A study on pathogenicity and mosquito transmission success in the rodent malaria parasite Plasmodium chabaudi adami

We investigated the parasitology, pathogenicity (virulence) and infectivity to mosquitoes of blood infections in mice, of two strains, DS and DK, of the rodent malaria parasite Plasmodium chabaudi adami. Blood infections of DS were found to be highly pathogenic; the asexual parasites in these infections were fast-growing and showed no evidence of selectivity in their infection of host erythrocytes. In contrast to DS, blood infections of DK were much less pathogenic; the asexual parasites were slower-growing and showed a moderate degree of selectivity to a subset of erythrocytes which were not reticulocytes. In both DS and DK infections, infectivity to mosquitoes was highest before the peak of asexual parasitaemia had occurred; usually this did not coincide with the time when gametocyte numbers in the blood were highest. Infections with the pathogenic DS strain in CBA mice produced fewer gametocytes than did the less pathogenic DK strain. The DS strain infections in both CBA and C57 mice were also significantly much less infective to mosquitoes than the DK strain. Investigations by others on the related rodent malaria parasite subspecies, Plasmodium chabaudi chabaudi, have indicated that the mosquito infectivity of blood infections in mice tended to be higher in the more pathogenic (virulent) and lower in the less pathogenic strains of this parasite subspecies. This is the converse of the finding of the present investigation of blood infections of P. c. adami in mice in which a more pathogenic, or virulent, strain (DS) of these parasites was significantly much less infective to mosquitoes than was a less pathogenic strain (DK).     

Understanding and predicting strain-specific patterns of pathogenesis in the rodent malaria Plasmodium chabaudi

Despite considerable success elucidating important immunological and resource-based mechanisms that control the dynamics of infection in some diseases, little is known about how differences in these mechanisms result in strain differences in patterns of pathogenesis. Using a combination of data and theory, we disentangle the role of ecological factors (e.g., resource abundance) in the dynamics of pathogenesis for the malaria species Plasmodium chabaudi in CD4+ T cell-depleted mice. We build a series of nested models to systematically test a number of potential regulatory mechanisms and determine the "best" model using statistical techniques. The best-fit model is further tested using an independent data set from mixed-clone competition experiments. We find that parasites preferentially invade older red blood cells even when they are more fecund in younger reticulocytes and that inoculum size has a strong effect on burst size in reticulocytes. Importantly, the results suggest that strain-specific differences in virulence arise from differences in red blood cell age-specific invasion rates and burst sizes, since these are lower for the less virulent strain, as well as from differences in levels of erythropoesis induced by each strain. Our analyses highlight the importance of model selection and validation for revealing new biological insights.     

On the contribution of the rodent model Plasmodium chabaudi for understanding the genetics of drug resistance in malaria

Malaria is a devastating disease that still claims over half a million lives every year, mostly in sub–Saharan Africa. One of the main barriers to malaria control is the evolution and propagation of drug-resistant mutant parasites. Knowing the genes and respective mutations responsible for drug resistance facilitates the design of drugs with novel modes of action and allows predicting and monitoring drug resistance in natural parasite populations in real-time. The best way to identify these mutations is to experimentally evolve resistance to the drug in question and then comparing the genomes of the drug-resistant mutants to that of the sensitive progenitor parasites. This simple evolutive concept was the starting point for the development of a paradigm over the years, based on the use of the rodent malaria parasite Plasmodium chabaudi to unravel the genetics of drug resistance in malaria. It involves the use of a cloned parasite isolate (P. chabaudi AS) whose genome is well characterized, to artificially select resistance to given drugs through serial passages in mice under slowly increasing drug pressure. The end resulting parasites are cloned and the genetic mutations are then discovered through Linkage Group Selection, a technique conceived by Prof. Richard Carter and his group, and/or Whole Genome Sequencing. The precise role of these mutations can then be interrogated in malaria parasites of humans through allelic replacement experiments and/or genotype-phenotype association studies in natural parasite populations. Using this paradigm, all the mutations underlying resistance to the most important antimalarial drugs were identified, most of which were pioneering and later shown to also play a role in drug resistance in natural infections of human malaria parasites. This supports the use of P. chabaudi a fast-track predictive model to identify candidate genetic markers of resistance to present and future antimalarial drugs and improving our understanding of the biology of resistance.     

Facilitation through altered resource availability in a mixed‐species rodent malaria infection

A major challenge in disease ecology is to understand how co‐infecting parasite species interact. We manipulate in vivo resources and immunity to explain interactions between two rodent malaria parasites, Plasmodium chabaudi and P. yoelii. These species have analogous resource‐use strategies to the human parasites Plasmodium falciparum and P. vivax: P. chabaudi and P. falciparum infect red blood cells (RBC) of all ages (RBC generalist); P. yoelii and P. vivax preferentially infect young RBCs (RBC specialist). We find that: (1) recent infection with the RBC generalist facilitates the RBC specialist (P. yoelii density is enhanced ~10 fold). This occurs because the RBC generalist increases availability of the RBC specialist's preferred resource; (2) co‐infections with the RBC generalist and RBC specialist are highly virulent; (3) and the presence of an RBC generalist in a host population can increase the prevalence of an RBC specialist. Thus, we show that resources shape how parasite species interact and have epidemiological consequences.     

Virulence genotypes and drug resistance of Helicobacter pylori from Vladivostok, Russia: another feature in the Far East

Helicobacter pylori in Vladivostok, Far Eastern Russia, was investigated during 2004 to 2009. The genotype cagA(+) vacA(+) (s1/m1 or m2) accounted for 74.7%, with cagA(-) vacA(+) (s2/m2) at 11.2%. The CagA EPIYA type was mainly Western ABC, with minor types (ABCCC and novel AAABC) or non-Western/non-East Asia type (AB). Regarding drug resistance, metronidazole resistance was the highest, with a marked decrease in 6 years (from 71.4% to 30.8%); in contrast, levofloxacin and clarithromycin resistance increased. The data indicate that in Vladivostok, H. pylori was mainly the Western (not East Asian) type and dynamic changes in drug resistance occurred during 6 years.     

Within-host competition and drug resistance in the human malaria parasite Plasmodium falciparum

Infections with the malaria parasite Plasmodium falciparum typically comprise multiple strains, especially in high-transmission areas where infectious mosquito bites occur frequently. However, little is known about the dynamics of mixed-strain infections, particularly whether strains sharing a host compete or grow independently. Competition between drug-sensitive and drug-resistant strains, if it occurs, could be a crucial determinant of the spread of resistance. We analysed 1341 P. falciparum infections in children from Angola, Ghana and Tanzania and found compelling evidence for competition in mixed-strain infections: overall parasite density did not increase with additional strains, and densities of individual chloroquine-sensitive (CQS) and chloroquine-resistant (CQR) strains were reduced in the presence of competitors. We also found that CQR strains exhibited low densities compared with CQS strains (in the absence of chloroquine), which may underlie observed declines of chloroquine resistance in many countries following retirement of chloroquine as a first-line therapy. Our observations support a key role for within-host competition in the evolution of drug-resistant malaria. Malaria control and resistance-management efforts in high-transmission regions may be significantly aided or hindered by the effects of competition in mixed-strain infections. Consideration of within-host dynamics may spur development of novel strategies to minimize resistance while maximizing the benefits of control measures.     

The fitness of drug-resistant malaria parasites in a rodent model: multiplicity of infection

Malaria infections normally consist of more than one clonally replicating lineage. Within-host interactions between sensitive and resistant parasites can have profound effects on the evolution of drug resistance. Here, using the Plasmodium chabaudi mouse malaria model, we ask whether the costs and benefits of resistance are affected by the number of co-infecting strains competing with a resistant clone. We found strong competitive suppression of resistant parasites in untreated infections and marked competitive release following treatment. The magnitude of competitive suppression depended on competitor identity. However, there was no overall effect of the diversity of susceptible parasites on the extent of competitive suppression or release. If these findings generalize, then transmission intensity will impact on resistance evolution because of its effect on the frequency of mixed infections, not because of its effect on the distribution of clones per host. This would greatly simplify the computational problems of adequately capturing within-host ecology in models of drug resistance evolution in malaria.     

Virulence not only costs but also benefits the transmission of a fungal virus

Current theory suggests that cost-benefit relationships govern the evolution of parasite virulence. The cost of virulence is expected to be high for fungal viruses, which are obligate parasites and completely dependent on their hosts. The majority of fungal viruses infect their hosts without any apparent symptoms. Cryphonectria hypovirus 1 (CHV-1), in contrast, is virulent and debilitates its host, Cryphonectria parasitica. However, the virulence of CHV-1 is associated with high costs for virus transmission, such as an attenuated fungal growth and reduced production of the fungal spores spreading the virus. In this study, we tested the hypothesis that virulence may not only have costs but also benefits for transmitting CHV-1 across vegetative incompatibility barriers between fungi. We investigated viruses with low, medium, and high virulence, and determined their transmission rate per host-to-host contact (transmissibility). The average transmission rate across all combinations tested was 53% for the most virulent virus, 37% for the virus with intermediate virulence, and 20% for the virus with lowest virulence. These results showed that increased virulence was strongly correlated with increased transmissibility, potentially counterbalancing virulence costs. This association of virulence and transmissibility may explain why CHV-1 spread widely and evolved higher virulence than most other fungal viruses.     

Within-host competition in genetically diverse malaria infections: parasite virulence and competitive success

Humans and animals often become coinfected with pathogen strains that differ in virulence. The ensuing interaction between these strains can, in theory, be a major determinant of the direction of selection on virulence genes in pathogen populations. Many mathematical analyses of this assume that virulent pathogen lineages have a competitive advantage within coinfected hosts and thus predict that pathogens will evolve to become more virulent where genetically diverse infections are common. Although the implications of these studies are relevant to both fundamental biology and medical science, direct empirical tests for relationships between virulence and competitive ability are lacking. Here we use newly developed strain-specific real-time quantitative polymerase chain reaction protocols to determine the pairwise competitiveness of genetically divergent Plasmodium chabaudi clones that represent a wide range of innate virulences in their rodent host. We found that even against their background of widely varying genotypic and antigenic properties, virulent clones had a competitive advantage in the acute phase of mixed infections. The more virulent a clone was relative to its competitor, the less it suffered from competition. This result confirms our earlier work with parasite lines derived from a single clonal lineage by serial passage and supports the virulence-competitive ability assumption of many theoretical models. To the extent that our rodent model captures the essence of the natural history of malaria parasites, public health interventions which reduce the incidence of mixed malaria infections should have beneficial consequences by reducing the selection for high virulence.     

Parasite transmission modes and the evolution of virulence

A mathematical model is presented that explores the relationship between transmission patterns and the evolution of virulence for horizontally transmitted parasites when only a single parasite strain can infect each host. The model is constructed by decomposing parasite transmission into two processes, the rate of contact between hosts and the probability of transmission per contact. These transmission rate components, as well as the total parasite mortality rate, are allowed to vary over the course of an infection. A general evolutionarily stable condition is presented that partitions the effects of virulence on parasite fitness into three components: fecundity benefits, mortality costs, and morbidity costs. This extension of previous theory allows us to explore the evolutionary consequences of a variety of transmission patterns. I then focus attention on a special case in which the parasite density remains approximately constant during an infection, and I demonstrate two important ways in which transmission modes can affect virulence evolution: by imposing different morbidity costs on the parasite and by altering the scheduling of parasite reproduction during an infection. Both are illustrated with examples, including one that examines the hypothesis that vector-borne parasites should be more virulent than non-vector-borne parasites (Ewald 1994). The validity of this hypothesis depends upon the way in which these two effects interact, and it need not hold in general.     

Variation in relapse frequency and the transmission potential of Plasmodium vivax malaria

There is substantial variation in the relapse frequency of Plasmodium vivax malaria, with fast-relapsing strains in tropical areas, and slow-relapsing strains in temperate areas with seasonal transmission. We hypothesize that much of the phenotypic diversity in P. vivax relapses arises from selection of relapse frequency to optimize transmission potential in a given environment, in a process similar to the virulence trade-off hypothesis. We develop mathematical models of P. vivax transmission and calculate the basic reproduction number R₀ to investigate how transmission potential varies with relapse frequency and seasonality. In tropical zones with year-round transmission, transmission potential is optimized at intermediate relapse frequencies of two to three months: slower-relapsing strains increase the opportunity for onward transmission to mosquitoes, but also increase the risk of being outcompeted by faster-relapsing strains. Seasonality is an important driver of relapse frequency for temperate strains, with the time to first relapse predicted to be six to nine months, coinciding with the duration between seasonal transmission peaks. We predict that there is a threshold degree of seasonality, below which fast-relapsing tropical strains are selected for, and above which slow-relapsing temperate strains dominate, providing an explanation for the observed global distribution of relapse phenotypes.     

The effects of mosquito transmission and population bottlenecking on virulence, multiplication rate and rosetting in rodent malaria

Malaria parasites vary in virulence, but the effects of mosquito transmission on virulence phenotypes have not been systematically analysed. Using six lines of malaria parasite that varied widely in virulence, three of which had been serially blood-stage passaged many times, we found that mosquito transmission led to a general reduction in malaria virulence. Despite that, the between-line variation in virulence remained. Forcing serially passaged lines through extreme population bottlenecks (<5 parasites) reduced virulence in only one of two lines. That reduction was to a level intermediate between that of the virulent parental and avirulent ancestral line. Mosquito transmission did not reverse the increased parasite replication rates that had accrued during serial passage, but it did increase rosetting frequencies. Re-setting of asexual stage genes during the sexual stages of the life cycle, coupled with stochastic sampling of parasites with variable virulence during population bottlenecks, could account for the virulence reductions and increased rosetting induced by mosquito transmission.     

Plasmodium berghei: histology, immunocytochemistry, and ultrastructure of the placenta in rodent malaria

The pathological changes associated with malarial infection in pregnancy were studied in rats and mice infected with Plasmodium berghei at different stages of gestation. Histopathological and ultrastructural studies of infected placentae near term in both species revealed disruption of architecture with gross thickening and necrosis of cells in the labyrinthine zone and fibrosis of the trilaminar trophoblast separating the maternal and fetal circulations. In the mouse, the extent of histopathological alterations in infected placentae ranged from the presence of immature erythrocytes in the fetal circulation in low grade maternal infection, to the marked deposition of fibrinoid material on the trilaminar trophoblast and inflammatory masses in severely infected placentae. In the rat, histopathological aberrations in the placentae were marked by placental stroma edema, fibrosis, and cellular infiltration. Immunohistological studies of cryostat sections of placentae from infected animals showed more parasites and pigment in infected mouse placentae than in the corresponding rat organ, but in both species parasites and pigment were largely confined to the maternal blood spaces and were only occasionally found in necrotic areas of trophoblast. No clear differences were observed between infected and control placentae in terms of the amount of IgG, IgM, or IgA which were each present in various amounts. These observations and the rarity of congenital malaria in the animals indicate that the placenta constitutes a major barrier to infection of the fetus. However, the pathological aberrations in the infected placentae may impose a biochemical stress upon the fetus which may account for the low birthweight, the increased frequency of abortion, and the greatly increased maternal and fetal death rates observed in malaria.     

Virulence evolution and the trade‐off hypothesis: history,current state of affairs and the future

It has been more than two decades since the formulation of the so‐called ‘trade‐off’ hypothesis as an alternative to the then commonly accepted idea that parasites should always evolve towards avirulence (the ‘avirulence hypothesis’). The trade‐off hypothesis states that virulence is an unavoidable consequence of parasite transmission; however, since the 1990s, this hypothesis has been increasingly challenged. We discuss the history of the study of virulence evolution and the development of theories towards the trade‐off hypothesis in order to illustrate the context of the debate. We investigate the arguments raised against the trade‐off hypothesis and argue that trade‐offs exist, but may not be of the simple form that is usually assumed, involving other mechanisms (and life‐history traits) than those originally considered. Many processes such as pathogen adaptation to within‐host competition, interactions with the immune system and shifting transmission routes, will all be interrelated making sweeping evolutionary predictions harder to obtain. We argue that this is the heart of the current debate in the field and while species‐specific models may be better predictive tools, the trade‐off hypothesis and its basic extensions are necessary to assess the qualitative impacts of virulence management strategies.     

Endocrine phenotype,reproductive success and survival in the great tit,Parus major

A central goal in evolutionary ecology is to characterize and identify selection patterns on the optimal phenotype in different environments. Physiological traits, such as hormonal responses, provide important mechanisms by which individuals can adapt to fluctuating environmental conditions. It is therefore expected that selection shapes hormonal traits, but the strength and the direction of selection on plastic hormonal signals are still under investigation. Here, we determined whether, and in which way, selection is acting on the hormones corticosterone and prolactin by characterizing endocrine phenotypes and their relationship with fitness in free‐living great tits, Parus major. We quantified variation in circulating concentrations of baseline and stress‐induced corticosterone and in prolactin during the prebreeding (March) and the breeding season (May) for two consecutive years, and correlated these with reproductive success (yearly fledgling number) and overwinter survival in female and male individuals. In both years, individuals with high baseline corticosterone concentrations in March had the highest yearly fledgling numbers; while in May, individuals with low baseline corticosterone had the highest yearly reproductive success. Likewise, individuals that displayed strong seasonal plasticity in baseline corticosterone concentrations (high in March and low in May) had the highest reproductive success in each year. Prolactin concentrations were not related to reproductive success, but were positively correlated to the proximity to lay. Between‐year plasticity in stress‐induced corticosterone concentrations of males was related to yearly variation in food abundance, but not to overall reproductive success. These findings suggest that seasonally alternating directional selection is operating on baseline corticosterone concentrations in both sexes. The observed between‐year consistency in selection patterns indicates that a one‐time hormone sample in a given season can allow the prediction of individual fitness.     

Antimicrobial Resistance,Virulence Factor-Encoding Genes,and Biofilm-Forming Ability of Community-Associated Uropathogenic Escherichia Coli in Western Saudi Arabia

To explore the prevalence of multidrug-resistant community-associated uropathogenic Escherichia coli (UPEC) and their virulence factors in Western Saudi Arabia. A total of 1,000 urine samples were examined for the presence of E. coli by selective plating on MacConkey, CLED, and sheep blood agar. Antimicrobial susceptibility patterns were determined using Vitek^® 2 Compact (MIC) and the disc diffusion method with Mueller-Hinton agar. Genes encoding virulence factors (kpsMTII, traT, sat, csgA, vat, and iutA) were detected by PCR. The overall prevalence of UTI-associated E. coli was low, and a higher prevalence was detected in samples of female origin. Many of the isolates exhibited resistance to norfloxacin, and 60% of the isolates showed resistance to ampicillin. No resistance to imipenem, meropenem, or ertapenem was detected. In general, half of the isolates showed multiple resistance patterns. UPEC exhibited a weak ability to form biofilms, where no correlation was observed between multidrug resistance and biofilm-forming ability. All uropathogenic E. coli isolates carried the kpsMTII, iutA, traT, and csgA genes, whereas the low number of the isolates harbored the sat and vat genes. The diversity of virulence factors harbored by community-associated UPEC may render them more virulent and further explain the recurrence/relapse cases among community-associated UITs. To the best of our knowledge, this study constitutes the first exploration of virulence, biofilm-forming ability, and its association with multidrug resistance among UPEC isolates in Saudi Arabia. Further investigations are needed to elucidate the epidemiology of community-associated UPEC in Saudi Arabia. Open in a separate window     

MHC diversity, malaria and lifetime reproductive success in collared flycatchers

Major histocompatibility complex (MHC) genes encode proteins involved in the recognition of parasite-derived antigens. Their extreme polymorphism is presumed to be driven by co-evolution with parasites. Host-parasite co-evolution was also hypothesized to optimize within-individual MHC diversity at the intermediate level. Here, we use unique data on lifetime reproductive success (LRS) of female collared flycatchers to test whether LRS is associated with within-individual MHC class II diversity. We also examined the association between MHC and infection with avian malaria. Using 454 sequencing, we found that individual flycatchers carry between 3 and 23 functional MHC class II B alleles. Predictions of the optimality hypothesis were not confirmed by our data as the prevalence of blood parasites decreased with functional MHC diversity. Furthermore, we did not find evidence for an association between MHC diversity and LRS.