Primary exoerythrocytic schizonts of a mammalian Plasmodium as a source of gametocytes

Comparison of microscopy, card agglutination test for Trypanosoma evansi, and real-time PCR in the diagnosis of trypanosomosis in dromedary camels of the Abu Dhabi Emirate, UAE

Introduction

Trypanosomosis is an important disease of dromedary camels caused by the pathogenic protozoan Trypanosoma evansi. This study aimed to compare three different tests for its diagnosis in this species: conventional microscopy, the card agglutination test for trypanosomosis/T. evansi (CATT/T. evansi) and real-time PCR.

Material and Methods

Whole blood and serum samples collected from 77 dromedary camels of Abu Dhabi, United Arab Emirates, were analysed with the test methods stated. Statistical analysis was done using McNemar’s chi-squared test, and Cohen’s kappa index (κ) was calculated.

Results

We obtained results with positivity of 18% (14/77) by microscopy, 22% by CATT (17/77) and 60% (46/77) by real-time PCR, with the chain reaction detecting at a respectively three- and two-fold greater rate than the other techniques. Analysis of the data revealed a relative sensitivity of 30.4% and 37.0% for microscopy and CATT, respectively, compared to real-time PCR. The difference between the real-time PCR’s sensitivity and those of the other methods was statistically significant, with X² values of 30.03 and 20.1, respectively (df = 1 and P = 0.05 in both cases). Agreement of microscopy results with those of with CATT was good (κ = 0.72; 95% CI = 0.62–0.82). Cohen’s kappa index showed fair agreement of real-time PCR with microscopy (κ = 0.26; 95% CI = 0.16–0.36) whereas it was in poor agreement with CATT (κ = 0.09; 95% CI = 0.02–0.15).

Conclusion

Real-time PCR was found to be more sensitive than microscopy and CATT.

Rodent Malaria Erythrocyte Preference Assessment by an Ex Vivo Tropism Assay

Circulating red blood cells consist of young erythrocytes (early and late reticulocytes) and mature erythrocytes (normocytes). The human malaria parasites, Plasmodium falciparum and P. vivax, have a preference to invade reticulocytes during blood-stage infection. Rodent malaria parasites that also prefer reticulocytes could be useful tools to study human malaria reticulocyte invasion. However, previous tropism studies of rodent malaria are inconsistent from one another, making it difficult to compare cell preference of different parasite species and strains. In vivo measurements of cell tropism are also subjected to many confounding factors. Here we developed an ex vivo tropism assay for rodent malaria with highly purified fractions of murine reticulocytes and normocytes. We measured invasion into the different erythrocyte populations using flow cytometry and evaluated the tropism index of the parasite strains. We found that P. berghei ANKA displayed the strongest reticulocyte preference, followed by P. yoelii 17X1.1, whereas P. chabaudi AS and P. vinckei S67 showed mixed tropism. These preferences are intrinsic and were maintained at different reticulocyte and normocyte availabilities. Our study shed light on the true erythrocyte preference of the parasites and paves the way for future investigations on the receptor-ligand interactions mediating erythrocyte tropism.

Gametocytogenesis in malaria parasite: commitment, development and regulation

Malaria parasites have evolved a complicated life cycle alternating between two hosts. Gametocytes are produced in the vertebrate hosts and are obligatory for natural transmission of the parasites through mosquito vectors. The mechanism of sexual development in Plasmodium has been the focus of extensive studies. In the postgenomic era, the advent of genome-wide analytical tools and genetic manipulation technology has enabled rapid advancement of our knowledge in this area. Patterns of gene expression during sexual development, molecular distinction of the two sexes, and mechanisms underlying subsequent formation of gametes and their fertilization have been progressively elucidated. However, the triggers and mechanism of sexual development remain largely unknown. This article provides an update of our understanding of the molecular and cellular events associated with the decision for commitment to sexual development and regulation of gene expression during gametocytogenesis. Insights into the molecular mechanisms of gametocyte development are essential for designing proper control strategies for interruption of malaria transmission and ultimate elimination.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

The biology of Plasmodium falciparum transmission stages

The most important function of any parasite is to secure transmission to new hosts. The gametocyte, the stage which has become developmentally committed to the sexual cycle, provides a critical link in the transmission of Plasmodium falciparum from the human host to the anopheline mosquito vector. It is therefore imperative that our determination to understand the biology of the gametocyte is greater than the technical obstacles which have resulted in the gametocyte being left very much out of the limelight by the intensive investigation of the asexual bloodstream parasite. Here we explore the areas of gametocyte biology which by nature of their relevance to control and pathology as well as basic biology, are the subjects of investigation in our laboratory. We also point out areas in need of particular attention.

The complete development in vitro of the vertebrate phase of the mammalian malarial parasite Plasmodium berghei

All three 'vertebrate' stages of the rodent malarial parasite Plasmodium berghei berghei were grown in vitro in the absence of the vertebrate host. The parasite was introduced into culture from infected mosquitoes and 2 in vitro culture methods were used sequentially to complete the 'vertebrate' phases of development in hepatoma and erythrocyte host cells. The resultant blood infection produced mature schizonts and male and female gametocytes. The protocol, which is now being extended to the human pathogen P. falciparum, may assist future studies on this important group of parasites.

Synchronized erythrocytic schizogony and gametocytogenesis of Plasmodium berghei in vivo and in vitro

Both asexual and sexual development of Plasmodium berghei was synchronized without chemical intervention using in vitro culture techniques. Combined in vivo and in vitro experiments were performed on the relationship between age, morphology and maturity of gametocytes. Schizogony took 22-23 h in the experiments. At 26 h post-invasion (p.i.) the first males became capable of exflagellation. By 20 h p.i. the first gametocytes were recognizable in Giemsa-stained smears but the sex was hardly distinguishable until maturity (26 h p.i.). Survival time of gametocytes was estimated at 26 h in vitro (half-life 13 h) and the same survival time was suggested for gametocytes in vivo. Schizonts of P. berghei apparently disappeared from the peripheral circulation upon maturity, rupturing almost immediately. Mature schizonts in vitro persisted up to 48 h p.i. in non-agitated cultures. No evidence was collected for sequestration of any sub-population of gametocytes.

Parasitic protozoa of the blood of rodents: a revision of Plasmodium berghei

Lowland populations of the Plasmodium berghei group are compared with strains from the highlands of Katanga, Republic of Zaire, and it is concluded that the former warrant separate specific status. It is proposed that Plasmodium berghei yoelii of the Central African Republic be raised to a species, P. yoelii, and the lowland subspecies from the CAR, Brazzaville and Nigeria be moved to this species as P. y. yoelii, P. y. killicki and P. y. nigeriensis. P. berghei from Katanga would then revert to a monotypic species.

Differences between P. berghei and P. yoelii are in distribution, hosts, optimum temperatures of sporogony, sizes of mature oocysts and of sporozoites, rates of growth and minimum maturation times of tissue schizonts in the liver of the white rat, the forms of six enzymes and the DNA.

Differences between three subspecies of P. yoelii are in distribution, the sizes of mature oocysts and of sporozoites, sizes of tissue schizonts in the liver of the white rat, the effect of tissue forms on the nuclei of infected parenchymal cells and the electrophoretic forms of the enzyme GDH.

Electron microscope study of the blood stages of Plasmodium tropiduri, a lizard malaria parasite

The study of the ultrastructure of the blood stages of P.tropiduri Aragao & Neiva, 1909 shows close similarities between this species and P.floridense, another lizard malaria parasite. The absence of a rounded structure in the merozoites of P.tropiduri, and their tendency not to project beyond the original area occupied by the schizont are the ultrastructural characteristics that differentiate the two species.

The close relationships of homology between P.tropiduri and the Plasmodium of birds are emphasized, and new evidence on their phylogenetic relationships is offered.

P.tropiduri has a mode of ingestion and digestion similar to that of P. elongatum. When the size of the cytostome, the size of the boluses produced by the phagotrophy, and the size of the digestive vacuoles are compared, they are seen to be nearly identical in the two species.

The comparative study of the structure of the cytostomes of the merozoites and the trophozoites, and of their relationships with the membranes that limit the parasites, gives new evidence that the external membrane of the trophozoites schizonts, and gametocytes is derived from the host cell.

An exflagellatory apparatus is described from the microgametocytes of P.tropiduri, and its relationships with similar structures described from the gametocytes of avian Plasmodium and from Coccidia are discussed.