Cryptic Plasmodium falciparum parasites in clinical P. vivax blood samples from Thailand

Polymerase chain reaction detection revealed cryptic Plasmodium falciparum infections in 21 of 160 samples collected from Thai patients diagnosed (by microscopy) with vivax malaria. The clinical and biological significance of these mixed infections is discussed in the context of chloroquine resistance and the low inoculation rates which characterize malaria epidemiology in Thailand.     

Pf332-C231-reactive antibodies affect growth and development of intra-erythrocytic Plasmodium falciparum parasites

The Plasmodium falciparum antigen 332 (Pf332), is a megadalton parasite protein expressed at the surface of infected red cells during later stages of the parasite's developmental cycle. Antibodies to different parts of this antigen have been shown to inhibit parasite growth and adherence to host cells with or without ancillary cells. However, the mechanisms involved in these inhibitions remain largely unknown. We further analysed the activities of specific antibodies with regard to their specific mechanisms of action. For these analyses, affinity purified human antibodies against epitopes in the C-terminal fragment of Pf332 (Pf332-C231) were employed. All purified antibodies recognized Pf332-C231 both by immunofluorescence and ELISA. IgG was the main antibody isotype detected, although all sera investigated had varying proportions of IgG and IgM content. All the antibodies showed a capacity to inhibit parasite growth in P. falciparum cultures to different extents, mainly by acting on the more mature parasite stages. Morphological analysis revealed the antibody effects to be characterized by the presence of a high proportion of abnormal schizonts (15-30%) and pyknotic parasites. There was also an apparent antibody effect on the red cell integrity, as many developing parasites (up to 10% of trophozoites and schizonts) were extracellular. In some cases, the infected red cells appeared to be disintegrating/fading, staining paler than surrounding infected and uninfected cells. Antigen reversal of inhibition confirmed that these inhibitions were antigen specific. Furthermore, the growth of parasites after 22-42 h exposure to antibodies was investigated. Following the removal of antibody pressure, a decreased growth rate of these parasites was seen compared to that of control parasites. The present study confirms the potential of Pf332 as a target antigen for parasite neutralizing antibodies, and further indicates that epitopes within the C231 region of Pf332 should constitute important tools in the dissection of the role of Pf332 in the biology of the malaria parasite, as well as in the design of a malaria vaccine.     

The interrelationship of Plasmodium falciparum and P. vivax in the Punjab

Analysis of data obtained from Giemsa-stained blood films from patients with mixed Plasmodium vivax and P. falciparum parasitaemias seen in outpatient clinics held over 20 consecutive months in 4 villages in the Pakistani Punjab suggest that infections with P. falciparum and P. vivax were independent of each other. There was no evidence to support the hypothesis that P. falciparum suppressed P. vivax. A likely explanation for the seasonal variation in species parasitaemia rates in the Punjab is that P. vivax was predominant early in the transmission season due to late relapses, while P. falciparum was predominant later in the transmission season because of community-wide development of immunity to P. vivax.     

A cell biologist's view of host cell recognition and invasion by malarial parasites

The migration of various Apicomplexan parasites through host cells and tissues is examined. Two approaches are taken. First a comparative examination of the invasive stages from an ultrastructural and biochemical viewpoint. Second a critical review of the mechanism of recognition and invasion of host cells. The ultrastructural organization of the invasive stages is highly conserved and invariably includes three classes of organelle whose probable functions can be deduced. These organelle classes are: The rhoptry/microneme/osmiophilic body/microsphere complex. These electron-dense, membrane-limited organelles are situated close to the parasite plasmalemma, and are often concentrated or specifically localized within the apical complex. Their contents are probably secreted by the parasite and induce various modifications in the host cell plasmalemma. However, it is concluded that the exact role of these organelles has not yet been determined nor their mechanism of action adequately evaluated. The microtubule cytoskeleton and associated structures (e.g., apical rings and conoid, etc.). These are structural organelles, and the microtubules probably direct the locomotion of the parasites. Locomotion is possibly achieved by a directed capping reaction. The plasmalemma and inner membrane vacuoles. The plasmalemma has a glycocalyx of varying complexity which includes molecules capable of host cell recognition and binding. It is speculated that host cell invasion is achieved by the directed capping of these molecules. Whilst appreciating the very specific nature of host cell recognition and invasion in some parasite stages (e.g., RBC invasion by the malarial merozoite), a marked contrast is observed in the ability of other stages of the malarial life-cycle (sporozoite and ookinete) to invade, migrate through, and escape from varied types of host cell.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasmodium falciparum and P. malariae epidemiology in a West African village.

Transmission of Plasmodium falciparum and P. malariae was studied in a village in Burkina Faso. Consecutive captures of mosquitos were organized twice a month over a year and the species of the mosquitos identified. Also, the prevalences and densities of Plasmodium spp. were determined every 2 months in a sample of children who lived in the village. Anopheles gambiae, A. funestus, and A. nili were the local vectors, but only the first two played a predominant role in both P. falciparum and P. malariae transmission. The parasitological sporozoite index (SI) was 4.48% for A. gambiae and 4.22% for A. funestus. The immunological SIs were higher: 5.82% of A. gambiae were infected with P. falciparum and only 0.16% with P. malariae; the corresponding proportions for A. funestus were 6.45% and 0.41%. Transmission of Plasmodium spp. by A. gambiae was important during the rainy season (July-October) and by A. funestus at the beginning of the dry season (September-November). Each child in the study village could receive about 396 P. falciparum-infected bites per year but only 22 of P. malariae. The P. falciparum parasite indices were maximum during the middle of the rainy season (August), while those for P. malariae reached a peak during the dry season (February).     

Concurrent dengue and malaria due to Plasmodium falciparum and P. vivax

Concurrent infections of dengue and malaria are rare. We report a case of dengue fever with acute malaria due to Plasmodium falciparum and P. vivax in which the presence of mixed infection with P. vivax was overlooked and confirmed later on during recurrence of the fever that had initially responded to conventional antimalarial treatment and symptomatic treatment for dengue fever. We suggest that in concurrent infections of dengue and malaria, possibility of mixed infection with various Plasmodium species should be excluded to ensure a better treatment outcome.     

The purification of gametocytes of Plasmodium falciparum and P. yoelii nigeriensis by colloidal silica (Percoll) gradient centrifugation

Percoll gradient centrifugation has been applied to Plasmodium falciparum- and P. yoelii-infected blood to allow rapid concentration of gametocytes. In the case of cultured P. falciparum, stage IV and V gametocytes were separated from immature stage II and III forms and concentrated from initial gametocytaemias of 1 to 3% to an average of greater than 80% and a maximum of 100% purity in a fraction which collected at the 1.064/1.083 g/ml interface; the rounded gametocytes of P. yoelii-infected mouse blood were also concentrated in this fraction. A much larger enrichment in microgametocyte concentration (average of 160-fold) was recorded in a fraction that collected at the 1.047/1.064 g/ml interface. The large increases in gametocyte concentration were paralleled by similarly large increases in exflagellation activity, thus confirming the viability of both preparations.     

The fine structure of Plasmodium falciparum and its host erythrocytes in natural malarial infections in man

Electron micrographs of ultra-thin sections of erythrocytes taken from two Liberian children ill with Plasmodium falciparum malaria show that the appliqué forms of this parasite are clearly within the host cell. The general fine structure of the parasites resembles that of other mammalian malaria parasites, as does the mode of ingestion of host cell material by pinocytosis. Granules of haemozoin were usually found in small vesicles pinched off from the large food vacuole. Scattered through the cytoplasm of infected red cells were narrow clear clefts bounded on each side by two unit membranes. These probably represent the Maurer''s clefts seen in light microscopy. The surface of infected erythrocytes was notably distorted, a phenomenon which may have a bearing on the stickiness of the infected red cells in human falciparum malaria and the segregation of these cells in the capillaries. Many uninfected erythrocytes showed a multiple alveolar, blister-like abnormality of a portion of the cell membrane; this was not seen in otherwise comparable blood from a case of P. ovale infection.     

Temporal relationships on macromolecular synthesis during the asexual cell cycle of Plasmodium falciparum

The over-all synthesis of DNA, RNA and protein was studied during the asexual cell cycle of Plasmodium falciparum. A method for stringent synchronization of the culture was developed. Rates of synthesis of the three macromolecules were determined every four hours by labelling the parasites with radioactive precursors during 30 min pulses. There was a peak of synthesis of DNA at 44 hours. Between 33 and 45 hours the rate of synthesis of DNA was exponential. Maximum RNA synthesis was reached at 36 hours. For proteins there were two peaks, one at 24 and the other at 40 hours. Partial degradation of the RNA synthesized by schizonts was detected.     

Plasmodium falciparum or P. malariae parasitemia in carriers of sickle cell trait in various Benin biotypes]

The prevalence of malaria and the frequency of gene S were surveyed in two different regions of Benin, savana and coastal lacustrine regions. In both regions, prevalence of malaria was not significantly different between Hb AA people and Hb AS people. Gene S prevalence was not modified by age, excepted for Hb SS which was not found in people upper than 25 years. In holoendemic area, i.e. lacustrine region, means of P. falciparum parasitaemia were significantly lower in Hb AS children than in Hb AA children. Sickle cell trait did not reduce the prevalence of malaria but seemed to decrease the level of parasitaemia.     

Studies of Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) protein peptides specifically binding to human RBC

Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) peptides used in normal red blood cell (RBC) binding assays revealed that peptides 33426 (79NINILSSVHRKGRILYDSF97) and 33460 (777HKKREKSISPHSYQKVSTKVQ797) bound with high activity, presenting nanomolar affinity constants. Such high binding activity peptides (HABPs) displayed helicoid and random coil structures as determined by circular dichroism. HABPs inhibited P. falciparumin vitro invasion of normal RBC by up to 61% (depending on concentration), suggesting that some RAMA protein regions could be involved in P. falciparum invasion of RBC. The nature and localisation of receptors on RBC surface responsible for HABP binding were studied using enzyme-treated erythrocytes and structural analysis.     

Electrophoretic variants of enzymes in isolates of Plasmodium falciparum, P. malariae and P. vivax from Thailand

A new electrophoretic variant of glucose phosphate isomerase (GPI), which we now denote GPI-3, has been found in isolates of Plasmodium falciparum from 6 patients, all of whom acquired the infection in the same region (in or near Prachinburi province) of Thailand. In other regions, from which 453 isolates have been tested, only GPI-1 and/or GPI-2 have been found. Two isolates of P. malariae from patients at Kanchanaburi showed a band of GPI activity on cellulose acetate gels at a cathodal position quite distinct from that of any previously known GPI variants in other human malaria parasites. Thirty-nine isolates of P. vivax from 3 regions of Thailand have been examined for variants of GPI and lactate dehydrogenase (LDH). Three forms of GPI were found, corresponding approximately in band positions to GPI-1, 2 and 3 of P. falciparum. The position of the band of LDH activity in P. vivax was the same in all the isolates examined, and different from that of LDH-1 in P. falciparum.     

Modelling the development of resistance of Plasmodium falciparum to anti-malarial drugs.

A hybrid transmission/genetics model for the development of resistance by Plasmodium falciparum to anti-malarial drugs is set forth. Field data on pyrimethamine resistance are used to calibrate and test the model. Simulated consequences of different drug dosage and population coverage strategies are presented to illustrate an ethical dilemma. A strategy of maximally effective treatment for all infected individuals in the short term can be deleterious--in terms of rapid development of drug resistance--for a community in the longer term.     

Plasmodium falciparum gametocytes: The effect of chloroquine on their development.

Asexual erythrocytic parasites of Plasmodium falciparum are killed by chloroquine, whilst mature gametocytes are not. The gametocytes of P. falciparum take 10 days to develop to maturity and their sensitivity to chloroquine during this time was studied in vitro to investigate when the switch from susceptibility to insusceptibility occurred and to compare the responses of asexual and immature sexual parasites to the drug. 45 to 50% of asexual parasites and immature gametocytes less than one day old survived in 0.1n. mols of chloroquine per ml but 0.3n. mols of drug per ml was lethal to both. Chloroquine at 1.0n mols per ml was lethal to developing gametocytes during their first six days of growth probably due, at least in part, to the drug disorganizing the parasite's digestion of host erythrocyte haemoglobin. The drug clumped the pigment of developing gametocytes. Only immature gametocytes in the final stage of development (stage 4) survive in high chloroquine concentrations.