Unstable Malaria Transmission Research Articles

Malaria morbidity, mortality and pregnancy outcome in areas with different levels of malaria transmission in Uganda: a hospital record-based study

A retrospective hospital record-based study for Uganda was conducted in Hoima district (an area of stable malaria transmission) and in Kabale district (an area of unstable malaria transmission). In-patient and maternity ward registers for January 1997–December 1998 were reviewed and malaria admissions, mortality, abortions, stillbirths and birthweights were recorded. Data were analysed in relation to rainfall patterns. Admissions due to malaria were significantly higher at Hoima compared to Kabale hospital in 1997 ( P < 0·0001). However, the situation reversed in 1998 with significantly more malaria cases registered at Kabale compared to Hoima hospital ( P < 0·0001). The increase at Kabale hospital in 1998 was attributed to increased and prolonged rains during 1997 (El Niño). Pregnancy was a risk factor for admission with malaria at Kabale, but not at Hoima. Anaemia was significantly more common among patients admitted to hospital at Hoima compared to Kabale in 1997 and 1998 ( P < 0·0001 and P = 0·02, respectively). The rate of low birthweight (birthweight < 2·5 kg) was significantly higher among primigravidae and multigravidae at Hoima hospital compared to Kabale hospital ( P < 0·0001). There were significantly more stillbirths at Kabale compared to Hoima hospital ( P < 0·0001). Routine hospital data such as birthweight and number of malaria cases can provide information on the level of malaria transmission useful for the health services to target appropriate malaria interventions and allocate resources to control outbreaks of malaria epidemics.

Chronic Plasmodium falciparum infections in an area of low intensity malaria transmission in the Sudan.

Chronic Plasmodium falciparum malaria infections in a Sudanese village, in an area of seasonal and unstable malaria transmission, were monitored and genetically characterized to study the influence of persistent infection on the immunology and epidemiology of low endemicity malaria. During the October-December malaria season of 1996, 51 individuals out of a population of 420 had confirmed and treated P. falciparum malaria in the village of Daraweesh in eastern Sudan. In a cross-sectional survey carried out in December 1996, an additional 6 individuals were found to harbour a microscopically negative but polymerase chain reaction (PCR)-positive P. falciparum infection. On 1 January 1997, a cohort of 43 individuals aged from 9 to 53, recruited from this group of recently malaria-infected individuals agreed to donate fortnightly blood samples for the next 9 months, the first 6 of which constitute the long Sudanese dry season when transmission falls to undetectable levels. Each blood sample was tested for the presence of persistent malaria infection by microscopy and PCR. Parasite-positive samples were genotyped using PCR assays that detect allelic polymorphism at the MSP-1, MSP-2 and GLURP marker gene loci. Of 43 individuals 16 were found to maintain chronic P. falciparum infections which were continuously genetically characterized.

Nine-year longitudinal study of antibodies to variant antigens on the surface of Plasmodium falciparum-infected erythrocytes.

PfEMP1 is an antigenically variable molecule which mediates the adhesion of parasitized erythrocytes to a variety of cell types and which is believed to constitute an important target for naturally acquired protective immune responses in malaria. For 9 years we have monitored individuals living in an area of low-intensity, seasonal, and unstable malaria transmission in eastern Sudan, and we have used this database to study the acquisition, specificity, and duration of the antibody response to variant parasitized erythrocyte surface antigens. Both the levels and the spectrum of reactivity of these antibodies varied considerably among individuals, ranging from low levels of antibodies recognizing only few parasitized erythrocyte surface antigens to high levels of broad-specificity antibodies. In general, episodes of clinical malaria were associated with increases in the levels of parasitized erythrocyte surface-specific antibodies that subsided within months of the attack. This response was often, but not always, specific for the antigenic variants expressed by the parasite isolate causing disease. Our study provides evidence that Palciparum falciparum malaria is associated with a short-lived, variant-specific antibody response to PfEMP1-like antigens exposed on the surface of parasitized erythrocytes. Furthermore, our data suggest that the antigenic repertoires of variant antigens expressed by different parasite isolates show considerable overlapping, at least under Sahelian conditions of low-intensity, seasonal, and unstable malaria transmission. Finally, we demonstrate the existence of persistent differences among individuals in the capacity to mount antibody responses to variant surface antigens.

Overlapping antigenic repertoires of variant antigens expressed on the surface of erythrocytes infected by Plasmodium falciparum.

Antibodies against variable antigens expressed on the surface of Plasmodium falciparum-infected erythrocytes are believed to be important for protection against malaria. A target for these antibodies is the P. falciparum erythrocyte membrane protein 1, PfEMP1, which is encoded by around 50 var genes and undergoes clonal variation. Using agglutination and mixed agglutination tests and flow cytometry to analyse the recognition of variant antigens on parasitized erythrocytes by plasma antibodies from individuals living in Daraweesh in eastern Sudan, an area of seasonal and unstable malaria transmission, we show that these antibodies recognize different variant antigens expressed by parasites of different genotype. Comparing the levels and acquisition of antibody to variant antigens in pairs of parasite isolates expressing different variant types, there is a correlation between the acquisition of antibodies to some combinations of variant antigens but not to others. These results indicate that (1) a single infection will induce the production of antibodies recognizing several variants of surface-expressed antigens, (2) the repertoire of variable antigens expressed by different parasites is overlapping and the degree of overlap differs between isolates, and (3) the expression of at least some variant antigens is genetically linked.

A longitudinal study of human antibody responses to Plasmodium falciparum rhoptry-associated protein 1 in a region of seasonal and unstable malaria transmission.

Rhoptry-associated protein 1 (RAP1) of Plasmodium falciparum is a nonpolymorphic merozoite antigen that is considered a potential candidate for a malaria vaccine against asexual blood stages. In this longitudinal study, recombinant RAP1 (rRAP1) proteins with antigenicity similar to that of P. falciparum-derived RAP1 were used to analyze antibody responses to RAP1 over a period of 4 years (1991 to 1995) of 53 individuals naturally exposed to P. falciparum malaria. In any 1 year during the study, between 23 and 39% of individuals who had malaria developed immunoglobulin G (IgG) antibodies detectable with at least one rRAP1 protein. However, the anti-RAP1 antibody responses were detected only during or shortly after clinical malarial infections. RAP1 antibody levels declined rapidly (within 1 to 2 months) following drug treatment of the infections. No anti-RAP1 antibodies were usually detected a few months after the end of malaria transmission, during the dry season, or by the start of the next malaria season. Thus, RAP1 IgG responses were very short-lived. The short duration of RAP1 antibody response may explain the apparent lack of response in a surprisingly high proportion of individuals after clinical malarial infections. For some individuals who experienced more than one malarial infection, a higher anti-RAP1 antibody response to subsequent infections than to earlier infections was observed. This suggested secondary responses to RAP1 and thus the development of immunological memory for RAP1.

Serologic responses to recombinant Plasmodium vivax Duffy binding protein in a Colombian village.

The Plasmodium vivax Duffy binding protein (DBP) is essential during merozoite invasion into human erythrocytes. Because of its biological importance, the DBP is also seen to have potential use as a malaria blood-stage vaccine. We have used a soluble recombinant DBP (rDBP) containing the functional ligand domain to assess the natural immunogenicity of DBP in a low-endemic vivax malaria region. Human sera from adult residents from a Colombian village with unstable vivax malaria transmission reacted specifically with the rDBP as determined by ELISA. There was a significant positive correlation between increased antibody response (average, median, and percent positives) and age of patients, although the level of responses did vary considerably in their reactivity to the rDBP from negative to very high level within each age group. These data confirm previous findings on the serologic reactivity of the DBP in exposed populations and that immunologic boosting to the DBP occurs in malaria-endemic regions even with low-level transmission.

Epidemic of malaria in north-eastern Kenya

From January to May, 1998, a major epidemic of Plasmodium falciparum malaria occurred in a non-immune population in north-eastern Kenya, an area of unstable malaria transmission. This epidemic happened after low transmission of P falciparum during 1996 and 1997 droughts, followed by major rainfall and floods in November to December, 1997, due to El Niño. Such an outbreak had not been reported since 1952 in this area.

Seasonal variation in agglutination of Plasmodium falciparum-infected erythrocytes.

Agglutination and rosette formation are in vitro characteristics of Plasmodium falciparum-infected erythrocytes, which have been associated with host protective immune responses and also with parasite virulence. The present study was carried out in an area of seasonal and unstable malaria transmission in eastern Sudan. Plasma samples were obtained before, during, and after the transmission season from a volunteer cohort of 64 individuals seven years of age and older. These plasmas were assayed for their ability to agglutinate cultured parasitized erythrocytes originally obtained from acute malaria infection samples taken from five of the cohort members. Our data show that the capacity of donor plasma samples to agglutinate parasitized cells depended largely on the time of sampling relative to the transmission season, at least within this epidemiologic setting. Thus, although less than half of the pretransmission season samples could agglutinate any of the five lines of cultured parasites, all post-transmission season samples could agglutinate at least one of the parasite lines, with 74% agglutinating two or more lines. This increase in the agglutination capacity of individual plasma samples after the transmission season occurred essentially regardless of whether an individual had experienced a clinical malaria attack during the transmission season. The study thus confirms the acquisition of agglutinating antibodies following episodes of clinical malaria, but also demonstrates that such acquisition can take place in the absence of disease, presumably as a consequence of subclinical infection. This is the first demonstration of marked seasonal fluctuations in the capacity of individuals' sera to agglutinate parasitized red blood cells. Possible explanations for this effect include a decrease in the levels of agglutinating antibodies between seasons, or shifts in the antigens being recognized by such antibodies from one transmission season to the next. Finally, we showed the existence of marked seasonal fluctuation in the levels of agglutinating antibodies, either because levels of such antibodies are not sustained between seasons or because the antigens recognized change from one season to the next.

Soluble haemoglobin is a marker of recent Plasmodium falciparum infections

Monoclonal antibodies (Mab) were raised against haemoglobin (Hb) associated with Plasmodium falciparum protein and used to develop an ELISA, measuring circulating levels of released Hb. This assay was evaluated in different malaria patients in parallel with ELISA assays for C-reactive protein (CRP) and haptoglobin. Levels of Hb were negatively associated with levels of haptoglobin. Increased levels of serum Hb and CRP and decreased levels of haptoglobin were seen in Danish malaria patients. Consecutive studies showed that increased Hb levels were detectable 3–7 days after initiation of treatment probably because of drug induced destruction of infected erythrocytes. Increased levels of CRP were measured 0–3 days after initiation of treatment. The Hb assay was used in an epidemiological study of malaria in an area of Sudan with unstable malaria transmission. The proportion of Sudanese adults with detectable soluble Hb was higher in the rainy season with malaria transmission compared to the dry season. Hb levels in the rainy season were negatively associated with levels of haptoglobin. Most adults had increased levels of soluble Hb and decreased levels of haptoglobin 7 and 30 days after their treatment of P. falciparum malaria compared to the levels during acute disease. Thus, both soluble Hb and haptoglobin appear to be markers of recent P. falciparum infections. Very high levels of CRP protein were measured in some of the malaria patients at the day of treatment while lower levels were recorded 7 and 30 days after treatment. Soluble Hb levels were associated with malariometric parameters in a similar fashion to haptoglobin. The new Mab-based assay for measuring soluble Hb in the peripheral blood of malaria patients may be useful for future epidemiological studies of malaria.

Humoral immune response to the 72 kDa heat shock protein from Plasmodium falciparum in populations at hypoendemic areas of malaria in western Brazilian Amazon

The heat-shock protein Pf72/Hsp70-1 from the human malaria parasite Plasmodium falciparum has been suggested as a potential candidate antigen for a multivalent vaccine. We have investigated the prevalence and levels of IgG antibodies to the recombinant protein PfR44, derived from Pf72/Hsp70-1, in individuals from different age groups living in Candeias do Jamari, an Amazonian town characterized by unstable and hypoendemic malaria transmission. Blood were collected from a household-based random sample comprising 241 people and the sera were comparatively tested against recombinant antigen PfR44 and a detergent-soluble extract of P. falciparum (PfAg-T). The prevalence and levels of IgG antibodies to both recombinant and total P. falciparum antigens were positively correlated with cumulative exposure to malaria, as estimated by the age of the individuals and the duration of their stay in the study area. Nevertheless, correlations between antibody responses to Pf72/Hsp70-1 and the acquisition of protective anti-malarial immunity could not be derived from our data.

High Proportion of Subclinical Plasmodium falciparum Infections in an Area of Seasonal and Unstable Malaria in Sudan

In the present longitudinal study, a cohort (n = 98) of children and adults 5-30 years of age living in an area of highly seasonal and unstable malaria transmission were followed for malaria morbidity during several successive transmission seasons. Based on morbidity surveillance during 1993 and measurements of antibody titers to the Plasmodium falciparum ring-infected erythrocyte surface antigen (Pf155/RESA), the cohort was divided into three groups: those who had at least one episode of clinical malaria (Group 1, n = 31), those who did not suffer from clinical malaria but had (Group 2, n = 63) or had not (Group 3, n = 4) a significant increase in antibody titers against the Pf155/RESA antigen. This increase was defined as equal to or greater than a four-fold increase in antibody titer in samples from same individuals taken at the beginning and the end of the malaria transmission season. Such increases in specific antibody levels suggested that the donors had been exposed to a P. falciparum blood-stage infection. Measurements of antibody titers to a peptide derived from the glutamate-rich protein exoantigen gave data parallel to those for Pf155/RESA. A surprisingly high fraction of individuals in the study cohort (approximately 66%) showed evidence of infection without ensuing clinical disease (Group 2).

Antibody response against Plasmodium falciparum exoantigens and somatic antigens: A longitudinal survey in a rural community in Rondônia, Western Brazilian Amazon

Three clinical and sero-epidemiological cross-sectional surveys involving 50 subjects were performed at six-month intervals in Urupá, a rural community characterized by unstable malaria transmission, situated in Rondônia State, Western Brazilian Amazon. Between the surveys, a clinically and parasitologically passive surveillance was established in this community and 48 malaria attacks (28 due to Plasmodium falciparum and 20 due to Plasmodium vivax) were recorded in this cohort of 50 subjects. Serum samples were collected at each survey and tested by enzyme immunoassay (ELISA) for IgG, IgG subclass and IgM antibodies against P. falciparum exoantigens isolated from culture supernatants and detergent-soluble somatic antigens. As expected, both anti-malarial IgG and IgM antibody titres were shown to rise after a malaria outbreak observed during the follow-up period. Nevertheless, in marked contrast with the profile of anti-malarial IgG subclasses described for semi-immune Africans, in this Amazonian community IgG 2 antibodies (that are non-cytophilic) against both antigens were shown to predominate over other IgG subclasses. Such overall predominance of IgG 2 subclass titres was statistically significant concerning exoantigens, but was of borderline significance in relation to IgG 1 antibodies against somatic antigens ( p=0.052). Moreover, highly variable patterns of boosting were observed in antibody responses against both antigens among the patients who suffered P. falciparum malaria attack during the study.

Malaria: a publication of the Tropical Programme of the Wellcome Trust.

Part 1 Malaria - the problem: what is malaria? four species of malaria parasite infect humans malaria is a difficult disease to study the life cycle of the genus the malaria mosquito anopheles malaria afflicts most of the world's people human communities and malaria problems of cure problems of control. Part 2 Plasmodium - the parasite: an introduction to ultrastructure phase 1 - from gametocyte to ookinete phase 2 - from ookinete to sporozoite phase 3 - from sporozoite to hepatic merozoite phase 4 - from hepatic merozoite to gametocyte metabolic pathways in the molecular biology of plasmodium falciparum antigens. Part 3 Malaria in the community: stable and unstable malaria transmission of malaria. Part 4 Control of mosquitoes and malaria: methods of malaria control methods used to study mosquitoes mosquito control in Papua, New Guinea malaria eradication in Sichuan imported malaria in Europe prospects. Appendices: severe malaria - a medical emergency sequestration in severe malaria clinical features of severe malaria intensive care of severe malaria treatment schedules for severe malaria.

Loss of cellular immune reactivity during acutePlasmodium falciparummalaria

Sixteen patients suffering from acute Plasmodium falciparum malaria were studied. All were residents of an area of unstable malaria-transmission in Eastern Sudan. Blood-samples were drawn at diagnosis, and 7 and 30 days later. Blood-samples from thirteen donors, drawn outside the malaria transmission season 5 months prior to the attack, were included in the study. Lymphoproliferative responsiveness to purified soluble malarial antigens and to the unrelated antigen PPD was lost during the acute phase of the disease in most donors, but was regained during convalescence, except in four donors recrudescing or reinfected by day 30. In contrast to the suppression of antigenic responses, cellular responses to phytohaemagglutinin (PHA) remained virtually unaffected. All donors showed elevated plasma-levels of soluble IL-2 receptor during the acute phase of the disease which normalized during convalescence. Five donors examined by fluorescence-activated cell sorting (FACS) showed no increase in surface expression of IL-2 receptor on peripheral lymphocytes. The data indicate that acute P. falciparum malaria causes a depletion of antigen-reactive T-cells from the peripheral circulation, probably due to homing of this cell-population to lymphoid tissues. It was also found that acute-phase plasma was suppressive to PPD-induced proliferative responses, indicating an additional suppressive mechanism operating in vivo.

Loss of cellular immune reactivity during acute Plasmodium falciparum malaria

Sixteen patients suffering from acute Plasmodium falciparum malaria were studied. All were residents of an area of unstable malaria-transmission in Eastern Sudan. Blood-samples were drawn at diagnosis, and 7 and 30 days later. Blood-samples from thirteen donors, drawn outside the malaria transmission season 5 months prior to the attack, were included in the study. Lymphoproliferative responsiveness to purified soluble malarial antigens and to the unrelated antigen PPD was lost during the acute phase of the disease in most donors, but was regained during convalescence, except in four donors recrudescing or reinfected by day 30. In contrast to the suppression of antigenic responses, cellular responses to phytohaemagglutinin (PHA) remained virtually unaffected. All donors showed elevated plasma-levels of soluble IL-2 receptor during the acute phase of the disease which normalized during convalescence. Five donors examined by fluorescence-activated cell sorting (FACS) showed no increase in surface expression of IL-2 receptor on peripheral lymphocytes. The data indicate that acute P. falciparum malaria causes a depletion of antigen-reactive T-cells from the peripheral circulation, probably due to homing of this cell-population to lymphoid tissues. It was also found that acute-phase plasma was suppressive to PPD-induced proliferative responses, indicating an additional suppressive mechanism operating in vivo.

Malaria during pregnancy in an area of unstable endemicity

A prospective study of malaria during pregnancy was conducted between September 1986 and December 1989 in an area of unstable (mesoendemic) malaria transmission on the Thai-Burmese border. Antenatal clinics were set up in camps for displaced persons of the Karen ethnic minority and 1358 pregnant women were enrolled at a mean estimated gestational age of 23 weeks (standard deviation 5·7 weeks) and were followed weekly until delivery. Malaria developed in 505 women (37·2%); 80·2% of infections were Plasmodium falciparum, 17·1% were P. vivax, and 2·7% were mixed. Primigravidae were infected more commonly than multigravidae: 153 322 (47·5%) compared with 318 953 (33·3%) ( P < 0·001). The incidence of malaria declined from the 20th week of gestation (12%) towards term (4·4%). Most infections were detected before symptoms developed, and there were no deaths associated with malaria. Despite this, malaria was associated with an overall 123 g reduction in birthweight (95% confidence interval [CI] 34–212 g). This reduction was largely accounted for by lower birthweights of babies born to infected primigravidae (mean reduction 151 g, 95% CI 21–282 g) and women in their 2nd and 3rd pregnancies (mean reduction 185 g, 95% CI 84–286 g). The incidence of anaemia requiring treatment was higher in women who developed malaria, 149 420 (35·4%) compared with 191 670 (28·5%), and was proportional to the number of parasitaemic episodes. Thus, despite regular antenatal clinic attendance with prompt detection and treatment of malaria (the currently employed antimalarial strategy in areas with multidrug-resistant P. falciparum), malaria still had a significant adverse effect on pregnancy. Effective prophylaxis or avoidance of exposure are the only antimalarial measures likely to protect pregnant women and their babies from the harmful effects of malaria in these areas.

Malaria transmission potential of Anopheles mosquitoes in the Mwea-Tebere irrigation scheme, Kenya

1. Anopheles arabiensis Patton and An. funestus Giles were identified as vectors of Plasmodium falciparum malaria in the Mwea-Tebere irrigation scheme, Kenya. An. arabiensis was the only member of the An. gambiae complex identified from chromosome characteristics. Other Anopheles species found included An. pharoensis Theobald, An. rufipes Gough and An. coustani Laveran. Survival rates per gonotrophic cycle for An. arabiensis averaged 0.37 during the short rains (October-November), 0.49 during the dry season (February) and 0.78 during the long rains (May-June). Vectorial capacities were correspondingly low due to low survival rates and a high degree of zoophily. The average duration of infective life for P. falciparum was 0.2 days for both An. arabiensis and An. funestus. In contrast, entomological inoculation rates were comparatively high: 6-8 infective bites/man/month. An. pharoensis averaged 110 bites/man/night during the short rains; 1/999 (0.1%) was positive by ELISA for P. falciparum circumsporozoite antigen, but the ELISA evidence is not conclusive for vector incrimination. In correspondence with clinical observations, the transmission of P. malariae and P. ovale is unlikely due to the low vector survival rates. The observed anomaly between low vectorial capacities and high entomological inoculation rates demonstrates the importance of accurately estimating vector sporozoite rates to monitor unstable malaria transmission in irrigated areas.

Reduced cellular immune reactivity in healthy individuals during the malaria transmission season

Antigen-induced cellular immune responses are suppressed during acute malaria. The present study engages the possibility that malaria-induced alterations in cellular immune reactivity extend beyond the clinical disease. Thus, lymphoproliferative responses of healthy individuals were diminished during the malaria transmission period in individuals living in an area of highly seasonal, unstable malaria transmission. This finding may have important implications for the design of studies of stimulatory properties of antigens using lymphocytes of endemic origin.

Cell‐mediated immune responses to soluble Plasmodium falciparum antigens in residents from an area of unstable malaria transmission in the Sudan

This paper describes immune responses to P. falciparum infection in individuals living in an area of highly seasonal, unstable malaria transmission. The in vitro cellular immune responses of peripheral blood mononuclear cells (PBMC) from 36 Sudanese donors to a complex of affinity purified soluble P. falciparum antigens (SPag) and two components thereof (Ag1 and Ag7) were examined and compared to humoral immune parameters. In 29/36 Sudanese donors, SPag induced a significant lymphoproliferative response in vitro. In contrast only 3/27 Danish donors never exposed to malaria responded to SPag. Ag1 and Ag7 induced significant lymphoproliferation in 9/34 and 13/36 Sudanese donors respectively, whereas no Danish donors responded. Significant interferon-gamma production was observed in 16/27, 1/5 and 3/12 Sudanese donors when stimulated by SPag, Ag1 and Ag7 respectively. Lymphoproliferative responses to SPag correlated with proliferative responses to Ag1 and Ag7, and with Spag-induced interferon-gamma production. These results indicate that T-cell clones recognizing epitopes on Ag1 and Ag7 have been expanded in the studied population as a result of exposure to P. falciparum infection. The T-cell parameters did not correlate with the presence of antibodies to SPag, Pf155/RESA or a crude parasite sonicate or with the schizont IFA titers of the plasma. This indicates that parameters outside the degree of exposure to P. falciparum influence the cellular immune responses to malaria.

Modeling malaria vaccines I: New uses for old ideas

Starting from a modification of the model of malaria transmission developed for the Garki project, this paper develops a model containing variables relevant to the simulation of malaria vaccination programs. Modifications include (1) integration of maintenance of immunity dependent on boosting and the possibility of loss immunity; (2) introduction of a boosting factor distinct from susceptibility to infection; (3) reinterpretation of the epidemiological compartments of positive immunes and nonimmunes in terms of severity of disease rather than just infection; (4) interpretation of the different stage-specific levels of immunity; (5) discrimination between different susceptibilities for the immune and nonimmune classes; (6) reformulation of the expression for acquisition of immunity to be biologically more acceptable. Simulations using the Garki model, Nedelman's modification of it, and our Basic model compare the similarities and differences in the predictive behavior of the models. Simulations using the Basic model reproduce observed periodic fluctuations of malaria attributed to the interplay of transmission-blocking immunity and loss of immunity in the absence of boosting in areas of unstable malaria transmission.