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      Using serological measures to monitor changes in malaria transmission in Vanuatu

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          Abstract

          Background

          With renewed interest in malaria elimination, island environments present unique opportunities to achieve this goal. However, as transmission decreases, monitoring and evaluation programmes need increasingly sensitive tools to assess Plasmodium falciparum and Plasmodium vivax exposure. In 2009, to assess the role of serological markers in evaluating malaria transmission, a cross-sectional seroprevalence study was carried out in Tanna and Aneityum, two of the southernmost islands of the Vanuatu archipelago, areas where malaria transmission has been variably reduced over the past few decades.

          Methods

          Malaria transmission was assessed using serological markers for exposure to P. falciparum and P. vivax. Filter blood spot papers were collected from 1,249 people from Tanna, and 517 people from Aneityum to assess the prevalence of antibodies to two P. falciparum antigens (MSP-1 19 and AMA-1) and two P. vivax antigens (MSP-1 19 and AMA-1). Age-specific prevalence was modelled using a simple catalytic conversion model based on maximum likelihood to generate a community seroconversion rate (SCR).

          Results

          Overall seropositivity in Tanna was 9.4%, 12.4% and 16.6% to P. falciparum MSP-1 19, AMA-1 and Schizont Extract respectively and 12.6% and 15.0% to P. vivax MSP-1 19 and AMA-1 respectively. Serological results distinguished between areas of differential dominance of either P. vivax or P. falciparum and analysis of age-stratified results showed a step in seroprevalence occurring approximately 30 years ago on both islands, indicative of a change in transmission intensity at this time. Results from Aneityum suggest that several children may have been exposed to malaria since the 2002 P. vivax epidemic.

          Conclusion

          Seroepidemiology can provide key information on malaria transmission for control programmes, when parasite rates are low. As Vanuatu moves closer to malaria elimination, monitoring changes in transmission intensity and identification of residual malaria foci is paramount in order to concentrate intervention efforts.

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          Most cited references33

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          Vivax malaria: neglected and not benign.

          Plasmodium vivax threatens almost 40% of the world's population, resulting in 132-391 million clinical infections each year. Most of these cases originate from Southeast Asia and the Western Pacific, although a significant number also occurs in Africa and South America. Although often regarded as causing a benign and self-limiting infection, there is increasing evidence that the overall burden, economic impact, and severity of disease from P. vivax have been underestimated. Malaria control strategies have had limited success and are confounded by the lack of access to reliable diagnosis, emergence of multidrug resistant isolates, the parasite's ability to transmit early in the course of disease and relapse from dormant liver stages at varying time intervals after the initial infection. Progress in reducing the burden of disease will require improved access to reliable diagnosis and effective treatment of both blood-stage and latent parasites, and more detailed characterization of the epidemiology, morbidity, and economic impact of vivax malaria. Without these, vivax malaria will continue to be neglected by ministries of health, policy makers, researchers, and funding bodies.
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            The neglected burden of Plasmodium vivax malaria.

            We estimate that the global burden of malaria due to Plasmodium vivax is approximately 70-80 million cases annually. Probably approximately 10-20% of the world's cases of P. vivax infection occur in Africa, south of the Sahara. In eastern and southern Africa, P. vivax represents around 10% of malaria cases but 50% of all malaria cases. About 80-90% of P. vivax outside of Africa occurs in the Middle East, Asia, and the Western Pacific, mainly in the most tropical regions, and 10-15% in Central and South America. Because malaria transmission rates are low in most regions where P. vivax is prevalent, the human populations affected achieve little immunity to this parasite; as a result, in these regions, P. vivax infections affect people of all ages. Although the effects of repeated attacks of P. vivax through childhood and adult life are only rarely directly lethal, they can have major deleterious effects on personal well-being, growth, and development, and on the economic performance at the individual, family, community, and national levels. Features of the transmission biology of P. vivax give this species greater resilience than the less robust Plasmodiumfalciparum in the face of conditions adverse to the transmission of the parasites. Therefore, as control measures become more effective, the residual malaria burden is likely increasingly to become that of P. vivax.
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              Serology: a robust indicator of malaria transmission intensity?

              To estimate the burden of malarial disease, and evaluate the likely effects of control strategies, requires reliable predictions of malaria transmission intensity. It has long been suggested that antimalarial antibody prevalences could provide a more accurate estimate of transmission intensity than traditional measures such as parasite prevalence or entomological inoculation rates, but there has been no systematic evaluation of this approach. Now, the availability of well characterized malarial antigens allows us to test whether serological measurements provide a practical method for estimating transmission. Here we present a suggested methodology, highlight the advantages and shortcomings of serological measurements of malaria transmission and identify areas in which further work is desirable.
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                Author and article information

                Journal
                Malar J
                Malaria Journal
                BioMed Central
                1475-2875
                2010
                16 June 2010
                : 9
                : 169
                Affiliations
                [1 ]London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK
                [2 ]School of Population Health, University of Queensland, Herston, Qld 4006, Australia
                [3 ]Ministry of Health, Port Vila, Vanuatu
                [4 ]Queensland Institute of Medical Research, Herston, Qld 4006, Australia
                [5 ]School of Medicine, University of Queensland, Herston, Qld 4006, Australia
                Article
                1475-2875-9-169
                10.1186/1475-2875-9-169
                2904786
                20553604
                0cfa0ff4-9015-43a5-b2fe-b8f2b6b17e04
                Copyright ©2010 Cook et al; licensee BioMed Central Ltd.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 23 February 2010
                : 16 June 2010
                Categories
                Research

                Infectious disease & Microbiology
                Infectious disease & Microbiology

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