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PubMed1998

Nonmanipulative parasites in manipulated hosts: 'hitch-hikers' or simply 'lucky passengers'?

Manipulation of intermediate host behavior to favor parasitic transmission has been demonstrated in a wide of range of parasitic taxa. Recent advances in parasitology have suggested that nonmanipulative parasite species can obtain a high probability of transmission simply by infecting hosts already manipulated ('hitch-hiker' parasites). In this study, from a field collection of Gammarus aequicauda (Amphipoda, second intermediate host), we analyzed the ecological association between the manipulative trematode Microphallus papillorobustus and the 2 nonmanipulative trematodes Microphallus hoffmanni and Levinseniella tridigitata. Although these 2 nonmanipulative parasites should be a priori advantaged when infecting manipulated gammarids, there was no significant ecological association between parasite species. We discuss the possible reasons why these 2 nonmanipulative parasites are only 'lucky passengers' rather than 'hitch-hikers.'

Animals↗
PubMed1994

The evolution of virulence in parasites and pathogens: reconciliation between two competing hypotheses.

According to conventional wisdom, parasites and pathogens should evolve reduced virulence to their hosts, because more virulent parasites and pathogens are more likely to drive their hosts, and themselves, to extinction. But this view has been criticized for its reliance on group selection. According to an alternative perspective, selection will favor whatever level of virulence maximizes the rate of increase of the parasite or pathogen. This optimum virulence depends on the functional relationship between a parasite or pathogen's transmissibility and its effect on host mortality, with selection often favoring an intermediate degree of virulence. The thesis of this paper is that models in which intermediate levels of virulence are favored lead quite naturally to the further conclusion that parasites and pathogens should-up to a point-become less virulent over time, once the feedbacks between ecological and evolutionary processes are incorporated into the analysis. As a consequence of successive adaptations by the parasite or pathogen, the density of susceptible hosts is reduced, thereby altering the balance between selective forces so as to favor reduced virulence. However, the evolutionarily stable strategy that is achieved is bounded away from complete avirulence. We conclude that models in which intermediate virulence is favored do not necessarily contradict the conventional wisdom in the long run; in fact, these models provide a simple mechanistic explanation for the evolution of reduced virulence.

Animals↗
PubMed2003

Deterministic extinction effect of parasites on host populations.

Experimental studies have shown that parasites can reduce host density and even drive host population to extinction. Conventional mathematical models for parasite-host interactions, while can address the host density reduction scenario, fail to explain such deterministic extinction phenomena. In order to understand the parasite induced host extinction, Ebert et al. (2000) formulated a plausible but ad hoc epidemiological microparasite model and its stochastic variation. The deterministic model, resembles a simple SI type model, predicts the existence of a globally attractive positive steady state. Their simulation of the stochastic model indicates that extinction of host is a likely outcome in some parameter regions. A careful examination of their ad hoc model suggests an alternative and plausible model assumption. With this modification, we show that the revised parasite-host model can exhibit the observed parasite induced host extinction. This finding strengthens and complements that of Ebert et al. (2000), since all continuous models are likely break down when all population densities are small. This extinction dynamics resembles that of ratio-dependent predator-prey models. We report here a complete global study of the revised parasite-host model. Biological implications and limitations of our findings are also presented.

Animals↗
PubMed2000

Parasite communities of the Salzhaff (Northwest Mecklenburg, Baltic Sea) II. Guild communities, with special regard to snails, benthic crustaceans, and small-sized fish.

Metazoan parasites of guilds of benthic snails and crustaceans and of four fish families--Gobiidae, Gasterosteidae, Syngnathidae, and Zoarcidae--were investigated off the brackish Salzhaff area (Southwest Baltic) in the semienclosed Salzhaff and the near Rerik Riff in the free Baltic. Comparisons revealed greater similarities in parasite populations and communities within the fish guilds than between them. According to an evaluation of the core-/satellite-species concept using abundance values, the most important parasites of fish were some generalists, such as Cryptocotyle spp., Podocotyle atomon, and Diplostomum spathaceum, as well as some specialists, such as Acanthostomum balthicum, Thersitina gasterostei, and Aphalloides timmi. These specialists revealed high degrees of prevalence in their main hosts and lower degrees in one or two by-hosts. Additional importance is assigned to parasites that cause harm to their hosts due to their large size, e.g., Schistocephalus spp., or via massive infestation, e.g., several digenean metacercariae. Because specialists were more prominent in snails and fish from the Rerik Riff, the correlation of host numbers with prevalence resulted in only a slight increase instead of a more rapid rise in regression among crustaceans and fish from the entire Salzhaff, where the generalists were more prevalent. The selected host guilds demonstrated the entire life cycles of three digeneans (P. atomon, A. balthicum, A. timmi), one acanthocephalan (Echinorhynchus gadi), and one nematode (Hysterothylacium sp.). The prevalence increased in these cycles from host level to host level and attained relatively high values in all guilds. The parasite fauna of the Salzhaff area is influenced by eutrophication stress, which leads to a high level of productivity and, consequently, to great densities in primary consumers such as snails and crustaceans. These are attractive for several secondary consumers such as fish and birds, which is the reason for the existence of at least 24 autogenic and 20 allogenic parasite species at this locality. The slight surplus of the first category indicates a yet-balanced environment in the investigation area.

Animals↗
PubMed2006

Diagnostic methods for parasitic infections in livestock.

Parasitic infections are a primary cause of lost productivity in livestock world-wide. Accurate detection of parasites depends on many factors, including collection, storage, and transport of the sample, as well as the method of laboratory evaluation. However, the presence of a particular parasite does not always indicate the presence of parasitic disease. For many parasites, there exists a level at which the effect on production characteristics is balanced by the effect on the development of immunity. Interpretation of test results, therefore, should also consider such factors as the age of the animal or animals, clinical history, nutrition, local epidemiology of the parasites prevalent in the area, and any treatments that have been implemented.

Animals↗
PubMed2003

Exploiting natural immunity to helminth parasites for the development of veterinary vaccines.

The development of subunit vaccines against most parasitic helminth infections will require a better understanding of the different components of a natural rejection process including (1) recognition of parasite antigens; (2) induction of protective immune response phenotypes; and (3) activation of appropriate immune effector mechanisms. While novel technologies have allowed significant progress to be made in the identification of candidate vaccine antigens, the large scale production of these antigens and their presentation to the host with appropriate adjuvant systems remains a major problem in vaccine research. Identification of the molecular interactions involved in the innate immune response to helminth infections and the application of new genomic and proteomic technologies are likely to lead to major advances in these research fields. Gastrointestinal nematode parasites and liver fluke are the most important helminth parasites of production animals. In recent years, a lot of new knowledge has been gathered on the immunobiology of the host-parasite interactions in these two infection systems, which has allowed new vaccination strategies to be considered. Functional genomic technologies such as gene expression analysis by microarrays, promise to further advance our understanding of the molecular pathways leading to protection against parasite infections. This will not only have implications for vaccine research, but also provide novel targets for drug development and genetic selection.

Animals↗
PubMed1990

A model of dynamic behavior of a parasite species assemblage.

A BASIC computer simulation model was constructed to mimic the dynamic behavior of an assemblage of parasite species over a range of abiotic conditions. Computer hosts sampled a parasite supra-assemblage consisting of seven parasite types which differed in their relative probabilities of infection. In this model, it was possible for the hosts to unsuccessfully sample the supra-assemblage, as must happen in nature. Parasite population aggregation, competition, and co-occurrence were also simulated. A single simulation run produced a sample of 24 host individuals with their respective parasite assemblages. Species density, mean infra-assemblage diversity, sample assemblage diversity, and mean prevalence were used as descriptors of assemblage structure. The model data were compared to a field data set consisting of 33 samples of the fish, Fundulus zebrinus, and its parasites, taken during a 7 year period from a single location in a fluctuating river. The results suggested that over the long term, field data did not depart significantly from that predicted by a null model. Results also suggested that if aggregation and co-occurrence operated to influence assemblage structure, then the correlation between sample assemblage diversity values, and those of the other descriptors (species density, infra-assemblage diversity, and mean prevalence) should be reduced. Competition had no effect on the long term dynamics of the computer assemblage.

Animals↗
PubMed1997

Parasite immune evasion and exploitation: reflections and projections.

Recent developments in parasite immune evasion and exploitation are reviewed with special reference to the papers presented in this volume. Parasites, broadly defined, of animals with good immune responses have evolved many strategies that adapt them to survive and reproduce. These strategies may be passive, or may involve active intervention with host immune regulation, and can be categorized as immune evasion, immune exploitation and molecular piracy. The concept of immune evasion began with Paul Ehrlich's demonstration of antigenic variation in African trypanosomes and was reinforced by later ideas on molecular mimicry. Molecular mimicry is updated in the light of recent discoveries about degeneracy and plasticity of TCR/MHC-peptide recognition. Possible connections between two of its postulated consequences, evasion and autoimmunity, are discussed. Another putative consequence of molecular mimicry, host antigenic polymorphism, is also updated. The concept of exploitation of host immune responses by parasites has been reinforced by new data on its first known examples, especially the immune dependence of schistosome egg excretion. Newer examples include use of host cytokines as parasite growth factors, virokines, viroreceptors and helminth pseudocytokines. Finally, questions of host gene capture by viruses and possible horizontal gene transfer between host and parasite mediated by retroviruses are examined. The latter is compared with molecular conservation as a source of molecular mimicry and other aspects of host--parasite coevolution.

Animals↗
PubMed2003

Parasitic exploitation as an engine of diversity.

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant-pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Animals↗
PubMed1995

Coinfection and the evolution of parasite virulence.

Analyses of the selection pressures acting on parasite virulence are made more complicated when individual hosts can simultaneously harbour many different strains or genotypes of a parasite. Here we explore the evolutionary dynamics of host-parasite associations in which individual hosts can be coinfected with many different parasite strains. (We take coinfection to mean that each strain transmits at a rate unaffected by the presence of others in the same host.) This study thus represents the opposite extreme to our earlier work on superinfection in which there is a dominance hierarchy such that only the most virulent strain present in a host is transmitted. For highly diverse populations of parasite strains, we find that such coinfection leads to selection for strains whose virulence-levels lie in a relatively narrow band close to the maximum consistent with the parasite's basic preproductive ratio, R0, exceeding unity.

Animals↗
PubMed2005

Evolution of virulence: coinfection and propagule production in spore-producing parasites.

BACKGROUND: The evolution of within-host growth rates by parasites is expected to depend on a trade-off between propagule production and virulence. The presence of coinfections, however, is thought to alter this trade-off, and hence alter the evolutionarily stable strategy (ESS) for the parasite. Here I consider a model wherein the number of coinfections that are identical by descent can depend on the parasite's reproductive strategy. Transmission success was treated as being either a negative-linear or a negative-exponential function of the total number of propagules produced by all coinfections. RESULTS: Increasing the number of unrelated coinfections either selected for a decrease in reproductive output by the parasite (linear case), or had no effect on the ESS (exponential case). Nonetheless, the total number of propagules produced within each host increased in both cases. Increasing the relatedness among coinfections, however, selected for reductions in parasite reproduction in both cases. CONCLUSION: Unrelated coinfection may increase overall parasite virulence, but the result stems from adding more infections rather than to more aggressive growth by the individual infections. However, all else being equal, if the coinfections are more related than expected by chance alone, then the total reproductive output by all coinfections would be expected to be reduced, resulting in reduced virulence.

Adaptation, Biological↗
PubMed2000

The prevalence of internal and external parasites in pigs of different ages and sexes in Southeast District, Botswana.

Botswana imports most pig-based products from neighbouring countries. Pig farming is limited by, among other things, the negative effect of parasites and diseases on production. The object of this study was to determine the prevalence of ecto- and endoparasites in pigs of different ages and sexes in the Southeast District of Botswana. Thirty-nine pigs were sampled for endoparasites and 19 for ectoparasites during a period of 2 1/2 months. Of all the pigs sampled, 54,55% were infected with Ascaris suum, 20,45% with Trichostrongylus spp. and 6,82% with Trichuris suis. Ascaris suum was found to be the most common endoparasite infesting both mature, i.e. 12 months and older, and young, i.e. less than 12 months old, pigs. Although not significantly different (P > 0,05), the prevalence of this parasite species was slightly higher (68,42% with an average of 1,023 +/- 545 eggs per gram (EPG) of faeces per pig) in mature than in young pigs (55% with an of average 1,500 +/- 846 EPG of faeces per pig). The prevalence of Trichostrongylus spp. was lower in mature (5,26% with 20 +/- 14 EPG of faeces per pig) than in young pigs (25% with 22 +/- 9 EPG of faeces per pig). The prevalence of T. suis was also lower in mature (0% infection) than in young pigs (15% with 9 +/- 4 EPG of faeces per pig). The prevalence of the three endoparasite species was not significantly different between the sexes A. suum (1,020 +/- 883 v. 1,503 +/- 522 EPG of faeces per pig), Trichostrongylus spp. (24 +/- 14 v. 18 +/- 8 EPG of faeces per pig) and T. suis (11 +/- 6 v. 2 +/- 4 EPG of faeces per pig) for male and female pigs respectively. Sarcoptes scabiei was the only ectoparasite identified on the pigs sampled for external parasites. It infested 40% of all pigs but the infestation on young pigs (70%) was higher than on the mature ones (33,33%). Since the infection of internal and external parasites was similar in young and old pigs of both sexes, controlling parasites is of great importance since these generally lead to reduced production and are also of public health concern. It is recommended that a further study be carried out to investigate the effect of internal and external parasites on productivity.

Age Factors↗
PubMed1980

[The problem of parasites in game animals in Austria].

The parasitical diseases are among the game diseases the most wide spread. Since the interest of keeping game healthy is of great universal importance, the problem of the prevention of serious losses caused by parasites arises. Today this is in many cases possible by the exploration of all present prophylactical and therapeutical avenues. The presupposition in this respect is the thorough knowledge of parasite and host. Only then antiparasitical drugs can be employed at the right time. Furthermore the biotop, the agricultural and the forestry conditions have to be considered. Although population losses caused by parasites can be very heavy, a game population can never be decimated so far that its continuance will be endangered. Owing to the hitherto existing knowledge a qualitative population regulation is not caused by the parasites. Detrimental effects with domestic animals caused by game parasites or vice versa in hunting grounds is certainly possible but dependent upon the species greatly different, and practically without essential meaning in Austria.

Animal Diseases↗
PubMed1996

Culture of malaria parasites in two different red blood cell populations using biotin and flow cytometry.

A novel culture system using biotin/streptavidin and flow cytometry was developed to compare maturation and growth rates in Plasmodium falciparum malaria parasites in two distinct red blood cell (RBC) populations. Biotin was used to label a selected RBC population which was then mixed with another distinct unbiotinylated RBC population. P. falciparum-infected RBCs were used to initiate co-cultures followed over 2-3 schizogonic growth cycles. Co-cultures were harvested and stained with streptavidin-fluorescein isothiocyanate (FITC) followed by fixation and staining of parasite DNA. The combination of biotin/streptavidin-FITC and DNA fluorochrome enabled simultaneous flow cytometric analysis of the two different RBC populations and of the parasitemias in each RBC population. We then used this system to study the in vitro susceptibility of RBCs from individuals with hemoglobin H (Hb H) disease to infection and growth of P. falciparum. Significant reduction in parasite multiplication was found in Hb H RBCs as compared with that in normal RBCs. This novel malaria culture system offers two major innovations: a method to compare directly the relative ability of any two red blood cell populations to support malaria parasite invasion and development under identical conditions, and a critical reduction in the volume of blood and reagents needed to assess parasite growth. The application of biotin-labeled RBCs in the flow cytometric analysis of parasite development may offer new insights in studies of the relationship between RBC defects and susceptibility to malaria parasites.

Animals↗
PubMed1999

Gastrointestinal parasitic infection, anthropometrics, nutritional status, and physical work capacity in Colombian boys.

This article tests the hypothesis that the presence of gastrointestinal parasites in Colombian boys is negatively associated with anthropometric characteristics, physical work capacity, blood hemoglobin (Hb) levels, and nutritional status. Anthropometric, Hb, &Vdot;O(2) max, and parasite load data were collected on 1,016 boys in Cali, Colombia. The boys were classified as lower socioeconomic class (SEC) from either urban or rural environments, and upper SEC from an urban environment. Sixty-three percent of the boys were infected with gastrointestinal parasites and, of the infected boys, 80-95% had light parasite loads. Parasites found included Necator americanus, Ascaris lumbricoides, Entamoeba histolytica, Trichuris trichiura, Giardia spp., and Enterobius vermicularis. Infected boys had significantly lower weight, stature, weight-for-height (among 6-9-year-old boys), Hb levels, and &Vdot;O(2) max (ANCOVA, controlling for age and SEC). In terms of nutritional status, infected boys were 1.47 times more likely to be classified as iron deficient than noninfected boys (chi-square, P < 0.001), and 1.61 times more likely to be classified as stunted (P < 0.001). Infection was not associated with wasting in any SEC group. In conclusion, light to moderate gastrointestinal parasite loads were associated with significantly lower weight, stature, weight-for-height (in 6-9-year-old boys), Hb levels, and &Vdot;O(2) max, and a significantly higher frequency of IDA and stunting. These data suggest that comprehensive analyses of the nutritional status of populations in regions endemic for parasitic infection should include testing for the presence of infection. Am. J. Hum. Biol. 11:763-771, 1999. Copyright 1999 Wiley-Liss, Inc.

Journal Article↗
PubMed1999

Export of parasite proteins to the erythrocyte cytoplasm: secretory machinery and traffic signals.

To the malaria parasite, the prospect of setting up residence within a human erythrocyte represents a formidable challenge. The mature human erythrocyte is essentially a bag of haemoglobin with no internal organelles and no protein synthesis machinery. The parasite needs, therefore, to assemble all the essential amenities--foundations, plumbing and furnishings--from scratch. The parasite remodels its adopted home by exporting proteins to the erythrocyte membrane. To reach their final destinations, the exported proteins must cross the parasite plasma membrane, the parasitophorous vacuole membrane and the erythrocyte cytosol. To further understand this unusual and complex trafficking pathway, we have searched for proteins that may form part of the trafficking machinery of the infected erythrocyte. We have identified an ER-located, calcium-binding homologue of reticulocalbin (PfERC) that co-localizes with the ER molecular chaperone, PfGRP. We have also identified a homologue of the GTP-binding protein, Sar1p, a small GTPase that, in other eukaryotic cells, is thought to play a crucial role in trafficking proteins between the ER and the Golgi. PfSar1p is located in discrete structures near the periphery of the parasite cytoplasm that may represent specialized export compartments. PfSar1p is exported to structures outside the parasite in the erythrocyte cytoplasm. The malaria parasite appears to be capable of elaborating components of the 'classical' vesicle mediated trafficking machinery outside the boundaries of its own plasma membrane.

Animals↗
PubMed2002

Parasitism of Lacanobia oleracea (lepidoptera) by the ectoparasitic wasp, Eulophus pennicornis, disrupts the cytoskeleton of host haemocytes and suppresses encapsulation in vivo.

Parasitism of Lacanobia oleracea larvae by the ectoparasitic wasp Eulophus pennicornis suppressed host haemocyte-mediated encapsulation of Sephadex DEAE A-25 beads in vivo. Beads dissected out of parasitized larvae had fewer haemocytes associated with them. Moreover, those haemocytes that were associated with the beads tended to retain a rounded configuration and rarely flattened. Similar results were obtained using in vitro encapsulation assays. SDS PAGE indicated that for parasitized and PBS injected larvae, there were some differences in the plasma proteins that bound to Sephadex DEAE A-25 beads, suggesting that parasitism-mediated changes to host plasma proteins might contribute to the differences in the encapsulation response occurring in these larvae. However, in vitro encapsulation assays using beads that had been pre-incubated in plasma from parasitized and unparasitized larvae, demonstrated that major differences in the extent of encapsulation did not occur. These results, plus in vitro haemocyte attachment and spreading assays, suggest that parasitism-mediated suppression of encapsulation is primarily due to reductions in the ability of host haemocytes to attach to (i.e., recognize) and flatten over non-self surfaces and other haemocytes. This proposal is corroborated by staining of actin in the haemocyte cytoskeleton by FITC-labelled phalloidin, which indicated that parasitism disrupts the formation of stress fibers and focal adhesions in plasmatocytes. By contrast, experimental injection of adult female wasp venom into unparasitized L. oleracea larvae had no significant effect on in vivo encapsulation responses or the haemocyte cytoskeleton. Arch. Insect Biochem. Physiol. 49:108-124, 2002. Published 2002 Wiley-Liss, Inc.

Animals↗
PubMed2006

Distribution of substance P reveals a novel subdivision in the hippocampus of parasitic South American cowbirds.

Parasitic cowbirds monitor potential hosts' nests and return to lay when appropriate, a task that is likely to involve spatial recall. Seasonal and sexual behavioral variations in the cowbirds correlate with anatomical changes in the hippocampal formation. During the breeding season, parasites have larger hippocampal formations than nonparasites. In parasitic species in which females alone perform nest bookkeeping, females have larger hippocampal formations than males. We investigated the distribution of the neuropeptide substance P (SP) in three sympatric cowbirds: two obligate parasites (shiny cowbird and screaming cowbird) and one nonparasite (bay-winged cowbird). Distribution of SP was similar to that in other songbirds, except for a previously undescribed field of dense SP-rich terminals within the hippocampus that we call the hippocampal SP terminal field (SPh). We found robust species differences in the volume of this new area, measured relative to the remainder of the telencephalon. SPh was largest in the generalist parasite (shiny cowbird) and smallest in the nonparasitic species (bay-winged cowbird). In the specialist parasite (screaming cowbird), SPh was smaller than in the generalist parasite but larger than in the nonparasitic species. SPh overlaps with two subdivisions described in the pigeon that have been related to the mammalian dentate gyrus and subiculum. The area containing SPh receives a major input from the lateral mammillary nucleus, which is probably the avian equivalent of the mammalian supramammillary nucleus (SUM), the main source of extrinsic SP input to mammalian hippocampus. SPh may be the termination of a pathway homologous to the SP-rich projection from SUM to the hippocampus in mammals.

Animals↗