The evolution of tissue migration by parasitic nematode larvae

Ascaris suum infection in juvenile pigs elicits a local Th2 response in a setting of ongoing Th1 expansion

Ascaris spp. undergo extensive migration within the body before establishing patent infections in the small intestinal tract of humans and pigs. However, whether larval migration is critical for inducing efficient type 2 responses remains poorly understood. Therefore, we investigated systemic versus local adaptive immune responses along the hepato-tracheal migration of Ascaris suum during primary, single infections in conventionally raised pigs. Neither the initial invasion of gut tissue nor migration through the liver resulted in discernable Th2 cell responses. In contrast, lung-stage larvae elicited a Th2-biased pulmonary response, which declined after the larvae had left the lungs. In the small intestine, we observed an accumulation of Th2 cells upon the arrival of fourth-stage larvae (L4) to the small intestinal lumen. In parallel, we noticed robust and increasing Th1 responses in circulation, migration-affected organs, and draining lymph nodes. Phenotypic analysis of CD4+ T cells specifically recognizing A. suum antigens in the circulation and lung tissue of infected pigs confirmed that the majority of Ascaris-specific T cells produced IL-4 (Th2) and, to a much lesser extent, IL-4/IFN-g (Th2/1 hybrids) or IFN-g alone (Th1). These data demonstrate that lung-stage but not the early liver-stage larvae lead to a locally restricted Th2 response. Significant Th2 cell accumulation in the small intestine occurs only when L4 complete the body migration. In addition, Th2 immunity seems to be hampered by the concurrent, nonspecific Th1 bias in growing pigs. Together, the late onset of Th2 immunity at the site of infection and the Th1-biased systemic immunity likely enable the establishment of intestinal infections by sufficiently large L4 stages and pre-adult worms, some of which resist expulsion mechanisms.

Strongyloides questions-a research agenda for the future

The Strongyloides genus of parasitic nematodes have a fascinating life cycle and biology, but are also important pathogens of people and a World Health Organization-defined neglected tropical disease. Here, a community of Strongyloides researchers have posed thirteen major questions about Strongyloides biology and infection that sets a Strongyloides research agenda for the future. This article is part of the Theo Murphy meeting issue 'Strongyloides: omics to worm-free populations'.

Extra-intestinal complications of Ascaris lumbricoides infections in India: A systematic review and meta-analysis

The aberrant migration of Ascaris lumbricoides may cause extra-intestinal ascariasis (EIA) involving hepato-biliary-pancreatic (HBP) or other extra-gastro-intestinal (EGI) organs. We performed a systematic review and meta-analysis to study the risk factors and clinical presentations of EIA, and differences in HBP and EGI ascariasis. Medline, Web of Science and Embase were searched for cases of EIA in the English language from India. From 1204 articles, 86 studies (105 cases) were included. The majority of the cases involved the HBP system (78%). Among HBP ascariasis, the most commonly involved site was the bile duct (53.6%). Females had 11.3 times higher odds (95% CI 2.852 to 44.856; p=0.001) of HBP ascariasis, while the pediatric population had lower odds (OR=0.323). Previous gallbladder disease was significantly associated with HBP ascariasis in adults (p=0.046), while a significantly higher number of cases of EGI ascariasis were observed among pediatric patients (p=0.003). Ocular symptoms occurred exclusively in the pediatric population (p=0.017). Overall, death was reported in 3.8% of patients (n=4). This review emphasizes the importance of the complications of EIA. It encourages future research into issues such as the reasons of higher gall bladder ascariasis in females and the implications of Ascaris-related complications following biliary tract interventions. It also suggests considering Ascaris as a differential diagnosis for airway obstuction in intubated critically ill patients.

Study of the migration of Fasciola hepatica juveniles across the intestinal barrier of the host by quantitative proteomics in an ex vivo model

Fasciola hepatica is a trematode parasite that infects animals and humans causing fasciolosis, a worldwide-distributed disease responsible for important economic losses and health problems. This disease is of growing public health concern since parasite isolates resistant to the current treatment (triclabendazole) have increasingly been described. F. hepatica infects its vertebrate host after ingestion of the encysted parasite (metacercariae), which are found in the water or attached to plants. Upon ingestion, newly excysted juveniles of F. hepatica (FhNEJ) emerge in the intestinal lumen and cross the intestinal barrier, reach the peritoneum and migrate to the biliary ducts, where adult worms fully develop. Despite the efforts made to develop new therapeutic and preventive tools, to date, protection against F. hepatica obtained in different animal models is far from optimal. Early events of host-FhNEJ interactions are of paramount importance for the infection progress in fasciolosis, especially those occurring at the host-parasite interface. Nevertheless, studies of FhNEJ responses to the changing host environment encountered during migration across host tissues are still scarce. Here, we set-up an ex vivo model coupled with quantitative SWATH-MS proteomics to study early host-parasite interaction events in fasciolosis. After comparing tegument and somatic fractions from control parasites and FhNEJ that managed to cross a mouse intestinal section ex vivo, a set of parasite proteins whose expression was statistically different were found. These included upregulation of cathepsins L3 and L4, proteolytic inhibitor Fh serpin 2, and a number of molecules linked with nutrient uptake and metabolism, including histone H4, H2A and H2B, low density lipoprotein receptor, tetraspanin, fatty acid binding protein a and glutathione-S-transferase. Downregulated proteins in FhNEJ after gut passage were more numerous than the upregulated ones, and included the heath shock proteins HSP90 and alpha crystallin, amongst others. This study brings new insights into early host-parasite interactions in fasciolosis and sheds light on the proteomic changes in FhNEJ triggered upon excystment and intestinal wall crossing, which could serve to define new targets for the prevention and treatment of this widespread parasitic disease.

Fasciolosis caused by the helminth parasite Fasciola hepatica is a serious health and economic problem worldwide. Treatment and prevention of this disease pose several drawbacks that have so far not been solved. The definition of suitable parasite molecular targets to overcome such drawbacks should be based on thoroughly deciphering host-parasite interactions, and in this regard most studies have focused on the adult stages of F. hepatica. Nevertheless, in this context, the study of the transient juvenile stages of this parasite could be of higher utility due to the importance of early interactions with the host for parasite migration and the successful establishment of infection. In this work, we set-up an ex vivo model and performed a quantitative proteomics approach to study the changes in F. hepatica juveniles upon gut passage. We found that the parasite tegument and somatic compartments experienced deep changes in their composition and showed that the host triggers the expression of specific molecules that are important for parasite migration and survival at this stage. The molecules described here could serve to better understand host-parasite interactions and to define new targets against fasciolosis.

The equine ascarids: resuscitating historic model organisms for modern purposes

The equine ascarids, Parascaris spp., are important nematode parasites of juvenile horses and were historically model organisms in the field of cell biology, leading to many important discoveries, and are used for the study of chromatin diminution. In veterinary parasitology, Parascaris spp. are important not only because they can cause clinical disease in young horses but also because they are the only ascarid parasites to have developed widespread anthelmintic resistance. Despite this, much of the general biology and mechanisms of anthelmintic resistance are poorly understood. This review condenses known basic biological information and knowledge on the mechanisms of anthelmintic resistance in Parascaris spp., highlighting the importance of foundational research programs. Although two variants of this parasite were recognized based on the number of chromosomes in the 1870s and suggested to be two species in 1890, one of these, P. univalens, appears to have been largely forgotten in the veterinary scientific literature over the past 100 years. We describe how this omission has had a century-long effect on nomenclature and data analysis in the field, highlighting the importance of proper specimen identification in public repositories. A summary of important basic biology, including life cycle, in vitro maintenance, and immunology, is given, and areas of future research for the improvement of knowledge and development of new systems are given. Finally, the limited knowledge regarding anthelmintic resistance in Parascaris spp. is summarized, along with caution regarding assumptions that resistance mechanisms can be applied across clades.

Economies of parasite body size

Parasitism has independently evolved multiple times across the entire tree of life, and there are numerous parasitic representatives from every major eukaryote kingdom. In animals alone, parasitism has independently evolved at least 200 times. If there are any organisms that one might think would have access to limitless resources, it would be parasites. You would think that living in or on the body of their host, which serves as both a habitat and a food source, would provide parasites with bountiful resources to maximise every aspect of their existence, especially reproduction. But parasitism is not a loophole out of life history trade-offs. There is still a finite amount of resources that a parasite can obtain and allocate to its many needs. Living in a resource-rich environment has allowed many parasites to grow to sizes that are of multiple orders of magnitude larger than their free-living relatives. But that does not mean that the underlying economy of nature and its limitations are inapplicable to parasites.

Possible Roles of Specific Amino Acids in β-Tubulin Isotypes in the Growth and Maintenance of Neurons: Novel Insights From Cephalopod Mollusks

Microtubules, are formed of the protein tubulin, which is a heterodimer of α- and β-tubulin subunits. Both α- and β-tubulin exist as numerous isotypes, differing in amino acid sequence and tissue distribution. Among the vertebrate β isotypes, βIII has a very narrow distribution, being found primarily in neurons and in advanced cancers. The places in the amino acid sequence where βIII differs from the other β isotypes are highly conserved in evolution. βIII appears to be highly resistant to reactive oxygen species and it forms highly dynamic microtubules. The first property would be very useful in neurons, which have high concentrations of free radicals, and the high dynamicity would aid neurite outgrowth. The same properties make βIII useful in cancers. Examination of the amino acid sequences indicates a cysteine cluster at positions 124–129 in βIII (CXXCXC). This occurs in all βIII isotypes but not in βI, βII, or βIV. βIII also lacks the easily oxidized C239. Both features could play roles in free radical resistance. Many aggressive tumors over-express βIII. However, a recent study of breast cancer patients showed that many of them mutated their βI, βII, and βIV at particular places to change the residues to those found at the corresponding sites in βIII; these are all sites that are highly conserved in vertebrate βIII. It is possible that these residues are important, not only in the resistance to free radicals, but also in the high dynamicity of βIII. The cephalopod mollusks are well known to be highly intelligent and can remodel their own brains. Interestingly, several cephalopods contain the cysteine cluster as well as up to 7 of the 17 residues that are highly conserved in vertebrate βIII, but are not found in βI, βII, or βIV. In short, it is possible that we are looking at a case of convergent evolution, that a βIII-like isotype may be required for neuronal growth and function and that a structure-function study of the particular residues conserved between vertebrate βIII and cephalopod tubulin isotypes could greatly increase our understanding of the role of the various tubulin isotypes in neuronal growth and function and could aid in the development of novel anti-tumor drugs.

Host-Parasite Relationships in Porcine Ascariosis: Anticoagulant Potential of the Third Larval Stage of Ascaris suum as a Possible Survival Mechanism

Simple Summary

Ascaris suum parasitises pigs all over the world causing a disease responsible for producing reductions in weight gains and damages to several organs of the infected animals that incur huge economic losses for the swine industry. While adult worms of this parasite are located in the small intestine of the host, their larval stages migrate through the bloodstream as an evolutionary advantageous strategy within a hostile environment that confronts host responses such as blood clots formation. The aim of this work is to study the ability of A. suum larvae to inhibit blood coagulation as a possible mechanism to control blood clots formation and facilitate their migration. The results showed that these larvae inhibited host blood coagulation and possessed molecules similar to those responsible for inhibiting blood coagulation in pigs. The anticoagulant effect of A. suum larvae could constitute a potential survival mechanism for the parasite. Therefore, developing new control strategies directed at this and similar processes could avoid A. suum larval migration and the establishment of adult worms in their definitive location, which is necessary to confront the damages and economic losses produced by this parasitosis.

Abstract

In order to evade the response of their hosts, helminth parasites have evolved precise and highly regulated mechanisms, including migration strategies of the larval stages. In regard to porcine ascariosis caused by Ascaris suum, its infective third-stage larvae (AsL3) undergo a complex migratory route through the bloodstream of their host before establishing in the small intestine to reach maturation. Despite the benefits attributed to this migration, blood clots formation could compromise larvae survival. The aim of this work was to study the interaction between the cuticle and excretory/secretory antigens of AsL3 and the host coagulation cascade. Larvae were obtained after incubating and hatching A. suum eggs, after which the antigenic extracts were produced. Their ability to disrupt the coagulation cascade was studied using anticoagulation and chromogenic assays, and techniques based on electrophoresis. The obtained results showed that both antigenic extracts possessed anticoagulant potential, being able to inhibit the intrinsic, extrinsic and/or common pathways of the blood coagulation cascade as well as the activated factor X. Moreover, three A. suum serpin proteins were identified as candidates to inhibit this host coagulation factor. To the best of our knowledge, this study shows, for the first time, the anticoagulant potential of the infective larvae of A. suum, which could be used by the parasite as a mechanism to facilitate its invasion and survival in the host.

Insights into Fasciola hepatica Juveniles: Crossing the Fasciolosis Rubicon

Unraveling the molecular interactions governing the first contact between parasite and host tissues is of paramount importance to the development of effective control strategies against parasites. In fasciolosis, a foodborne trematodiasis caused mainly by Fasciola hepatica, these early interactions occur between the juvenile worm and the host intestinal wall a few hours after ingestion of metacercariae, the infectious stage of the parasite. However, research on these early events is still scarce and the majority of studies have focused on the adult worm. Here, we review current knowledge on the biology and biochemistry of F. hepatica juveniles and their molecular relationships with the host tissues and identify the research needs and gaps to be covered in the future.

Whipworm and roundworm infections

Trichuriasis and ascariasis are neglected tropical diseases caused by the gastrointestinal dwelling nematodes Trichuris trichiura (a whipworm) and Ascaris lumbricoides (a roundworm), respectively. Both parasites are staggeringly prevalent, particularly in tropical and subtropical areas, and are associated with substantial morbidity. Infection is initiated by ingestion of infective eggs, which hatch in the intestine. Thereafter, T. trichiura larvae moult within intestinal epithelial cells, with adult worms embedded in a partially intracellular niche in the large intestine, whereas A. lumbricoides larvae penetrate the gut mucosa and migrate through the liver and lungs before returning to the lumen of the small intestine, where adult worms dwell. Both species elicit type 2 anti-parasite immunity. Diagnosis is typically based on clinical presentation (gastrointestinal symptoms and inflammation) and the detection of eggs or parasite DNA in the faeces. Prevention and treatment strategies rely on periodic mass drug administration (generally with albendazole or mebendazole) to at-risk populations and improvements in water, sanitation and hygiene. The effectiveness of drug treatment is very high for A. lumbricoides infections, whereas cure rates for T. trichiura infections are low. Novel anthelminthic drugs are needed, together with vaccine development and tools for diagnosis and assessment of parasite control in the field.

Differences in Gene Expression Profiles of Seven Target Proteins in Third-Stage Larvae of Anisakis simplex (Sensu Stricto) by Sites of Infection in Blue Whiting (Micromesistius poutassou)

The third-stage larvae of the parasitic nematode genus Anisakis tend to encapsulate in different tissues including the musculature of fish. Host tissue penetration and degradation involve both mechanic processes and the production of proteins encoded by an array of genes. Investigating larval gene profiles during the fish infection has relevance in understanding biological traits in the parasite’s adaptive ability to cope with the fish hosts’ defense responses. The present study aimed to investigate the gene expression levels of some proteins in L3 of A. simplex (s.s.) infecting different tissues of blue whiting Micromesistius poutassou, a common fish host of the parasite in the NE Atlantic. The following genes encoding for Anisakis spp. proteins were studied: Kunitz-type trypsin inhibitor (TI), hemoglobin (hb), glycoprotein (GP), trehalase (treh), zinc metallopeptidase 13 (nas 13), ubiquitin-protein ligase (hyd) and sideroflexin 2 (sfxn 2). Significant differences in gene transcripts (by quantitative real-time PCR, qPCR) were observed in larvae located in various tissues of the fish host, with respect to the control. ANOVA analysis showed that relative gene expression levels of the seven target genes in the larvae are linked to the infection site in the fish host. Genes encoding some of the target proteins seem to be involved in the host tissue migration and survival of the parasite in the hostile target tissues of the fish host.

Pro-fibrinolytic potential of the third larval stage of Ascaris suum as a possible mechanism facilitating its migration through the host tissues

Background

Ascaris roundworms are the parasitic nematodes responsible for causing human and porcine ascariasis. Whereas A. lumbricoides is the most common soil-transmitted helminth infecting humans in the world, A. suum causes important economic losses in the porcine industry. The latter has been proposed as a model for the study of A. lumbricoides since both species are closely related. The third larval stage of these parasites carries out an intriguing and complex hepatopulmonary route through the bloodstream of its hosts. This allows the interaction between larvae and the physiological mechanisms of the hosts circulatory system, such as the fibrinolytic system. Parasite migration has been widely linked to the activation of this system by pathogens that are able to bind plasminogen and enhance plasmin generation. Therefore, the aim of this study was to examine the interaction between the infective third larval stage of A. suum and the host fibrinolytic system as a model of the host-Ascaris spp. relationships.

Methods

Infective larvae were obtained after incubating and hatching fertile eggs of A. suum in order to extract their cuticle and excretory/secretory antigens. The ability of both extracts to bind and activate plasminogen, as well as promote plasmin generation were assayed by ELISA and western blot. The location of plasminogen binding on the larval surface was revealed by immunofluorescence. The plasminogen-binding proteins from both antigenic extracts were revealed by two-dimensional electrophoresis and plasminogen-ligand blotting, and identified by mass spectrometry.

Results

Cuticle and excretory/secretory antigens from infective larvae of A. suum were able to bind plasminogen and promote plasmin generation in the presence of plasminogen activators. Plasminogen binding was located on the larval surface. Twelve plasminogen-binding proteins were identified in both antigenic extracts.

Conclusions

To the best of our knowledge, the present results showed for the first time, the pro-fibrinolytic potential of infective larvae of Ascaris spp., which suggests a novel parasite survival mechanism by facilitating the migration through host tissues.

The role of the liver in the migration of parasites of global significance

Many parasites migrate through different tissues during their life-cycle, possibly with the aim to enhance their fitness. This is true for species of three parasite genera of global importance, Ascaris, Schistosoma and Plasmodium, which cause significant global morbidity and mortality. Interestingly, these parasites all incorporate the liver in their life-cycle. The liver has a special immune status being able to preferentially induce tolerance over immunity. This function may be exploited by parasites to evade host immunity, with Plasmodium spp. in particular using this organ for its multiplication. However, hepatic larval attrition occurs in both ascariasis and schistosomiasis. A better understanding of the molecular mechanisms involved in hepatic infection could be useful in developing novel vaccines and therapies for these parasites.

Plastic and micro-evolutionary responses of a nematode to the host immune environment

Parasitic organisms have to cope with the defences deployed by their hosts and this can be achieved adopting immune evasion strategies or optimal life history traits according to the prevailing pattern of immune-mediated mortality. Parasites often encounter variable immune environments both within and between hosts, promoting the evolution of plastic strategies instead of fixed responses. Here, we explored the plasticity and micro-evolutionary responses of immunomodulatory mechanisms and life history traits to the immune environment provided by the host, using the parasitic nematode Heligmosomoides polygyrus. To test if the parasite responds plastically to the immune environment, we stimulated the systemic inflammatory response of mice and we assessed i) the expression of two genes with candidate immunomodulatory functions (Hp-Tgh2 and Hp-CPI); ii) changes in the number of eggs shed in the faeces. To test if the immune environment induces a micro-evolutionary response in the parasite, we maintained the nematode in mice whose inflammatory response was up- or down-regulated during four generations. We found that H. polygyrus plastically responded to a sudden rise of pro-inflammatory cytokines, up-regulating the expression of two candidate genes involved in the process of immune modulation, and enhancing egg output. At the micro-evolutionary level, parasites maintained in hosts experiencing different levels of inflammation did not have differential expression of Hp-Tgh2 and Hp-CPI genes when infecting unmanipulated, control, mice. However, parasites maintained in mice with an up-regulated inflammation shed more eggs compared to the control line. Overall, our study shows that H. polygyrus can plastically adjust the expression of immunomodulatory genes and life history traits, and responds to selection exerted by the host immune system.

Autonomy and integration in complex parasite life cycles

Complex life cycles are common in free-living and parasitic organisms alike. The adaptive decoupling hypothesis postulates that separate life cycle stages have a degree of developmental and genetic autonomy, allowing them to be independently optimized for dissimilar, competing tasks. That is, complex life cycles evolved to facilitate functional specialization. Here, I review the connections between the different stages in parasite life cycles. I first examine evolutionary connections between life stages, such as the genetic coupling of parasite performance in consecutive hosts, the interspecific correlations between traits expressed in different hosts, and the developmental and functional obstacles to stage loss. Then, I evaluate how environmental factors link life stages through carryover effects, where stressful larval conditions impact parasites even after transmission to a new host. There is evidence for both autonomy and integration across stages, so the relevant question becomes how integrated are parasite life cycles and through what mechanisms? By highlighting how genetics, development, selection and the environment can lead to interdependencies among successive life stages, I wish to promote a holistic approach to studying complex life cycle parasites and emphasize that what happens in one stage is potentially highly relevant for later stages.

Helminths in the lungs

Parasitic helminths infect well over one billion people and typically cause chronic and recurrent infections that exert a considerable toll on human health and productivity. A significant number of important intestinal- and tissue-dwelling helminth parasites have evolved a scripted migration through select organ systems. Of specific interest here are the helminth parasites that interact with respiratory tissues and the pulmonary immune system. This review will consider the nature of the interactions between helminth parasites and the lung environment, as well as the consequences of these interactions on the evolution of parasitism and host immunity.

The evolution of parasitism in Nematoda

Nematodes are abundant and diverse, and include many parasitic species. Molecular phylogenetic analyses have shown that parasitism of plants and animals has arisen at least 15 times independently. Extant nematode species also display lifestyles that are proposed to be on the evolutionary trajectory to parasitism. Recent advances have permitted the determination of the genomes and transcriptomes of many nematode species. These new data can be used to further resolve the phylogeny of Nematoda, and identify possible genetic patterns associated with parasitism. Plant-parasitic nematode genomes show evidence of horizontal gene transfer from other members of the rhizosphere, and these genes play important roles in the parasite-host interface. Similar horizontal transfer is not evident in animal parasitic groups. Many nematodes have bacterial symbionts that can be essential for survival. Horizontal transfer from symbionts to the nematode is also common, but its biological importance is unclear. Over 100 nematode species are currently targeted for sequencing, and these data will yield important insights into the biology and evolutionary history of parasitism. It is important that these new technologies are also applied to free-living taxa, so that the pre-parasitic ground state can be inferred, and the novelties associated with parasitism isolated.

Variability in the intensity of nematode larvae from gastrointestinal tissues of a natural herbivore

The migration of infective nematode larvae into the tissues of their hosts has been proposed as a mechanism of reducing larval mortality and increase parasite lifetime reproductive success. Given that individual hosts differ in the level of exposure, strength of immune response and physiological conditions we may expect the number of larvae in tissue to vary both between and within hosts. We used 2 gastrointestinal nematode species common in the European rabbit (Oryctolagus cuniculus) and examined how the number of larvae in the tissue changed with the immune response, parasite intensity-dependent constraints in the lumen and seasonal weather factors, in rabbits of different age, sex and breeding status. For both nematode species, larvae from the gastrointestinal tissue exhibited strong seasonal and host age-related patterns with fewer larvae recovered in summer compared to winter and more in adults than in juveniles. The number of larvae of the 2 nematodes was positively associated with intensity of parasite infection in the lumen and antibody responses while it was negatively related with air temperature and rainfall. Host sex, reproductive status and co-infection with the second parasite species contributed to increase variability between hosts. We concluded that heterogeneities in host conditions are a significant cause of variability of larval abundance in the gastrointestinal tissues. These findings can have important consequences for the dynamics of nematode infections and how parasite's life-history strategies adjust to host changes.

Benefits of fidelity: does host specialization impact nematode parasite life history and fecundity?

The range of hosts used by a parasite is influenced by macro-evolutionary processes (host switching, host–parasite co-evolution), as well as ‘encounter filters’ and ‘compatibility filters’ at the micro-evolutionary level driven by host/parasite ecology and physiology. Host specialization is hypothesized to result in trade-offs with aspects of parasite life history (e.g. reproductive output), but these have not been well studied. We used previously published data to create models examining general relationships among host specificity and important aspects of life history and reproduction for nematodes parasitizing animals. Our results indicate no general trade-off between host specificity and the average pre-patent period (time to first reproduction), female size, egg size, or fecundity of these nematodes. However, female size was positively related to egg size, fecundity, and pre-patent period. Host compatibility may thus not be the primary determinant of specificity in these parasitic nematodes if there are few apparent trade-offs with reproduction, but rather, the encounter opportunities for new host species at the micro-evolutionary level, and other processes at the macro-evolutionary level (i.e. phylogeny). Because host specificity is recognized as a key factor determining the spread of parasitic diseases understanding factors limiting host use are essential to predict future changes in parasite range and occurrence.

Determinants of Life-history Evolution in Nematodes

What are the determinants of parasite life-history evolution? Does life-history evolution of parasitic species differ from their free-living relatives? How and why do host and parasite life-history traits covary? Here, Serge Morand and Gabriele Sorci use recent comparative studies to investigate life-history evolution in nematodes which present both parasitic and free-living groups. Application of life-history theory to nematodes suggests that the conventional wisdom concerning the high fecundity of parasitic species could simply be the result of the larger body size of the latter when compared with free-living forms. The authors also emphasize, with the use of examples, that in most cases parasite life-history evolution depends on host features.

Associations between body composition and helminths of lesser snow geese during winter and spring migration

Costs of parasitism are predicted to be higher with greater parasite intensities and higher inter-parasite competition (diversity). We tested whether greater helminth intensities and diversity were associated with poorer body composition (whole-body fat, protein, mineral and true body mass) in lesser snow geese, Chen caerulescens caerulescens. As part of a larger study on nutritional ecology, 828 wintering or migrating geese were shot between January and May 1983 in 27 different date-locations (samples) during their northward migration through mid-continental North America. A large proportion of overall variation in body composition and parasite communities was among samples, so we analyzed data within each of the 27 samples, controlling for structural body size (the first principal component of 10 body size measurements), sex and the age of geese. There was no compelling evidence that cestodes, trematodes or helminth diversity were associated with variation in body composition but nematodes had several negative associations with fat reserves. However, negative associations between fat reserves and nematodes occurred most often in geese collected between March and May when nematode prevalences and intensities were relatively low. This suggests several possibilities: that the most common nematodes (Heterakis dispar and Trichostrongylus tenuis) were more virulent at this time, that infected individuals had been chronically infected and suffered cumulative nutrient deficits that lasted until late in the spring migration, or that geese became more vulnerable to the effects of parasites at this time of year, possibly because they redirected resources away from immunity toward fat storage in preparation for reproduction.

Macroparasite life histories

Parasites and parasitism is common. Worm macroparasites have evolved life-history traits that allow them to successfully transmit between spatially and temporally separated patches of host resource and to survive within these environments. Macroparasites have common life-history strategies to achieve this, but these general themes are modified in a myriad of ways related to the specific biology of their hosts. Parasite life histories are also dynamic, responding to conditions inside and outside of hosts, and they continue to evolve, especially in response to our attempts to control them and the harm that they cause.

Parasites induced skin allergy: a strategic manipulation of the host immunity

The absence of a consistent link between parasitoses and skin allergic symptoms in the clinical investigations contrasts to the fact that some parasites are the most potent inducers of immunoglobulin E that exist in nature. To shed some light into this question, this review is focused on the actual knowledge regarding parasites life cycle, interactions with host immunity, the influence on host behavior, and finally the role of all these factors on the skin allergy. The collected data demonstrate that parasites could manipulate the host behavior for its own benefit in different ways, altering its (epi)genetic, biochemical, immunologic or physiologic functions as well as altering its behavior and activity. In this context, skin allergy may be associated with certain stages of the parasites' life cycle and migration into biological barriers, but not necessarily with presence of the parasitosis in the host organism. As compared to T helper (Th) 1 response, the Th2 one, the eosinophilic infiltration and the complement inhibition could assure better conditions for the development of some parasites. Taken together, the suggested hypotheses could be a plausible explanation for the epidemiological puzzle regarding urticaria occurrence, Th2 response and parasitoses, but further studies are necessary to provide better-based conclusions.

KEYWORDS

Eosinophilic Infiltration; Host behavior; Parasites life cycle; Skin allergy; Th1/Th2 response.

The dynamics of macroparasite host-self-infection: a study of the patterns and processes of pinworm (Oxyuridae) aggregation

OBJECTIVES

Among parasites, Taylor's power law identifies a tight relationship in aggregation of macroparasite infection intensity with few exceptions; notably, the nematode family Oxyuridae tends to have higher than expected aggregation. Oxyuridae infect a wide range of mammalian hosts and have a unique reproductive strategy that involves conventional horizontal transmission, as well as re-infection of an already infected host. We asked the question, do the unique aspects of pinworm life-history explain an exception to the widely observed patterns of aggregation of parasite populations?

METHODS

We empirically examined the differences among Oxyuridae (genus: Syphacia) compared with other helminth (genus: Heligmosomoides) parasite aggregations in 2 rodent hosts with similar ecology: the yellow-necked mouse (Apodemus flavicollis) from Trento, Italy and the white-footed mouse (Peromyscus leucopus) from Pennsylvania, USA. To investigate the effects of pinworm life-history characteristics on generating aggregation, we present a stochastic model that explores aggregation under a range of host-self-infection, parasite death, and transmission scenarios.

RESULTS

Oxyuridae parasites had consistently greater aggregation compared to other nematodes regardless of host or parasite species identity, and pinworm aggregation exceeded the range of macroparasite aggregation described previously.

CONCLUSIONS

Our simulations demonstrate that host-self-infection, on its own, is sufficient to generate aggregation values greater than the predicted values.

Why do larval helminths avoid the gut of intermediate hosts?

In complex life cycles, larval helminths typically migrate from the gut to exploit the tissues of their intermediate hosts. Yet the definitive host's gut is overwhelmingly the most favoured site for adult helminths to release eggs. Vertebrate nematodes with one-host cycles commonly migrate to a site in the host away from the gut before returning to the gut for reproduction; those with complex cycles occupy sites exclusively in the intermediate host's tissues or body spaces, and may or may not show tissue migration before (typically) returning to the gut in the definitive host. We develop models to explain the patterns of exploitation of different host sites, and in particular why larval helminths avoid the intermediate host's gut, and adult helminths favour it. Our models include the survival costs of migration between sites, and maximise fitness (=expected lifetime number of eggs produced by a given helminth propagule) in seeking the optimal strategy (host gut versus host tissue exploitation) under different growth, mortality, transmission and reproductive rates in the gut and tissues (i.e. sites away from the gut). We consider the relative merits of the gut and tissues, and conclude that (i) growth rates are likely to be higher in the tissues, (ii) mortality rates possibly higher in the gut (despite the immunological inertness of the gut lumen), and (iii) that there are very high benefits to egg release in the gut. The models show that these growth and mortality relativities would account for the common life history pattern of avoidance of the intermediate host's gut because the tissues offer a higher growth rate/mortality rate ratio (discounted by the costs of migration), and make a number of testable predictions. Though nematode larvae in paratenic hosts usually migrate to the tissues, unlike larvae in intermediates, they sometimes remain in the gut, which is predicted since in paratenics mortality rate and migration costs alone determine the site to be exploited.

Ecological immunology of a tapeworms' interaction with its two consecutive hosts

Host-parasite interactions in parasites with complex life cycles have recently gained much interest. Here, we take an evolutionary ecologist's perspective and analyse the immunological interaction of such a parasite, the model tapeworm Schistocephalus solidus, with its two intermediate hosts, a cyclopoid copepod and the three-spined stickleback. We will be focussing especially on the parallel links between the different phases during an infection in the different hosts; the immunological interactions between host(s) and parasite; and their impact on parasite establishment, growth, host manipulation and parasite virulence in the next host in the cycle. We propose to extend the 'extended phenotype' concept and not only include the ultimate but also the proximate, physiological causes. In particular, parasite-induced host manipulation is suggested to be caused by the interactions of the parasite with the hosts' immune systems.

Atopic disorders and parasitic infections

This chapter examines the relationship between atopic disorders and parasitic infections. Atopy is an exaggerated IgE-mediated Type-1 immune response in predisposed individuals. Conflicting information exists in regard to the relationship of parasitic infections and the classic allergic diseases, that is, atopic dermatitis, allergic rhinitis and asthma. Attention is paid to the explanations for these discrepancies in the literature found within both human and animal studies on atopy with particular emphasis on helminthic infections. The factors that cause only a proportion of atopic individuals to develop clinical disease have not been defined although helminths confer protection in many studies examined. Early childhood infections help induce a Th1-biased immunity and prevent the induction of the Th2 system that causes atopy. Acute parasitic infections may increase manifestations of allergy, whereas chronic infections with parasites decrease atopic predisposition. Nonetheless, a causal association between geohelminth infection and atopic disorders has not been established. Some helminthic substances, especially the cytokines, have respiratory and anti-allergic effects, and may therefore become useful as therapeutic modalities for many atopic and allergic disorders.

Schistocephalus solidus: Establishment of tapeworms in sticklebacks – fast food or fast lane?

The penetration of the intestinal mucosal wall is supposed to be critical for helminth parasite infestation, but has rarely been analyzed in detail. We here studied the establishment process of Schistocephalus solidus tapeworms in their second intermediate host, the three-spined stickleback, from oral uptake after experimental exposure, to passage through the gastro-intestinal tract and arrival in the fish body cavity. Using histological techniques, we found tapeworms to penetrate the intestine within 14-24 h, spending most of the time in the stomach lumen and only a very short period in the intestine. Unexpectedly, tapeworms lost their outer layer, together with the cercomer, in the intestine lumen rather than later during intestine wall penetration. Once exposed, the underlying tegument with microtriches might serve to facilitate migration of the parasite into the body cavity.

Molecular phylogeny of clade III nematodes reveals multiple origins of tissue parasitism

Molecular phylogenetic analyses of 113 taxa representing Ascaridida, Rhigonematida, Spirurida and Oxyurida were used to infer a more comprehensive phylogenetic hypothesis for representatives of 'clade III'. The posterior probability of multiple alignment sites was used to exclude or weight characters, yielding datasets that were analysed using maximum parsimony, likelihood, and Bayesian inference methods. Phylogenetic results were robust to differences among inference methods for most high-level taxonomic groups, but some clades were sensitive to treatments of characters reflecting differences in alignment ambiguity. Taxa representing Camallanoidea, Oxyurida, Physalopteroidea, Raphidascarididae, and Skrjabillanidae were monophyletic in all 9 analyses whereas Ascaridida, Ascarididae, Anisakidae, Cosmocercoidea, Habronematoidea, Heterocheilidae, Philometridae, Rhigonematida and Thelazioidea were never monophyletic. Some clades recovered in all trees such as Dracunculoidea and Spirurina included the vast majority of their sampled species, but were non-monophyletic due to the consistent behaviour of one or few 'rogue' taxa. Similarly, 102 of 103 clade III taxa were strongly supported as monophyletic, yet clade III was paraphyletic due to the grouping of Truttaedacnitis truttae with the outgroups. Mapping of host 'habitat' revealed that tissue-dwelling localization of nematode adults has evolved independently at least 3 times, and relationships among Spirurina and Camallanina often reflected tissue predilection rather than taxonomy.

Helminths can protect themselves against rejection inhibiting hostile respiratory allergy symptoms

The ability of common environmental allergens to stimulate IgE responses and thus to produce allergic diseases has tended to overshadow the fact that helminthic parasites are possibly the most potent inducers of this immunoglobulin that exists in nature. Although it has been well established that during these infections there is a stimulation of IgE against their own antigens as well as a strong induction of nonspecific TH2/IL-4 polyclonal IgE, similarly to the allergic processes, many authors debate if the presence of these infections correlates inversely or not with the rate prevalence of atopy or respiratory allergy. Interpreting this relationship, we suggest that sometimes the intensive infections of hosts, especially with soil helminths which migrate in the respiratory ways or use there as entrance, can induce the production of some mediators ('helminth(k)ines'), to reduce the possibility of their reactive expulsion from the host. The ability to suppress hostile allergic symptoms despite the simultaneous induction of IgE response and local inflammation maybe is established due to the selective evolution, to assure for the parasites better chances for an effective life and reproduction within their mammalian hosts.

Tissue migration by parasitic helminths – an immunoevasive strategy?

Migration through host tissues has major costs for parasitic helminths in terms of energy expenditure, risks of attrition and the need to adapt to varying physicochemical environments. Nevertheless, such migratory phases seem to confer a specific survival advantage. One reason for this might be the avoidance of specific host immune-defence mechanisms designed to protect against threats at mucosal surfaces.

Prevalence, intensity, and differential development of Pseudodelphis oligocotti (Nematoda: Dracunculoidea) in sympatric fish hosts of the northeastern Pacific coast

Counter to expectations of coevolved parasite-host relationships, parasites frequently infect hosts that never contribute to their reproduction, making the identification of a parasite's true host-specificity problematic. Pseudodelphis oligocotti (Nematoda: Dracunculoidea) infects several coastal Pacific fishes, but its course of development appears highly variable, suggesting that incidence does not reflect effective host range. To determine the host range of P. oligocotti and describe its relationship to various potential hosts, 24 fish species were examined from several British Columbia localities for prevalence, intensity, and extent and tissue location of parasite development. Pseudodelphis oligocotti infects 9 species of fishes from 5 orders, of which penpoint gunnel, Apodichthys flavidus, showed the highest prevalence and intensity, up to 80% and 19 (+/- 17.1 SD) worms per host, respectively. Although subadult and adult P. oligocotti occurred in all 9 fishes, larvigerous P. oligocotti only occurred in A. flavidus and rarely in the northern clingfish, Gobiesox maeandricus. Infective first-stage larvae were recovered from gill tissue of A. flavidus. Thus, at most only 2 of the 9 host species infected by P. oligocotti actually contribute to its transmission. The occurrence of P. oligocotti in diverse hosts may be accounted for by the parasite's indiscriminant mode of transmission via ingestion of free-living intermediate copepod hosts, where highly exposed or more suitable fishes (or both) are closely related by diet and microhabitat. This study demonstrates how parasite transmission and host ecology can greatly affect observed host range and ultimately its potential for expansion.

The effect of the host immune response on the parasitic nematode Strongyloides ratti

The host immune response has profound effects on parasitic nematode infections. Here we have investigated how a range of infection parameters are affected by host immune responses and by their suppression and enhancement. The infection parameters considered were the number of parasitic females, their size, per capita fecundity and intestinal position. We found that in immunosuppressive treatments worms persist in the gut, sometimes with a greater per capita fecundity, maintain their size and have a more anterior gut position, compared with worms from control animals. In immunization treatments there are fewer worms in the gut, sometimes with a lower per capita fecundity and they are shorter and have a more posterior gut position, compared with worms from control animals. Worms from animals immunosuppressed by corticosteroid treatment reverse their changes in size and gut position. This description of these phenomena pave the way for a molecular biological analysis of how these changes in infection parameters are brought about by the host immune response.

Exposure of leopard frogs to a pesticide mixture affects life history characteristics of the lungworm Rhabdias ranae

We tested the hypothesis that exposure of leopard frogs ( Rana pipiens) to agricultural pesticides can affect the infection dynamics of a common parasite of ranid frogs, the lungworm Rhabdias ranae. After a 21-day exposure to sublethal concentrations of a pesticide mixture composed of atrazine, metribuzin, aldicarb, endosulfan, lindane and dieldrin, or to control solutions (water, dimethyl sulfoxide), parasite-free juvenile frogs were challenged with 30 infective larvae of R. ranae. Approximately 75% of the larvae penetrated the skin and survived in both exposed and control animals, suggesting that pesticides did not influence host recognition or penetration components of the transmission process. Rather, we found that the migration of R. ranae was significantly accelerated in hosts exposed to the highest concentrations of pesticides, leading to the establishment of twice as many adult worms in the lungs of frogs 21 days post-infection. Pesticide treatment did not influence the growth of lungworms but our results indicate that they matured and reproduced earlier in pesticide-exposed frogs compared to control animals. Such alterations in life history characteristics that enhance parasite transmission may lead to an increase in virulence. Supporting evidence shows that certain components of the frog immune response were significantly suppressed after exposure to the pesticide mixture. This suggests that the immune system of anurans exerts a control over lungworm migration and maturation and that agricultural contaminants can interfere with these control mechanisms. Our results also contribute to the ongoing debate regarding the role that anthropogenic factors could play in the perplexing disease-related die-offs of amphibians observed in several parts of the world.

Molecular phylogenetic analysis of the genus Strongyloides and related nematodes

Strongyloides spp., parasitic nematodes of humans and many other terrestrial vertebrates, display an unusual heterogonic lifecycle involving alternating parasitic and free-living adult reproductive stages. A number of other genera have similar lifecycles, but their relationships to Strongyloides have not been clarified. We have inferred a phylogeny of 12 species of Strongyloides, Parastrongyloides, Rhabdias and Rhabditophanes using small subunit ribosomal RNA gene (SSU rDNA) sequences. The lineage leading to Strongyloides appears to have arisen within parasites of terrestrial invertebrates. Inferred lifecycle evolution was particularly dynamic within these nematodes. Importantly, the free-living Rhabditophanes sp. KR3021 is placed within a clade of parasitic taxa, suggesting that this species may represent a reversion to a non-parasitic lifecycle. Species within the genus Strongyloides are very closely related, despite the disparity of host species parasitised. The highly pathogenic human parasite Strongyloides fuelleborni kelleyi is not supported as a subspecies of the primate parasite S. fuelleborni fuelleborni, but is most likely derived from a local zoonotic source.

Alternative migration routes of Ascaris suum in the pig

Experiments were conducted to investigate possible alternative routes of extraintestinal migration of Ascaris suum larvae in the pig. Pigs were infected with A. suum via injection of newly hatched larvae into cecal veins (i.v.), into cecal lymph nodes (LN), or intraperitoneally (i.p.), and control animals were inoculated orally with infective eggs (p.o.). Two pigs per inoculation route were necropsied on days 1, 4, and 13 postinoculation. The numbers of liver lesions and the percentage of larvae recovered was considerably greater in pigs inoculated i.v. or p.o. on each necropsy day. However, irrespective of inoculation route, at least a proportion of larvae passed through the livers and were able to complete migration to the small intestine by day 13. The results indicate that larval penetration of the intestinal wall is not necessary for liver-lung migration and that passage through the liver may be favorable for migrating A. suum larvae, although a delayed arrival in the small intestine cannot be ruled out for larvae following alternative routes.

Mass dynamics of the spleen and other organs in geese: measures of immune relationships to helminths?

The spleen is an important organ of avian immune systems. We examined whether helminth loads were related to spleen mass in the lesser snow goose, Chen caerulescens caerulescens. On 27 collecting occasions, 744 geese were obtained at 13 different locations in a south-north gradient in midcontinental North America. The masses of the spleen, caecum, small intestine, large intestine, pancreas, heart, and crop of all geese were determined, and intestinal and caecal helminths were counted. Seventy-eight percent of geese harbored at least one helminth species. For analyses, helminths were grouped as cestodes (26% prevalence), trematodes (19% prevalence), and nematodes (70% prevalence). After sample location and time, host age, host sex, and host body size were controlled for in a multivariate analysis of covariance, nematodes were the only helminth group associated with variation in organ masses. Greater nematode loads were weakly associated with lower spleen, higher caecum, lower large intestine, and lower heart masses. When uninfected individuals were excluded from the analysis, greater nematode loads were no longer associated with variation in spleen size but were associated with higher crop mass, and greater cestode loads were associated with higher heart mass. In neither of these analyses were any other cestode-organ or trematode-organ associations significant. Geese carrying two or more helminth groups had lower spleen masses than did geese infected with no or one helminth group. When we interchanged response and explanatory variables from the preceding analyses and retained the same covariates, the same organ mass - helminth associations tended to remain significant. Nonetheless, the small variation in helminth loads explained by variation in spleen mass (or vice versa) provided only weak support for the hypothesis that intraspecifically, wild individuals with lower investment in immunity are more susceptible to nematode infections.

Parasites, biodiversity, and population dynamics in an ecosystem in the high arctic

The abomasa of 163 Svalbard reindeers (Rangifer tarandus platyrhynchus) from Nordenskiöld Land, Spitsbergen (78 degrees N, 15 degrees E) were examined for adult and juvenile nematodes. Spitsbergen has midnight sun from late April to late August, arctic night from late October to mid-February, permafrost, and snow cover from October to June. Plant growth is restricted to 6-8 weeks, usually starting mid-June. In the reindeer calves, which are born in June prevalence and intensity of infection increased over the winter. All young (1 year) and adult (older than 1 year) reindeers were infected. Variations in worm burden, fraction of adult nematodes, male to female ratio in the nematodes, number of adult nematodes, number of juvenile nematodes, and distribution of juvenile nematodes on length groups were analysed for relationships with sex and age of host, and time of the year. The material was further analysed for relationships between the reindeer's content of stored fat and worm burden and its elements. It is concluded that transmission of nematodes to the reindeer continues over the winter and that arrested larval development has not been strongly selected for in this High Arctic locality. Infection continued to increase in adult males but not in adult females with age. This is explained as a balance between gain and loss of worms in the females connected to levelling off of food intake at end of somatic growth. In adult male reindeers there was a negative trend between stored fat and infection at all seasons. In reproducing females there were significant negative relationships in winter-spring, while in yeld females no significant relationship was found. The nematodes belonged to the following taxa: Ostertagia gruehneri m. gruehneri, O. gruehneri m. arcticus, Marshallagia marshalli m. marshalli, M. marshalli. m. occidentalis, Teladorsagia circumcincta m. circumcincta, T. circumcincta. m. trifurcata, and T. circumcincta. m. davtiani. There appears not to be any specially adapted species among the helminth parasites of reindeer (Rangifer tarandus) and muskoxen (Ovibos moschatus) in the High Arctic. In the Low Arctic Uminmakstrongylus pallikuukensis of muskoxen may be an example of such adaptation.

Molecular analysis of nematode diversity and the evolution of parasitism

A thorough and coherent classification of the phylum Nematoda is essential if the evolution of countless phenotypes is to be understood. Here, Mark Dorris, Paul De Ley and Mark Blaxter discuss how the application of molecular phylogenetics is helping to resolve some of the inconsistencies in morphological classification and phylogeny by establishing relationships between free-living and parasitic groups, showing possible patterns underlying the origins of parasitism and placing key nematode species in an evolutionary context for comparative study.