Temporal analysis of IgG antibody responses to Plasmodium falciparum antigens in relation to changing malaria epidemiology in a West African setting.

BACKGROUND: Coordinated scaled-up malaria control interventions have substantially contributed to the dramatic decrease of malaria-related morbidity and mortality in several endemic countries, including Senegal. However, the impacts of a given malaria control intervention on vector and parasite populations, acquired immunity, and disease burden remain very poorly documented largely due to the lack of continuous surveys. This study took advantage of the sera bank established as part of the Dielmo longitudinal project to investigate the dynamics of IgG antibody responses that accompanied the epidemiological changes resulting from malaria control interventions. Schizonts crude extract of a local strain of Plasmodium falciparum (Pfsch07/03) was used in ELISA to measure and compare seroprevalence and magnitude of IgG antibody responses from 2000 to 2012. RESULTS: The prevalence of Pfsch07/03 IgG antibody responses progressively decreased from 97.25% in 2000 to 57.3% in 2012. The prevalence of Pfsch07/03 antibodies categorized between three different age groups (<7, 7-15, and >15 years) revealed increased seroprevalence with age ranging from 47.19 to 62.67 and 89.45%, respectively in (<7, 7-15, and >15 years) old age groups. A marked drop in seroprevalence was observed after 2008 and was significant in the younger (<7 years) and intermediate (7-15 years) age groups, unlike older individuals aged >15 years (p = 1.00). CONCLUSIONS: The study revealed a substantial contribution of all malaria control interventions to the decrease of IgG antibodies responses to Pfsch07/03 throughout prevention of human-mosquitos contacts, or reduction of parasite biomass. The present study demonstrates the wider potential of sero-epidemiological analysis in monitoring changes in malaria transmission resulting from a given malaria control intervention.

Modelling dynamic change of malaria transmission in holoendemic setting (Dielmo, Senegal) using longitudinal measures of antibody prevalence to Plasmodium falciparum crude schizonts extract.

BACKGROUND: Evaluation of local Plasmodium falciparum malaria transmission has been investigated previously using the reversible catalytic model based on prevalence of antibody responses to single antigen to estimate seroconversion rates. High correlations were observed between seroconversion rates and entomological inoculation rates (EIR). However, in this model, the effects of malaria control interventions and clinical episodes on serological measurements were not assessed. This study monitors the use of antibody responses to P. falciparum crude extracts for assessing malaria transmission, compares seroconversion rates estimated from longitudinal data to those derived from cross-sectional surveys and investigates the effects of malaria control interventions on these measures in an area of declining malaria transmission. In addition, the validity of this model was evaluated by comparison with the alternative model. METHODS: Five cross-sectional surveys were carried out at the end of the wet season in Dielmo, a malaria-endemic Senegalese rural area in 2000, 2002, 2008, 2010 and 2012. Antibodies against schizonts crude extract of a local P. falciparum strain adapted to culture (Pf 07/03) were measured by ELISA. Age-specific seroprevalence model was used both for cross-sectional surveys and longitudinal data (combined data of all surveys). RESULTS: A total of 1504 plasma samples obtained through several years follow-up of 350 subjects was used in this study. Seroconversion rates based on P. falciparum schizonts crude extract were estimated for each cross-sectional survey and were found strongly correlated with EIR. High variability between SCRs from cross-sectional and longitudinal surveys was observed. In longitudinal studies, the alternative catalytic reversible model adjusted better with serological data than the catalytic model. Clinical malaria attacks and malaria control interventions were found to have significant effect on seroconversion. DISCUSSION: The results of the study suggested that crude extract was a good serological tool that could be used to assess the level of malaria exposure in areas where malaria transmission is declining. However, additional parameters such as clinical malaria and malaria control interventions must be taken into account for determining serological measurements for more accuracy in transmission assessment.

Dramatic declines in seropositivity as determined with crude extracts of Plasmodium falciparum schizonts between 2000 and 2010 in Dielmo and Ndiop, Senegal.

BACKGROUND: Programmes of pre-elimination of malaria have been implemented in Senegal since 2010, and the burden of malaria has decreased substantially. These changes in the epidemiology should be monitored with effective tools that allow changes in patterns of transmission to be estimated. In Dielmo and Ndiop, two villages of Senegal with different malaria endemicity, infections have been followed longitudinally for 20 years, during which time there have been several control interventions leading to substantial decreases of transmission. This study aimed to compare malaria antibody responses of the inhabitants of these two villages, between 2000 and 2010, using schizont crude extracts of a local strain of P. falciparum (Pf Sch07/03). METHODS: Sera collected from inhabitants of the two villages (141 from Dielmo and 79 from Ndiop in 2000; 143 from Dielmo and 79 from Ndiop in 2010) were used to assess the prevalence of antibodies against crude schizont extracts of Pf Sch07/03. Three ages groups were defined: [5-9] yrs, [10-14] yrs and [15-19] yrs. Statistical comparisons were performed. Seroprevalence and the magnitude of antibody responses were compared between age groups, villages and periods. RESULTS: Overall seroprevalence to P.fSch07/03 decreased between 2000 and 2010 in both villages: from 94.4% to 44.4% in Dielmo and from 74.4% to 34.6% in Ndiop. The difference between Dielmo and Ndiop was highly significant in 2000 (p<0.001) but not in 2010 (p >0.20). The decrease in seroprevalence was larger in younger (more than 40%) than older (less than 19%) inhabitants. Longitudinal monitoring of the younger group showed that seroprevalence decreased between 2000 and 2010 in Dielmo from 98.7 to 79.3, but not in Ndiop from 67.6 to 66.7. The magnitude of antibody responses in seropositive individuals was significantly higher in 2000 than 2010 for both villages. CONCLUSIONS: Crude extracts of P. falciparum are appropriate tools for evaluating malaria prevalence at different periods, and in both low and high endemic area. Using crude extracts from local strains to assess transmission may allow efficient evaluation of the consequences of control programs on malaria transmission.