Serologic Markers for Detecting Malaria Figure 1. Seroprevalence data for antibodies against A) Plasmodium falciparum merozoite surface protein 119 (MSP-119), B) P. falciparum apical membrane antigen 1 (AMA-1), C) P. vivax MSP-119, and D) P. vivax AMA-1 by age in the study population, Somalia, 2008. Gray lines indicate 95% conﬁdence intervals. Seroconversion rates (95% conﬁdence intervals) were as follows: P. falciparum MSP-119 0.0082 (0.0068–0.097); AMA-1 0.0053 (0.0042–0.0066); P. vivax MSP-119 0.0086 (0.0055–0.0133); AMA-1 0.0075 (0.0050–0.0112). the study sample, thereby minimizing bias caused by local cross-reactive antigens. Although this method does not rule out cross-reactive antigens, it makes it unlikely. Antibody responses in young children who are unlikely to have ac- quired infections outside the study area, and for whom no recent travel history was reported, also suggest recent ma- laria transmission. In our study area, several children <5 years of age had antibody titers >200 to P. falciparum (n = 17) or P. vivax (n = 6). The presence of strong antibody re- sponses (indirect ﬂuorescent antibody titer >20) in children <15 years of age was used as evidence for active transmis- sion of malaria in area of low endemicity in Middle Amer- ica (Costa Rica) (25,26). The indication for local malaria transmission we pro- vide in this study is relevant for local health workers who should be prepared for fever investigations with standard parasitologic techniques (microscopy and RDT). Malaria should be considered as a plausible cause of febrile illness, particularly in an epidemic form. Low-intensity malaria transmission and the presence of malaria vectors make the area susceptible to malaria epidemics, which can have a high mortality rate in resource-poor areas (29), especially if outbreak detection systems (30) are not feasible because of a poor health infrastructure. We observed heterogeneity in seroreactivity within the study area. Although the 3 villages had low transmission intensity and showed no difference in microscopic parasite carriage, serologic markers showed variation in malaria exposure. Antibody prevalence against P. falciparum and, less markedly, P. vivax were lowest in Xuunshaley, which was furthest from seasonal rivers. Combined P. falciparum MSP-119 and AMA-1 antibody prevalence was 2× higher in Badahabo and Ceel-Bardaale than in Xuunshaley. SaTScan analysis indicated heterogeneity in malaria exposure at a microepidemiologic level. We observed 1 statistically sig- niﬁcant cluster of persons with higher seroreactivity against Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 3, March 2010 397