André Spiegel

Combating malaria in Africa

The spread of antimalarial drug resistance has major consequences for malaria control in tropical Africa. Here, the impact of chloroquine resistance on the burden of malaria is analyzed and its implications for the Roll Back Malaria initiative are examined. Malaria mortality has increased at least twofold during the past two decades. Combination therapy should be available for home treatment of young children. The potential toxicity of most antimalarials will require special surveillance programs. The main contribution to malaria control using methods to reduce the entomological inoculation rate is expected in areas with low or unstable transmission. Classic vector-control methods could potentially eliminate malaria in most urban areas and such programs deserve high priority.

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.

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.

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.

Similar feeding preferences of Anopheles gambiae and A. arabiensis in Senegal

This study in Senegal compared the feeding preferences of Anopheles gambiae and A. arabiensis while controlling for equal accessibility to hosts located outdoors under bed net traps. All fed A. gambiae complex females were identified with the aid of the polymerase chain reaction and their blood meal sources were identified by enzyme-linked immunosorbent assay. A total of 605 anophelines, including 281 A. gambiae and 301 A. arabiensis, were captured, 32.2% in the human-baited traps and 67.8% in bovine-baited traps. 30.3% of A. gambiae fed in the former and 69.7% fed in the latter; the corresponding figures for A. arabiensis were 29.6% and 70.4%. Thus, when the hosts were located outdoors and made equally available, the feeding preferences of A. gambiae and A. arabiensis were similar (P = 0.81). These results suggest that biases existed in previous studies, most of which suggested that A. arabiensis was more zoophilic than A. gambiae. Alternatively, the feeding behaviour of these 2 species may differ in various parts of Africa.

In vitro activities of ferrochloroquine against 55 Senegalese isolates of Plasmodium falciparum in comparison with those of standard antimalarial drugs

The in vitro activities of ferrochloroquine, chloroquine, quinine, mefloquine, halofantrine, amodiaquine, artesunate, atovaquone, cycloguanil and pyrimethamine were evaluated against Plasmodium falciparum isolates from Senegal (Dielmo, Ndiop), using an isotopic micro‐drug susceptibility test. The IC50 values for ferrochloroquine ranged from 0.55 to 28.2 nM and the geometric mean IC50 for the 55 isolates was 7.9 nM (95% CI, 6.5–9.7 nM). Ferrochloroquine was 35 times more active than chloroquine (35‐fold greater against chloroquine‐resistant isolates), quinine, mefloquine, amodiaquine, cycloguanil and pyrimethamine. Weak positive correlations were observed between the responses to ferrochloroquine and that to chloroquine, quinine, and amodiaquine, but not compulsorily predictive of cross‐resistance. There was no significant correlation between the response to ferrochloroquine and that to mefloquine, halofantrine, artesunate, atovaquone, cycloguanil and pyrimethamine. Ferrochloroquine may be an important alternative drug for the treatment of chloroquine‐resistant malaria.

Epidemiological and clinical aspects of blackwater fever among African children suffering frequent malaria attacks

Blackwater fever (BWF), one of the commonest causes of death of Europeans living in Africa at the beginning of the twentieth century, but rarely diagnosed since the 1950s, is related to Plasmodium falciparum malaria but there is considerable debate and controversy about its aetiology. From 1990 to 2000, the whole population of Dielmo, a village in Senegal, was involved in a prospective study of malaria. Three cases of BWF occurred in 3 children aged 4, 7 and 10 years, belonging to a subgroup of children who suffered malaria attacks every 4 to 6 weeks over many years, who had received repeated quinine treatment. The spread of chloroquine resistance, by increasing the use of more toxic alternative drugs, may expose endemic populations to a high incidence of severe side effects of antimalarials.

Regulation of Antigen-Specific Immunoglobulin G Subclasses in Response to Conserved and Polymorphic Plasmodium falciparum Antigens in an In Vitro Model

ABSTRACT Cytophilic antibodies (Abs) play a critical role in protection against Plasmodium falciparum blood stages, yet little is known about the parameters regulating production of these Abs. We used an in vitro culture system to study the subclass distribution of antigen (Ag)-specific immunoglobulin G (IgG) produced by peripheral blood mononuclear cells (PBMCs) from individuals exposed to P. falciparum or unexposed individuals. PBMCs, cultivated with or without cytokines and exogenous CD40/CD40L signals, were stimulated with a crude parasite extract, recombinant vaccine candidates derived from conserved Ags (19-kDa C terminus of merozoite surface protein 1 [MSP119], R23, and PfEB200), or recombinant Ags derived from the polymorphic Ags MSP1 block 2 and MSP2. No P. falciparum-specific Ab production was detected in PBMCs from unexposed individuals. PBMCs from donors exposed frequently to P. falciparum infections produced multiple IgG subclasses when they were stimulated with the parasite extract but usually only one IgG subclass when they were stimulated with a recombinant Ag. Optimal Ab production required addition of interleukin-2 (IL-2) and IL-10 for all antigenic preparations. The IgG subclass distribution was both donor and Ag dependent and was only minimally influenced by the exogenous cytokine environment. In vitro IgG production and subclass distribution correlated with plasma Abs to some Ags (MSP119, R23, and MSP2) but not others (PfEB200 and the three MSP1 block 2-derived Ags). Data presented here suggest that intrinsic properties of the protein Ag itself play a major role in determining the subclass of the Ab response, which has important implications for rational design of vaccine delivery.

A cohort study of Plasmodium falciparum diversity during the dry season in Ndiop, a Senegalese village with seasonal, mesoendemic malaria

Prolonged carriage of Plasmodium falciparum in humans during the dry season is critical for parasite survival, as the infected subjects constitute a major reservoir in the absence of transmission. Yet, very little is known about the host/parasite interactions contributing to parasite persistence. In order to study the characteristics of P. falciparum infections during the dry season, we have genotyped parasites collected from untreated, asymptomatic individuals during 3 cross-sectional surveys conducted during the dry season in Ndiop, a Senegalese village with seasonal, mesoendemic malaria. Monthly entomological surveillance did not detect any transmission during that period. Parasite prevalence decreased markedly in the children aged < 7 years after 7 months of undetected transmission, but was stable in older children and adults throughout the dry season. In all chronically infected individuals, infection complexity remained stable, but there were substantial fluctuations of individual genotype(s), reflecting complex dynamics of multiple-clone infections during chronic asymptomatic parasite carriage. This fluctuation resulted in changes in the msp1 and msp2 allelic distribution within the cohort after 7 months of undetected transmission, contrasting with the stability observed during the preceding rainy season in that village.