Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “parasite”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31Linked to original sources
PubMed2004

Echinococcus multilocularis proliferation in mice and respective parasite 14-3-3 gene expression is mainly controlled by an alphabeta CD4 T-cell-mediated immune response.

The role of specific B lymphocytes and T-cell populations in the control of experimental Echinococus multilocularis infection was studied in micro MT, nude, T-cell receptor (TCR)-beta(-/-), major histocompatibility complex (MHC)-I(-/-) and MHC-II(-/-) mice. At 2 months postinfection, the parasite mass was more than 10 times higher in nude, TCR-beta(-/-) and MHC-II(-/-) mice than in infected C57BL/6 wild-type (WT) mice, and these T-cell-deficient mice started to die of the high parasite load at this time-point. In contrast, MHC-I(-/-) and micro MT mice exhibited parasite growth rates similar to those found in WT controls. These findings clearly point to the major role that CD4(+) alphabeta(+) T cells play in limiting the E. multilocularis proliferation, while CD8(+) T and B cells appeared to play a minor role in the control of parasite growth. In the absence of T cells, especially CD4(+) or alphabeta(+) T cells, the cellular immune response to infection was impaired, as documented by the lack of hepatic granuloma formation around the parasite and by a decreased splenocyte responsiveness to concanavalin A (Con A) and parasite antigen stimulation. Surprisingly, in T-cell-deficient mice, the ex vivo expression of interferon-gamma (IFN-gamma) and other inflammatory cytokines (except for interleukin-6) were increased in association with a high parasite load. Thus, the relative protection mediated by CD4(+) alphabeta(+) T cells against E. multilocularis infection seemed not be IFN-gamma dependent, but rather to rely on the effector's function of CD4(+) alphabeta(+) T cells. The local restriction of parasite germinal cell proliferation was reflected by a regulatory effect on the expression of 14-3-3 protein within the parasite tissue in T-cell-deficient mice. These results provide a strong indication that the CD4(+) alphabeta(+) T-cell-mediated immune response contributes to the control of the parasite growth and to the regulation of production of the parasite 14-3-3 protein in metacestode tissues.

14-3-3 Proteins↗
PubMed2002

Malaria parasites giving rise to recrudescence in vitro.

Recrudescences were simulated in vitro with drug treatment to examine how drug-sensitive parasites survive the treatment. Various numbers of cultured parasites were treated with lethal doses of pyrimethamine or mefloquine for various lengths of time. Recrudescences were observed in parasite populations with larger initial numbers of parasites when the treatment duration was prolonged. Equal numbers of parasitized erythrocytes were treated with various concentrations of pyrimethamine or mefloquine. There was no clear linear relationship between the incidence of recrudescence and the drug concentration. Parasites that had recrudesced were continuously allowed to recrudesce in the succeeding recrudescence experiments. Both the duration from the cessation of treatment to the time at which the recrudescent parasitemia level reached 1% and the growth rate of recrudescent parasites were equal among these recrudescences. The recrudescent parasites in these experiments were as sensitive to the drugs as the parasites tested before treatment were. These results suggest that a parasite culture may contain parasites in some phases that are not killed by drug for up to 10 days, which explains the recrudescences that occur even after treatment.

Animals↗
PubMed2003

Mortality in immatures of the floodwater mosquito Ochlerotatus albifasciatus (Diptera: Culicidae) and effects of parasitism by Strelkovimermis spiculatus (Nematoda: Mermithidae) in Buenos Aires Province, Argentina.

Life tables were constructed for six cohorts of immature stages of the floodwater mosquito Ochlerotatus albifasciatus (Macquart) in a park in Buenos Aires, highlighting the mortality attributable to the parasitic nematode, Strelkovimermis spiculatus Poinar & Camino. Two cohorts were selected to compare parasite incidence in all mosquito stages when low and high parasitism occurred. Development time of Oc. albifasciatus from first instar to adult was 7.7-10 days in the spring, 6 days in the summer, and 10.9-21.9 days in the fall. Survival was estimated as 0-1.4% in the spring, 2% in the summer and 0.2-4.4% in the fall. The highest "K" value (Killing power) occurred during a fall cohort when prevalence of the parasite was 86.9%, and the lowest in a spring cohort. Parasitism occurred during all seasons, but S. spiculatus persisted to adult only in the summer and fall, when adult mosquitoes developed from parasitized third and fourth instars larvae. The abundance of S. spiculatus differed between old and young larvae only when parasite prevalence was the highest. Although pupae and adults of Oc. albifasciatus were parasitized, no pupal mortality attributable to parasitism was recorded. The proportion of parasitized adults ranged from 14.2% and 5.7% in the two cohorts compared. Pupal wet weight and adult wing lengths did not differ between parasitized and unparasitized individuals.

Animals↗
PubMed2001

The role of parasitic diseases as causes of mortality in small ruminants in a high-potential farming area in central Kenya.

A 15-year retrospective study was performed to determine the role of parasitic diseases in causing mortalities in small ruminants. In total, 115 (32 %) sheep were diagnosed as having been killed by parasitic diseases out of 366 that died as a result of disease. The major cause of mortality was helminthosis (63 % of all parasitic cases). Most of the helminthosis cases were attributed to haemonchosis (40% of parasitic cases). Heartwater was the second most important parasitic disease (27% of all parasitic cases). Ninety-five (26%) goats were diagnosed to have been killed by parasitic diseases out of 365 cases presented at the post mortem facility. Helminthosis was the most frequent cause of mortality (55% of the total parasitic diseases). Twenty-six goats were killed by haemonchosis (27% of all parasitic diseases). Heartwater was the second most important parasitic disease, accounting for about 20% of all parasitic diseases. These findings indicate that viable helminth and tick control strategies should be devised in order to reduce mortality caused by helminthosis and heartwater and thereby achieve improved productivity.

Animals↗
PubMed1994

Antimalarial action of hydrophilic drugs: involvement of aqueous access routes to intracellular parasites.

The antimalarial action and intracellular distribution of the hydrophilic agents phloridzin (PHL) (a bioflavonoid glycoside) and desferrioxamine (DFO) (an iron chelator) were studied in cultures of Plasmodium falciparum-infected human erythrocytes. When added to cultures, these agents arrested parasite growth with IC50 values of 12 microM (PHL) and 22 microM (DFO). At 37 degrees, PHL (40 microM) was virtually impermeant to uninfected cells but permeated with a mean t1/2 of 1.5 hr in trophozoites (30% accessible cell volume) and 8 hr in rings (10% of accessible cell volume). PHL, in analogy with DFO, was demonstrably permeant to infected cells harboring mature forms of the parasites. Permeation was restricted to only a fraction of the infected cell volume. PHL elicited inhibition of nucleic acid synthesis within 1 hr of exposure of trophozoites to PHL (40 microM) and in > 8 hr of exposure of rings. Red cell containers into which millimolar concentrations of PHL or DFO were encapsulated demonstrably supported parasite invasion and subsequent parasite growth and maturation (48-hr incubation). Under culture conditions, uninfected or parasite-infected red cell containers that were loaded with either agent retained the drugs for at least 42 hr at hundred-micromolar concentrations. The agent present in the cells was fully active after release from cells and administration to test cultures of parasites. PHL added to parasite cultures was active at micromolar concentrations, but when present intracellularly it was virtually inactive even at millimolar concentrations. The data presented are consistent with direct access of hydrophilic agents from medium to parasite, a process referred to as fenestration. Permeation into parasites might constitute the rate-limiting step in drug uptake and drug-mediated arrest of parasite growth by PHL and DFO. The putative role of the parasitophorous duct in providing aqueous access routes from medium to parasites is discussed.

Animals↗
PubMed2004

Associations between fish reproductive cycle and the dynamics of metazoan parasite infection.

The parasite fauna of the stone loach (Barbatula barbatula) was investigated monthly from May to December 2001. Four parasite species were recorded: Gyrodactylus spp. (Monogenea), Tylodelphys clavata (Digenea), Proteocephalus sagittus (Cestoda), and Raphidascaris acus (Nematoda). The changes in the seasonal dynamics of parasite infection were investigated throughout the 8 months. The potential effect of the reproductive investment of the host, measured by gonad mass and gonado-somatic index (GSI), on the parasite infection was tested against the prediction that, during periods of high reproductive investment (beginning of the breeding period or forming gonads after breeding), the fish are more susceptible to parasite infection. Differences between parasite loads between genders were also hypothesized. Seasonal differences in infection were observed for all parasite species studied. The values of GSI showed a pattern of energy accumulation in the pre-reproductive period and at the beginning of breeding, a decrease during breeding, and an increase in the post-breeding period. A similar pattern was observed for parasite abundance, a strong or weak increase in spring and/or autumn and a decrease during summer (July and August). Positive correlations between the abundance of Gyrodactylus spp. and R. acus and both gonad mass and GSI were found in females after eliminating the effect of fish weight. Our results suggest that stone loach females are more susceptible to parasite infection in periods of higher reproductive investment. The main factor determining the infection of T. clavata was fish size. The abundance of P. sagittus was positively correlated with GSI in the total fish sample with no detectable effect of sex or fish weight. The increase in cestode infection in spring supports the hypothesis that the parasite life cycle could by synchronized with the beginning of host reproduction, probably induced by increasing fish hormone levels in the spring.

Animals↗
PubMed1994

Vaccination against animal parasites.

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.

Animals↗
PubMed1998

Parasite diversity/host age and size relationship in three coral-reef fishes from French Polynesia.

The parasite communities of three coral-reef fish species (Stegastes nigricans, Dascyllus aruanus and Cephalopholis argus) were on Tiahura reef, French Polynesia. The age and growth of each fish was analysed by otolith increment counts and a significant correlation between these variables was found. Stegastes nigricans was parasitised by six adult parasite species, D. aruanus by two adult parasite species and C. argus by five adult parasite species. The most common parasite species were found in all fish size classes. Ectoparasites showed a positive relationship between their abundance and host body length for all three reef fish species. A positive relationship was found only between host size and parasite abundance for common endoparasite species. Parasite species richness, Brillouin's diversity index, and host size and age were positively related. Finally, we discuss the influence of different biological (host diet, host immune response, parasite life-cycle) and ecological factors on parasite community structure in these three reef fishes. Host diet quality seems to be one of the major factors affecting the endoparasite community structure in these reef fishes. Ectoparasite communities seem to be influenced more by biological factors such as, for example, host immunity for the caligid larvae or parasite life-cycle for the gnathiid praniza larvae. In addition, the effect of ecological factors such as cleaning symbiosis on these ectoparasites cannot be dismissed.

Aging↗
PubMed2003

Introduced species and their missing parasites.

Damage caused by introduced species results from the high population densities and large body sizes that they attain in their new location. Escape from the effects of natural enemies is a frequent explanation given for the success of introduced species. Because some parasites can reduce host density and decrease body size, an invader that leaves parasites behind and encounters few new parasites can experience a demographic release and become a pest. To test whether introduced species are less parasitized, we have compared the parasites of exotic species in their native and introduced ranges, using 26 host species of molluscs, crustaceans, fishes, birds, mammals, amphibians and reptiles. Here we report that the number of parasite species found in native populations is twice that found in exotic populations. In addition, introduced populations are less heavily parasitized (in terms of percentage infected) than are native populations. Reduced parasitization of introduced species has several causes, including reduced probability of the introduction of parasites with exotic species (or early extinction after host establishment), absence of other required hosts in the new location, and the host-specific limitations of native parasites adapting to new hosts.

Adaptation, Physiological↗
PubMed1996

Antigenic variation and the within-host dynamics of parasites.

Many parasites exhibit antigenic variation within their hosts. We use mathematical models to investigate the dynamical interaction between an antigenically varying parasite and the host's immune system. The models incorporate antigenic variation in the parasite population and the generation of immune responses directed against (i) antigens specific to individual parasite variants and (ii) antigens common to all the parasite variants. Analysis of the models allows us to evaluate the relative importance of variant-specific and cross-reactive immune responses in controlling the parasite. Early in the course of infection within the host, when parasite diversity is below a defined threshold value (the value is determined by the biological properties of the parasite and of the host's immune response), the variant-specific immune responses are predominant. Later, when the parasite diversity is high, the cross-reactive immune response is largely responsible for controlling the parasitemia. It is argued that increasing antigenic diversity leads to a switch from variant-specific to cross-reactive immune responses. These simple models mimic various features of observed infections recorded in the experimental literature, including an initial peak in parasitemia, a long and variable duration of infection with fluctuating parasitemia that ends with either the clearance of the parasite or persistent infection.

Animals↗
PubMed2003

Potential interactions between metazoan parasites of the Mayan catfish Ariopsis assimilis and chemical pollution in Chetumal Bay, Mexico.

The effect of pollutants on the intensity of infection of metazoan parasites in the Mayan catfish, Ariopsis assimilis was investigated. Data were collected on pollutants and metazoan parasites from 76 catfish from five localities in Chetumal Bay in October, 1996. Nineteen pollutants (pesticides, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)) were found in the catfish livers. Heavy metal content was not determined. Nineteen metazoan parasite species were recovered. After controlling for fish length and sampling station, there was a significant negative linear relationship between the intensity of the larval digenean Mesostephanus appendiculatoides and 1,1,1,-trichloro-2,2-bis (4-chlorophenyl) ethane (DDT) concentrations. This negative relationship may be explained either by the effect of the pesticide on the mortality of (i) free-living larval forms, (ii) metacercariae in the fish, (iii) infected fish or (iv) intermediate host snails. There were significant differences between fish parasitized and not parasitized with M. appendiculatoides with respect to their DDT concentrations. There were also significant differences between the variances of the mean Clark's coefficient of condition values between catfish parasitized and not parasitized by M. appendiculatoides, with the variance of non-parasitized catfish being significantly larger. The results provided statistical evidence that DDT has a detrimental effect on M. appendiculatoides infection intensity. Furthermore, the significantly larger variance value of Clark's coefficient for non-parasitized fish suggested that DDT affects both the parasite and general host condition.

Animals↗
PubMed2004

Parasites and the evolutionary diversification of primate clades.

Coevolutionary interactions such as those between hosts and parasites have been regarded as an underlying cause of evolutionary diversification, but evidence from natural populations is limited. Among primates and other mammalian groups, measures of host diversification rates vary widely among lineages, but comparative studies have not yet identified a reliable explanation for this variation. In this study, we used a comprehensive data set of disease-causing organisms from free-living primates to illustrate how phylogenetic comparative methods can be used to examine mammalian lineage diversity in relation to parasite species richness. Our results provide evidence that the phylogenetic diversity of primate clades is correlated positively with the number of parasite species harbored by each host and that this pattern is largely independent of other host traits that have been shown to influence diversification rates and parasite species richness in primates. We investigated two possible mechanisms that could explain this association, namely that parasites themselves drive host evolutionary diversification through processes linked with sexual selection and that host shifts or host sharing increases parasite species richness among diverse primate clades. Neither parasite species richness nor host diversification is related to measures of sexual selection in primates. Further, we found only partial evidence that more rapidly diversifying host lineages produced increased opportunities for host sharing or host shifting by parasites through mechanisms involving species' geographic range overlap. Thus, our analyses provide evidence for an important link between the evolutionary diversification of primates and the richness of their parasite communities, but other mechanisms, particularly those related to reciprocal selection or coextinction of hosts and parasites, require further investigation.

Animals↗
PubMed2026

Genomic contingence beneath ecological convergence: the tempo and mode of gene loss in parasitic bilaterians.

Parasitism has independently evolved hundreds of times among metazoans. Nonetheless, parasites have explored only a limited range of ecologies, and they display frequent convergence in morphological, behavioral, and life-history traits. Although gene loss in particular parasitic species has been documented, it is not known if gene loss converges along the same lines as these other traits. To test for convergent gene loss, we characterized the housekeeping, regulatory, and DNA-repair complements of 48 bilaterian species, including 20 parasites belonging to 6 different bilaterian phyla. We found that different parasitic strategies do not display characteristic tempos or modes of gene loss. Further, the accelerated rates of gene loss seen in some parasites were almost always shared with their free-living relatives, indicating that the increased rate of loss preceded the rise of parasitism. Therefore, the convergent ecological strategies and adaptations that have arisen in distantly related parasitic lineages overlay contingent gene losses, which largely reflect their phylogenetic history. These results have important implications for how ecologists and evolutionary biologists should model the acquisition of parasitism, especially regarding the long-held assumption that reversion from a parasitic to a free-living state is impossible.

Animals↗
PubMed2003

Parasite-mediated predation between native and invasive amphipods.

Parasites can structure biological communities directly through population regulation and indirectly by processes such as apparent competition. However, the role of parasites in the process of biological invasion is less well understood and mechanisms of parasite mediation of predation among hosts are unclear. Mutual predation between native and invading species is an important factor in determining the outcome of invasions in freshwater amphipod communities. Here, we show that parasites mediate mutual intraguild predation among native and invading species and may thereby facilitate the invasion process. We find that the native amphipod Gammarus duebeni celticus is host to a microsporidian parasite, Pleistophora sp. (new species), with a frequency of infection of 0-90%. However, the parasite does not infect three invading species, G. tigrinus, G. pulex and Crangonyx pseudogracilis. In field and laboratory manipulations, we show that the parasite exhibits cryptic virulence: the parasite does not affect host fitness in single-species populations, but virulence becomes apparent when the native and invading species interact. That is, infection has no direct effect on G. d. celticus survivorship, size or fecundity; however, in mixed-species experiments, parasitized natives show a reduced capacity to prey on the smaller invading species and are more likely to be preyed upon by the largest invading species. Thus, by altering dominance relationships and hierarchies of mutual predation, parasitism strongly influences, and has the potential to change, the outcome of biological invasions.

Amphipoda↗
PubMed2004

Immunological investments reflect parasite abundance in island populations of Darwin's finches.

The evolution of parasite resistance can be influenced by the abundance of parasites in the environment. However, it is yet unresolved whether vertebrates change their investment in immune function in response to variation in parasite abundance. Here, we compare parasite abundance in four populations of small ground finches (Geospiza fuliginosa) in the Galapagos archipelago. We predicted that populations exposed to high parasite loads should invest more in immune defence, or alternatively use a different immunological defence strategy. We found that parasite prevalence and/or infection intensity increased with island size. As predicted, birds on large islands had increased concentrations of natural antibodies and mounted a strong specific antibody response faster than birds on smaller islands. By contrast, the magnitude of cell-mediated immune responses decreased with increasing parasite pressure, i.e. on larger islands. The data support the hypothesis that investments into the immune defence are influenced by parasite-mediated selection. Our results are consistent with the hypothesis that different immunological defence strategies are optimal in parasite-rich and parasite-poor environments.

Analysis of Variance↗
PubMed2001

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.

Animals↗
PubMed2003

Evidence for strong host clone-parasite species interactions in the Daphnia microparasite system.

Organisms are often confronted with multiple enemy species. Defenses against different parasite species may be traded off against each other. However, if resistance is based on (potentially costly) general defense mechanisms, it may be positively correlated among parasites. In an experimental study, we confronted 19 clones from one Daphnia magna population with two bacterial and three microsporidian parasite species. All parasites were isolated from the same pond as the hosts. Host clones were specific in their susceptibility towards different parasite species, and parasite species were host-clone specific in their infectivity, spore production, and virulence, resulting in highly significant host-parasite interactions. Since the Daphnia's resistance to different parasite species showed no obvious correlation, neither general defense mechanisms nor trade-offs in resistance explain our findings. None of the Daphnia clones were resistant to all parasite species, and the average level of resistance was quite similar among clones. This may reflect a cost of defense, so that the cumulative cost of being resistant to all parasite species might be too high.

Analysis of Variance↗
PubMed1995

The art of parasite survival.

Parasites develop and survive in an environment which is often hostile to them. When facing aggressive conditions parasites are able to use various and complex strategies. Echinococcus granulosus, Toxocara canis, Pneumocystis carinii, Entamoeba or Toxoplasma gondii are able to seclude from the environment when stressed by surrounding (immunologic or non-immunologic) aggressive factors. Specific antigens which exert a functional activity during a short period of time appear to be concealed from the immune attack at this crucial moment. This is the case for rhoptry or dense granule antigens of Plasmodium or Toxoplasma sporozoa involved in the formation of the parasitophorous vacuole which are released in a space perfectly isolated from the outside and therefore from antibodies. Some parasites like Schistosoma mansoni or Trypanosoma brucei reveal an amazing opportunistic behavior when they use cytokines of host origin induced by the infectious process for their own development. Leishmania, Toxoplasma and Trypanosoma cruzi are able to invade immunologically competent macrophages and to avoid the triggering of killing mechanisms of these cells. Parasites also take advantage of the genetic restriction of the immune response and it has been observed for Plasmodia that some high molecular weight antigens are unable to induce an immune response in particular strains of mice. Parasite receptors involved in the invasion of host cells by parasites can function in the presence of antibodies which can explain the failure of vaccination attempts targeting this type of molecules. Among the mechanisms developed by parasites to resist to drugs it appears that transmembrane transporters described in many protozoa or helminth parasites could play a role. Moreover, the description of parasite-specific enzymes able to protect them against the damaging effects of oxygen radicals suggests that parasites are potentially able to develop a resistance phenomenon against drugs acting via an oxidative burst.

Adaptation, Physiological↗