Serologic Markers for Detecting Malaria Table 1. Characteristics of persons included in cross-sectional survey for Plasmodium falciparum and P. vivax infection, Somalia, 2008* Characteristic Dry season, Mar–Apr End of wet season, Aug–Sep No. persons 1,178 1,128 Age, y, median (IQR) 17 (6–36) 15 (6–37) Female 48.6 (573/1,178) 50.8 (573/1,128) Reported regular bed net use 1.9 (22/1,158) 2.2 (25/1,128) Reported fever in 14 d preceding survey 4.8 (57/1,179) 0.6 (7/1,128) Temperature >37.5°C at time of survey 0.8 (10/1,177) 1.1 (12/1,124) Positive rapid diagnostic test result 0 (0/1,173) 0 (0/1,106) Plasmodium falciparum parasite prevalence† 0 (0/1,173) 0 (0/1,106) P. vivax parasite prevalence† 0 (0/1,173) 0 (0/1,106) *IQR, interquartile range (25th 75th percentile). Values are % (no. positive/no. tested) unless otherwise indicated. †Determined by screening 200 high-power microscopic fields. sion rates for MSP-119 and AMA-1 (11) was <0.1 infec- tious bites/person/year. When MSP-119 and AMA-1 data were combined, 17.9% (179/1,001) of the persons tested showed reactivity against P. falciparum (i.e., had antibod- ies against P. falciparum MSP-119, AMA-1, or both) and 19.3% (202/1,044) against P. vivax. There was a signiﬁcant positive association between reactivity against P. falcipar- um and P. vivax (p<0.001). However, only 39.8% (66/166) of persons with antibodies against P. falciparum also re- sponded against P. vivax antigens, and there was no appar- ent correlation between antibody titers against antigens of the 2 malaria species (p>0.58). Spatial Patterns in Seroreactivity P. falciparum antibody prevalence was 9.4% (23/244) in Xuunshaley, 21.7% (30/138) in Badahabo (p = 0.001), and 20.4% (126/619) in Ceel-Bardaale (p<0.001) (Table 2). P. vivax antibody prevalence was 16.1% (40/248) in Xuunshaley, 21.0% (31/148) in Badahabo (p = 0.11), and 20.2% (131/648) in Ceel-Bardaale (p = 0.13) (Table 2). Age-adjusted P. falciparum seroreactivity was signiﬁ- cantly increased in a cluster of 18 households (108 persons) in Ceel-Bardaale (p = 0.002) (Figure 2). In Xuunshaley, there was a small cluster of 6 households (27 persons) with a higher age-adjusted P. vivax seroreactivity (p = 0.005). Factors Associated with Seroreactivity Seroreactivity data were analyzed for villages sepa- rately because villages were >7 km apart and were therefore likely to have their own transmission characteristics. In all 3 villages, P. falciparum antibody prevalence increased with age (Table 3). For Ceel-Bardaale, an independent negative association was found between P. falciparum antibody re- sponses and distance to the nearest seasonal river (OR 0.94, 95% CI 0.88–0.99, p = 0.03) after adjustment for age and correlation between observations from the same household. Within the group of persons who had a positive antibody response against P. falciparum, the titer increased with age in Xuunshaley (β = 1.74, SE = 0.81, p = 0.031) and Ceel- Bardaale (β = 11.48, SE = 3.49, p = 0.001). Similar to P. falciparum, P. vivax antibody prevalence increased with age in all 3 villages (Table 3). For Ceel- Bardaale, distance to the nearest seasonal river was nega- tively associated with P. vivax immune response (OR 0.93, Table 2. Immune responses against Plasmodium falciparum and P. vivax in study participants, by village, Somalia, 2008* Village† Characteristic Xuunshaley Badahabo Ceel-Bardaale No. persons 271 160 697 Median age, y (IQR) 20 (7–40) 17.5 (5–35) 13 (6–35) P. falciparum immune response Combined 9.4 (23/244) 21.7 (30/138) 20.4 (126/619) MSP-1 5.1 (13/254) 13.4 (19/142) 15.0 (95/634) 251.2 (155.0–285.3) 214.8 (169.8–275.5) 248.5 (190.5–397.2) AMA-1 4.8 (12/252) 9.2 (13/141) 8.9 (58/653) 169.1 (137.0–190.9) 189.3 (158.4–225.5) 233.7 (170.7–487.0) P. vivax immune response Combined MSP-1 16.1 (40/248) 21.0 (31/148) 20.2 (131/648) 11.9 (30/252) 13.9 (21/151) 10.4 (58/648) 333.9 (271.4–463.9) 342.4 (280.1–374.8) 291.5 (248.7–393.3) AMA-1 5.2 (13/252) 7.4 (11/149) 12.8 (85/665) 151.5 (146.0–202.1) 183.5 (155.7–275.7) 227.1 (184.6–390.8) *IQR, interquartile range (25th–75th percentile); MSP-1, merozoite surface protein 1; AMA-1, apical membrane antigen 1. †Values are % prevalence (no. positive/no. tested) or approximate median titer (IQR) only for seropositive persons unless otherwise indicated. ‡Adjusted for age and correlations between observations from the same household, when applicable. p value‡ 0.04 <0.001 <0.001 0.12 0.02 0.12 0.33 0.33 0.39 0.001 0.10 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 3, March 2010 395