Malaria remains an important cause of mortality and morbidity in many parts of the world and it could have adverse impact on the population, both from a health and a socio-economic attitudes. In malaria endemic areas, clinical manifestation of Plasmodium infection varies from asymptomatic to severe and fatal malaria. In high transmission areas, continuous exposures to Plasmodium parasites lead to partial immunity and consequently, create asymptomatic carriers in a given population . In addition, asymptomatic cases provide a fundamental reservoir of parasites and they might become gametocyte carriers, contributing in the persistence of malaria transmission . Therefore, the presence of asymptomatic cases is a big challenge for the management of the elimination programme in any malaria endemic area. In order to achieve a successful elimination, detection of all parasite carriers by active case detection and then treatment of all cases must be considered to interrupt the malaria transmission in endemic areas.
Asymptomatic malaria infections were frequently described in high and intermediate transmission areas including Ghana [3, 4], Kenya , Senegal [5, 6], Gabon [7, 8], Nigeria [9, 10], Uganda , Thailand , Burma  and Yemen . However, in recent years, such cases have also been reported from low endemic areas such as Amazon region of Brazil and Peru [15–23], Colombia , Solomon Island  and Principe . Notably, John and colleagues  reported that administration of different malaria control interventions reduced the asymptomatic malaria cases in an unstable malaria transmission area of Kenya and also in high transmission endemic area of Sri Lanka . Since symptomless malaria consequences in the persistence of the parasite reservoirs and increases malaria transmission in human population, it can interfere with malaria elimination strategies. Therefore, to achieve successful elimination and finally eradication of malaria from the world, survey on the presences and the prevalence of asymptomatic cases in diverse malaria settings is recommended.
In Iran, a country located in the south-west of Asia, malaria was a major health problem with approximately 30-40% of the total mortality during 19211949 . The National Malaria Eradication Programme was initiated in 1957 and remarkable achievements were obtained in most parts of the country. However, due to different obstructions in the south and south-east regions, malaria transmission was maintained with more than 90% of the total malaria cases in these areas. Therefore, in these particular areas the eradication programme was re-oriented to a control programme in 1980 . Interestingly, afterward, disease burden has significantly reduced in the south and south-eastern parts of the country, due to successful interventions in controlling malaria. Consequently, since 2009, Iran, with 10 other countries entered the malaria pre-elimination programme with the technical support from the World Health Organization [30, 31].
Simultaneously, indoor residual spraying, long-lasting impregnated bed nets (LLINs), active case detection and case management with the first-line recommended therapy for uncomplicated malaria, artemisinin combination therapy (ACT), combined with improved diagnostic capacities in all health facilities were all employed with a greater rate in comparison with the malaria control programme in Iran. In consequence, towards the reduction in transmission, based on the report by the Center for Diseases Management and Control (CDMC), the total malaria cases in Iran gradually dropped from 11,460 cases in 2008 to 6,122 in 2009 and as low as 3,031 cases in 2010 (Iranian CDMC, surveillance report, unpublished).
It should be noted that prior to applying an elimination strategy, the targeting countries need to occupy some constructions of active case detection for appropriate treatment of the asymptomatic parasite carriers to prevent major cause of sustained disease transmission. Therefore, it is prerequisite to detect parasite carriers that are undiagnosed by the light microscopy method or persist in population after anti-malarial treatment caused by drug resistance. To facilitate and complete the elimination efforts in Iran, this investigation was designed to assess the presence of the parasite carriers free of clinical symptoms in two malaria endemic districts with a history of malaria transmission in the south of Iran, Bashagard and Ghale-Ganj districts of Hormozgan and Kerman provinces, where transmission has been drastically reduced in recent years. The results of this study demonstrate the effectiveness and feasibility of implementing malaria elimination interventions in these malaria endemic areas of Iran.