Hindawi Publishing Corporation Journal of Parasitology Research Volume 2013, Article ID 247273, 5 pages http://dx.doi.org/10.1155/2013/247273 Clinical Study Prevalence and Level of Antibodies Anti-Plasmodium spp. in Travellers with Clinical History of Imported Malaria Rita Medina Costa, Karina Pires de Sousa, Jorge Atouguia, Luis Távora Tavira, and Marcelo Sousa Silva Unidade de Ensino e Investigac ¸˜ ao de Cl´ ınica Tropical, Centre for Malaria and Tropical Diseases, Instituto de Higiene e Medicina Tropical, Universidade Nova de Lisboa, Rua da Junqueira 100, 1349-008 Lisbon, Portugal Correspondence should be addressed to Marcelo Sousa Silva; mssilva@ihmt.unl.pt Received 10 January 2013; Revised 18 March 2013; Accepted 18 March 2013 Academic Editor: Dave Chadee Copyright © 2013 Rita Medina Costa et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In this study, we show that 40.29% of travellers with a possible history of malaria exposure were positive for anti-Plasmodium spp. antibodies, while these individuals were negative by microscopy. Te antibody test described here is useful to elucidate malaria exposure in microscopy-negative travellers from endemic countries. 1. Introduction Malaria is an infectious disease caused by a protozoan parasite of the genus Plasmodium, which is transmitted between humans by the bite of infected female Anopheles mosquitoes. Tese parasites have a complex life cycle, both in the invertebrate vector and vertebrate hosts. In the human body, parasites multiply in hepatocytes, and then invade red blood cells (RBCs), initiating blood stage infection, which corresponds to the symptomatic period of the disease [1]. Malaria remains one of the most serious public health problems not only in endemic countries, where 2 billion people (approximately 40% of the world’s population) are at risk of contracting the disease, but also in nonendemic areas, where the increasing number of imported malaria cases is worrying [2]. In developed countries, imported malaria predominates in tourists and immigrants who travel to their home countries to visit friends and relatives. Every year, approximately 125 million international travellers visit malaria endemic areas, and 30,000 of them contract the disease [3, 4]. In Portugal, the occurrence of 50 such cases per year [5] is estimated according to the National Public Health System. Following infection with any of the fve species of Plas- modium that are capable of infecting humans, P. falciparum, P. ovale, P. vivax, P. malariae, and P. knowlesi, specifc antibodies are produced one or two weeks afer the initial infection and persist for three to six months afer parasite clearance [6]. Tese antibodies may endure for months or years in semi- immune patients in endemic countries where reinfection is frequent. However, in a na¨ ıve patient, antibody levels fall more rapidly. Reinfection or relapse leads to a secondary response with a high and rapid rise in antibody titres [6, 7]. Tus, in the present study, we aim to evaluate the prevalence and the level of anti-Plasmodium spp. antibodies in serum samples from travellers with possible clinical signals and symptoms of malaria. Using an ELISA-based commercial immunoassay kit to measure antimalarial antibodies, we determined the raw serological profle of these individuals. Additionally, we compare the latter serological profle with the gold-standard laboratory diagnosis, based on direct microscopy. 2. Materials and Methods 2.1. Study Population. Te population for this study consisted of 335 individuals with possible clinical history of malaria and 23 healthy individuals (healthy Portuguese individuals who have never been in malaria-endemic countries). All of the 435 subjects who have had potential exposure to Plasmodium spp. travelled back to Portugal from malaria-endemic regions of