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

New member of Plasmodium (Vinckeia) and Plasmodium cyclopsi discovered in bats in Sierra Leone - nuclear sequence and complete mitochondrial genome analyses

Malaria remains the most important arthropod-borne infectious disease globally. The causative agent, Plasmodium, is a unicellular eukaryote that develops inside red blood cells. Identifying new Plasmodium parasite species that infect mammalian hosts can shed light on the complex evolution and diversity of malaria parasites. Bats feature a high diversity of microorganisms including seven separate genera of malarial parasites. Three species of Plasmodium have been reported so far, for which scarce reports exist. Here we present data from an investigation of Plasmodium infections in bats in the western Guinean lowland forest in Sierra Leone. We discovered a new Plasmodium parasite in the horseshoe bat Rhinolophus landeri. Plasmodium cyclopsi infections in a member of leaf-nosed bats, Doryrhina cyclops, exhibited a high prevalence of 100%. Phylogenetic analysis of complete mitochondrial genomes and nine nuclear markers recovered a close relationship between P. cyclopsi and the new Plasmodium parasite with the rodent species Plasmodium berghei, a widely used in vivo model to study malaria in humans. The data suggests that the "rodent/bat" Plasmodium (Vinckeia) clade represents a diverse group of malarial parasites that would likely expand with a systematic sampling of small mammals in tropical Africa. Identifying the bat Plasmodium repertoire is central to our understanding of the evolution of Plasmodium parasites in mammals.

The origins, isolation, and biological characterization of rodent malaria parasites

Rodent malaria parasites have been widely used in all aspects of malaria research to study parasite development within rodent and insect hosts, drug resistance, disease pathogenesis, host immune response, and vaccine efficacy. Rodent malaria parasites were isolated from African thicket rats and initially characterized by scientists at the University of Edinburgh, UK, particularly by Drs. Richard Carter, David Walliker, and colleagues. Through their efforts and elegant work, many rodent malaria parasite species, subspecies, and strains are now available. Because of the ease of maintaining these parasites in laboratory mice, genetic crosses can be performed to map the parasite and host genes contributing to parasite growth and disease severity. Recombinant DNA technologies are now available to manipulate the parasite genomes and to study gene functions efficiently. In this chapter, we provide a brief history of the isolation and species identification of rodent malaria parasites. We also discuss some recent studies to further characterize the different developing stages of the parasites including parasite genomes and chromosomes. Although there are differences between rodent and human malaria parasite infections, the knowledge gained from studies of rodent malaria parasites has contributed greatly to our understanding of and the fight against human malaria.

Grammomys surdaster, the Natural Host for Plasmodium berghei Parasites, as a Model to Study Whole-Organism Vaccines Against Malaria

AbstractInbred mice are commonly used to test candidate malaria vaccines, but have been unreliable for predicting efficacy in humans. To establish a more rigorous animal model, we acquired African woodland thicket rats of the genus Grammomys, the natural hosts for Plasmodium berghei. Thicket rats were acquired and identified as Grammomys surdaster by skull and teeth measurements and mitochondrial DNA genotyping. Herein, we demonstrate that thicket rats are highly susceptible to infection by P. berghei, and moderately susceptible to Plasmodium yoelii and Plasmodium chabaudi: 1-2 infected mosquito bites or 25-100 sporozoites administered by intravenous injection consistently resulted in patent parasitemia with P. berghei, and resulted in patent parasitemia with P. yoelii and P. chabaudi strains for at least 50% of animals. We then assessed efficacy of whole-organism vaccines to induce sterile immunity, and compared the thicket rat model to conventional mouse models. Using P. berghei ANKA radiation-attenuated sporozoites, and P. berghei ANKA and P. yoelii chemoprophylaxis vaccination approaches, we found that standard doses of vaccine sufficient to protect laboratory mice for a long duration against malaria challenge, are insufficient to protect thicket rats, which require higher doses of vaccine to achieve even short-term sterile immunity. Thicket rats may offer a more stringent and pertinent model for evaluating whole-organism vaccines.

Do mixed infections matter? Assessing virulence of mixed-clone infections in experimental human and murine malaria

BACKGROUND

Malaria parasites within an individual infection often consist of multiple strains (clonal populations) of a single species, which have the potential to interact both with one another, and with the host immune system. Several effects of these interactions have been measured in different parasite systems including competition and mutualism; however, direct observation of these effects in human malaria has been limited by sampling complexities and inherent ethical limitations.

METHODS

Using multiple complementary epidemiological models, we propose a suite of analyses to more fully utilize data from challenge experiments, and re-examine historical human challenge studies with mixed-strain Plasmodium vivax inocula. We then compare these results with murine model systems using mixed-strain Plasmodium yoelii or Plasmodium chabaudi, to explore the utility of these methods in fully utilizing these data, including the first quantitative estimates of effect sizes for mixed-strain parasitemia. These models also provide a method to assess consistency within these animal model systems.

RESULTS

We find that amongst a limited set of P. vivax (incubation time) and P. yoelii infections (time-to-mortality), survival times at a study population-level are intermediate between each single-clone infection, and are not dominated by the more virulent clone; in P. vivax relapses, mixed clone infections also show intermediate survival curves. In these infections, the results strongly suggest that highly virulent clones have their virulence attenuated by the presence of less-virulent clones. The analysis of multiple experiments with P. chabaudi suggests greater nuances in the interactions between strains, and that mortality and time-to-event in mixed-strain infections are both indistinguishable from single infections with the more virulent strain.

CONCLUSIONS

These divergent dynamics support earlier work that suggested drivers of virulence may differ in fundamental ways between malaria species that are reticulocyte-specific and those that readily infect all red blood cell stages which should be studied in greater detail. The effect sizes (magnitude of biological effects) from these analyses are significant, and suggest the potential for important gains in malaria control by greater incorporation of evolutionary epidemiology theory. Moreover, we suggest that using these epidemiological models may generally allow fuller use of data from experimentally challenging animal model experiments.

Implications of Glutathione Levels in the Plasmodium berghei Response to Chloroquine and Artemisinin

Malaria is one of the most devastating parasitic diseases worldwide. Plasmodium drug resistance remains a major challenge to malaria control and has led to the re-emergence of the disease. Chloroquine (CQ) and artemisinin (ART) are thought to exert their anti-malarial activity inducing cytotoxicity in the parasite by blocking heme degradation (for CQ) and increasing oxidative stress. Besides the contribution of the CQ resistance transporter (PfCRT) and the multidrug resistant gene (pfmdr), CQ resistance has also been associated with increased parasite glutathione (GSH) levels. ART resistance was recently shown to be associated with mutations in the K13-propeller protein. To analyze the role of GSH levels in CQ and ART resistance, we generated transgenic Plasmodium berghei parasites either deficient in or overexpressing the gamma-glutamylcysteine synthetase gene (pbggcs) encoding the rate-limiting enzyme in GSH biosynthesis. These lines produce either lower (pbggcs-ko) or higher (pbggcs-oe) levels of GSH than wild type parasites. In addition, GSH levels were determined in P. berghei parasites resistant to CQ and mefloquine (MQ). Increased GSH levels were detected in both, CQ and MQ resistant parasites, when compared to the parental sensitive clone. Sensitivity to CQ and ART remained unaltered in both pgggcs-ko and pbggcs-oe parasites when tested in a 4 days drug suppressive assay. However, recrudescence assays after the parasites have been exposed to a sub-lethal dose of ART showed that parasites with low levels of GSH are more sensitive to ART treatment. These results suggest that GSH levels influence Plasmodium berghei response to ART treatment.

Malaria parasite genetics: doing something useful

Genetics has informed almost every aspect of the study of malaria parasites, and remains a key component of much of the research that aims to reduce the burden of the disease they cause. We describe the history of genetic studies of malaria parasites and give an overview of the utility of the discipline to malariology.

Murine infection models for vaccine development: the malaria example

Vaccines are developed and eventually licensed following consecutive human clinical trials. Malaria is a potential fatal vector-borne infectious disease caused by blood infection of the single-cell eukaryote Plasmodium. Pathogen stage conversion is a hallmark of parasites in general and permits unprecedented vaccine strategies. In the case of malaria, experimental human challenge infections with Plasmodium falciparum sporozoites can be performed under rigorous clinical supervision. This rare opportunity in vaccinology has permitted many small-scale phase II anti-malaria vaccine studies using experimental homologous challenge infections. Demonstration of safety and lasting sterile protection are central endpoints to advance a candidate malaria vaccine approach to phase II field trials. A growing list of antigens as targets for subunit development makes pre-selection and prioritization of vaccine candidates in murine infection models increasingly important. Preclinical assessment in challenge studies with murine Plasmodium species also led to the development of whole organism vaccine approaches. They include live attenuated, metabolically active parasites that educate effector memory T cells to recognize and inactivate developing parasites inside host cells. Here, opportunities from integrating challenge experiments with murine Plasmodium parasites into malaria vaccine development will be discussed.

Anopheles gambiae pathogen susceptibility: the intersection of genetics, immunity and ecology

Mosquitoes are the major arthropod vectors of human diseases such as malaria and viral encephalitis. However, each mosquito species does not transmit every pathogen, owing to reasons that include specific evolutionary histories, mosquito immune system structure, and ecology. Even a competent vector species for a pathogen displays a wide range of variation between individuals for pathogen susceptibility, and therefore efficiency of disease transmission. Understanding the molecular and genetic mechanisms that determine heterogeneities in transmission efficiency within a vector species could help elaborate new vector control strategies. This review discusses mechanisms of host-defense in Anopheles gambiae, and sources of genetic and ecological variation in the operation of these protective factors. Comparison is made between functional studies using Plasmodium or fungus, and we call attention to the limitations of generalizing gene phenotypes from experiments done in a single genetically simple colony.

Wave expansion of CD34+ progenitor cells in the spleen in rodent malaria

Defense against malaria depends upon amplification of the spleen structure and function for the clearance of parasitized red blood cells (pRBC). We studied the distribution and amount of CD34(+) cells in the spleens of mice infected with rodent malaria. We sought to identify these cells in the spleen and determine their relationship to infection. C57BL/6J mice were infected with self-resolving, Plasmodium chabaudi CR, or one of the lethal rodent malaria strains, P. chabaudi AJ and P. berghei ANKA. We then recorded parasitemia, mortality, and the presence of CD34(+) cells in spleen, as determined by immunohistochemistry and flow cytometry. In the non-lethal strain, the spleen structure was maintained during amplification, but disrupted in lethal models. The abundance of CD34(+) cells increased in the red pulp on the 4th and 6th days p.i. in all models, and subsided on the 8th day p.i. Faint CD34(+) staining on the 8th day p.i., was probably due to differentiation of committed cell lineages. In this work, increase of spleen CD34(+) cells did not correlate with infection control.

Congenicity and genetic polymorphism in cloned lines derived from a single isolate of a rodent malaria parasite

Many of the most commonly studied lines of the rodent malaria parasite Plasmodium yoelii yoelii originated from a single parasite isolate designated 17X. Amongst these lines, however, are parasites that exhibit variation in genotype and phenotype (e.g. growth rate). We describe here the results of a comparative genetic analysis between cloned lines of 17X that differ in growth rate, using nucleotide sequences of specific genes and patterns of genome-wide amplified fragment length polymorphism (AFLP). Our findings indicate that the original stock of 17X comprises two unrelated genotypes. Genotype-1 is represented by parasites with a slow growth phenotype (e.g. 17X (NIMR)) and a fast growth phenotype (e.g. 17XYM). Within this genotype, there are also genomic differences manifest as a small number of AFLP bands that differentiate the fast- and slow-growing lines from each other. The other genotype, genotype-2, is represented only by parasites with a slow growth phenotype (e.g. 17XA).

The phylogeny of rodent malaria parasites: simultaneous analysis across three genomes

Species of Plasmodium that naturally infect wild rodents but can also be maintained in laboratory mice have long been used as model systems in which to study the biology of malaria parasites. Several of these rodent parasites are now providing useful genomic comparisons to those species that cause malaria in humans. Here we examined the phylogenetic relationships of 19 strains of rodent malaria parasites including four species native to African thicket rats (Plasmodium berghei, Plasmodium chabaudi, Plasmodium vinckei, and Plasmodium yoelii) and one from a porcupine (Plasmodium atheruri) using DNA sequence data collected from seven genes from each of the three parasite genomes. These included the nuclear dihydrofolate reductase gene and a cysteine protease gene, mitochondrial cytochrome b and cytochrome oxidase I genes, and the elongation factor tufA, caseinolytic protease C, and "open reading frame 470" genes from the apicoplast genome, for a combined total of 5049 nucleotides. Using simultaneous analysis, a method of combining each of the gene partitions into a super-matrix, two equally parsimonious trees were recovered. Bayesian analysis of the dataset produced the same topology. The basic species groups were well supported, with the exception of the placement of P. atheruri within the P. vinckei clade. Named subspecies showed a wide array of genetic differentiation, but fell into monophyletic groups.

Haemogregarines of the genus Hepatozoon (Apicomplexa: Adeleina) in rodents from northern Europe

We studied the prevalence and distribution of Hepatozoon infections in small rodents from Finland and other areas in northern Europe. Hepatozoon infections were more common in voles (Arvicolinae) than mice (Murinae) and more prevalent in voles of the genus Clethrionomys than in voles of the genus Microtus. Transmission electron microscopical examination of Hepatozoon erhardovae Krampitz, 1964 from bank voles Clethrionomys glareolus (Schreber) showed that intracellular lung meronts were located in alveolar septa. Meronts consisted of varying numbers of merozoites packed with amylopectin vacuoles inside electron-lucent parasitophorous vacuole. The size of the meronts was approximately 19 x 14 microm. Monozoic or dizoic cysts were frequent findings in the lung alveoles; the size of cysts was approximately 10 x 6 microm. Gametocytes were found inside eosinophilic granulocytes in the capillaries of lung tissue. Ultrastructurally, micronemes, microtubules, mitochondria, nuclei and lipid droplets were visible.

Nitric oxide and reactive nitrogen intermediates during lethal and nonlethal strains of murine malaria

The virulence of Plasmodia depends partly on the strain of parasite and partly on the host. In this study, Plasmodium berghei N/13/1A/4/203 caused the death of mice, whereas Plasmodium chabaudi chabaudi AS was not lethal. Current opinion is that nitric oxide (NO) and other reactive nitrogen intermediates (RNI) are produced in several host organs during malaria to resist infection or produce tissue damage. NO and RNI production in blood or plasma, brain, liver and spleen in MF1 mice was investigated during P. berghei and P. c. chabaudi infection, in order to help determine whether changes in NO production are beneficial or detrimental to the host in vivo. NO production was measured both directly and indirectly as nitrites and nitrates, to represent RNI. No changes in blood NO were detected in P. berghei infected mice, but increases were observed in brain, liver and spleen. In P. c. chabaudi infected mice, rises in NO concentration were observed in blood and spleen, whereas a decline in liver NO was seen, but there were no changes in brain. Liver contained the highest concentration of RNI, but increasing concentrations were seen in both plasma and spleen in both P. berghei and P. c. chabaudi infected mice. These results show that NO and RNI production alters during murine malaria. The changes depend upon the tissue, the day of infection, the degree of parasitaemia, the strain of Plasmodia and the method of measuring NO biosynthesis. Lethal P. berghei induced NO production in the mid and late stages of infection in mice when parasitaemia was high, whereas in nonlethal P. c. chabaudi infection, NO production was increased in the early and late stages when parasitaemia was low. These data are consistent with a role for NO in the protection of the MF1 mouse against Plasmodia. Failure to clear the parasite is associated with evidence of increased NO production in brain and liver, which may contribute to the pathology of malaria, but this hypothesis requires confirmation from other experimental approaches.

Description and epidemiology of Theileria youngi n. sp. from a northern Californian dusky-footed woodrat (Neotoma Fuscipes) population

An epidemiologic study designed to identify the small mammal reservoir for the zoonotic WA1-type babesial parasite resulted in the discovery of a small, intraerythrocytic piroplasm in smeared blood from dusky-footed woodrats (Neotoma fuscipes) in northern California. The woodrat parasites were isolated and compared to other piroplasm parasites based on their morphology, antigenicity, and genetic characteristics. These studies indicated that the woodrat parasites were not the WA1-type babesial agent but were of the genus Theileria. We accordingly named it Theileria youngi. The prevalence in the woodrat population was high (61%). Infection was unrelated to gender or age of the woodrats. Potential vectors for this tick-transmitted parasite included 3 species of ticks recovered from the woodrats. Dermacentor occidentalis, Ixodes woodi, and Ixodes pacificus. Mostly larval or nymphal stages were recovered, suggesting transstadial transmission is possible. This is the first piroplasm fully characterized from a dusky-footed woodrat.

Plasmodium yoelii: identification of rhoptry proteins using monoclonal antibodies

Thirteen monoclonal antibodies, obtained after immunization of mice with Plasmodium yoelii schizonts, were selected using immunofluorescence assay: they all presented typical fluorescence patterns of rhoptries. This antigen localization was confirmed by immunoelectron microscopy. The molecular weights of the recognized antigens are 68, 80, 105, 130 and 140 kDa as determined by immunoprecipitation and immunoblot under reducing and nonreducing conditions. These values are very similar to these of the low and high molecular weight complex components of Plasmodium falciparum. Furthermore, these antigens are soluble like P. falciparum rhoptry proteins. Interestingly, our monoclonal antibodies also reacted with two other Plasmodium species (Plasmodium berghei NKK173 strain and P. yoelii nigeriensis 798 VK strain), giving sometimes more complex labeling with apical, membranous, nuclear, or/and cytoplasmic localizations. Finally, none of the monoclonal antibodies stained the rhoptries of P. falciparum FCCE-1/Niger strain.

The complete sequence of Plasmodium berghei merozoite surface protein-1 and its inter- and intra-species variability

The complete gene for merozoite surface protein-1 (MSP-1) from Plasmodium berghei has been cloned and sequenced. Comparison of the P. berghei MSP-1 sequence with MSP-1 from other rodent parasites reveals five conserved domains interrupted by four variable blocks. These variable blocks exhibit no sequence homology but do have similar amino acid compositions. Primary proteolytic processing sites are located near the boundaries between the conserved domains and the variable blocks. Sequencing of the variable blocks from several P. berghei isolates shows that the predominant intra-species difference is in the number of tandem repeats. The inter- and intra-species differences suggest that the variable blocks are localized areas with relatively high levels of slipped-strand mispairing, unequal crossing-over, or other intragenic recombination activity. MSP-1 from P. berghei exhibits more repetitiveness than MSP-1 from other species suggesting that P. berghei experiences a higher intrinsic level of events producing variable numbers of tandem repeats or a lower level of events leading to the degeneration of tandem repeats.

Amplification of cytoadherence in cerebral malaria: towards a more rational explanation of disease pathophysiology

Cerebral malaria in man and in mice is the consequence of a cascade of events, involving the production of toxins by the parasite and cytokines by the host, and eventually leading to the amplification of the expression of the receptors for cytoadherence on brain capillary endothelial cells. Variations in the intrinsic characteristics of parasite isolates or the genetic make-up of the host and the degree of antimalarial immunity can modulate this sequence of events. A working hypothesis is proposed in which two features of the parasite, the ability to cytoadhere and to produce toxins, are clearly dissociated and where the amplification of cytoadherence receptors is considered crucial. This hypothesis, illustrated by new data from human malaria and rodent models, suggests that cerebral malaria may occur when these features occur together during an infection, while not necessarily within the same parasite clone.

The effects of chloroquine on the infectivity of chloroquine-sensitive and -resistant populations of Plasmodium yoelii nigeriensis to mosquitoes

Subtherapeutic doses of chloroquine have been reported to enhance infectivity of drug-resistant Plasmodium species to their vectors. In this investigation, Plasmodium yoelii nigeriensis N67 strain showed enhanced infectivity to mosquitoes when stimulated by chloroquine. Both sensitive and resistant clones were derived from the N67 strain by dilution, showing that this strain is polymorphic for the resistant trait. A resistant subline was derived by selection under drug pressure from a chloroquine-sensitive clone, but neither the sensitive nor the resistant clones or sublines showed enhancement of infectivity in the presence of chloroquine. This suggests that the enhancement of infectivity shown by the N67 strain is a response to chloroquine stimulation shown only by certain of the genotypes within it, and that this response to chloroquine is not a trait causally connected with the genes coding for chloroquine resistance.

Infectivity of Plasmodium berghei sporozoites measured with a DNA probe

A 2.3 kb, 32P-labeled repetitive DNA probe of Plasmodium berghei was used to measure the amount of parasite DNA in the liver of Norway Brown rats and mice infected with sporozoites. Standard hybridization curves were obtained by probing different amounts (100 pg to 1 microgram) of P. berghei DNA immobilized on nitrocellulose filters. Host DNA did not interfere with hybridization specificity and sensitivity. A 100-fold increase in hepatic parasite DNA was detected between 25 h post-infection and the peak of parasite proliferation, detected at 44 h. The amount of parasite DNA increased with the number of injected sporozoites. At 5 h post-infection, a large proportion of parasite DNA was found in the spleen. However, this diminished with time and was negligible in amount at 25 h. A significant number of viable sporozoites were probably cleared in the spleen, since considerably more parasite DNA was found in the livers of splenectomized rats than in sham-operated counterparts. Although older rats develop much lower parasitemias upon inoculation of sporozoites, no significant differences were observed in the amount of parasite DNA in rats, 43 and 152 days old, injected with equal numbers of sporozoites. The higher resistance to malaria displayed by older rats is probably controlled by post-hepatic events. The infectivity of sporozoites for A/J mice was calculated to be about 1/20th that of Norway Brown rats.

Fine structure of exoerythrocytic merozoite formation of Plasmodium berghei in rat liver

The fine structure of exoerythrocytic merogony of Plasmodium berghei was studied after perfusion-fixation of rat livers from 51 h post-inoculation onwards. Meroblast formation was effected by clefts originating from the parasite plasmalemma and by fusion of vacuoles with each other. Invaginations at the periphery resulted in labyrinthine structures providing the parasites with an enormous increase in surface area, which might facilitate exchange of metabolites. When the parasitophorous vacuole membrane collapsed, the newly formed merozoites were lying free in the hepatocytic cytoplasm, which degenerated until the merozoites were sticking together by a stroma, obviously a remnant of the host hepatocyte. Groups of merozoites, still kept together by the spongy stroma, were subsequently released in the bloodstream. At 53 h most of the developmental stages leading to the release of merozoites could be found and thereafter parasite numbers decreased while large granulomas became apparent.

Protection of athymic (Nu/Nu) BALB/c mice against Plasmodium berghei by splenocytes from normal (Nu/+) BALB/c mice

Athymic BALB/c (Nu/Nu) mice died at 7-13 days after inoculation (DAI) of Plasmodium berghei NK65, whereas their heterozygous (Nu/+) littermates died at 7-8 DAI. Nude (Nu/Nu) mice, reconstituted with 2 x 10(7) splenocytes from uninfected heterozygous (Nu/+) littermates at 20 days before parasite inoculation (DBI), died about 2 days earlier than control nude mice; nude mice reconstituted at 10 or 2 DBI lived 2 to 4 days longer than control nudes; and nude mice reconstituted 2 DAI lived even longer and some survived. These findings indicate that P. berghei NK65 induces at least two T-cell dependent immune phenomena, one suppressive and the other stimulatory. Reconstitution of nude mice with T-cells from BALB/c (Nu/+) mice appeared to reduce or bypass suppressive T-cell activities which allowed the formation of a protective immune response by some of the nude mice.

Histochemical observations on the exoerythrocytic malaria parasite Plasmodium berghei in rat liver

Enzyme histochemical methods were performed on sporozoite infected liver tissue of rats in order to gain insight into the nutrition and metabolism of exoerythrocytic forms of Plasmodium berghei. The following enzymes were demonstrated in the hepatocytic stages of the parasites, obtained 41 and 48 h after inoculation of sporozoites: acid phosphatase, cytochrome oxidase, NADH-tetrazolium reductase, succinate dehydrogenase, NAD+ and NADP+ dependent isocitrate dehydrogenase, NADP+-dependent malate dehydrogenase, lactate dehydrogenases, 6-phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenases and alpha-glycerol-phosphate dehydrogenase. The results suggest that a conventional Embden-Meyerhoff pathway, pentose phosphate pathway and Krebs' citric acid cycle may in part be present in these exoerythrocytic parasites. Alkaline phosphatase, nucleoside polyphosphatase, 5' nucleotidase, glucose-6-phosphatase, alpha-glucan phosphorylase, NAD+ dependent malate dehydrogenase, amino-peptidase M and non-specific esterases were not detected by our techniques in the parasite. The enzyme distribution of this intrahepatocytic malaria parasite revealed by histochemistry is compared with the enzyme distribution in the other phases of the parasite's life cycle.

Observations on the morphology and electrophoretic variation of enzymes of the rodent malaria parasites of Cameroon, Plasmodium yoelii, P. chabaudi and P. vinckei

Six samples of rodent blood infected with malaria parasites were isolated from Cameroon. Of these, 2 contained mixed infections of Plasmodium vinckei and P. yoelii, 3 contained P. vinckei alone and 1 P. chabaudi alone. Each isolate was cloned and the resulting lines examined for morphology of blood and mosquito forms, and for electrophoretic variation in enzymes. The P. chabaudi and P. yoelii lines were morphologically and enzymically identical to isolates of the Central African Republic. Similarly, 1 P. vinckei line was identical to an isolate of the Central African Republic. The remaining 4 P. vinckei lines showed considerable variation, some enzymes being like those in isolates of surrounding regions, while others were unique to Cameroon.

The genetic basis of diversity in malaria parasites

The principal findings of the P. falciparum surveys are given below. Considerable diversity of enzymes, antigens, drug sensitivity and other characters is seen among P. falciparum isolates. Cloning studies show that certain isolates contain mixtures of parasites which may be diverse in one or more of these characters. No obvious regional distribution is seen in the enzymic and antigenic characters examined, although differences in the frequencies of certain enzymes appear to exist. Variations in drug sensitivity are seen among parasites from different regions, the occurrence of resistant forms usually being correlated with the extent of use of the drug concerned.

An ultrastructural study of developing stages of exo-erythrocytic Plasmodium berghei in rat hepatocytes

The ultrastructure of immature exo-erythrocytic forms of Plasmodium berghei in rat hepatocytes was studied at stages between 25 and 51 h of development. A new method was successfully applied to localize the parasites in a small portion of the liver by temporary ligature of blood vessels to the majority of the liver, and from the spleen and the pancreas. Nuclear profiles appeared to be part of a highly lobed nuclear reticulum. Peripheral vesiculation and vacuolization of the cytoplasm was increasingly prominent and ushered in the formation of pseudocytomeres. Spacing between host- and parasite-derived membranes could first be observed after 43 h of development. In general, organellar development was found to follow closely that described for plasmodial oocysts.

Identification of economically important parasites

The taxonomy of a parasite can be an important guide to its pathogenic characteristics. A wide range of anatomical, biochemical and behavioural tests is now being developed to define different strains and subspecies of the main tropical parasites.

Studies on enzyme variation in the murine malaria parasites Plasmodium berghei, P. yoelii, P. vinckei and P. chabaudi by starch gel electrophoresis

Electrophoretic variation of the enzymes glucose phosphate isomerase, 6-phosphogluconate dehydrogenase, lactate dehydrogenase and glutamate dehydrogenase (NADP-dependent) has been studied in the African murine malaria parasites Plasmodium berghei, P. yoelii, P. vinckei and P. chabaudi and their subspecies. Horizontal starch gel electrophoresis was used throughout. The number of isolates examined in each subspecies varied from 1 (P. y. nigeriensis) to 24 (P. c. chabaudi). Extensive enzyme variation was found among isolates of most of the subspecies from which more than two such isolates were available for study. It is clear that the phenomenon of enzyme polymorphism is of common occurrence among malaria parasites. With the exception of P. berghei and P. yoelii, of which all isolates share an identical electrophoretic form of lactate dehydrogenase, no enzyme forms are shared between any of the 4 species of murine plasmodia. By contrast, within each species common enzyme forms are shared among each of the subspecies. The subspecies are nevertheless, distinguished from each other by the electrophoretic forms of at least one enzyme. The distribution and reassortment of enzyme variation among isolates of a single subspecies is in accordance with the concept of malaria parasites as sexually reproducing organisms. The study of variation among parasites present in individual wild-caught rodent hosts demonstrates that natural malarial infections usually comprise genetically heterogeneous populations of parasites. Nevertheless, the number of genetically distinct types of parasite of any one species present in a single infected host appears to be small. Generally not more than 2 or 3 clones of parasite of distinct genetic constitution are present in a single infected animal.

Influence of rodent malaria on the course of Leishmania enriettii infection of hamsters

Plasmodium yoelii infection was established in hamsters, and the effect of this type of malaria on concurrent Leishmania enriettii infection was examined. It was found that the course of the L. enriettii infection was affected by P. yoelii and that this effect depended on the relative timing of the two infections. A chronic malarial infection with Plasmodium berghei was also established in hamsters, and this was found to affect the course of a concurrent L. enriettii infection in a similar manner to P. yoelii. These results are discussed in relation to current knowledge of the immunosuppressive effects of plasmodia.

A genetic investigation of virulence in a rodent malaria parasite

The genetic basis of virulence in a line (YM) of Plasmodium yoelii yoelii was investigated in a cross with a mild line (A/C). The blood forms of the virulent line developed extensively in mature erythrocytes of mice, causing death of the host within 7 days; infections with the mild line were normally restricted to reticulocytes, infected animals recovering after three weeks. Lines YM and A/C differed additionally in enzyme and drug-sensitivity markers. Studies on infections established from each line alone from sporozoite mixtures of the two lines and from the cross between the lines showed that the appearance of virulence had been caused by a genetic change in the parasite, and not by other factors such as a concurrent infection with another organism. An analysis of the characters of 56 clones derived from the cross showed that the virulence character had undergone recombination with the other markers, and appeared to be inherited in Mendelian fashion. Three clones exhibited atypical virulence, although it was not clear whether this had been produced by genetic recombination.

New observations on the malaria parasites of rodents of the Central African Republic - Plasmodium vinckei petteri subsp. nov. and Plasmodium chabaudi Landau, 1965

The morphology and enzyme forms of malaria parasites isolated from 50 wild caught specimens of Thamnomys rutilans from the Central African Republic have been studied and three distinct species of Plasmodium identified. One species has been confirmed as Plasmodium yoelii yoelii. Morphological features of both the remaining species correspond to those given by Landau (1965) in her original description of Plasmodium chabaudi. For one of these species the name P. chabaudi has been retained; the other species, which closely resembles subspecies of Plasmodium vinckei from other regions of Africa, has been designated Plasmodium vinckei petteri subsp. nov. P. chabaudi and P.v. petteri are distinguished from each other both in their morphology in the blood and sporogonic stages, and in their enzyme forms.