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PubMed1988

The use of enzymopathic human red cells in the study of malarial parasite glucose metabolism.

The in vitro growth of Plasmodium falciparum malaria parasites was assayed in mutant red cells deficient in either diphosphoglycerate mutase (DPGM) or phosphoglycerate kinase (PGK). In addition, cDNA probes developed for human DNA sequences coding for these enzymes were used to examine the parasite genome by means of restriction endonuclease digestion and Southern blot analysis of parasite DNA. In both types of enzymopathic red cells, parasite growth was normal. In infected DPGM deficient red cells, no DPGM activity could be detected, and in normal red cells, DPGM activity declined slightly in a manner suggestive of parasite catabolism of host protein. However, in infected PGK deficient red cells, there was a 100-fold increase in PGK activity, and in normal red cells, a threefold increase in PGK activity was observed. Parasite PGK could be recovered from isolated parasites, and a marked increase in heat instability of parasite PGK as compared with the host cell enzyme was noted. Neither cDNA probe was found to cross-react with DNA sequences in the parasite genome. It is concluded that the parasite has no requirement for DPGM, and probably has no gene for this enzyme. On the other hand, the parasite does require PGK, (an adenosine triphosphate [ATP] generating enzyme) and synthesizes its own enzyme, which must have been encoded in the parasite genome. The parasite PGK gene most likely lacks sufficient homology to be detected by a human cDNA probe. Enzymopathic red cells are useful tools for elucidating the glycolytic enzymology of parasites and their co-evolution with their human hosts.

Animals↗
PubMed2003

Potential chemotherapeutic targets in the purine metabolism of parasites.

Parasites are responsible for a wide variety of infectious diseases in human as well as in domestic and wild animals, causing an enormous health and economical blight. Current containment strategies are not entirely successful and parasitic infections are on the rise. In the absence of availability of antiparasitic vaccines, chemotherapy remains the mainstay for the treatment of most parasitic diseases. However, there is an urgent need for new drugs to prevent or combat some major parasitic infections because of lack of a single effective approach for controlling the parasites (e.g., trypanosomiasis) or because some serious parasitic infections developed resistance to presently available drugs (e.g., malaria). The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Some of the most striking differences between parasites and their mammalian host are found in purine metabolism. Purine nucleotides can be synthesized by the de novo and/or the so-called "salvage" pathways. Unlike their mammalian host, most parasites studied lack the pathways for de novo purine biosynthesis and rely on the salvage pathways to meet their purine demands. Moreover, because of the great phylogenic separation between the host and the parasite, there are in some cases sufficient distinctions between corresponding enzymes of the purine salvage from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Furthermore, the specificities of purine transport, the first step in purine salvage, diverge significantly between parasites and their mammalian host. This review highlights the unique transporters and enzymes responsible for the salvage of purines in parasites that could constitute excellent potential targets for the design of safe and effective antiparasitic drugs.

Animals↗
PubMed1990

Immunization with Theileria parva parasites from buffaloes results in generation of cytotoxic T cells which recognize antigens common among cells infected with stocks of T. parva parva, T. parva bovis, and T. parva lawrencei.

Immunity to infection by the protozoan parasite Theileria parva in cattle is partially attributable to cytotoxic T cells, which kill lymphocytes infected with the schizont stage of the parasite. Here we evaluated five stocks of buffalo-derived T. parva lawrencei parasites and two stocks of cattle-derived T. parva parva parasites for their ability to induce in vivo cytotoxic T cells which can kill lymphocytes infected with a wide variety of strains of T. parva parasites. A group of seven full-sibling cattle, produced by embryo transfer and matched for at least one major histocompatibility complex class I haplotype, were immunized by infection and treatment with the parasite stocks. Target cells used in in vitro cytotoxicity assays were infected with five buffalo-derived parasite stocks and five cattle-derived parasite stocks, including T. parva parva and T. parva bovis. Immunization with any of the seven parasite stocks resulted in the generation of cytotoxic T cells which recognized parasite antigens on most if not all of the target cell lines tested, although the T. parva bovis stock was the least effective at doing so. Further in-depth analyses performed with peripheral blood mononuclear cells from one of the cattle immunized with T. parva lawrencei parasites showed that the pattern of killing of the panel of target cells was altered when either cells infected with different parasite stocks or clones of infected cells were used as stimulator cells in vitro, suggesting the presence of more than one population of parasite-specific cytotoxic effector cells in the peripheral blood mononuclear cells. However, clones of these cytotoxic effector cells recognized common or cross-reactive antigen epitopes expressed by the entire panel of infected target cells. These T-cell clones will be useful for identifying common T-cell antigen epitopes of T. parva and the parasite genes encoding them.

Animals↗
PubMed2002

Parasite communities of freshwater fish under flood conditions.

The parasite communities of the freshwater fish species roach ( Rutilus rutilus), bitterling ( Rhodeus sericeus amarus) and perch ( Perca fluviatilis) were investigated during a major flood. Differences in parasite community structure due to changes in the host environment were expected. We therefore tested for differences in parasite species abundance and diversity as a consequence of the flood. Potential changes in parasite community structure due to seasonal influences were also considered. We found differences in the composition of parasite species and the proportion of ecto- and endoparasites in the three host species. The parasite community of roach was richer in species than those of perch and bitterling. Roach harboured more ectoparasite than endoparasite species. Parasite communities of both perch and bitterling were richer in endoparasite species and had a lower number of ectoparasite species. Parasite communities of the three different host species responded in different ways to environmental change. Considering all parasite species, no effect of either season or flood was found on the species diversity in either perch or bitterling. The flood caused an increase in the parasite species diversity in roach. For roach and perch, higher parasite abundance were found before the flood compared to after flood. However, no difference was found between the different seasons after the flood. By contrast, bitterling had the highest parasite abundance in the season after the flood. When analysing ecto- and endoparasites separately, a decrease in ectoparasite abundance was found in roach and perch, probably related to the flood, and due mainly to a decrease in monogenean species. A significant increase in the abundance of endoparasites was found in perch. In bitterling, the post-flood increase in ectoparasites was due to a high abundance of Gyrodactylus spp. We conclude that the parasite communities of the three fish species responded in different ways to the flood. This may be due to changes in the host life history strategy or in the immune response in conditions of stress.

Animals↗
PubMed1988

Impact of internal parasites on beef cattle.

Internal parasitism is a pervasive constant that reduces returns in beef cattle production. Parasitism may influence production sufficiently so that data derived investigating performance response will be erroneous. The interaction of internal parasites of cattle with other facets of their lives makes it imperative that more parasite research be done on cattle to consider their impact. Results of trials designed to determine forage production at various stocking densities may not reflect the nutritive value of the forage, but instead the severity of parasite exposure. Relative resistance or susceptibility to certain parasites vary with the breed of livestock and the species of parasite. Extensive work to evaluate the ability of various breeds and sires to influence parasitic numbers has been conducted in sheep but not in cattle. Some parasite trials have ignored the effects of nutrition on the establishment and retention of parasites. This becomes especially important where multiple parasite exposure may occur. Susceptible cattle, even on an optimal diet, will become parasitized, but if reexposed to the same species of parasite, cattle on an optimal diet may be able to resist reinfection, whereas those on deficient diets will not.

Animals↗
PubMed2002

Effects of brood parasitism on host reproductive success: evidence from larval interactions among dung beetles.

This paper investigates the effect of brood parasitism in a dung beetle assemblage in an arid region of Spain. The study was conducted during the spring season (March-May 1994-1998) using mesh cylinders buried into the ground, filled with sand and with sheep dung on top. We quantified the proportion of nests containing larvae of parasitic beetles and their effect on host larvae survival. Experiments on the effect of parasitic larvae on host-larvae survival were conducted by placing scarab brood masses (raised from captive scarabs in the laboratory) in containers with and without aphodiid larvae. During the spring, dung desiccation is rapid, preventing aphodiids nesting in the dung, and forcing these species to adopt brood parasitism as a nesting strategy. Parasitic aphodiids were found in 12-47% of scarab nests of three species. The incidence of brood parasitization was positively related with the number of brood masses contained in the nests, being also higher in the most abundant species. Field data and experiments showed that brood parasites significantly reduced host larvae survival from 74.8% in non-parasitized nests to 8.8% in parasitized nests. Because different rates of nest parasitization and mortality were caused by parasites, brood parasitism had a differential effect on different host species. Thus, brood parasitism constitutes an important mortality factor reducing the reproductive success of the host species and potentially affecting the beetle abundance in the area.

Animals↗
PubMed1985

The population dynamics of competition between parasites.

A number of published studies of competition between parasite species are examined and compared. It is suggested that two general levels of interaction are discernible: these correspond to the two levels of competition recognized by workers studying free-living animals and plants: 'exploitation' and 'interference' competition. The former may be defined as the joint utilization of a host species by two or more parasite species, while the latter occurs when antagonistic mechanisms are utilized by one species either to reduce the survival or fecundity of a second species or to displace it from a preferred site of attachment. Data illustrating both levels of interaction are collated from a survey of the published literature and these suggest that interference competition invariably operates asymmetrically. The data are also used to estimate a number of population parameters which are important in determining the impact of competition at the population level. Theoretical models of host-parasite associations for both classes of competition are used to examine the expected patterns of population dynamics that will be exhibited by simple two-species communities of parasites that utilize the same host population. The analysis suggests that the most important factor allowing competing species of parasites to coexist is the statistical distribution of the parasites within the host population. A joint stable equilibrium should be possible if both species are aggregated in their distribution. The size of the parasite burdens at equilibrium is then determined by other life-history parameters such as pathogenicity, rates of resource utilization and antagonistic ability. Comparison of these theoretical expectations with a variety of sets of empirical data forms the basis for a discussion about the importance of competition in natural parasite populations. The models are used to assess quantitatively the potential for using competing parasite species as biological control agents for pathogens of economic or medical importance. The most important criterion for identifying a successful control agent is an ability to infect a high proportion of the host population. If such a parasite species also exhibits an intermediate level of pathology or an efficient ability to utilize shared common resources, antagonistic interactions between the parasite species contribute only secondarily to the success of the control. Competition in parasites is compared with competition in free-living animals and plants. The comparison suggests further experimental tests which may help to assess the importance of competition in determining the structure of more complex parasite-host communities.

Animals↗
PubMed2001

Solute flux into parasitic plants.

Parasitic plants form intimate contacts with host tissue in order to gain access to host solutes. There are a variety of cell types within the host which parasitic plants could access to extract solutes. Depending on the degree to which the parasite has embraced the parasitic lifestyle, the extent of solute flux and the pathways used to transfer solutes from host to parasite will vary. To date, a variety of experimental approaches argue for diversity in the mechanisms and the routes by which parasites accumulate host solutes. Contact between host and parasite ranges from direct lumen-to-lumen links between host and parasite xylem and continuity between the sieve elements of host and parasite, to the involvement of transfer cells between host and parasite. Progress has been slow since Solms-Laubach distinguished types of parasitic plants that fed from host phloem or xylem in 1867, but advances in clearly delineating the pathways that link host and parasite should now be possible using fluorescent proteins expressed and restricted to particular cell types of the host. This will initially necessitate using Arabidopsis, but should allow the types of connection, i.e. symplasmic or apoplasmic, to be determined and then the identification of parasite transporters responsible for solute flux.

Adaptation, Physiological↗
PubMed1996

[Role of host behavior in the life cycle of parasites].

Parasite is an organism which, at least in a part of its ontogeny uses another living organism as a proper environment for its life. In the "parasite-host" relationships, formed by both these components, the parasite itself bears the burden of formation and maintenance of these relationships in a balance. Three factors play the main role in this process: (1) physiological adaptations of the parasite, enabling survival in the host body and resistance against defence reactions of the host; (2) morphological adaptations, leading to changes in the body structure of the parasite, dependent on location in the host body; and (3) behavioural adaptations assuring contacts of parasites with their hosts. In the process of evolution most groups of parasites evolved a complicated life cycle, with change of host and outer environment, succeeding in maturation of the parasite and production of the offspring. To pursuit this aim the parasite takes advantage from the behaviour of its potential host, its food preferences (e.g. by inclusion into a food chain), periodic or circadian migrations, and generally from its mode of life. The parasite modifies behaviour of its hosts, sometimes to a high degree, especially the behaviour of intermediate hosts, making them more conspicuously displayed for predators, the most often their final hosts. The parasite itself changes also its behaviour to be more attractive for a potential host or to enhance the possibility of finding a proper host. Finally, in a host population the parasite bears upon the position of particular host individuals by degradation of dominants and shifting them from reproduction. This phenomenon may be considered as a self-defence of the host population against reproduction of ill individuals, weaken by a burden of parasites.

Animals↗
PubMed1997

Community ecology of the metazoan parasites of grey mullets, Mugil platanus (Osteichthyes: Mugilidae) from the littoral of the state of Rio de Janeiro, Brazil.

One hundred and fifty specimens of grey mullets, Mugil platanus Günther, collected from the littoral of the State of Rio de Janeiro, Brazil, between June 1984 to August 1988, were necropsied to study their infracommunities of metazoan parasites. Twenty-five species of metazoan parasites were collected. One hundred and forty-nine (99.3%) fishes were parasitized by one or more metazoan species. According to their importance values, 13 species were considered dominant, 7 species codominant and 2 species subordinate (digeneans were the majority of the total number of parasite specimens collected, with 63.7%). The presence of larval stages of digeneans and cestodes suggest that M. platanus is at an intermediate level in the marine trophic web. The Simpson index for all parasite species was low (0.059) indicating lack of dominance by a particular species in the parasite community. The parasites of M. platanus showed a typical overdispersed pattern of distribution. The capture localities of the hosts were consolidated into 3 groups and its parasite communities showed high qualitative and quantitative similarity, with high values for the Sorenson similarity coefficient and Percent similarity coefficient. Six species had a positive correlation between the host's total body length and parasite intensity, while three species had a negative correlation. Four species had a positive correlation between the host's total body length and parasite prevalence and one species had negative correlation. Five parasite species had differences in prevalence and intensity in relation to sex of host, but these differences were confounded with the total length of male and female fishes. The mean diversity in the infracommunities of M. platanus was H' = 0.936 +/- 0.477, without correlation with the host's total length and without significant difference between male and female fishes. In the ectoparasites, the infracommunities showed overall positive association, 6 pairs of copepod species had a significant positive association. One pair of species showed a negative covariation between parasite intensities. Infracommunities of endoparasites also showed overall positive association, and four species pairs of digeneans had a significant positive association. Only one had a negative association. The specialist components with low prevalence values predominated in the parasite community, which, based on these factors, was considered closer to the isolationist type.

Animals↗
PubMed1997

[The fate of parasites of animal origin transmitted to humans].

The fate of a parasite transmitted from an animal to man depends on the ability of the contaminating agent to reach a place where it can thrive, to find necessary nutrients, and to resist host defense mechanisms. The purpose of this study was to evaluate the incidence of transmission of parasites from animals to man and to determine to what extent transmission is followed by development. Stenoxenic parasites whose life cycle requires transmission from animals to man obviously develop in man and then return to animals. These parasites cause holozoonoses of the cyclozoonosis type. Some euryxenic parasites can develop as well in man as in animals. These parasites can cause holozoonoses of the amphixenoses type. Other presumably euryxenic parasites can be transmitted from animals to man but not vice versa. These parasites are hemizoonoses agents. Non-transmission back from man to animals can be observed under several circumstances: incomplete development in man with failure to reach the stage at which transmission back to animals is possible; full development but with immaturity or sterility of the elements of dissemination necessary for transmission back to animals; full development but no way of evacuating elements of dissemination; full development and evacuation but with failure of elements of dissemination to survive. In these four cases man constitutes a dead-end for the parasite. A fifth possibility is that the parasite reaches full development but transmission back to animals cannot occur because man is not preyed upon by a carnivorous animal. In this case parasites are potential agents of holozoonoses and man is a cul-de-sac for the involved parasites.

Angiostrongylus↗
PubMed1999

Egg-laying behaviour by shiny cowbirds parasitizing brown-and-yellow marshbirds.

We studied the laying behaviour of shiny cowbirds, Molothrus bonariensis, parasitizing brown-and-yellow marshbirds, Pseudoleistes virescens. Shiny cowbirds lay two egg morphs, spotted and white immaculate. Brown-and-yellow marshbirds eject the white egg morph but accept the spotted morph. The incidence of parasitism in this host was 66.5%, and half of the parasitized nests had more than one shiny cowbird egg. There was a positive relationship between the number of parasitic events and the availability of nests in the laying stage, but parasitic events reached a plateau when the availability of nests was high. The distribution of parasitic eggs per nest was more clumped than expected by chance. Shiny cowbird females synchronized parasitism with host laying in 80% of the cases. They seldom parasitized nests before the host started laying or after the nest had been deserted or predated. The majority of multiply parasitized nests were parasitized by more than one female. Females that lay white eggs did not avoid parasitizing brown-and-yellow marshbird nests. Egg pecking by cowbird females resulted, on average, in one egg lost per parasitic event and the probability of being broken was greater for host eggs when host and parasitic eggs where both in the nest. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1997

Heme biosynthesis by the malarial parasite. Import of delta-aminolevulinate dehydrase from the host red cell.

The mouse and human malarial parasites, Plasmodium berghei and Plasmodium falciparum, respectively, synthesize heme de novo following the standard pathway observed in animals despite the availability of large amounts of heme, derived from red cell hemoglobin, which is stored as hemozoin pigment. The enzymes, delta-aminolevulinate dehydrase (ALAD), coproporphyrinogen oxidase, and ferrochelatase are present at strikingly high levels in the P. berghei infected mouse red cell in vivo. The isolated parasite has low levels of ALAD and the data clearly indicate it to be of red cell origin. The purified enzyme preparations from the uninfected red cell and the parasite are identical in kinetic properties, subunit molecular weight, cross-reaction with antibodies to the human enzyme, and N-terminal amino acid sequence. Immunogold electron microscopy of the infected culture indicates that the enzyme is present inside the parasite and, therefore, is not a contaminant. The parasite derives functional ALAD from the host and the enzyme binds specifically to isolated parasite membrane in vitro, suggestive of the involvement of a receptor in its translocation into the parasite. While, ALAD, coproporphyrinogen oxidase, and ferrochelatase from the parasite and the uninfected red cell supernatant have identical subunit molecular weights on SDS-polyacrylamide gel electrophoresis and show immunological cross-reaction with antibodies to the human enzymes, as revealed by Western analysis, the first enzyme of the pathway, namely, delta-aminolevulinate synthase (ALAS) in the parasite, unlike that of the red cell host, does not cross-react with antibodies to the human enzyme. However, ALAS enzyme activity in the parasite is higher than that of the infected red cell supernatant. We therefore conclude that the parasite, while making its own ALAS, imports ALAD and perhaps most of the other enzymes of the pathway from the host to synthesize heme de novo, and this would enable it to segregate this heme from the heme derived from red cell hemoglobin degradation. ALAS of the parasite and the receptor(s) involved in the translocation of the host enzymes into the parasite would be unique drug targets.

Animals↗
PubMed1998

Acute Plasmodium chabaudi chabaudi malaria infection induces antibodies which bind to the surfaces of parasitized erythrocytes and promote their phagocytosis by macrophages in vitro.

CBA/Ca mice infected with 5 x 10(4) Plasmodium chabaudi chabaudi AS-parasitized erythrocytes experience acute but self-limiting infections of relatively short duration. Parasitemia peaks ( approximately 40% infected erythrocytes) on day 10 or 11 and is then partially resolved over the ensuing 5 to 6 days, a period referred to as crisis. How humoral and cellular immune mechanisms contribute to parasite killing and/or clearance during crisis is controversial. Humoral immunity might be parasite variant, line, or species specific, while cellular immune responses would be relatively less specific. For P. c. chabaudi AS, parasite clearance is largely species and line specific during this time, which suggests a primary role for antibody activity. Accordingly, acute-phase plasma (APP; taken from P. c. chabaudi AS-infected mice at day 11 or 12 postinfection) was examined for the presence of parasite-specific antibody activity by enzyme-linked immunosorbent assay. Antibody binding to the surface of intact, live parasitized erythrocytes, particularly those containing mature (trophozoite and schizont) parasites, was demonstrated by immunofluorescence in APP and the immunoglobulin G (IgG)-containing fraction thereof. Unfractionated APP (from P. c. chabaudi AS-infected mice), as well as its IgG fraction, specifically mediated the opsonization and internalization of P. c. chabaudi AS-parasitized erythrocytes by macrophages in vitro. APP from another parasite line (P. c. chabaudi CB) did not mediate the same effect against P. c. chabaudi AS-parasitized erythrocytes. These results, which may represent one mechanism of parasite removal during crisis, are discussed in relation to the parasite variant, line, and species specificity of parasite clearance during this time.

Acute Disease↗
PubMed1974

Surface properties of extracellular malaria parasites: morphological and cytochemical study.

Morphological and cytochemical surface characteristics of isolated malaria parasites (Plasmodium berghei) and host erythrocytes were compared by electron microscopy by using thin section and carbon replica techniques. Erythrocytes were uniform in shape and had fine, granular surfaces. In contrast, free parasites exhibited a variety of sizes, shapes, and surface textures. Fine surface stippling was a common topographical feature of isolated parasites. Small, infective forms often had patterned surfaces resulting from the protuberance of an underlying thick intermediate layer. Results of cytochemical analysis using a sialophilic colloidal iron stain indicated that the malaria parasite's surface lacked exposed sialic acid groups which would normally give rise to a net negative surface charge common to erythrocytes. Biochemical assay demonstrated that malaria parasites contained about one-half the amount of sialic acid per unit weight as did control red cell extracts. Similarly, external acidic mucopolysaccharide coats of free parasites, as revealed by ruthenium red staining were extremely thin as compared with the thick glycocalyx layer of red cells. Lipid plaques at the surface of parasites and red cells were localized by lipophilic iron colloid staining. Although the gross patchwork distribution of plaques was somewhat similar for the two cell types, the parasites were stained more intensely and had a closer-knit patchwork pattern than those exhibited by the erythrocytes. Such findings indicate that there are slight differences in the arrangement of phospholipids at the surfaces of limiting membranes of host cells and parasites. The significance of the above cytochemical surface properties of the malaria parasite (which are seemingly akin to those of intracellular organelles is discussed in relation to certain host-parasite interactions, such as parasite adhesion to target cells and enhanced clearance of extracellular parasites.

Cell Membrane↗
PubMed2001

Altered dietary nutrient intake maintains metabolic homeostasis in parasitized larvae of the insect Manduca sexta L.

Manduca sexta larvae exhibited altered food selection over a 2- or 3-day feeding period when parasitized by Cotesia congregata, and offered a choice of two chemically defined diets, one containing casein without sucrose and a second with sucrose but no casein. While normal larvae consumed the diets in a ratio of approximately 2:1 protein:carbohydrate (w/w), parasitized insects consumed a ratio of approximately 1:1. The altered nutrient ratio consumed by parasitized insects was principally due to a decrease in consumption of the protein diet, and was only partially explained by their lower growth. Conditioning larvae for 1 day to either one of the choice diets had little effect on subsequent dietary intake over a 2-day feeding period. Conditioned larvae, regardless of parasitism, initially fed on the opposite diet immediately after conditioning. Although this suggests that the altered nutrient intake displayed by parasitized insects was not due to any failure in their capacity for dietary selection, these results do not definitively demonstrate an altered nutrient intake target by parasitized larvae. Rather, parasitism may compromise dietary selection, resulting in random feeding. When parasitized larvae were maintained on several isocaloric diets with a varying ratio of casein and sucrose, those larvae feeding on the diet with a ratio of 1:1 of these nutrients supported the largest parasite population. Previous investigation of larvae maintained on a single artificial diet established that parasitized insects display an aberrant induction of gluconeogenesis, so that haemolymph trehalose is maintained at a level equivalent to that of normal insects. In contrast, the present results demonstrated that parasitized larvae offered a choice of diets, and feeding at the altered nutrient ratio above, maintain haemolymph sugar but have the same level of gluconeogenesis as normal larvae given the same dietary choice. These investigations suggest that altered food selection by parasitized M. sexta larvae maintains metabolic homeostasis and, moreover, may be adaptive for C. congregata, potentially maximizing the number of parasites developing in a single host larva.

Animal Nutritional Physiological Phenomena↗
PubMed1984

Parasite enzymes as potential targets for antiparasitic chemotherapy.

I have thus far listed a total of 10 potential targets for antiparasitic chemotherapeutic consideration. This is by no means a completed list. Many more will be added to it with time and with more future findings. Among these 10 targets (summarized in Table I), however, one may gain some insight and see a few interesting general trends: (1) Nucleic acid metabolism and carbohydrate-energy metabolism in protozoan parasites appear to be targets for fruitful chemotherapeutic attacks. Their being useful targets results generally from the deficient metabolism in the protozoan parasites. Thus, the main vulnerability among the protozoan parasites is closely associated with their parasitic nature. (2) Microtubules and nervous systems appear to be the main chemotherapeutic targets in helminths. They differ from those in the host not because of their parasitic nature but, more likely, because of the evolutionary distance separating the mammalian hosts and the primitive metazoa. Thus, free-living nematodes, such as Caenorhabditis elegans, have their microtubules just as susceptible to the benzimidazole anthelmintics as those from the parasitic worms. The motoneuronal map of C. elegans is identical with that of Ascaris lumbricoides. Both worms are similarly immobilized by levamisole, piperazine, avermectins, etc. The dual insecticidal and antiexoparasite activities found in the avermectins and milbemycins may also suggest that the free-living insects and the ticks and lice may have the same GABA nervous system. This main discrepancy between protozoan parasites and metazoan parasites may be partly attributable to the higher mutation rates and higher frequencies of genetic recombination among the protozoa, evidenced by the higher rates of development of drug resistance among them. The fast adaptation to a new environment may be essential for survival, but it would also lead to metabolic deficiencies after the protozoa lived in a luxurious environment for a while. This revelation may suggest that future chemotherapeutic studies on parasitic helminths can utilize free-living helminths as models to eliminate many unnecessary technical difficulties. Also, there perhaps could be a further classification among the parasites to term the protozoa "true parasites" and the helminth "pseudo-parasites" from the viewpoint of chemotherapy.

Animals↗
PubMed1996

The contagion indicator hypothesis for parasite-mediated sexual selection.

Hamilton and Zuk [Hamilton, W. D. & Zuk, M. (1982) Science 218, 384-387] proposed that females choosing mates based on the degree of expression of male characters obtain heritable parasite resistance for their offspring. Alternatively, the "contagion indicator" hypothesis posits that females choose mates based on the degree of expression of male characters because the latter indicate a male's degree of infestation of parasites and thus the risk that choosing females and their offspring will acquire these parasites. I examined whether parasite transmittability affects the probability that parasite intensity and male mating success are negatively correlated in intraspecific studies of parasite-mediated sexual selection. When females risk infection of themselves or their future offspring as a result of mating with a parasitized male, negative relationships between parasite intensity and male mating success are significantly more likely to occur than when females do not risk such infection. The direct benefit to females of avoiding parasitic infection is proposed to lead to the linkage between variable secondary sexual characters and the intensity of transmittable parasites. The direct benefits of avoiding associatively transmittable parasites should be considered in future studies of parasite-mediated sexual selection.

Adaptation, Biological↗