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PubMed1996

The effect of parasites on wildlife.

Populations of animals which live in the wild are regulated by many biotic and abiotic factors. Parasites are one of the biotic factors. Parasites may influence their hosts in different ways. They may cause the death of the host due to a direct lethal effect or an indirect effect. Direct lethal effects may occur if killing is a part of the life cycle of the parasite or if hosts and parasites have not developed an equilibrium. The introduction of hosts or parasites into a new environment with suitable hosts or parasites is an example. Death by parasitism may also be caused by a combination of the emaciating effects of parasites combined with factors such as bad weather conditions, environmental pollution or human handling. Parasites may also influence the behaviour of their hosts. If the hosts are intermediate hosts in the life cycle of the parasites, the alterations in behaviour may make them an easier prey for their predators, the final hosts. Parasites may also influence the reproductive success of the hosts. In this respect the relationship between the red grouse (Lagopus lagopus scoticus) and the caecal nematode Trichostrongylus tenuis has been well worked out.

Animals↗
PubMed1996

Critical resources that influence habitat selection decisions by gastrointestinal helminth parasites.

Habitat selection may be the basis of some of the most exciting questions in behavioural ecology today, but parasites are being excluded from this debate. Parasites are not aberrant; they form a large proportion of the diversity of life on earth, and one estimate suggests that parasitism is more common than all other feeding strategies combined. We still do not understand the adaptive value of habitat selection behaviours in these organisms, even though the literature is full of examples of parasites migrating and navigating through hosts to their specific habitats. Parasites must make the same decisions that every animal has to make regarding food acquisition, shelter and reproduction. However, we cannot even make reasonable guesses on the habitat selection strategies and critical resources that influence their decision-making. The purpose of this review is to provide examples of experiments and methods of incorporating critical resources into the ecological analyses of habitat selection by gastrointestinal parasites. Information on parasite resources is simply not available for most parasites, and these ideas might stimulate and guide future research. In addition, parasites are ideal models to test theoretical assumptions of habitat selection. Experimental manipulations of parasites are ideal models to test theoretical assumptions of habitat selection. Experimental manipulations of parasite populations are simple, and habitats of endoparasites can be precisely altered by surgical methods. Few tests of habitat selection theory have been attempted in free-living environments because of the difficulty of assessing the correlations between environmental variations and organismal success in real-world situations, but this is not a problem with parasites because their habitats are replicated exactly in each host.

Animals↗
PubMed2003

Phospholipids in parasitic protozoa.

Parasitic protozoa are surrounded by membrane structures that have a different lipid and protein composition relative to membranes of the host. The parasite membranes are essential structurally and also for parasite specific processes, like host cell invasion, nutrient acquisition or protection against the host immune system. Furthermore, intracellular parasites can modulate membranes of their host, and trafficking of membrane components occurs between host membranes and those of the intracellular parasite. Phospholipids are major membrane components and, although many parasites scavenge these phospholipids from their host, most parasites also synthesise phospholipids de novo, or modify a large part of the scavenged phospholipids. It was recently shown that some parasites like Plasmodium have unique phospholipid metabolic pathways. This review will focus on new developments in research on phospholipid metabolism of parasitic protozoa in relation to parasite-specific membrane structures and function, as well as on several targets for interference with the parasite phospholipid metabolism with a view to developing new anti-parasitic drugs.

Animals↗
PubMed1989

Antigens of parasitic helminths in diagnosis, protection and pathology.

A thorough study of parasitic helminth antigens is a pre-requisite for control programmes based on accurate immunochemical diagnosis, protection by vaccination and perhaps immune modulation to diminish pathological sequelae. Studies should be directed at the identification of those stage- or age-specific surface, secreted and somatic antigens which are involved in the host-parasite interactions responsible for immunity and/or pathology. Current methods of diagnosis of parasitic infections often fail to detect low-level patent infections, which incurs the risk of having a reservoir capable of perpetuating infections. There is, then, an urgent requirement for accurate immunochemical diagnosis, to be used in association with, and for the evaluation of, drug treatment and vector elimination, in parasite control programmes. Given the high sensitivity of current immunoassay technology, the only bar to establishing the necessary immunological tests is the choice of suitably specific antigen/antibody systems. Assays designed to detect parasite products or antigens are a major priority, as they indicate current infection, whereas those which detect antibody only indicate exposure to infection, which may or may not be current. Surface and secreted antigens are the most likely targets for protective immune responses and thus form a logical focus for vaccine design. The cestodes, which present such strong evidence for immunity following natural infection, are likely to yield effective vaccines by modern procedures. Certain antigens must, however, stimulate the humoral and/or cellular responses which are responsible for the undesirable immunopathological consequences of many helminthic diseases. The nematodes and trematodes furnish some extreme examples of such pathology. The ultimate objective in identifying these particular antigens is to utilize them in the appropriate down-regulation of the immune response responsible for such pathology. As an illustration, we have presented an interesting correlation between one particular clinical condition of onchocerciasis (Sowda) and the serological response, defined both in terms of the parasite antigens and an immunoglobulin class-restricted antibody response. Finally, the complexity of these parasite systems and the host response to the parasite should not be underestimated. Modern analytical techniques allow their detailed analysis in terms of the humoral antibody responses and afford the possibility of the future development of control and disease management procedures tailored to each individual host-parasite system. However, novel systems are required to complete the analysis of the cellular components of the immune response to parasite antigens, and functional studies are needed to determine the role that these parasite antigens play in the complex interaction between parasite and host.

Animals↗
PubMed1991

Modification of host cell membrane lipid composition by the intra-erythrocytic human malaria parasite Plasmodium falciparum.

The phospholipid and fatty acid compositions of the host infected erythrocyte plasma membrane (IEPM) have been determined for erythrocytes infected with the human malaria parasite Plasmodium falciparum. IEPM were prepared by selective lysis of the host erythrocyte (but not of the parasite membranes) with 0.1% saponin, followed by differential centrifugation. The purity of the IEPM was determined by measuring the membrane-specific enzyme markers acetylcholinesterase, glutamate dehydrogenase and lactate dehydrogenase, and by immunoelectron microscopy using monoclonal antibodies specific for human erythrocyte glycophorin A (4E7) and for a 195 kDa parasite membrane glycoprotein (Pf6 3B10.1). Both approaches demonstrated that the host erythrocyte plasma membrane preparation was free from contamination by parasite membranes. During intra-erythrocytic development of the parasite, the phospholipid composition of the erythrocyte membrane was strikingly altered. IEPM contained more phosphatidylcholine (38.7% versus 31.7%) and phosphatidylinositol (2.1% versus 0.8%) and less sphingomyelin (14.6% versus 28.0%) than normal uninfected erythrocytes. Similar alterations in phospholipid composition were determined for erythrocyte membranes of parasitized cells isolated by an alternative method utilizing polycationic polyacrylamide microbeads (Affigel 731). The total fatty acid compositions of the major phospholipids in IEPM were determined by g.l.c. The percentage of polyunsaturated fatty acids in normal erythrocyte phospholipids (39.4%) was much higher than in phospholipids from purified parasites (23.3%) or IEPM (24.0%). The unsaturation index of phospholipids in IEPM was considerably lower than in uninfected erythrocytes (107.5 versus 161.0) and was very similar to that in purified parasites (107.5 versus 98.5). Large increases in palmitic acid (C16:0) (from 21.88% to 31.21%) and in oleic acid (C18:1) (from 14.64% to 24.60%), and major decreases in arachidonic acid (C20:4) (from 17.36% to 7.85%) and in docosahexaenoic acid (C22:6) (from 4.34% to 1.8%) occurred as a result of infection. The fatty acid profiles of individual phospholipid classes from IEPM resembled in many instances the fatty acid profiles of parasite phospholipids rather than those of uninfected erythrocytes. Analysis of IEPM from P. falciparum-infected erythrocytes (trophozoite stage) revealed that, during intra-erythrocytic maturation of the parasite, the host erythrocyte phospholipid composition was markedly refashioned. These alterations were not dependent on the method used to isolate the IEPM, with similar results obtained using either a saponin-lysis method or binding to Affigel beads. Since mature erythrocytes have negligible lipid synthesis and metabolism, these alterations must occur as a result of parasite-directed metabolism of erythrocyte lipids and/or trafficking of lipids between the parasite and erythrocyte membranes.

Acetylcholinesterase↗
PubMed1993

The proteases and pathogenicity of parasitic protozoa.

Protozoan parasites are among the most prevalent pathogens worldwide. Diseases like malaria, leishmaniasis, amebiasis, and trypanosomiasis affect hundreds of millions of people. Recent advances in our understanding of the biochemistry and molecular biology of these organisms has focused attention on specific parasite molecules that are key to the parasite life cycle or the pathogenesis of the diseases they produce. One group of enzymes that plays myriad roles in these processes are the parasite-derived proteases. Different types of proteases are frequently expressed at different stages of the parasite life cycle to support parasite replication and metamorphosis. Intracellular parasites such as those that produce malaria and Chagas' disease express high levels of protease activity to efficiently degrade host proteins like hemoglobin. In other instances, such as infection with Entamoeba histolytica, the causative agent of amebiasis, proteases released by the parasite can damage host cells and tissues, contributing to host tissue damage and parasite invasion. Detailed studies of these enzymes have led to model systems for the study of parasite gene regulation, parasite metabolism, and the host-parasite interplay. In some instances, proteases appear to be promising targets for the development of new antiparasitic chemotherapy.

Amino Acid Sequence↗
PubMed1991

The need for live parasites for long-term immunity in malaria.

All of the results of the various experiments support a role for living, proliferating parasites in the efficient induction of anti-parasitic as well as anti-disease (CM) immunity. Non-proliferating parasites or material from disrupted parasites are poor or non-antigens in this respect. Three possibilities as to why living parasites are important in immunity could be considered: 1. circulating parasites contain insufficient antigen to induce protective immunity, but sufficient antigen can be produced during proliferation; 2. only circulating parasites arrive at critical places (e.g. parts of the white pulp of the spleen) for the presentation of the important antigen or induction of appropriate signals. 3. Architectural changes are needed (i.e. formation of barrie-cell-complexes) for the immune response to be effective. The first possibility explains why exoantigens, as well as live, proliferating parasites are efficient inducers of anti-CM immunity. Since these immunizations have no effect on parasitemia, additional/other immune reaction(s) are needed for anti-parasitic immunity. The important role of the spleen in malaria and malaria immunity is well-known. The second possibility includes the idea that live, proliferating parasites circulate through the spleen continuously where unsatisfactory or infected erythrocytes are removed rather than in the liver. Injected killed parasites or material from them when present in the circulation is to a larger extent taken up by the Kupffer cells from the liver rather than the spleen. Presence and uptake of parasites in the spleen may provide the critical confrontation and/or delivery of signals necessary for the development of immunity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Analysis of human T cell clones specific for conserved peptide sequences within malaria proteins. Paucity of clones responsive to intact parasites.

T cells are thought to be of central importance in malaria immunity. Peptides copying malaria protein sequences often stimulate human CD4+ T cells and it was thought that they represented T cell epitopes present in the parasite and may thus have particular relevance to malaria vaccine development. To verify whether synthetic peptides representing highly conserved regions of parasite Ags may contribute to a malaria vaccine, we searched the data bank for conserved regions of Plasmodium falciparum malaria proteins that were not homologous to known self (human) proteins. We synthesized 24 such peptides representing 11 of the cloned and sequenced malaria asexual stage Ags, which were predicted by algorithms to represent T cell epitopes, and 6 peptides not predicted to be T cell epitopes and used these to generate T cell clones from individuals with an extensive previous history of malaria exposure. The T cell clones responded vigorously to many peptides but only a single clone, specific for a peptide within merozoite surface protein-1, 20-39, VTHESYQELVKKLEALEDAV, and not previously defined to be a T cell epitope responded to malaria parasites by proliferation and secretion of IFN-gamma. This epitope was not revealed by studying parasite-induced T cell lines and is thus subdominant. The clone was able to significantly inhibit parasite growth in vitro. The final step in the inhibition of parasite growth appears to be nonspecific because other activated clones (not specific for malaria sequences) can inhibit parasite growth. Our data suggest that few conserved peptides within malaria parasites can be processed from the intact parasite. However, such peptides that can be processed from malaria parasites may be expected to stimulate parasite-specific T cells that could inhibit parasite growth and as such may be lead candidates for a vaccine aimed at inducing cellular immunity to malaria.

Adult↗
PubMed2002

Food webs and the transmission of parasites to marine fish.

Helminth parasites of fish in marine systems are often considered to be generalists, lacking host specificity for both intermediate and definitive hosts. In addition, many parasites in marine waters possess life cycles consisting of long-lived larval stages residing in intermediate and paratenic hosts. These properties are believed to be adaptations to the long food chains and the low densities of organisms distributed over broad spatial scales that are characteristic of open marine systems. Moreover, such properties are predicted to lead to the homogenization of parasite communities among fish species. Yet, these communities can be relatively distinct among marine fishes. For benthos, the heterogeneous horizontal distribution of invertebrates and fish with respect to sediment quality and water depth contributes to the formation of distinct parasite communities. Similarly, for the pelagic realm, vertical partitioning of animals with depth will lead to the segregation of parasites among fish hosts. Within each habitat, resource partitioning in terms of dietary preferences of fish further contributes to the establishment of distinct parasite assemblages. Parasite distributions are predicted to be superimposed on distributional patterns of free-living animals that participate as hosts in parasite life cycles. The purpose of this review is first, to summarize distribution patterns of invertebrates and fish in the marine environment and relate these patterns to helminth transmission. Second, patterns of transmission in marine systems are interpreted in the context of food web structure. Consideration of the structure and dynamics of food webs permits predictions about the distribution and abundance of parasites. Lastly, parasites that influence food web structure by regulating the abundance of dominant host species are briefly considered in addition to the effects of pollution and exploitation on food webs and parasite transmission.

Animals↗
PubMed2000

Richness patterns in the parasite communities of exotic poeciliid fishes.

Three species of poeciliids (Gambusia holbrooki, Xiphophorus helleri and X. maculatus) and 15 species of ecologically similar native freshwater fishes (mainly eleotrids, ambassids, melanotaeniids and retropinnids) were examined for parasite richness to investigate parasite flux, qualitative differences, quantitative differences and the structuring factors in parasite communities in the 2 fish types in Queensland, Australia. Theory suggests that poeciliids would harbour depauperate parasite communities. Results supported this hypothesis; poeciliids harboured more species-poor parasite infracommunities and regional faunas than natives (P < 0.0001), despite greater sampling effort for the former. Cluster analysis of presence/absence data for poeciliids and the 6 most-sampled native fishes revealed that parasite communities of the 2 fish groups are qualitatively distinct; the proportion of parasite species with complex life-cycles was lower in poeciliids than in native species, and Myxosporea, Microspora, Coccidia and parasitic Crustacea were all absent from poeciliids. Limited exchange of parasite species has occurred between natives and poeciliids. Logistic ordinal regression analysis revealed that fish origin (exotic or native), environmental disturbance and host sex were all significant determinants of parasite community richness (P < 0.05). Theoretical modelling suggests that poeciliids are at a competitive advantage over native fishes because of their lack of parasites.

Animals↗
PubMed1985

Nutritional aspects of parasitic infection.

The nutritional basis of the ecological relationship between parasites and their hosts is reviewed using examples of the parasitic infections of man whenever possible. Two important points are discussed first: the distinction between parasitic infection and parasitic disease, and the concepts of synergism or antagonism between undernutrition and parasitic disease. The effects of parasites on the nutritional status of the host are examined in four ways. First, in terms of the ways in which parasites can disturb nutritive processes by effects on physical activity to obtain food, and by effects on food consumption, digestion and absorption. Secondly, in terms of the nutritional cost of an infection to a parasitised host. Thirdly, in terms of the feeding, nutrition and metabolism of parasites. Finally, in terms of damage to the tissues of the host caused by parasites. Two other sections deal briefly with the transmission of parasites in food and the effects of food on parasites.

Animals↗
PubMed1996

Comparative analysis of the metazoan parasite communities of leatherjackets, Oligoplites palometa, O. saurus, and O. saliens (Osteichthyes:Carangidae) from Sepetiba Bay, Rio de Janeiro, Brazil.

One hundred and fifty-seven specimens of leatherjackets, 84 specimens of Oligoplites palometa (Cuvier), 37 specimens of O. saurus (Bloch & Schneider), and 36 specimens of O. saliens (Bloch) captured at the Sepetiba Bay, State of Rio de Janeiro, between March of 1991 and November of 1992, were necropsied to study their communities of metazoan parasites. All fishes studied were parasitized by one or more metazoan species. Eighteen parasite species were found in O. palometa and 13 parasite species were found in O. saurus and O. saliens, respectively. The digeneans were the majority of the parasite specimens collected, with 90.3%, 82.2%, and 87.7% in O. palometa, O. saurus, and O. saliens, respectively. The Simpson index was lower than 0.25 for the 3 host species, showing an absence of concentration for dominance. The similarity coefficients among the parasite communities of the 3 fish species was above 66%. The parasites of the 3 host species showed typical overdispersed pattern of distribution. Four cases of positive correlation between host's total length and prevalence and parasite intensity were found. The majority of the species did not show influence of host sex. The 3 host species had similar parasite diversity, which was not correlated with the host's total body length and there was no significant difference between male and female hosts. Oligoplites palometa and O. saurus each had one pair of ectoparasite species sharing a positive association and with a positive correlation between their intensities. No pairs of associated species were found in O. saliens. Parasites were found along the entire extension of the gastrointestinal tract of the 3 host species. The parasite communities of O. palometa, O. saurus, and O. saliens are defined as isolationists because there are few evidences of interspecific association or covariation. The presence of larval stages of cestodes and nematodes suggests that the three species of Oligoplites are at an intermediate level in the marine trophic web.

Animals↗
PubMed2003

Interactions between parasitized and unparasitized conspecifics: parasitoids modulate competitive dynamics.

Parasitism influences many aspects of a host's behavior and physiology. Therefore, parasitism is also likely to influence the competitive ability of the host. Field populations of phytophagous insects are often a mix of parasitized and unparasitized conspecifics and the inclusion of parasitism in their competitive dynamics may alter expected outcomes. We investigated the influence of parasitism by the hymenopteran parasitoid Phanerotoma franklini Gahan on the competitive interactions among larvae of its host Acrobasis vaccinii Riley. We found that parasitized larvae were poorer competitors and required less food to complete development compared to unparasitized larvae. To examine the influence of parasitism on the competitive dynamics of this system, we constructed an individual-based model parameterized with our laboratory data. The model examined the role of resource availability and parasitism rate on larval survival. The model suggests that parasitized larvae (and, hence parasitoids) experience higher levels of mortality from competition than unparasitized larvae. Further, the model also suggests that the decreased consumption of resources by parasitized larvae results in a decline in the occurrence of competition as the parasitism rate increases. We suggest that these observations may be general to many parasitoid-host systems.

Animals↗
PubMed1999

The optimal strategy for brood-parasitism: how many eggs should be laid in the host's nest?

We consider the optimal strategy for intra-specific brood-parasitism, especially with respect to the number of eggs laid by the parasitic individual in the nest of non-parasitic individual, in particular, a host that does not reject the parasite's eggs. With a fundamental mathematical model, assuming that the survival probability of the parasite's offspring in the nest of the host is significantly smaller than that in parasite's own nest, we determine the optimal number of eggs laid in the nest of host that maximizes the expected reproductive fitness of the parasite. We show that the invasion success of brood-parasitism could significantly depend on the total number of eggs laid by the parasite in a breeding season, and that the successfully invading brood-parasitism could realize maximum fitness with a specific number of parasite's eggs laid in the nest of the host.

Animals↗
PubMed2003

Habitat overlap and gastrointestinal parasitism in sympatric African bovids.

Gastrointestinal parasite infections are widespread among wild ungulates. Because many of these parasites infect multiple host species, inter-specific interactions among hosts potentially play an important role in parasite transmission dynamics in ungulate communities. In this study, the effects of inter-specific contact on parasitism rates in 11 sympatric African bovids was examined using habitat overlap among species as a measure of cross-species contact rates. Across individual hosts, strongyle nematode abundance increased with increasing numbers of bovid species occupying a habitat. Furthermore, comparative analyses show a positive association between strongyle prevalence and level of habitat overlap across taxa. These findings suggest that among sympatric bovids, contact between species contributes significantly to the transmission of generalist nematode parasites. For a more host-specific parasite group, coccidia, parasite abundance and individual probability of infection declined in hosts living in bovid rich habitats. This pattern may reflect enhanced interspecific competition among parasites in these areas. Finally, similar to strongyle abundance, individual parasite richness also increased among hosts occupying habitats with higher numbers of bovid species. No association between habitat overlap and parasite richness was detected at higher taxonomic scales, however, which suggests that contact between host species may not contribute to parasite colonization of new host taxa.

Animals↗
PubMed1990

Effects of host age, host density and parent age on reproduction of the filth fly parasite Urolepis rufipes (Hymenoptera: Pteromalidae).

Urolepis rufipes Ashmead, a pteromalid wasp, was recently discovered parasitizing house fly and stable fly pupae in eastern Nebraska dairies. Studies have been conducted on the biology of this parasite to evaluate its potential as a biological control agent of stable flies (Stomoxys calcitrans (L.] and house flies (Musca domestica L.). House fly pupae were suitable as hosts for U.rufipes at all ages; however, significantly higher parasitism occurred on host pupae aged 96-120 h. Parasite-induced mortality (host mortality without progeny production) was higher than for other pteromalid parasites of filth flies under similar conditions. Parasitism increased with parasite--host ratio at 20 degrees C; however, the opposite was noted at 30 degrees C for parasite--host ratios ranging from 5:50 to 50:50. Fly eclosion decreased as parasite--host ratio increased at 20 degrees C, and no host eclosion occurred at the highest parasite--host ratios (20:50 and 50:50) at 30 degrees C. Females produced an average of 18.6 female and 7.6 male progeny. 88% of the progeny were produced during the first 6 days post parental eclosion. The short life span, low progeny emergence rate and high per cent host eclosion, in comparison with other parasite species, suggests that the Nebraska strain of U.rufipes may not an effective biological control agent of house flies.

Age Factors↗
PubMed2002

Modulating the modulators: parasites, neuromodulators and host behavioral change.

Neuromodulators can resculpt neural circuits, giving an animal the behavioral flexibility it needs to survive in a complex changing world. This ability, however, provides parasites with a potential mechanism for manipulating host behavior. This paper reviews three invertebrate host-parasite systems to examine whether parasites can change host behavior by secreting neuromodulators. The parasitic wasp, Cotesia congregata, suppresses host feeding partly by inducing the host (Manduca sexta) to increase the octopamine concentration in its hemolymph. The increased octopamine concentration disrupts the motor pattern produced by the frontal ganglion, preventing the ingestion of food. Polymorphus paradoxus (Acanthocephalan) alters the escape behavior of its host, Gammarus lacustris (Crustacea), possibly through an effect on the host's serotonergic system. The trematode Trichobilharzia ocellata inhibits egg-laying in its snail host (Lymnaea stagnalis), partly by inducing the host to secrete schistosomin. Schistosomin decreases electrical excitability of the caudodorsal cells. The parasite also alters gene expression for some neuromodulators within the host's central nervous system. In at least two of these three examples, it appears that the host, not the parasite, produces the neuromodulators that alter host behavior. Producing physiologically potent concentrations of neuromodulators may be energetically expensive for many parasites. Parasites may exploit indirect less energetically expensive methods of altering host behavior. For example, parasites may induce the host's immune system to produce the appropriate neuromodulators. In many parasites, the ability to manipulate host behavior may have evolved from adaptations designed to circumvent the host's immune system. Immune-neural-behavioral connections may be pre-adapted for parasitic manipulation.

Animals↗
PubMed1997

Fitness of parasites: pathology and selection.

Parasites improve their fitness as a result of the selection of traits which determine their relationships with their hosts. Some of these relationships are examined briefly. There is a cost of virulence for parasites, paralleling the cost of resistance for hosts, which implies that the good health of the host can be a component of parasite fitness; conversely, some transmission modes imply that the host be markedly weakened by the parasite. Pathogenicity can be influenced by characters such as a transmission of the parasite from parents to offspring, or the demographic characteristics of the host populations. Important components of parasite fitness are: the complexity of the life-cycle; the degree of specialization for a more or less open host range; the conspicuousness or discretion of the infective and parasitic stages. However, the best possible adaptation to a particular host is not always selected: when a parasite exploits several host species, the gene flows between parasites which have developed in different hosts may be responsible for "maladaptation". This may be important for an understanding of the pathogenicity of certain human parasitic diseases.

Animals↗