Adama Tall

Genome-wide and fine-resolution association analysis of malaria in West Africa.

We report a genome-wide association (GWA) study of severe malaria in The Gambia. The initial GWA scan included 2,500 children genotyped on the Affymetrix 500K GeneChip, and a replication study included 3,400 children. We used this to examine the performance of GWA methods in Africa. We found considerable population stratification, and also that signals of association at known malaria resistance loci were greatly attenuated owing to weak linkage disequilibrium (LD). To investigate possible solutions to the problem of low LD, we focused on the HbS locus, sequencing this region of the genome in 62 Gambian individuals and then using these data to conduct multipoint imputation in the GWA samples. This increased the signal of association, from P = 4 × 10−7 to P = 4 × 10−14, with the peak of the signal located precisely at the HbS causal variant. Our findings provide proof of principle that fine-resolution multipoint imputation, based on population-specific sequencing data, can substantially boost authentic GWA signals and enable fine mapping of causal variants in African populations.

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.

Long-term clinical protection from falciparum malaria is strongly associated with IgG3 antibodies to merozoite surface protein 3.

Background Surrogate markers of protective immunity to malaria in humans are needed to rationalize malaria vaccine discovery and development. In an effort to identify such markers, and thereby provide a clue to the complex equation malaria vaccine development is facing, we investigated the relationship between protection acquired through exposure in the field with naturally occurring immune responses (i.e., induced by the parasite) to molecules that are considered as valuable vaccine candidates. Methods and Findings We analyzed, under comparative conditions, the antibody responses of each of six isotypes to five leading malaria vaccine candidates in relation to protection acquired by exposure to natural challenges in 217 of the 247 inhabitants of the African village of Dielmo, Senegal (96 children and 121 older adolescents and adults). The status of susceptibility or resistance to malaria was determined by active case detection performed daily by medical doctors over 6 y from a unique follow-up study of this village. Of the 30 immune responses measured, only one, antibodies of the IgG3 isotype directed to merozoite surface protein 3 (MSP3), was strongly associated with clinical protection against malaria in all age groups, i.e., independently of age. This immunological parameter had a higher statistical significance than the sickle cell trait, the strongest factor of protection known against Plasmodium falciparum. A single determination of antibody was significantly associated with the clinical outcome over six consecutive years in children submitted to massive natural parasite challenges by mosquitoes (over three parasite inoculations per week). Finally, the target epitopes of these antibodies were found to be fully conserved. Conclusions Since anti-MSP3 IgG3 antibodies can naturally develop along with protection against P. falciparum infection in young children, our results provide the encouraging indication that these antibodies should be possible to elicit by vaccination early in life. Since these antibodies have been found to achieve parasite killing under in vitro and in vivo conditions, and since they can be readily elicited by immunisation in naïve volunteers, our immunoepidemiological findings support the further development of MSP3-based vaccine formulations.

Increased frequency of malaria attacks in subjects co-infected by intestinal worms and Plasmodium falciparum malaria.

The influence of intestinal worm infections on malaria was studied in individuals from Dielmo, Senegal in 1998. Results suggest that, compared with those infected, individuals free of helminths had the same degree of protection against malaria as that provided by sickle-cell trait, the most potent factor of resistance to malaria identified to date.

Incidence of tick-borne relapsing fever in west Africa: longitudinal study

BACKGROUND The ongoing drought in sub-Saharan countries has led to the colonisation of west African Savanna by Ornithodoros sonrai; this tick acts as a vector for Borrelia crocidurae, which causes tick-borne relapsing fever (TBRF). Our aim was to ascertain the incidence of TBRF in west Africa. METHODS From 1990 to 2003, we monitored the incidence of TBRF in Dielmo, Senegal, by daily clinical surveillance and by blood testing of individuals with a fever. From 2002 to 2005, we investigated the presence of O sonrai in 30 villages in Senegal, Mauritania, and Mali, and measured by PCR the prevalence of B crocidurae. FINDINGS The average incidence of TBRF over 14 years was 11 per 100 person-years (range from 4 in 1990 to 25 in 1997). All age-groups presented a high incidence of the disease. In addition to relapses, repeated infections in the same individuals were common, with some affected by up to six distinct infections during the study period. Epidemiological studies indicated that 26 of the 30 studied villages (87%) were colonised by the vector tick O sonrai and that the average B crocidurae infection rate of the vector was 31%. INTERPRETATION The incidence of TBRF at the community level is the highest described in Africa for any bacterial disease. The presence of the vector tick in most villages investigated and its high infection rate suggest that TBRF is a common cause of fever in most rural areas of Senegal, Mauritania, and Mali.

5. Variation of Plasmodium falciparum msp1 block 2 and msp2 allele prevalence and of infection complexity in two neighbouring Senegalese villages with different transmission conditions

To investigate the impact of transmission on the development of immunity to malaria and on parasite diversity, longitudinal surveys have been conducted for several years in Dielmo and Ndiop, 2 neighbouring Senegalese villages with holo- and mesoendemic transmission conditions, respectively. We analysed Plasmodium falciparum msp1 block 2 and msp2 genotypes of isolates collected from 58% of the Dielmo villagers during the same week as those studied recently from Ndiop. Allele frequencies differed in both villages, indicating considerable microgeographical heterogeneity of parasite populations. The complexity of the infections, estimated using individual or combined msp1 and msp2 genotyping, in Dielmo was more than double that in Ndiop and it was age-dependent in Dielmo but not in Ndiop. Thus, this study confirmed the influence of age on the complexity of asymptomatic malaria infections in a holoendemic area. The age distribution of complexity in Dielmo substantiates the interpretation that the number of parasite types per isolate reflects acquired antiparasite immunity. This cross-sectional survey also confirms that the sickle cell trait has no impact on complexity but influences the distribution of P. falciparum genotypes.

Increased susceptibility to malaria during the early postpartum period.

BACKGROUND Pregnancy is associated with increased susceptibility to malaria. It is generally agreed that this increased risk ends with delivery, but the possible persistence of increased susceptibility during the puerperium had not been investigated. METHODS From June 1, 1990, to December 31, 1998, we monitored exposure to malaria, parasitemia, and morbidity among the residents of a village in Senegal in which the rate of transmission of malaria was high. In this population we analyzed 71 pregnancies in 38 women from the year before conception and through one year after delivery. RESULTS Among the 38 women, there were 58 episodes of clinical Plasmodium falciparum malaria during 61,081 person-days of observation. The incidence of malaria was 20.2 episodes per 1000 person-months during the year preceding conception and 12.0 episodes per 1000 person-months during the period from 91 to 365 days after delivery. The incidence of episodes of malaria increased significantly during the second and third trimesters of pregnancy and reached a maximum of 75.1 episodes per 1000 person-months during the first 60 days after delivery. The adjusted relative risk of an episode of malaria was 4.1 (95 percent confidence interval, 1.8 to 9.5) during the first 60 days post partum, as compared with the year preceding pregnancy. The duration of fever during the episodes of malaria was longer and the prevalence and density of asymptomatic malarial parasitemia were significantly higher during pregnancy and the early postpartum period than during the other periods. CONCLUSIONS Among women who live in areas with high rates of transmission of malaria, the susceptibility to malaria is highest during the second and third trimesters of pregnancy and the early postpartum period.

Four years' entomological study of the transmission of seasonal malaria in Senegal and the bionomics of Anopheles gambiae and A. arabiensis

From 1993 to 1996, an entomological survey was conducted in the village of Ndiop, Senegal, as part of a research programme on malaria epidemiology and the mechanisms of protective immunity. Mosquitoes were captured on human bait and by indoor spraying. Species from the Anopheles gambiae complex were identified using the polymerase chain reaction, and Plasmodium falciparum infections were detected by enzyme-linked immunosorbent assay for circumsporozoite protein. The vector species identified were A. gambiae (33.9%), A. arabiensis (63.2%), A. melas (0.3%) and A. funestus (2.5%). Similar proportions of A. gambiae (74.2%) and A. arabiensis (73.8%) contained human blood; 27.0% of A. gambiae and 28.3% of A. arabiensis had fed on cattle. The sporozoite rates were similar for A. gambiae (3.2%) and A. arabiensis (3.7%). The annual entomological inoculation rates varied greatly depending on the year. There were 63, 17, 37 and 7 infected bites per person per year in 1993, 1994, 1995 and 1996 respectively. Transmission was highly seasonal, from July to October. A. arabiensis was responsible for 66% of malaria transmission, A. gambiae for 31%, and A. funestus for 3%.

Rickettsia felis-associated uneruptive fever, Senegal.

During November 2008–July 2009, we investigated the origin of unknown fever in Senegalese patients with a negative malaria test result, focusing on potential rickettsial infection. Using molecular tools, we found evidence for Rickettsia felis–associated illness in the initial days of infection in febrile Senegalese patients without malaria.

Antibodies to the Conserved C-Terminal Domain of the Plasmodium falciparum Merozoite Surface Protein 1 and to the Merozoite Extract and Their Relationship with In Vitro Inhibitory Antibodies and Protection against Clinical Malaria in a Senegalese Village

Antibodies to Plasmodium falciparum C-terminal merozoite surface protein 1 (PfMSP-1p19) have been correlated with protection against malaria, but this association may apply to many merozoite antigens. To address this question, we conducted a prospective serological study of 205 individuals in an active 5-month clinical survey in a Senegalese village where malaria is mesoendemic. Before the 2000 rainy season, antibody responses specific for recombinant baculovirus PfMSP-1p19 or merozoite extracts were compared with 2 in vitro functional antibody activities (inhibition of parasite grown and erythrocyte invasion) and with the number of clinical episodes during 5 months of follow-up. Antibody levels to PfMSP-1p19 and merozoite extract correlated, respectively, with erythrocyte invasion and parasite growth inhibition. Although antibody levels to both antigen preparations were associated with age, functional parameters were not. High levels of anti-PfMSP-1p19 immunoglobulin G were associated with reduced malaria in an age-adjusted multivariate analysis. These results support baculovirus PfMSP-1p19-based vaccine development.