Village Of Dielmo Research Articles

Use of HRP-2-based rapid diagnostic test for Plasmodium falciparum malaria: assessing accuracy and cost-effectiveness in the villages of Dielmo and Ndiop, Senegal

BackgroundIn 2006, the Senegalese National Malaria Control Programme (NMCP) has recommended artemisinin-based combination therapy (ACT) as the first-line treatment for uncomplicated malaria and, in 2007, mandated testing for all suspected cases of malaria with a Plasmodium falciparum HRP-2-based rapid diagnostic test for malaria (RDT(Paracheck®). Given the higher cost of ACT compared to earlier anti-malarials, the objectives of the present study were i) to study the accuracy of Paracheck® compared to the thick blood smear (TBS) in two areas with different levels of malaria endemicity and ii) analyse the cost-effectiveness of the strategy of the parasitological confirmation of clinically suspected malaria cases management recommended by the NMCP.MethodsA cross-sectional study was undertaken in the villages of Dielmo and Ndiop (Senegal) nested in a cohort study of about 800 inhabitants. For all the individuals consulting between October 2008 and January 2009 with a clinical diagnosis of malaria, a questionnaire was filled and finger-prick blood samples were taken both for microscopic examination and RDT. The estimated costs and cost-effectiveness analysis were made considering five scenarios, the recommendations of the NMCP being the reference scenario. In addition, a sensitivity analysis was performed assuming that all the RDT-positive patients and 50% of RDT-negative patients were treated with ACT.ResultsA total of 189 consultations for clinically suspected malaria occurred during the study period. The sensitivity, specificity, positive and negative predictive values were respectively 100%, 98.3%, 80.0% and 100%. The estimated cost of the reference scenario was close to 700€ per 1000 episodes of illness, approximately twice as expensive as most of the other scenarios. Nevertheless, it appeared to us cost-effective while ensuring the diagnosis and the treatment of 100% of malaria attacks and an adequate management of 98.4% of episodes of illness. The present study also demonstrated that full compliance of health care providers with RDT results was required in order to avoid severe incremental costs.ConclusionsA rational use of ACT requires laboratory testing of all patients presenting with presumed malaria. Use of RDTs inevitably has incremental costs, but the strategy associating RDT use for all clinically suspected malaria and prescribing ACT only to patients tested positive is cost-effective in areas where microscopy is unavailable.

Coxiella burnetii in humans and ticks in rural Senegal.

BackgroundQ fever is a worldwide zoonotic disease caused by Coxiella burnetii. Epidemiologically, animals are considered reservoirs and humans incidental hosts.Methodology/Principal FindingsWe investigated Q fever in rural Senegal. Human samples (e.g., sera, saliva, breast milk, feces) were screened in the generally healthy population of two villages of the Sine-Saloum region. Ticks were collected in four regions. Seroprevalence was studied by immunofluorescence, and all other samples were tested by two qPCR systems for detection of C. burnetii. Positive samples were genotyped (multispacer typing) by amplification and sequencing of three spacers. Strains were isolated by cell culture. We found that the seroprevalence may be as high as 24.5% (59 of 238 studied) in Dielmo village. We identified spontaneous excretion of C. burnetii by humans through faeces and milk. Hard and soft ticks (8 species) were infected in 0–37.6%. We identified three genotypes of C. burnetii. The previously identified genotype 6 was the most common in ticks in all studied regions and the only one found in human samples. Three strains of genotype 6 of C. burnetii were also recovered from soft tick Ornithodoros sonrai. Two other genotypes found in ticks, 35 and 36, were identified for the first time.Conclusions/SignificanceQ fever should be considered a significant public health threat in Senegal. Humans, similar to other mammals, may continuously excrete C. burnetii.

Toward the Rational Design of a Malaria Vaccine Construct Using the MSP3 Family as an Example: Contribution of Antigenicity Studies in Humans

Plasmodium falciparum merozoite surface protein (MSP3) is a main target of protective immunity against malaria that is currently undergoing vaccine development. It was shown recently to belong, together with MSP6, to a new multigene family whose C-terminal regions have a similar organization, contain both homologous and divergent regions, and are highly conserved across isolates. In an attempt to rationally design novel vaccine constructs, we extended the analysis of antigenicity and function of region-specific antibodies, previously performed with MSP3 and MSP6, to the remaining four proteins of the MSP3 family using four recombinant proteins and 24 synthetic peptides. Antibodies to each MSP3 family antigen were found to be highly prevalent among malaria-exposed individuals from the village of Dielmo (Senegal). Each of the 24 peptides was antigenic, defining at least one epitope mimicking that of the native proteins, with a distinct IgG isotype pattern for each, although with an overall predominance of the IgG3 subclass. Human antibodies affinity purified upon each of the 24 peptides exerted an antiparasite antibody-dependent cellular inhibition effect, which in most cases was as strong as that of IgG from protected African adults. The two regions with high homology were found to generate a broad network of cross-reactive antibodies with various avidities. A first multigenic construct was designed using these findings and those from related immunogenicity studies in mice and demonstrated valuable immunological properties. These results indicate that numerous regions from the MSP3 family play a role in protection and provide a rationale for the tailoring of new MSP3-derived malaria vaccines.

Evidence for Multiple B- and T-Cell Epitopes inPlasmodium falciparumLiver-Stage Antigen 3

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.

Long-Term Clinical Protection from Falciparum Malaria Is Strongly Associated with IgG3 Antibodies to Merozoite Surface Protein 3

BackgroundSurrogate 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 FindingsWe 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.ConclusionsSince 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.

AN EPIDEMIOLOGIC MODEL OF THE INCIDENCE OF ACUTE ILLNESS IN PLASMODIUM FALCIPARUM MALARIA

We propose a stochastic model for simulating malaria tolerance. The model relates the probability of a clinical attack of malaria to the peripheral parasite densities via a pyrogenic threshold that itself responds dynamically to the parasite load. The parameters of the model have been estimated by fitting it to the relationship between incidence of clinical episodes and the entomologic inoculation rate, using age-specific incidence data from two villages in Senegal and one village in Tanzania. The model reproduces the shifts in age distribution of clinical episodes associated with variation in transmission intensity, and in keeping with the data, predicts a slightly higher lifetime number of episodes in the mesoendemic village of Ndiop than in the holoendemic village of Dielmo. This model provides a parsimonious explanation of counter-intuitive relationships between the overall incidence of clinical malaria and transmission intensity. In contrast to the theory of endemic stability, recently proposed to apply to P. falciparum, it does not assume any intrinsic age dependence in the outcome of infection. This model can be used to explore the consequences for predictions of the effects of different anti-malarial interventions on the incidence of clinical malaria.

RELATIONSHIPS BETWEEN THE OUTCOME OF PLASMODIUM FALCIPARUM INFECTION AND THE INTENSITY OF TRANSMISSION IN AFRICA

Establishing the relationship between transmission intensity and health outcomes is crucial for the planning of long-term malaria control programs. Unfortunately this is fraught with methodologic difficulties. In this report, we address some of these problems by considering some important parameters that have previously been ignored. One important consideration is that the incidence of infection for Plasmodium falciparum malaria is much lower than entomologic inoculation rates (EIRs), especially at higher transmission levels. Moreover, biting rates of malaria vectors per host depend on his or her biomass and thus age. We propose an algorithm for estimating human infection rates from the EIR with allowance for these two factors. We then re-analyzed 1) data on EIR and age-specific incidence of clinical malaria in two villages in Senegal and 2) a survey of infant and child mortality rates across Africa. In each case, we review analyses of incidence in relation to the EIR and carry out a new analysis of morbidity and mortality rates in relation to the estimated incidence of infection (h). Reduction in malaria transmission may result in a shift of acute malaria attacks to older ages, and thus have little impact on life-time risk of clinical attacks. However, our analysis of the Senegalese data indicates that the peak incidence rate of disease relative to infection rates is in the youngest age groups in both the villages of Dielmo (EIR = 200 infectious bites per year) and Ndiop (EIR = 20). This suggests that simple models of acquired clinical immunity can explain age-incidence profiles better when incidence is expressed in relation to h, than when expressed in relation to the EIR. Relationships of malaria transmission intensity (in endemic areas) with overall mortality are very different from those with acute morbidity. Infant mortality rates (IMRs) decrease substantially when the EIR is reduced, probably largely because of prevention of indirect mortality. However, we were are not yet able to draw strong conclusions about the shape of relationships between the IMR and h because many of the available data points have similar values of h. The effects of transmission reduction on mortality rates in older age groups are also uncertain. However, it is clear that reduction of exposure during infancy is not reflected in increased mortality at older ages.

Spatial and temporal variations of the chromosomal inversion polymorphism of Anopheles funestus in Senegal.

The polymorphism of paracentric inversions of An. funestus polytene chromosomes was studied along a transect in Senegal in order to assess their variations at the spatial and temporal level. There was an increase in the degree of chromosomal polymorphism from the West to South-East. At the geographical level the variations in inversion frequencies were highly significant whatever the chromosomal arm considered. However, the variations in the chromosomal inversion frequencies did not change significantly over either seasons or years, except for inversion 3b in the village of Dielmo. Such geographical variability within a relatively limited area, associated to temporal stability, suggest a restricted gene flow between the populations studied, probably due to discontinuities in the An. funestus distribution and to its bioecology.

Plasmodium falciparum clinical malaria in Dielmo, a holoendemic area in Senegal: no influence of acquired immunity on initial symptomatology and severity of malaria attacks.

Six hundred eighty-nine Plasmodium falciparum malaria attacks were observed during a three-year period among 226 inhabitants of the village of Dielmo, Senegal, an area of high malaria transmission. Malaria attacks were defined as clinical episodes with fever (body temperature > or = 38.0 degrees C) or reporting of fever or headache or vomiting, associated with a parasite:leukocyte ratio above an age-dependent pyrogenic threshold identified in this population. The symptom frequencies were tested against age, gender, and parasite density using a random-effect logistic regression model and the study of distinguishable clinical presentations was carried out by multi-correspondence analysis. There was little difference between the severity of symptoms during the initial course of attacks in young children and adults, and this severity was not correlated with the duration of the pathologic episode. It was not possible to distinguish objectively different malaria attack types according to the severity of clinical manifestations. In contrast, the duration of fever, symptoms, and parasite clearance were significantly longer among the youngest children than among the oldest children and adults. These findings suggest that of the two components of protective immunity, anti-parasite immunity and anti-toxic immunity, only the first would play a major role as age increases. They suggest also that the initial clinical presentation of malaria attacks is not predictive of the level of protective immunity.

Plasmodium ovale in a highly malaria endemic area of Senegal

During 4 months, from June to September 1990, the population of Dielmo village, Senegal, an area of intense and perennial malaria transmission, was enrolled in a follow-up study including daily clinical surveillance and bi-weekly malaria parasitaemia monitoring. Thick blood film examinations indicated that 48·5% of children ( 49 101 ) and 32·4% of adults ( 34 105 ) were infected at least once by Plasmodium ovale during the study period; 148 distinct episodes of patent parasitaemia were observed, with estimated maximum durations of 3–115 d. The mean duration at first decreased significantly with age, from 11·4 d in children under 5 years old to 4·2 d in adults aged 40·59 years, but then increased in older adults to 7·0 d. In all age groups, most infections were asymptomatic. Only high parasitaemias were significantly associated with fever; 3 clinical malaria attacks due to P. ovale were seen during the study period.

Incidence of clinical malaria in pregnant women exposed to intense perennial transmission

The interaction between pregnancy and malaria attacks was investigated from 1990 to 1994 among women in the village of Dielmo, a holoendemic area in Senegal where malaria transmission is intense and perennial. Clinical and parasitological data collected during the daily follow-up of 48 pregnancies among 31 women were compared with those collected from the same women using the same methods during the year which preceded or followed their pregnancy. The parasite prevalence, mean and maximum parasite density in Plasmodium falciparum infections were significantly higher during pregnancy. The incidence rate of malaria attacks was, on average, 4·2 times higher during pregnancy than during the control period. Although most pregnancies were not associated with a malaria attack and the incidence of malaria attacks decreased as the number of previous pregnancies increased, a significant increase in risk of malaria attacks among multigravidae was noted until the fifth pregnancy.

Revisiting host/parasite interactions: molecular analysis of parasites collected during longitudinal and cross-sectional surveys in humans.

This paper summarizes the first conclusions arising from an analysis of parasite diversity in blood samples collected during longitudinal surveys conducted in Senegal. Parasite typing was carried out using a PCR-based molecular analysis of allelic polymorphism. The parasite populations circulating in the village of Dielmo during periods of intense transmission (when the inoculation rate was 0.5-1 infective bite/night) are characterized by a considerable allelic diversity of the MSP-1, MSP-2 and TRAP loci. A large proportion of blood samples contained several MSP-1 or MSP-2 alleles. In asymptomatic carriers, the complexity of the infections (number of alleles and genetic diversity of these alleles) was age-dependent. In children, the trend was for a reduced complexity during clinical episodes. Molecular typing showed that successive clinical episodes experienced by children were caused by genetically distinct parasites. Longitudinal analysis of asymptomatic carriers indicated that in the absence of transmission, the same parasite types were carried for long periods, while rapidly changing profiles were observed during intense transmission season. The consequences of these findings on our understanding of acquired anti-parasite immunity in humans living in endemic are discussed.

Rapid turnover of Plasmodium falciparum populations in asymptomatic individuals living in a high transmission area.

A polymerase chain reaction (PCR) typing technique, based on the amplification of polymorphic regions from the merozoite surface protein 1 (MSP-1) and MSP-2 Plasmodium falciparum genes, was used to characterize parasites collected in a longitudinal study of asymptomatic carriers of malaria parasites living in two distinct epidemiologic situations. Blood samples were collected from children and adults living in the village of Dielmo, Senegal, when malaria transmission was 3-6 infective bites/week/individual. For each individual, every sample collected at two-week intervals over a period of three months showed a specific PCR pattern. Changes involved both appearance and disappearance of specific alleles. Analysis of blood samples collected at a few-days interval showed that modifications of the PCR patterns occurred rapidly. Most alleles were detected over a period of 2-3 weeks, but some alleles could be detected only for a few days. The frequent modifications of the PCR patterns indicate significant changes in allelic balance over time, and importantly, this was observed both in children and adults. These results strongly contrast with the stability of the parasite types harbored by asymptomatic individuals living in Pikine, Senegal during a period in which malaria transmission was interrupted, and therefore suggest that the rapid turnover observed in Dielmo may reflect the introduction of new parasite populations by mosquitoes.

Biology of the vectors and transmission of Plasmodium falciparum, P. malariae and P. ovale in a village in the savanna of west Africa (Dielmo, Senegal)

From April 1990 to March 1992 a longitudinal entomological study was carried out in Dielmo village, Senegal, an area of Sudan-type savanna. Mosquitoes were sampled by night-bite collections and pyretnrum spray collections. Seven anopheles species were identified: An. gambiae s.s. An. arabiensis, An. funestus, An. pharoensis, An. rufipes, An. squamosus and An. ziemanni. Present throughout the year, An. gambiae s.l. and An. funestus represented more than 98% of anopheles captured on man. A yearly wave of An. gambiae s.l. was observed in the rainy season and An. funestus was generally more abundant in the dry season. The sporozoite rate was 1.5% and 1.3%, respectively, for these two species. Sporozoite typing by monoclonal antibodies indicated that the proportion of infected salivary glands was 92.7% P. falciparum, 18.2% P malariae and 8.2% P. ovale. The inoculation rate was calculated to be respectively 111, 21 and 8 infective bites per human for P. falciparum, P. malariae and P. ovale during the first year. Transmission was highest in the second year, with respectively 272, 54 and 25 infective bites per human.

Malaria attacks in children exposed to high transmission: who is protected?

In persons naturally exposed to high transmission, how significant, in terms of immune protection, is the occurrence or non-occurrence of a malaria attack during a given observation period? This question was studied in a West African village where Plasmodium falciparum malaria is holoendemic with intense and perennial transmission. A cohort of 94 children aged 4 months-14 years from Dielmo village, Sénégal, was studied over 4 months, June-September 1990. 41 children had no malaria attack and 53 children suffered between one and 6 attacks. The average number of attacks was 1·80, 2·25, 1·87, 0·29 and 0·07, respectively, in children aged 4–11 months, 1–2, 3–6, 7–10 and 11–14 years. The transmission level was 75 infective bites per person. Analysis of the distribution of the number of attacks in individual children suggested that all children within the same age group had either the same, or a very similar, level of protection. This suggests that the acquisition of clinical protection in areas where malaria is highly endemic involves a progressive and homogeneous decrease of the probability of having a malaria attack (attacks occur less frequently as age increases, in all children), rather than the acquisition of complete protection by an increasing number of children. The differences between the number of clinical attacks observed in young children and their presumed exposure suggest that protective mechanisms become effective from the first reinfection onwards, and are independent of the cumulative exposure to a great variety of antigens.