RESEARCH Table 3. Factors associated with Plasmodium falciparum or P. vivax seroprevalence in 3 villages, Somalia, 2008* P. falciparum P. vivax Village Factor OR (95% CI) p value OR (95% CI) Xuunshaley Age 1.02 (1.00–1.04) 0.029 1.04 (1.02–1.06) Badahabo Age 1.03 (1.01–1.05) 0.002 1.03 (1.01–1.05) Ceel-Bardaale Age 1.03 (1.02–1.04) <0.001 1.03 (1.02–1.04) Distance to river† 0.94 (0.88–0.99) 0.028 0.93 (0.88–0.99) *OR, odds ratio; CI, confidence interval. Estimates are adjusted for correlation between observations from the same household. †Nearest seasonal river. p value <0.001 0.006 <0.001 0.016 95% CI 0.87–0.99, p = 0.02) after adjustment for age and correlation between observations from the same household. P. vivax antibody titer did not increase with age or any oth- er factor in those persons who were seropositive. House- hold factors, socioeconomic factors, distance to the nearest livestock enclosure, and use of mosquito netting were not independently associated with immune responses against P. falciparum or P. vivax. Although seroprevalence and antibody titers were higher in older age groups, seroreactivity was also observed in young children. P. falciparum antibodies were detected in 22 children <5 years of age (median titer 216.5, IQR 173.2–248.5); 10 had antibodies against P. vivax (median titer 220.1, IQR 190.4–262.4), and 2 of these children had antibodies against P. falciparum and P. vivax. Thirty chil- dren <5 years of age who responded to malaria antigens were from all 3 villages (3 from Xuunshaley, 7 from Bada- habo, and 20 from Ceel-Bardaale). Travel to areas in which malaria was highly endemic in the past 3 months was not reported for any of these children with antibodies against P. vivax, P. falciparum, or both. In children <5 years of age, a response against P. falciparum antigens was not related to a response against P. vivax antigens (p = 0.30). Discussion We showed that serologic markers can be used to de- tect heterogeneity in malaria transmission in the Gebiley District of Somalia where malaria transmission occurs at levels too low to be detected by microscopy. None of the slides or rapid diagnostic tests showed parasite carriage in the population, and MSP-119 and AMA-1 seroprevalence data showed a clear increase in seroreactivity with age and evidence for variation in exposure to malaria between and within villages. Malaria is perceived as a public health problem in the study area (22), and the 2 slide-conﬁrmed malaria cases con- ﬁrm local clinical malaria episodes. Malaria transmission in the Gebiley District could not be conﬁrmed by micros- copy or RDT in 2 large cross-sectional surveys in the gen- eral population. However, our serologic ﬁndings conﬁrmed the occurrence of malaria transmission in the area. Using a validated model to relate age-speciﬁc seroconversion rates to EIR (11), we estimated that P. falciparum transmission intensity in this area in Somalia was low (EIR <0.1 infec- tious bites/person/year). Because of the longevity of anti- body responses, this estimate should be interpreted as an average EIR experienced over several years. The low EIR appeared to be supported by examination of breeding sites at the end of the wet season, which conﬁrmed the presence of malaria vectors at a low density. We did not directly de- termine the EIR by sampling adult mosquitoes because the low density of mosquitoes would have required intensive sampling over different seasons (6,23). Serologic data showed a clear age-dependency in malaria-speciﬁc immune responses, which suggested ex- posure-driven age acquisition of antibody response. Once acquired, antibody responses to MSP-119 and AMA-1 will persist for several years (L.C. Okell, unpub. data) (12), and the rate of acquisition in younger age groups is therefore critical for determining current malaria transmission in- tensity. The maximum seroprevalence for individual ma- laria antigens did not exceed 25% in the oldest age groups, which is comparable to areas of low malaria endemicity in northeastern Tanzania (12). Because of the longevity of antibody responses, sero- reactivity may not necessarily be the result of recent ex- posure or exposure in the study area (11,24–26). Consid- erable changes in transmission intensity in the study area would have been detected by the model (14). However, especially in adults, exposure to parasites earlier in life and a history of traveling to malaria-endemic areas can obscure immune responses resulting from recent local transmission (25). Our data indicate that although antibodies may have been acquired outside the study area, ongoing local ma- laria transmission at a low intensity is likely. Elimination of false-positive results to reliably detect low-level local malaria transmission is necessary. Cross-reactivity between immune responses to malaria and other parasites have been reported (27,28) but are ex- pected to be more pronounced when whole parasite extract is used instead of recombinant proteins representing single antigens. The chance of cross-reactive antibody responses may be minimized by using sera at a minimum dilution of 1:80 (27). Our serum samples were tested at considerably higher dilutions and we observed no relation in antibody titers between the homologous antigens of P. falciparum and P. vivax. Moreover, our method for calculating sero- positivity derives its seronegative population from within 396 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 3, March 2010