RESEARCH Serologic Mar kers for Detecting Malaria in Areas of Low Endemicity, Somalia, 2008 Teun Bousema, Randa M. Youssef, Jackie Cook, Jonathan Cox, Victor A. Alegana, Jamal Amran, Abdisalan M. Noor, Robert W. Snow, and Chris Drakeley Areas in which malaria is not highly endemic are suitable for malaria elimination, but assessing transmission is difﬁ- cult because of lack of sensitivity of commonly used meth- ods. We evaluated serologic markers for detecting variation in malaria exposure in Somalia. Plasmodium falciparum or P. vivax was not detected by microscopy in cross-sectional surveys of samples from persons during the dry (0/1,178) and wet (0/1,128) seasons. Antibody responses against P. falciparum or P. vivax were detected in 17.9% (179/1,001) and 19.3% (202/1,044) of persons tested. Reactivity against P. falciparum was signiﬁcantly different between 3 villages (p<0.001); clusters of seroreactivity were present. Distance to the nearest seasonal river was negatively associated with P. falciparum (p = 0.028) and P. vivax seroreactivity (p = 0.016). Serologic markers are a promising tool for detecting spatial variation in malaria exposure and evaluating malaria control efforts in areas where transmission has decreased to levels below the detection limit of microscopy. Scaused by Plasmodium falciparum. Almost all areas ub-Saharan Africa has the highest incidence of malaria where P. falciparum parasite prevalence is >50% in the general population are located in Africa (1). However, ma- Author afﬁliations: London School of Hygiene and Tropical Medi- cine, London, UK (T. Bousema, J. Cook, J. Cox, C. Drakeley); University of Alexandria, Alexandria, Egypt (R.M. Youssef); Kenya Medical Research Institute–Wellcome Trust Research Programme, Nairobi, Kenya (R.M. Youssef, V.A. Alegana, J. Amran, A.M. Noor, R.W. Snow); Roll Back Malaria–World Health Organization, Har- geisa, Somalia (J. Amran); and University of Oxford, Oxford, UK (A.M. Noor, R.W. Snow) DOI: 10.3201/eid1603.090732 laria is not uniformly distributed (1,2) and many parts of Africa are characterized by low transmission intensity of malaria (1). These areas are considered suitable for inten- sive malaria control and disease elimination (3,4). Assessing malaria transmission intensity and evaluat- ing interventions are complicated at low levels of malaria transmission. Assessing transmission intensity directly by determining the exposure to malaria-infected mosquitoes (entomologic inoculation rate [EIR]) is difﬁcult when mos- quito numbers are low, sometimes below the detection lim- its of commonly used trapping methods (5,6), and spatial and temporal variations in mosquito densities necessitate long-term intensive sampling (5,7,8). Determination of ma- laria parasite prevalence in the human population is a com- monly used alternative (9), but it also becomes less reliable as an indicator of transmission intensity when endemicity is low (3,9,10). Therefore, an alternative method is needed to assess transmission intensity, evaluate interventions, and obtain information for control programs in areas of low en- demicity. Prevalence of antibodies against malaria parasites has been explored as a means of assessing malaria transmis- sion intensity (11–13). Antibody seroconversion rates are less susceptible to seasonal ﬂuctuations in malaria expo- sure (11,12), show a tight correlation with EIR (12,13), and show potential to detect recent changes in malaria transmis- sion intensity (14). Serologic markers could be particularly useful in areas of low endemicity, where it may be easier to detect relatively long-lasting antibody responses than a low prevalence of malaria infections in the human population or infected mosquitoes. We used serologic markers of expo- sure to determine spatial variation in malaria transmission intensity in an area of low endemicity in Somalia (15). 392 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 3, March 2010