Aissatou Toure-Balde

Malaria morbidity and pyrethroid resistance after the introduction of insecticide-treated bednets and artemisinin-based combination therapies: a longitudinal study

BACKGROUND Substantial reductions in malaria have been reported in several African countries after distribution of insecticide-treated bednets and the use of artemisinin-based combination therapies (ACTs). Our aim was to assess the effect of these policies on malaria morbidity, mosquito populations, and asymptomatic infections in a west African rural population. METHODS We did a longitudinal study of inhabitants of Dielmo village, Senegal, between January, 2007, and December, 2010. We monitored the inhabitants for fever during this period and we treated malaria attacks with artesunate plus amodiaquine. In July, 2008, we offered longlasting insecticide (deltamethrin)-treated nets (LLINs) to all villagers. We did monthly night collections of mosquitoes during the whole study period, and we assessed asymptomatic carriage from cross-sectional surveys. Our statistical analyses were by negative binomial regression, logistic regression, and binomial or Fisher exact test. FINDINGS There were 464 clinical malaria attacks attributable to Plasmodium falciparum during 17,858 person-months of follow-up. The incidence density of malaria attacks averaged 5·45 (95% CI 4·90-6·05) per 100 person-months between January, 2007, and July, 2008, before the distribution of LLINs. Incidence density decreased to 0·41 (0·29-0·55) between August, 2008, and August, 2010, but increased back to 4·57 (3·54-5·82) between September and December, 2010--ie, 27-30 months after the distribution of LLINs. The rebound of malaria attacks were highest in adults and children aged 10 years or older: 45 (63%) of 71 malaria attacks recorded in 2010 compared with 126 (33%) of 384 in 2007 and 2008 (p<0·0001). 37% of Anopheles gambiae mosquitoes were resistant to deltamethrin in 2010, and the prevalence of the Leu1014Phe kdr resistance mutation increased from 8% in 2007 to 48% in 2010 (p=0·0009). INTERPRETATION Increasing pyrethroid resistance of A gambiae and increasing susceptibility of older children and adults, probably due to decreasing immunity, caused the rebound and age shift of malaria morbidity. Strategies to address the problem of insecticide resistance and to mitigate its effects must be urgently defined and implemented. FUNDING Institut de Recherche pour le Développement and the Pasteur Institute of Dakar.

The rise and fall of malaria in a west African rural community, Dielmo, Senegal, from 1990 to 2012: a 22 year longitudinal study

BACKGROUND A better understanding of the effect of malaria control interventions on vector and parasite populations, acquired immunity, and burden of the disease is needed to guide strategies to eliminate malaria from highly endemic areas. We monitored and analysed the changes in malaria epidemiology in a village community in Senegal, west Africa, over 22 years. METHODS Between 1990 and 2012, we did a prospective longitudinal study of the inhabitants of Dielmo, Senegal, to identify all episodes of fever and investigate the relation between malaria host, vector, and parasite. Our study included daily medical surveillance with systematic parasite detection in individuals with fever. We measured parasite prevalence four times a year with cross-sectional surveys. We monitored malaria transmission monthly with night collection of mosquitoes. Malaria treatment changed over the years, from quinine (1990-94), to chloroquine (1995-2003), amodiaquine plus sulfadoxine-pyrimethamine (2003-06), and finally artesunate plus amodiaquine (2006-12). Insecticide-treated nets (ITNs) were introduced in 2008. FINDINGS We monitored 776 villagers aged 0-101 years for 2 378 150 person-days of follow-up. Entomological inoculation rate ranged from 142·5 infected bites per person per year in 1990 to 482·6 in 2000, and 7·6 in 2012. Parasite prevalence in children declined from 87% in 1990 to 0·3 % in 2012. In adults, it declined from 58% to 0·3%. We recorded 23 546 fever episodes during the study, including 8243 clinical attacks caused by Plasmodium falciparum, 290 by Plasmodium malariae, and 219 by Plasmodium ovale. Three deaths were directly attributable to malaria, and two to severe adverse events of antimalarial drugs. The incidence of malaria attacks ranged from 1·50 attacks per person-year in 1990 to 2·63 in 2000, and to only 0·046 in 2012. The greatest changes were associated with the replacement of chloroquine and the introduction of ITNs. INTERPRETATION Malaria control policies combining prompt treatment of clinical attacks and deployment of ITNs can nearly eliminate parasite carriage and greatly reduce the burden of malaria in populations exposed to intense perennial malaria transmission. The choice of drugs seems crucial. Rapid decline of clinical immunity allows rapid detection and treatment of novel infections and thus has a key role in sustaining effectiveness of combining artemisinin-based combination therapy and ITNs despite increasing pyrethroid resistance. FUNDING Pasteur Institutes of Dakar and Paris, Institut de Recherche pour le Développement, and French Ministry of Cooperation.

Selection of glutamate-rich protein long synthetic peptides for vaccine development: antigenicity and relationship with clinical protection and immunogenicity

ABSTRACT Antibodies against three long synthetic peptides (LSPs) derived from the glutamate-rich protein (GLURP) of Plasmodium falciparum were analyzed in three cohorts from Liberia, Ghana, and Senegal. Two overlapping LSPs, LR67 and LR68, are derived from the relatively conserved N-terminal nonrepeat region (R0), and the third, LR70, is derived from the R2 repeat region. A high prevalence of antibody responses to each LSP was observed in all three areas of endemic infection. Levels of cytophilic immunoglobulin G (IgG) antibodies against both GLURP regions were significantly correlated with protection from clinical P. falciparum malaria. Protected children from the Ghana cohort possessed predominantly IgG1 antibodies against the nonrepeat epitope and IgG3 antibodies against the repeat epitope. T-cell proliferation responses, studied in the cohort from Senegal, revealed that T-helper-cell epitopes were confined to the nonrepeat region. When used as immunogens, the LR67 and LR68 peptides elicited strong IgG responses in outbred mice and LR67 also induced antibodies in mice of different H-2haplotypes, confirming the presence of T-helper-cell epitopes in these constructs. Mouse antipeptide antisera recognized parasite proteins as determined by immunofluorescence and immunoblotting. This indicates that synthetic peptides derived from relatively conserved epitopes of GLURP might serve as useful immunogens for vaccination against P. falciparum malaria.

A multiplex assay for the simultaneous detection of antibodies against 15 Plasmodium falciparum and Anopheles gambiae saliva antigens

BackgroundAssessment exposure and immunity to malaria is an important step in the fight against the disease. Increased malaria infection in non-immune travellers under anti-malarial chemoprophylaxis, as well as the implementation of malaria elimination programmes in endemic countries, raises new issues that pertain to these processes. Notably, monitoring malaria immunity has become more difficult in individuals showing low antibody (Ab) responses or taking medications against the Plasmodiumfalciparum blood stages. Commonly available techniques in malaria seroepidemiology have limited sensitivity, both against pre-erythrocytic, as against blood stages of the parasite. Thus, the aim of this study was to develop a sensitive tool to assess the exposure to malaria or to bites from the vector Anopheles gambiae, despite anti-malarial prophylactic treatment.MethodsAb responses to 13 pre-erythrocytic P. falciparum-specific peptides derived from the proteins Lsa1, Lsa3, Glurp, Salsa, Trap, Starp, CSP and Pf11.1, and to 2 peptides specific for the Anopheles gambiae saliva protein gSG6 were tested. In this study, 253 individuals from three Senegalese areas with different transmission intensities and 124 European travellers exposed to malaria during a short period of time were included.ResultsThe multiplex assay was optimized for most but not all of the antigens. It was rapid, reproducible and required a small volume of serum. Proportions of Ab-positive individuals, Ab levels and the mean number of antigens (Ags) recognized by each individual increased significantly with increases in the level of malaria exposure.ConclusionThe multiplex assay developed here provides a useful tool to evaluate immune responses to multiple Ags in large populations, even when only small amounts of serum are available, or Ab titres are low, as in case of travellers. Finally, the relationship of Ab responses with malaria endemicity levels provides a way to monitor exposure in differentially exposed autochthonous individuals from various endemicity areas, as well as in travellers who are not immune, thus indirectly assessing the parasite transmission and malaria risk in the new eradication era.

Evidence for multiple B- and T-cell epitopes in Plasmodium falciparum liver-stage antigen 3.

ABSTRACT Liver-stage antigen 3 (LSA-3) is a new vaccine candidate that can induce protection against Plasmodium falciparum sporozoite challenge. Using a series of long synthetic peptides (LSP) encompassing most of the 210-kDa LSA-3 protein, a study of the antigenicity of this protein was carried out in 203 inhabitants from the villages of Dielmo (n = 143) and Ndiop (n = 60) in Senegal (the level of malaria transmission differs in these two villages). Lymphocyte responses to each individual LSA-3 peptide were recorded, some at high prevalences (up to 43%). Antibodies were also detected to each of the 20 peptides, many at high prevalence (up to 84% of responders), and were directed to both nonrepeat and repeat regions. Immune responses to LSA-3 were detectable even in individuals of less than 5 years of age and increased with age and hence exposure to malaria, although they were not directly related to the level of malaria transmission. Thus, several valuable T- and B-cell epitopes were characterized all along the LSA-3 protein, supporting the antigenicity of this P. falciparum vaccine candidate. Finally, antibodies specific for peptide LSP10 located in a nonrepeat region of LSA-3 were found significantly associated with a lower risk of malaria attack over 1 year of daily clinical follow-up in children between the ages of 7 and 15 years, but not in older individuals.

Substantial asymptomatic submicroscopic Plasmodium carriage during dry season in low transmission areas in Senegal: Implications for malaria control and elimination

Background In the progress towards malaria elimination, the accurate diagnosis of low-density asymptomatic infections is critical. Low-density asymptomatic submicroscopic malaria infections may act as silent reservoirs that maintain low-level residual malaria transmission in the community. Light microscopy, the gold standard in malaria diagnosis lacks the sensitivity to detect low-level parasitaemia. In this study, the presence and prevalence of submicroscopic Plasmodium carriage were investigated to estimate the parasites reservoir among asymptomatic individuals living in low transmission areas in Dielmo and Ndiop, Senegal during the dry season. Methods A total of 2,037 blood samples were collected during cross-sectional surveys prior the malaria transmission season in July 2013 (N = 612), June 2014 (N = 723) and June 2015 (N = 702) from asymptomatic individuals living in Dielmo and Ndiop, Senegal. Samples were used to determine the prevalence of submicroscopic Plasmodium carriage by real time PCR (qPCR) in comparison to microscopy considered as gold standard. Results The prevalence of submicroscopic Plasmodium carriage was 3.75% (23/612), 12.44% (90/723) and 6.41% (45/702) in 2013, 2014 and 2015, respectively. No Plasmodium carriage was detected by microscopy in 2013 while microscopy-based prevalence of Plasmodium carriage accounted for only 0.27% (2/723) and 0.14% (1/702) in 2014 and 2015, respectively. Plasmodium falciparum accounted for the majority of submicroscopic infections and represented 86.95% (20/23), 81.11% (73/90) and 95.55 (43/45) of infections in 2013, 2014 and 2015 respectively. Conclusion Low-density submicroscopic asymptomatic Plasmodium carriage is common in the study areas during the dry season indicating that traditional measures are insufficient to assess the scale of parasite reservoir when transmission reaches very low level. Control and elimination strategies may wish to consider using molecular methods to identify parasites carriers to guide Mass screening and Treatment strategies.

Dramatic Changes in Malaria Population Genetic Complexity in Dielmo and Ndiop, Senegal, Revealed Using Genomic Surveillance

Dramatic changes in transmission intensity can impact Plasmodium population diversity. Using samples from 2 distant time-points in the Dielmo/Ndiop longitudinal cohorts from Senegal, we applied a molecular barcode tool to detect changes in parasite genotypes and complexity of infection that corresponded to changes in transmission intensity. We observed a striking statistically significant difference in genetic diversity between the 2 parasite populations. Furthermore, we identified a genotype in Dielmo and Ndiop previously observed in Thiès, potentially implicating imported malaria. This genetic surveillance study validates the molecular barcode as a tool to assess parasite population diversity changes and track parasite genotypes.