Frequent exposure to the malaria parasite is required to maintain an efficient immune response capable of controlling parasite load and reducing the severity of infection. Thus, a reduction in parasite transmission (that is, the spread of the parasite among people via the bite of infected mosquitoes) can, ironically, result in more severe infections if the parasite has not been completely eliminated from the region. Given the increased susceptibility to malaria infection during pregnancy, the authors decided to investigate the consequences of the reduction in malaria immunity among pregnant women.
To do so, they examined parasitological, immunological and clinical trends among pregnant women delivering in the Manhiça District Hospital in Southern Mozambique between 2003 and 2012, in the context of a clinical assay on intermittent preventive treatment during pregnancy. The results show that the prevalence in infections by P. falciparum decreased from 33% in 2003 to 2% in 2010 and increased to 6% in 2012, and that these trends were mirrored by the level of antimalarial antibodies, including those generated during pregnancy and that help regulate parasite density in the placenta. Parasite density in infected women was higher in 2010-2012, as compared to the 2003-2005 period of high transmission, and associated with worse delivery outcomes in terms of significantly reduced maternal haemoglobin levels and birth weight.
As the authors explain, this study demonstrates that the weakening of antimalarial immunity due to infrequent parasite exposure can increase the severity of infections if the parasite is not completely eliminated or reappears. Dr. Alfredo Mayor, researcher at ISGlobal and first author of the study, emphasizes: "these results underline the importance of sustaining efforts to avoid rebounds of malaria associated with reductions in naturally acquired immunity." In addition, they suggest that pregnant women could represent a useful group to monitor global malaria trends.
These results are particularly relevant in view of the efforts made by Mozambique to eliminate malaria in the southern region, an initiative supported by "la Caixa" Foundation through its program "la Caixa against Malaria". The study was funded by the Malaria Elimination Scientific Alliance (MESA), the Instituto de Salud Carlos III, the Banco de Bilbaco, Vizcaya, Argentaria (BBVA) Foundation and the European and the Developing Countries Clinical Trials Partnership (EDCTP).
A. Mayor, A.Bardají, E. Macete, T. Nhampossa, AM Fonseca, M.Sc.et al. Changing Trends in P. falciparum Burden, Immunity, and Disease in Pregnancy. NEJM, Oct 22, 2015.