- Research article
- Open Access
Gravidity and malaria trends interact to modify P. falciparum densities and detectability in pregnancy: a 3-year prospective multi-site observational study
BMC Medicine volume 20, Article number: 396 (2022)
Low-density Plasmodium falciparum infections prevail in low transmission settings, where immunity is expected to be minimal, suggesting an immune-independent effect on parasite densities. We aimed to describe parasite densities in pregnancy, and determine how gravidity and antibody-mediated immunity affect these, during a period of declining malaria transmission in southern Mozambique.
We documented P. falciparum infections at first antenatal care visits (n = 6471) between November 2016 and October 2019 in Ilha Josina (high-to-moderate transmission area), Manhiça (low transmission area), and Magude (pre-elimination area). Two-way interactions in mixed-effects regression models were used to assess gravidity-dependent differences in quantitative PCR-determined P. falciparum positivity rates (PfPRqPCR) and densities, in the relative proportion of detectable infections (pDi) with current diagnostic tests (≥ 100 parasites/μL) and in antimalarial antibodies.
PfPRqPCR declined from 28 to 13% in Ilha Josina and from 5–7 to 2% in Magude and Manhiça. In primigravidae, pDi was highest in Ilha Josina at the first study year (p = 0.048), which declined with falling PfPRqPCR (relative change/year: 0.41, 95% CI [0.08; 0.73], p = 0.029), with no differences in antibody levels. Higher parasite densities in primigravidae from Ilha Josina during the first year were accompanied by a larger reduction of maternal hemoglobin levels (− 1.60, 95% CI [− 2.49; − 0.72; p < 0.001), than in Magude (− 0.76, 95% CI [− 1.51; − 0.01]; p = 0.047) and Manhiça (− 0.44, 95% CI [− 0.99; 0.10; p = 0.112). In contrast, multigravidae during the transmission peak in Ilha Josina carried the lowest pDi (p = 0.049). As PfPRqPCR declined, geometric mean of parasite densities increased (4.63, 95% CI [1.28; 16.82], p = 0.020), and antibody levels declined among secundigravidae from Ilha Josina.
The proportion of detectable and clinically relevant infections is the highest in primigravid women from high-to-moderate transmission settings and decreases with declining malaria. In contrast, the falling malaria trends are accompanied by increased parasite densities and reduced humoral immunity among secundigravidae. Factors other than acquired immunity thus emerge as potentially important for producing less detectable infections among primigravidae during marked declines in malaria transmission.
The density of malaria parasites in an individual has implications for the ability to detect the infection, since standard diagnostic test such as microscopy and rapid diagnostic test (RDT) can miss low density infections . Parasite density is considered to be mainly shaped by host factors, in particular immunity . Naturally acquired immunity against P. falciparum develops after repeated exposure, with a faster acquisition in high transmission areas . However, most infections in low transmission settings, where immunity is expected to be minimal, are characterized by their low parasite densities . This observation challenges the idea that host immunity is the only determinant of epidemiological malaria features.
Parasite factors such as the number of co-infecting parasite variants that compete within the human host, intrinsic growth rate, virulence, sexual commitment, and fitness may also play a role in shaping parasite densities under different intensities of transmission [3,4,5]. Plasmodium parasites have developed sophisticated mechanisms to adjust to host conditions  and seasonal fluctuations . However, most of the evidence supporting this adaptability has been obtained from in vitro and animal models, leaving the role of parasite factors in shaping human infections and their potential to undermine the efficacy of malaria control programs to be determined . In addition, most of longitudinal data on malaria in Africa is based on symptomatic cases  and lacks precise estimations of parasite densities, which makes it difficult to study the epidemiological signatures of these parasite adaptations.
Pregnant women attending antenatal care (ANC) clinics constitute an attractive group for monitoring changes in malaria prevalence in the population . A few studies have explored parasite densities in this group, in particular in low transmission settings or during marked declines in transmission, but none have included direct measures of immunity. Infections acquired before pregnancy are maintained at low density by antimalarial immunity developed after life-long encounters with P. falciparum [9, 10]. These infections, together with those acquired in pregnancy, multiply to high densities in primigravidae from the end of the 12th week of gestation [9, 11] when the placenta selects P. falciparum parasites expressing VAR2CSA that binds to chondroitin sulfate A . Antibodies against VAR2CSA are acquired in a gravidity-dependent manner, and appear to protect multigravid women against severe malaria [9,10,11]. This pregnancy specific immune response reflects exposure in very specific time windows (i.e., the pregnancy period) and might be utilized to gain a better understanding of the role of immunity in controlling parasite densities.
There is little knowledge on the relationship between intensity of malaria transmission, parasite densities, and immunity, especially among pregnant women and in contexts of declining transmission. Here, we aimed to describe parasite densities in pregnancy, and how gravidity and humoral immunity affect these, during a period of declining malaria in southern Mozambique. To address this, we undertook a 3-year prospective observational study among pregnant Mozambican women at first ANC visits in southern Mozambique. We applied molecular methods to quantify P. falciparum infections and antibody levels in women from a historically moderate transmission setting (Ilha Josina) , a setting that rapidly transitioned from moderate to low malaria transmission after an elimination initiative (Magude) , and a historically low malaria transmission setting (Manhiça). Using an interaction analysis, we tested the hypothesis that pregnancy-specific immunity is the major factor controlling parasite densities in malaria-exposed multigravidae, whereas immune-independent parasitological factors dominate parasite densities in anti-VAR2CSA-naïve primigravidae.
Study area and population
This 3-year prospective observational study was conducted between November 2016 and November 2019 at ANC clinics in Manhiça Sede District Hospital, Ilha Josina Health Center (Manhiça District), and Magude Sede Health Center (Magude District), in Maputo Province, southern Mozambique (Additional File 1: Fig. S1). The region is characterized by its subtropical climate with a hot and rainy season from November to April and a cool and dry season during the rest of the year. Overall, malaria transmission is low in Manhiça District, with rapid diagnostic test (RDT)-based parasite rates (PRRDT) in the community of 6.1% and 1.4% in 2017  and 2018 (B. Galatas, personal communication), respectively. PRRDT in Ilha Josina, a river island in the confluences of the Incomati River with historically high-to-moderate transmission levels , was 18.1% in 2017 and 3.7% in 2018 (B. Galatas, personal communication). Magude District, located 102 km from Manhiça, is a low transmission area (PRRDT: 2.6% in 2017  and 1.4% in 2018 ) resulted from the deployment of an intensive package of malaria elimination interventions since 2015 . Indoor residual spraying was deployed annually in the study areas.
Recruitment and data collection
Pregnant women at their first routine ANC visit were invited to participate in the study if resident in the study area. A fingerprick blood drop was collected onto filter papers (dried blood spots [DBS]) with the participant’s consent. A brief form, including visit date, age, gravidity, gestational age based on the fundal height measurement, area of residence, and recent movements was completed. HIV status was recorded from the maternal health card. In case of an unavailable record, an HIV serological rapid test was done according to standard procedures for voluntary counseling and testing. Maternal hemoglobin levels were determined by HemoCue® Hb 201 + (HemoCue AB, Angelholm, Sweden).
DNA extracted from DBS was used for detection and quantification of P. falciparum in duplicate using real-time quantitative PCR (qPCR) assay targeting the 18S ribosomal RNA gene . Experiments were repeated if the qPCR efficiency or the positive control were out of the range obtained by the average ± 3 standard deviations (SD; Additional File 1: Fig. S2). Positive DBSs were used for Illumina next generation sequencing of PCR amplicons targeting 101 single nucleotide polymorphisms (SNPs) in the parasite genome . The 101-SNP barcode was used to estimate the number of distinct genotypes within the infection (complexity of infection [COI]) using the program COIL .
Immunoglobulins G determinations
Immunoglobulin G (IgG) eluted from 5760 available DBS were quantified using the xMAP technology and the Luminex 100/200 System (https://www.luminexcorp.com) . The multiplex suspension array panel included VAR2CSA antigens (Duffy binding-like recombinant domains DBL3-4  as well as peptides targeting the NTS region [P1] and ID1 [P8 and PD] ), non-pregnancy specific antigens (19-kDa fragment of the merozoite surface protein-1 [MSP119] , region II/F2 of erythrocyte-binding antigen-175 [EBA175] , full-length P. falciparum reticulocyte-binding homologue protein 2 and 5 (PfRH2  and PfRH5 )), and biomarkers of recent P. falciparum exposure  (gametocyte exported protein 18 [GEXP18], acyl CoA synthetase 5 [ACS5] ag3, early transcribed membrane protein 5 [ETRAMP5] ag1 and heat shock protein 40 [HSP40] ag1). Information about antigens, procedures for reconstitution of DBS and quality control, bead-based immunoassay, and data normalization are described in Additional File 1: Fig. S2 and Additional File 2: Supplementary Methods [26, 27].
Sample size calculation and data analysis
We estimated that the molecular detection of P. falciparum in all DBS collected from Ilha Josina (n = 250/year) and a random selection of approximately 2700 samples per year in both Magude and Manhiça would allow for the estimation of the 95% confidence interval (95%CI) of annual P. falciparum positivity rates between 20 and 5% in each of the 3 sites, with a margin of error lower than or equal to the expected positivity rate. Age was categorized as either younger than 18 years or 18 years and older, season as rainy (November to April) or dry (May to October) and year of recruitment from November to October in 2016–2017 as year 1, 2017–2018 as year 2,or 2018–2019 as year 3. Women reporting to have changed their residence area during their pregnancy, as well as women with missing information on gravidity, residence area or HIV, were dropped from the analysis. Detectable infections (Di) were defined as those with parasite densities equal or above 100 parasites/μL, which is the detection limit commonly used for standard RDTs , and expressed relative to the total of infections detected by qPCR (proportion of Di [pDi]).
Categorical and continuous data were compared between sites, gravidity groups, and time periods using Fisher’s exact test and Student t-test, respectively. Multilevel mixed-effects regression models with random intercept at site level were estimated to assess associations with qPCR positivity and pDi (logistic) and log-transformed parasite densities and mean fluorescence intensities (MFIs) as well as hemoglobin levels (linear). All models were estimated using transmission level (site) and gravidity (first pregnancy and two or more previous pregnancies) as independent variables and were adjusted for season, HIV status, residence in village or rural area, and place where molecular analysis was conducted (Mozambique or Spain). The hypothesis that the control mechanisms of P. falciparum infection in pregnancy are gravidity-dependent was assessed including an interaction term between gravidity and transmission levels (by site and period) in the models. Secondary analyses, which included genetic complexity of infections and secundigravidae, were assessed using linear and mixed-effects regression models, respectively. The significance level was set at 0.05. All analyses were performed using the Stata statistical program version 16 (Stata Corporation, College Station, TX, USA).
Characteristics of study participants
Among the 10,439 first ANC visits recorded in the three antenatal clinics from November 2016 to November 2019, 8745 (8464 women) were included in the study. Not residing in the study area was the main reason for exclusion (Additional File 1: Fig. S3). No major differences were observed between the women included in the study and the 6471 selected for the molecular analyses nor between the women from the 3 study centers (Additional File 3: Table S1). Overall, 8% (518/6471) of the pregnant women presented at the ANC clinic during their first trimester (mean gestational age: 20.4 weeks; SD: 5.5) and 29% (1872/6471) were HIV-infected. The proportion of women at their first, second, and third or more pregnancies was 27% (1754/6471), 26% (1675/6471), and 47% (3042/6471), respectively.
P. falciparum qPCR positivity rates
A total of 483 (7.5%) out of the 6471 blood samples were positive for P. falciparum by qPCR (26% [199/770] in Ilha Josina, 6% [169/3044] in Manhiça, and 4% [115/2657] in Magude, p < 0.001; Table 1). Fifty-five percent of the infections (268/483) were detected in the first year (November 2016 to October 2017). qPCR-confirmed P. falciparum infections were more likely in first pregnancies (adjusted odds ratio [aOR] 1.65, 95% CI [1.30; 2.11], p < 0.001; Fig. 1), during the rainy season (aOR 1.44, 95% CI [1.17; 1.78], p < 0.001; Fig. 1), and in women residing in rural settings (aOR 1.99, 95% CI [1.37; 2.87], p < 0.001). On average, PfPRqPCR declined annually by 42% (95% CI [30–55], p < 0.001) in Ilha Josina (from 28% in year 1 to 13% in year 3), 44% (95% CI [30–57], p < 0.001) in Magude (from 5 to 2%), and 45% (95% CI [33–56], p < 0.001) in Manhiça (from 7 to 2%; Additional File 3: Table S2). These declines were similar between primigravid and multigravid women (p for the interaction [pI] > 0.05; Fig. 2 and Additional File 3: Table S3). The genetic complexity of infections among primigravid women during the first year was lower in Manhiça (1.17, SD 0.39; n = 12) compared to Ilha Josina (1.57, SD 0.59; n = 23) and Magude (1.60, SD 0.55; n = 5; p = 0.039), while no differences were observed among multigravidae nor during second and third years (Additional File 1: Fig. S4).
P. falciparum densities and detectability
Geometric mean of parasite density was 36.7 parasites/μL (SD 118.9). Forty-four percent (212/483) of the qPCR-detected infections were equal or above 100 parasites/μL and thus considered as infections detectable by conventional RDTs. Parasite densities (p = 0.004) and the proportion of detectable infections (p = 0.001) were higher among HIV-infected than HIV-uninfected women. Densities were 5.4-fold (95%CI [2.7; 10.6], p < 0.001) higher in primigravidae than multigravidae (Table 1); this increase in density was larger in Ilha Josina (16.7, 95% CI [6.3, 33.3], p < 0.001) than in Magude (6.3, 95% CI [1.9, 20.0], p = 0.002) and Manhiça (1.9, 95% CI [0.7, 5.3], p = 0.195; Additional File 3: Table S2). Similar trends were observed in the pDi (Table 1 and Additional File 3: Table S2).
At the peak of P. falciparum positivity rates (November 2016 to October 2017), pDi was the highest among primigravidae in Ilha Josina (83%) compared to those in Magude (50%) and Manhiça (58%; p = 0.048; Fig. 1 and Additional File 3: Table S4). In contrast, pDi among multigravidae in the first year was the highest in Manhiça (44%) compared to Ilha Josina (21%) and Magude (25%, p = 0.049; pI = 0.008; Fig. 1 and Additional File 3: Table S4). Thereafter, pDi in Ilha Josina decreased among primigravidae (average proportional annual change of 0.41, 95% CI [0.08; 0.73]; p = 0.029) and tended to increase among multigravidae (1.47, 95% CI [0.68; 2.25]; p = 0.161; pI = 0.007; Fig. 2A and Additional File 3: Table S3), mainly driven by the increase among secundigravidae (2.43, 95% CI [0.87–6.76], p = 0.089; Fig. 2B and Additional File 3: Table S5). The pDi in Manhiça and Magude remained stable (0.94, 95% CI [0.63; 1.44]; p = 0.784) or tended to increase (1.57, 95% CI [0.95; 2.62]; p = 0.081), respectively, with no evidence of a modification by gravidity (Fig. 2A and Additional File 3: Table S4). Parasite densities followed the same trends as pDi (Figs. 1, 2 and Additional File 4: Supplementary Results).
Immunoglobulin G levels against P. falciparum antigens
Antibody levels were higher among women from Ilha Josina (all antigens), HIV-infected (all antigens except ACS5), those residing in rural settings (DBL3-4, RII/F2-EBA175, PfRH2, PfRH5, GEXP18, ACS5, ETRAMP, HSP40), and multigravidae (DBL3-4, P1, MSP119, RII/F2-EBA175, GEXP18 and ACS5; Fig. 3 and Additional File 1: Fig. S5). Antibody levels against DBL3-4 (p = 0.013), P8 (p = 0.003), P1 (p = 0.029), and GEXP18 (p = 0.048) declined during the 3-year study period. Since P. falciparum infection was associated with increased antibody levels against all the antigens (Additional File 1: Fig. S5), women with P. falciparum infection were excluded from further analysis to discard antibody boosting by active infections. Antibodies against DBL3-4 were similarly low in primigravidae women from three sites but increased in secundigravidae and multigravidae from Ilha Josina compared to Manhiça and Magude (pI < 0.001; Fig. 3A). In contrast, antibodies against non-pregnancy-specific antigens were similarly high in all women from Ilha Josina regardless of gravidity. Antibody levels remained constant throughout the 3-year study period in women from Magude and Manhiça, while a reduction in antibody levels was observed among secundigravidae from Ilha Josina (annual relative change of 0.83, 95% CI [0.72; 0.94], p = 0.004 for P8-VAR2CSA; 0.83, 95% CI [0.72; 0.96], p = 0.009 for Pd-VAR2CSA; 0.84, 95% CI [0.73; 0.98], p = 0.026 for PfRH2, and 0.89, 95% CI [0.0.79; 0.99], p = 0.026 for GEXP18; Fig. 3B and Additional File 3: Table S6).
Clinical impact of P. falciparum infections
P. falciparum infection was associated with a reduction of 0.65 g/dL of hemoglobin (95% CI [− 0.80; − 0.50], p < 0.001), which was larger among primigravidae (− 0.93 g/dL, 95% CI [− 1.19; − 0.69], p < 0.001) than multigravidae (− 0.49 g/dL, 95% CI [− 0.68; − 0.32], p < 0.001; pI = 0.004). Among primigravidae, the largest reduction was observed in women from Ilha Josina (− 1.60, 95% CI [− 2.49; − 0.72; p < 0.001), then Magude (− 0.76, 95% CI [− 1.51; − 0.01]; p = 0.047) and Manhiça (− 0.44, 95% CI [− 0.99; 0.10; p = 0.112; pI = 0.091; Fig. 4).
This is the first study to provide epidemiological and immunological evidence of non-immune factors that influence parasite densities when malaria burden falls from high-to-moderate to low levels. Between November 2016 and November 2019, P. falciparum qPCR-positivity rates at first ANC visit in southern Mozambique declined from 28% in Ilha Josina, 7% in Manhiça, and 5% in Magude to 13%, 2%, and 2%, respectively. Decreasing transmission in these three different epidemiological settings were followed by changes in parasite densities which are gravidity-dependent. Among secundigravidae in Ilha Josina, parasite densities and the relative abundance of detectable infections increased with declining malaria, as expected from les malaria exposure in the previous pregnancies. However, falling parasite rates were associated with declining parasite densities among anti-VAR2CSA-naïve primigravidae, without major changes in pregnancy-specific and general immunity. Therefore, factors other than acquired immunity produce less detectable, lower-density infections among pregnant women over short but marked declines in malaria burden.
Trends of parasite densities among multigravid women are consistent with the role of pregnancy-specific immunity in controlling parasite densities [16, 29]. The proportion of detectable infections was lower among multigravid women from areas with recent high-to-moderate transmission (Ilha Josina and Magude), compared to primigravidae in respective regions. As P. falciparum rates fall in Ilha Josina, parasite densities and detectability in secundigravidae increased to levels observed in primigravidae. These changes in a short period of 3 years are accompanied by reductions in antimalarial antibodies, both pregnancy-specific and generally, in secundigravidae , presumably reflecting a reduction in parasite exposure in the previous first pregnancy and a waning of acquired immunity . In contrast, no gravidity effect or change during the study period was observed in parasite densities and pregnancy-specific immunity among women from the historically low transmission setting in Manhiça.
Parasitemia in first pregnancy follows the opposite patterns of multigravid women, pointing to differences in the factors that determine parasite densities among women without pregnancy-specific immunity. When P. falciparum rates were at their highest levels (first year of the study), parasite densities and detectability, as well as the infection-associated reduction in hemoglobin, were higher among primigravidae from Ilha Josina, than those from Magude and Manhiça. Thereafter, parasite densities among primigravidae from Ilha Josina decreased as parasite rates dropped. This shift in parasite densities was not accompanied by changes in VAR2CSA-specific antibodies, which were similarly low among primigravid women in the three settings , as expected from the limited VAR2CSA exposure at the pregnancy onset (i.e., before the placenta develops [9, 11]). It can also not be explained by general anti-malarial immunity acquired before of pregnancy, as levels of antibodies against non-pregnancy-specific antigens were correlated with parasite rates, which was expected to affect densities reversely. More recent infections in Ilha Josina  are also unlikely to explain the higher observed parasite densities, as this effect is expected to manifest independently of gravidity and in the rainy season, when the force of infection is larger, compared to the dry season. Instead, the data suggests the role of factors other than acquired immunity in regulating parasite densities in primigravid women from Ilha Josina during marked epidemiological transitions. The higher number of genetically different parasites among infected primigravidae from Ilha Josina compared to those from Manhiça is in line with the increase in parasite clones and multiplication rates as transmission increases suggested from previous studies [3, 33]. The immunity developed during previous pregnancies may mask these non-immune factors in multigravid women. Further studies are needed to identify the molecular mechanisms of the immune-independent control of parasite densities and their potential impact on balancing parasite replication and gametocyte production, as suggested by evolutionary and experimental models [3, 4].
The results of this study have several public health implications. First, the proportion of P. falciparum infections that can be detected with currently available diagnostic tools varies substantially depending on the intensity of transmission and gravidity. The proportion of detectable infections in primigravidae increased from 50 to 83% in areas with a PfPRqPCR of 4% and 28%, respectively. In contrast, detectable infections in multigravidae decreased from 44 to 21% with increasing parasite rates. Therefore, levels and trends of malaria burden, as well as fertility rates, are expected to affect the sensitivity to detect P. falciparum infections and as a consequence the efficiency of screen-based strategies , as well as the contribution of pregnant women in sustaining transmission . Second, the adverse effects of P. falciparum infections over periods in which malaria prevalence declines may increase among multigravid women as immunity wanes and parasite densities increase, but decrease among primigravidae as non-immune factors reduce parasite densities. Third, the results of this study highlight the value of monitoring malaria in pregnancy as an independent measure of malaria burden  that can capture geographical and temporal malaria trends. Fourth, the identification of factors other than acquired immunity which determine densities as transmission changes  may allow the improvement of surveillance approaches and the development of new antimalarial interventions. Finally, incorporating non-immune factors in mathematical models of malaria in pregnancy [9, 34, 35] may improve their efficacy in describing the host-parasite biology and malaria transmission dynamics.
This analysis has several limitations. First, inaccessibility of the placenta at booking visits prevents the direct study of parasite dynamics in that organ; however, parasite densities in peripheral and placental blood have been shown to follow similar trends [16, 29]. Second, the low number of P. falciparum infections in the low transmission settings of Magude and Manhiça limited the ability to stratify women with two or more pregnancies in the analysis. Third, genotyping of parasite infections was not successful for low-density infections, which represents a large proportion of available samples. Fourth, age-dependent effects might have been attributed to pregnancy-specific immunity, given the correlation between age and gravidity. However, previous modeling data  suggests that pregnancy, rather than age, is the major determinant of the observed patterns. Fifth, it is not possible to generalize the observations of this study to non-pregnant individuals, although similar reductions in parasite densities with declining transmission have been observed in the general population [36, 37]. Finally, the study did not assess genetic, behavioral, socio-economic, and environmental factors which may contribute to the different malaria burden . However, these are not expected to vary substantially in the study populations, since they were relatively closely located in the south of Mozambique.
Parasite densities in anti-VAR2CSA-naïve primigravidae decrease following a drastic malaria decline, independently of acquired immunity. This is in contrast with the reductions of immunity among secundigravidae which is accompanied by increasing parasite densities. The effect observed in first-time pregnant women, who have not yet developed pregnancy-specific immunity, might be explained by factors other than acquired immunity that favor low densities in low transmission areas. These observations challenge the dogma that all malaria infections progress to symptomatic infections as immunity wanes with a declining transmission. Both acquired immune and non-immune factors contribute to the substantial heterogeneity in the detectability of P. falciparum infections in pregnancy, which is dependent on gravidity and changes in transmission intensity, affecting the sensitivity of current diagnostic tools as well as their role in malaria transmission. Finally, this study emphasizes the need to re-evaluate malaria conceptual frameworks, which are currently centered on the role of immunity, to better inform control and elimination strategies.
Availability of data and materials
A deidentified and restricted dataset, with accompanying data dictionary, can be provided by approved request after completion of a data use agreement, by emailing email@example.com with the subject line Attention: MiPMon Data.
Acyl CoA Synthetase 5
Adjusted odds ratio
Complexity of infection
Dried blood spot
Early transcribed membrane protein 5
Gametocyte exported protein 18
Heat shock protein 40
Mean fluorescence intensities
19-KDa fragment of the merozoite surface protein-1
Proportion of detectable infections
- PfPRqPCR :
Quantitative PCR-determined P. falciparum positivity rate
P. falciparum Reticulocyte-binding protein homologue
p for the interaction
- PfPRRDT :
Rapid diagnostic test-based P. falciparum parasite rate
Quantitative real-time polymerase chain-reaction
Single nucleotide polymorphism
Okell LC, Bousema T, Griffin JT, Ouedraogo AL, Ghani AC, Drakeley CJ. Factors determining the occurrence of submicroscopic malaria infections and their relevance for control. Nat Commun. 2012;3:1237.
Langhorne J, Ndungu FM, Sponaas AM, Marsh K. Immunity to malaria: more questions than answers. Nat Immunol. 2008;9(7):725–32.
Reece SE, Ramiro RS, Nussey DH. Plastic parasites: sophisticated strategies for survival and reproduction? Evol Appl. 2009;2(1):11–23.
Rono MK, Nyonda MA, Simam JJ, Ngoi JM, Mok S, Kortok MM, Abdullah AS, Elfaki MM, Waitumbi JN, El-Hassan IM, et al. Adaptation of Plasmodium falciparum to its transmission environment. Nat Ecol Evol. 2018;2(2):377–87.
Andrade CM, Fleckenstein H, Thomson-Luque R, Doumbo S, Lima NF, Anderson C, Hibbert J, Hopp CS, Tran TM, Li S, et al. Increased circulation time of Plasmodium falciparum underlies persistent asymptomatic infection in the dry season. Nat Med. 2020;26(12):1929–40.
Mancio-Silva L, Slavic K, Grilo Ruivo MT, Grosso AR, Modrzynska KK, Vera IM, Sales-Dias J, Gomes AR, MacPherson CR, Crozet P, et al. Nutrient sensing modulates malaria parasite virulence. Nature. 2017;547(7662):213–6.
Nkumama IN, O’Meara WP, Osier FHA. Changes in malaria epidemiology in Africa and new challenges for elimination. Trends Parasitol. 2017;33(2):128–40.
Mayor A, Menendez C, Walker PGT. Targeting pregnant women for malaria surveillance. Trends Parasitol. 2019;35(9):677–86.
Walker PG, Griffin JT, Cairns M, Rogerson SJ, van Eijk AM, ter Kuile F, Ghani AC. A model of parity-dependent immunity to placental malaria. Nat Commun. 2013;4:1609.
Tuikue Ndam N, Tornyigah B, Dossou AY, Escriou G, Nielsen MA, Salanti A, Issifou S, Massougbodji A, Chippaux JP, Deloron P. Persistent Plasmodium falciparum infection in women with an intent to become pregnant as a risk factor for pregnancy-associated malaria. Clin Infect Dis. 2018;67(12):1890–6.
Rogerson SJ, Desai M, Mayor A, Sicuri E, Taylor SM, van Eijk AM. Burden, pathology, and costs of malaria in pregnancy: new developments for an old problem. Lancet Infect Dis. 2018;18(4):e107–18.
Salanti A, Staalsoe T, Lavstsen T, Jensen AT, Sowa MP, Arnot DE, Hviid L, Theander TG. Selective upregulation of a single distinctly structured var gene in chondroitin sulphate A-adhering Plasmodium falciparum involved in pregnancy-associated malaria. Mol Microbiol. 2003;49(1):179–91.
Galatas B, Guinovart C, Bassat Q, Aponte JJ, Nhamussua L, Macete E, Saute F, Alonso P, Aide P. A prospective cohort study to assess the micro-epidemiology of Plasmodium falciparum clinical malaria in Ilha Josina Machel (Manhica, Mozambique). Malar J. 2016;15(1):444.
Galatas B, Saute F, Marti-Soler H, Guinovart C, Nhamussua L, Simone W, Munguambe H, Hamido C, Montana J, Muguande O, et al. A multiphase program for malaria elimination in southern Mozambique (the Magude project): A before-after study. PLoS Med. 2020;17(8):e1003227.
Galatas B, Mayor A, Gupta H, Balanza N, Jang IK, Nhamussua L, Simone W, Cistero P, Chidimatembue A, Munguambe H, et al. Field performance of ultrasensitive and conventional malaria rapid diagnostic tests in southern Mozambique. Malar J. 2020;19(1):451.
Mayor A, Bardaji A, Macete E, Nhampossa T, Fonseca AM, Gonzalez R, Maculuve S, Cistero P, Ruperez M, Campo J, et al. Changing trends in P. falciparum burden, immunity, and disease in pregnancy. N Engl J Med. 2015;373(17):1607–17.
Jacob CG, Thuy-Nhien N, Mayxay M, Maude RJ, Quang HH, Hongvanthong B, et al. Genetic surveillance in the Greater Mekong subregion and South Asia to support malaria control and elimination. Elife. 2021;10:e62997.
Galinsky K, Valim C, Salmier A, de Thoisy B, Musset L, Legrand E, Faust A, Baniecki ML, Ndiaye D, Daniels RF, et al. COIL: a methodology for evaluating malarial complexity of infection using likelihood from single nucleotide polymorphism data. Malar J. 2015;14:4.
Fonseca AM, Gonzalez R, Bardaji A, Jairoce C, Ruperez M, Jimenez A, Quinto L, Cistero P, Vala A, Sacoor C, et al. VAR2CSA serology to detect Plasmodium falciparum transmission patterns in pregnancy. Emerg Infect Dis. 2019;25(10):1851–60.
Chene A, Gangnard S, Dechavanne C, Dechavanne S, Srivastava A, Tetard M, Hundt S, Leroy O, Havelange N, Viebig NK, et al. Down-selection of the VAR2CSA DBL1–2 expressed in E. coli as a lead antigen for placental malaria vaccine development. NPJ Vaccines. 2018;3:28.
Mazumdar S, Sachdeva S, Chauhan VS, Yazdani SS. Identification of cultivation condition to produce correctly folded form of a malaria vaccine based on Plasmodium falciparum merozoite surface protein-1 in Escherichia coli. Bioprocess Biosyst Eng. 2010;33(6):719–30.
Pandey KC, Singh S, Pattnaik P, Pillai CR, Pillai U, Lynn A, Jain SK, Chitnis CE. Bacterially expressed and refolded receptor binding domain of Plasmodium falciparum EBA-175 elicits invasion inhibitory antibodies. Mol Biochem Parasitol. 2002;123(1):23–33.
Sahar T, Reddy KS, Bharadwaj M, Pandey AK, Singh S, Chitnis CE, Gaur D. Plasmodium falciparum reticulocyte binding-like homologue protein 2 (PfRH2) is a key adhesive molecule involved in erythrocyte invasion. PLoS ONE. 2011;6(2):e17102.
Reddy KS, Pandey AK, Singh H, Sahar T, Emmanuel A, Chitnis CE, Chauhan VS, Gaur D. Bacterially expressed full-length recombinant Plasmodium falciparum RH5 protein binds erythrocytes and elicits potent strain-transcending parasite-neutralizing antibodies. Infect Immun. 2014;82(1):152–64.
Helb DA, Tetteh KK, Felgner PL, Skinner J, Hubbard A, Arinaitwe E, Mayanja-Kizza H, Ssewanyana I, Kamya MR, Beeson JG, et al. Novel serologic biomarkers provide accurate estimates of recent Plasmodium falciparum exposure for individuals and communities. Proc Natl Acad Sci U S A. 2015;112(32):E4438-4447.
Taylor SM, Mayor A, Mombo-Ngoma G, Kenguele HM, Ouedraogo S, Ndam NT, Mkali H, Mwangoka G, Valecha N, Singh JP, et al. A quality control program within a clinical trial Consortium for PCR protocols to detect Plasmodium species. J Clin Microbiol. 2014;52(6):2144–9.
Oyola SO, Ariani CV, Hamilton WL, Kekre M, Amenga-Etego LN, Ghansah A, Rutledge GG, Redmond S, Manske M, Jyothi D, et al. Whole genome sequencing of Plasmodium falciparum from dried blood spots using selective whole genome amplification. Malar J. 2016;15(1):597.
Moody A. Rapid diagnostic tests for malaria parasites. Clin Microbiol Rev. 2002;15(1):66–78.
Feng G, Simpson JA, Chaluluka E, Molyneux ME, Rogerson SJ. Decreasing burden of malaria in pregnancy in Malawian women and its relationship to use of intermittent preventive therapy or bed nets. PLoS ONE. 2010;5(8):e12012.
Ampomah P, Stevenson L, Ofori MF, Barfod L, Hviid L. Kinetics of B cell responses to Plasmodium falciparum erythrocyte membrane protein 1 in Ghanaian women naturally exposed to malaria parasites. J Immunol. 2014;192(11):5236–44.
Staalsoe T, Megnekou R, Fievet N, Ricke CH, Zornig HD, Leke R, Taylor DW, Deloron P, Hviid L. Acquisition and decay of antibodies to pregnancy-associated variant antigens on the surface of Plasmodium falciparum-infected erythrocytes that protect against placental parasitemia. J Infect Dis. 2001;184(5):618–26.
Bousema T, Okell L, Felger I, Drakeley C. Asymptomatic malaria infections: detectability, transmissibility and public health relevance. Nat Rev Microbiol. 2014;12(12):833–40.
Hendry JA, Kwiatkowski D, McVean G. Elucidating relationships between P.falciparum prevalence and measures of genetic diversity with a combined genetic-epidemiological model of malaria. PLoS Comput Biol. 2021;17(8):e1009287.
Walker PGT, Cairns M, Slater H, Gutman J, Kayentao K, Williams JE, Coulibaly SO, Khairallah C, Taylor S, Meshnick SR, et al. Modelling the incremental benefit of introducing malaria screening strategies to antenatal care in Africa. Nat Commun. 2020;11(1):3799.
Goncalves BP, Walker PG, Cairns M, Tiono AB, Bousema T, Drakeley C. Pregnant women: an overlooked asset to Plasmodium falciparum malaria elimination campaigns? Trends Parasitol. 2017;33(7):510–8.
Koepfli C, Ome-Kaius M, Jally S, Malau E, Maripal S, Ginny J, Timinao L, Kattenberg JH, Obadia T, White M, et al. Sustained malaria control over an 8-year period in Papua New Guinea: the challenge of low-density asymptomatic Plasmodium infections. J Infect Dis. 2017;216(11):1434–43.
Morris U, Xu W, Msellem MI, Schwartz A, Abass A, Shakely D, Cook J, Bhattarai A, Petzold M, Greenhouse B, et al. Characterising temporal trends in asymptomatic Plasmodium infections and transporter polymorphisms during transition from high to low transmission in Zanzibar, 2005–2013. Infect Genet Evol. 2015;33:110–7.
We are grateful to the mothers that participated in the study, the clinical teams at the Manhiça District Hospital, the Magude Health Center, and the Ilha Josina Health center, and the personnel at CISM and ISGlobal (Maternal, Infant and Reproductive Health Team). We are also grateful to Dr. Deepak Gaur (Jawaharlal Nehru University, New Delhi, India; deceased in 2021) who offered very generously recombinant proteins produced in his lab.
This work was supported by the National institute of Health (1R01AI123050-01), the Departament d’Universitats i Recerca de la Generalitat de Catalunya (AGAUR; 2017 SGR 664), and Ministerio de Ciencia e Innovación (PID2020-118328RB-I00). ISGlobal is a member of the CERCA Programme, Generalitat de Catalunya (http://cerca.cat/en/suma/). CISM is supported by the Government of Mozambique and the Spanish Agency for International Development (AECID). We also acknowledge support from the Spanish Ministry of Science and Innovation through the “Centro de Excelencia Severo Ochoa 2019–2023” Program (CEX2018-000806-S). This research is part of ISGlobal’s Program on the Molecular Mechanisms of Malaria, which is partially supported by the Fundación Ramón Areces. This publication uses data from the MalariaGEN SpotMalaria project as described in “Jacob CG et al.; Genetic surveillance in the Greater Mekong Subregion and South Asia to support malaria control and elimination; eLife 2021;10:e62997 https://doi.org/10.7554/eLife.62997.” The project is coordinated by the MalariaGEN Resource Centre with funding from Wellcome (206194, 090770). The authors would like to thank the staff of Wellcome Sanger Institute Sample Management, Genotyping, Sequencing and Informatics teams for their contribution. The funders had no role in the study design, data collection, data analysis, data interpretation, or writing of this report. The corresponding author had full access to all the data in the study and had final responsibility for the decision to submit for publication.
Ethics approval and consent to participate
All study protocols were approved by CISM’s institutional ethics committee, Hospital Clínic of Barcelona’s Ethics Committee (HCB/2015/0787), and the Mozambican National Committee for Bioethics in Health (18/CNBS/2016). Written informed consent and assent (for 12- to 17-year old) was obtained from all participating women or an accompanying adult if she was younger than 18 years of age.
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Fig. S1. Map of Manhiça and Magude District in southern Mozambique where the study was conducted. Fig. S2: Performance of the qPCRs and quantitative suspension array assays along with the different experiments. Fig. S3. Study profile. Fig. S4. Genetic complexity of the infections by study area and period. Fig. S5. Antibody levels by studied factors.
Supplementary methods (Quantitative suspension array assay).
Characteristics of study participants. Table S2. Interactions between centers and study variables on parasitological outcomes. Table S3. Interactions between gravidity and temporal trends on parasitological outcomes. Table S4. P. falciparum parasite rates, density and detectability between centers. Table S5. Annual changes in parasitological outcomes in pregnant women from Ilha Josina. Table S6. Annual changes in antibody levels by site and gravidity group.
Supplementary results (P. falciparum parasite densities).
About this article
Cite this article
Matambisso, G., Brokhattingen, N., Maculuve, S. et al. Gravidity and malaria trends interact to modify P. falciparum densities and detectability in pregnancy: a 3-year prospective multi-site observational study. BMC Med 20, 396 (2022). https://doi.org/10.1186/s12916-022-02597-6
- Parasite densities