16, 95% IC 1.16-6.13); speaking onlyan autochthonous language (RM= 2.48, 95% IC 1.19-3.77);having poor knowledge about malaria (RM= 2.26 95% IC1.10-4.66 P< 0.02); the amount of vegetation around thehouse (RM= 20.43, 95% IC 5.98-70.87, P< 0.000; RM= 3.78,95% IC 1.21-11.80, for 60-100% and 30-59%, respectively); living in houses constructed with perishable materials (RM=2.85, 95% IC 1.62-5.01); living on the periphery of the town(RM= 6.23, 95% IC 3.50-11.0); sleeping on a dirt floor (RM=2.98, 95% IC 1.78-5.01) or with two or more people in the samebed (RM= 1.85, 95% CI 1.09-3.14); not using bed nets (RM=2.39, 95% IC 1.18-4.83, P< 0.003) or using bed nets with holes(RM= 13.93, 95 IC 2.48-78.01); traveling outside of the village(RM= 9.16, 95% IC 1.98-42.2); and previous malaria casesin the house (RM= 5.84, 95% IC 3.33-10.22). Conclusions.Risk of malaria infection was associated with socio-culturaland environmental factors exposing individuals to mosquitobites. A higher risk of malaria infection occurred outside thelocality and by intradomiciliar transmission probably as aresult of relapsing asymptomatic relatives.