Development Of Parasitic Nematodes Research Articles

Introduction to Strongyloides stercoralis Anatomy.

Strongyloides stercoralis, commonly known as the human threadworm, is a skin-penetrating gastrointestinal parasitic nematode that infects hundreds of millions of people worldwide. Like other Strongyloides species, S. stercoralis is capable of cycling through a single free-living generation. Although S. stercoralis and the free-living nematode Caenorhabditis elegans are evolutionarily distant, the free-living adults of S. stercoralis are similar enough in size and morphology to C. elegans adults that techniques for generating transgenics and knockouts in C. elegans have been successfully adapted for use in S. stercoralis. High-quality genomic and transcriptomic data are also available for S. stercoralis. Thus, one can use a burgeoning array of functional genomic tools in S. stercoralis to probe questions about parasitic nematode development, physiology, and behavior. Knowledge gained from S. stercoralis will inform studies of other parasitic nematodes such as hookworms that are not yet amenable to genetic manipulation. This review describes the basic anatomy of S. stercoralis.

Signaling in Parasitic Nematodes: Physicochemical Communication Between Host and Parasite and Endogenous Molecular Transduction Pathways Governing Worm Development and Survival.

Signaling or communication between host and parasite may occur over relatively long ranges to enable host finding and acquisition by infective parasitic nematode larvae. Innate behaviors in infective larvae transmitted from the soil that enhance the likelihood of host contact, such as negative geotaxis and hypermotility, are likely mediated by mechanoreception and neuromuscular signaling. Host cues such as vibration of the substratum, elevated temperature, exhaled CO2, and other volatile odorants are perceived by mechanosensory and chemosensory neurons of the amphidial complex. Beyond this, the molecular systems that transduce these external cues within the worm are unknown at this time. Overall, the signal transduction mechanisms that regulate switching between dauer and continuous reproductive development in Caenorhabditis elegans, and doubtless other free-living nematodes, have provided a useful framework for testing hypotheses about how the morphogenesis and development of infective parasitic nematode larvae and the lifespan of adult parasites are regulated. In C. elegans, four major signal transduction pathways, G protein-coupled receptor signaling, insulin/insulin-like growth factor signaling, TGFβ-like signaling and steroid-nuclear hormone receptor signaling govern the switch between dauer and continuous development and regulate adult lifespan. Parasitic nematodes appear to have conserved the functions of G-protein-coupled signaling, insulin-like signaling and steroid-nuclear hormone receptor signaling to regulate larval development before and during the infective process. By contrast, TGFβ-like signaling appears to have been adapted for some other function, perhaps modulation of the host immune response. Of the three signal transduction pathways that appear to regulate development in parasitic nematodes, steroid-nuclear hormone signaling is the most straightforward to manipulate with administered small molecules and may form the basis of new chemotherapeutic strategies. Signaling between parasites and their hosts' immune systems also occurs and serves to modulate these responses to allow chronic infection and down regulate acute inflammatory responses. Knowledge of the precise nature of this signaling may form the basis of immunological interventions to protect against parasitism or related lesions and to alleviate inflammatory diseases of various etiologies.

The Developmental Biology of Parasitic Nematodes.

My research focuses on understanding the factors that regulate the development of parasitic nematodes (roundworms) during the infective process, in which their larvae pass from the external environment into mammalian hosts, including humans. This research matters because these factors are essential to the survival of a group of pathogenic organisms that collectively cause debilitation, illness, and some deaths in over 1.5 billion people around the globe. In this enormous population—comprising almost one in five people on the planet—parasitic nematodes such as hookworms, roundworms, whipworms, and threadworms complicate pregnancy, impair both physical and cognitive development in children, and sap the productive energies of infected adults. Other parasitic nematodes, such as the Guinea worm and the filariae, cause pain and disfigurement of limbs or excruciating skin disease and potentially blinding lesions of the eye. This problem is underappreciated by both policy makers and the general public in relatively prosperous countries because the diseases involved occur under conditions of extreme poverty—their parasitic agents thrive in communities of people who are food-insecure and who live in substandard housing and with poor sanitation. Parasitic nematodes also contribute to food insecurity by causing anemia, gastroenteritis, and malnutrition in livestock and attendant decrements in milk and meat production. My primary research subject is the intestinal threadworm, Strongyloides stercoralis. Like many other parasitic nematodes, S. stercoralis develops in contaminated soil, through a series of noninfectious larval stages, into an infectious form, the third-stage larva, which penetrates the skin of its human or canine host and undertakes a complex migration to the small intestine. There, it can cause enteritis and diarrhea in uncomplicated infection and can cause overwhelming and potentially fatal fulminant infection in hosts with compromised immune systems. S. stercoralis is an important pathogen in its own right, but we study it because some peculiarities of its life cycle allow us to subject the worms to genetic study that would be impossible or impractical in other parasitic nematodes. One such genetic method that my research group has developed for S. stercoralis is transgenesis—the ability to transfer new genes, sometimes with strategic alterations in their functions, into the organism. Thus far, Strongyloides and some related worms are the only parasitic nematodes in which transgenesis has proven feasible. We were able to accomplish this important technical advancement because similarities in the body forms of Strongyloides and the free-living nematode Caenorhabditis elegans (which is widely used as a model for basic research) allowed us to adapt well-developed techniques for transgenesis in that worm to our parasites. The basic research questions we address with this method revolve around how Strongyloides uses molecular signaling mechanisms within its cells to control the formation of its infective larvae, the behaviors these larvae use to find and invade their hosts, and the processes by which they resume their development to adult parasites within the intestines of these hosts. We took our cues about which of the many signaling mechanisms operating in Strongyloides might be important in this infectious process from the wealth of knowledge about how such mechanisms control development of the free-living nematode, C. elegans. We and our colleagues recently identified a signaling pathway involving steroid hormones in Strongyloides that may point the way towards development of new drugs to combat infection by this parasite in humans—a crucial need, given the relatively small number of currently available drugs, and another reason why this research matters. Thus, my research draws heavily upon basic science developed with the C. elegans model, both for its conceptual underpinnings and for its methodology. This aspect of the research matters because it places the work within the profoundly beneficial movement towards translational biomedical science that has energized and accelerated the flow of vital information from the laboratory bench to the patient bedside, of late. Finally, the research matters to me personally because, after almost 35 years at it, I am still fascinated by the parasitic worms that I study. A large part of this is the thrill of glimpsing the unseen, of “seeing” the inner workings of organisms that spend some or all of their time living inside of other animals. I also find the worms to be beautiful in their own way. I am never happier than when I can take a precious hour or two away from the cares of administrative duties and teaching commitments to sit alone at the microscope and study them in detail in order to document changes resulting from our genetic manipulations that will constitute the next new finding. Far from a personal conceit, fascination of this type is a driving force behind the best basic science, and so, it matters very much in the fight to eradicate the scourge of parasitic disease from the globe. Image 1 James B. Lok.

Recent Advances in Elucidating Nematode Moulting - Prospects of Using Oesophagostomum dentatum as a Model.

There are major gaps in our knowledge of many molecular biological processes that take place during the development of parasitic nematodes, in spite of the fact that understanding such processes could lead to new ways of treating and controlling parasitic diseases via the disruption of one or more biological pathways in the parasites. Progress in genomics, transcriptomics, proteomics and bioinformatics now provides unique opportunities to investigate the molecular basis of key developmental processes in parasitic nematodes. The porcine nodule worm, Oesophagostomum dentatum, represents a large order (Strongylida) of socioeconomically important nematodes, and provides a useful platform for exploring molecular developmental processes, particularly given that this nematode can be grown and maintained in culture in vitro for periods longer than most other nematodes of this order. In this article, we focus on the moulting process (ecdysis) in nematodes; review recent advances in our understanding of molecular aspects of moulting in O. dentatum achieved by using integrated proteomic-bioinformatic tools and discuss key implications and future prospects for research in this area, also with respect to developing new anti-nematode interventions and biotechnological outcomes.

Biological characterization and pathogenicity of three Haemonchus contortus isolates in primary infections in lambs

The biological characterization and differential pathogenicity of three isolates of Haemonchus contortus, one autochthonous (Aran 99) and two allochthonous (Moredun Research Institute, MRI, and Merck Sharp and Dohme, MSD) were studied by primary experimental infection of Manchego lambs. Thus, six female lambs (5.5 months old) were infected with 12,000 L 3 larvae of each helminth isolate. Parasitological (pre-patent period, parasite egg shedding dynamics), biopathological (packed cell volume (PCV), haemoglobin concentration, plasma proteins, serum pepsinogen) and zootechnical parameters (live weight gain, thoracic perimeter) were measured throughout the study. After sacrifice (85 days post-infection (pi)), lamb carcasses were inspected for parasite burden and development (establishment rate, male/female ratio, degree of parasite development), and the average carcass weight of the experimental groups was compared. The autochthonous combination (Manchego lambs—Aran 99) had a longer pre-patent period (28 days) and a significantly different pattern of egg elimination (maximum elimination on day 80 pi). The establishment rate and parasite burden (average values of 8.18% and 988 adult helminths, respectively) were both low, with no significant differences between isolates. There were no significant differences in parasitic nematode development in terms of size and weight (1264.66 μm and 149.45 μg for male worms and 2093.33 μm and 411.46 μg for females, respectively), although Aran 99 females weighed less ( p < 0.05). All isolates induced a slight but significant reduction of PCV values from day 23 pi onwards. Inter-isolate differences were found, with the effects in the case of MSD being more pronounced. Variations of serum protein levels were minimal in all lamb groups. The live weight gain of MSD- and Aran 99-infected animals was significantly lower ( p < 0.05) than for MRI-infected lambs and uninfected control animals. Carcass yield from the lambs infected with the autochthonous isolate (Aran 99) was lower. The MSD isolate therefore showed a higher comparative pathogenicity.

Genomics of reproduction in nematodes: prospects for parasite intervention?

Understanding reproductive processes in parasitic nematodes has the potential to lead to the informed design of new anthelmintics and control strategies. Little is known, however, about the molecular mechanisms underlying sex determination, gametogenesis and reproductive physiology for most parasitic nematodes. Together with comparative analyses of data for the free-living nematode Caenorhabditis elegans, molecular investigations are beginning to provide insights into the processes involved in reproduction and development in parasitic nematodes. Here, we review recent developments, focusing on technological aspects and on molecules associated with sex-specific differences in adult nematodes.

Genetic regulation of arrested development in nematodes: are age-1 and daf-gene orthologs present in Dictyocaulus viviparus?

In opposite to the free-living soil nematode Caenorhabditis elegans, the genetic regulation of hypobiosis or inhibited or arrested development in parasitic nematodes is completely unknown. In C. elegans, the daf-genes or the age-1 gene are of major importance in signaling pathways regulating arrested development. To investigate if orthologs of these genes are present in the bovine lungworm Dictyocaulus viviparus, a PCR analysis with gene-specific primer combinations was performed. No orthologs of the age-1 or daf-genes could be identified in D. viviparus. The possible differences in the role of the daf-genes concerning arrested development in parasitic and free-living nematodes will be discussed.

The effects of birdsfoot trefoil ( Lotus corniculatus) and chicory ( Cichorium intybus) when compared with perennial ryegrass ( Lolium perenne) on ovine gastrointestinal parasite development, survival and migration

Two experiments were conducted to investigate the effects of birdsfoot trefoil and chicory on parasitic nematode development, survival and migration when compared with perennial ryegrass. In experiment one, sheep faeces, containing 10,385 Cooperia curticei eggs were added to 25 cm diameter pots containing birdsfoot trefoil, chicory or ryegrass, and the pots maintained under optimal conditions for nematode parasite development. Replicate pots of each forage type were destructively sampled on day 8, 16, 20, 28 and 37 to collect the nematode larvae. When forages were compared on a dry matter basis, by day 16 there were 31% and 19% fewer larvae on birdsfoot trefoil and chicory than on ryegrass, respectively ( P < 0.01). In the second experiment, replicate 1 m 2 field plots of birdsfoot trefoil, chicory and ryegrass were sub-sampled on day 14, 21, 35 and 49 for larval counts following the application of sheep faeces containing 585,000 Teladorsagia circumcincta eggs to each plot on day 0. Results showed there were a minimum of 58% and 63% fewer infective stage parasitic larvae on birdsfoot trefoil and chicory, respectively, compared with ryegrass on day 14 and 35 when forages were compared on a forage dry matter, plot area sampled and leaf area basis ( P < 0.01). Overall, these results indicate that the number of infective stage larvae on birdsfoot trefoil and chicory pasture was reduced by the effect of their sward structure on the development/survival/migration of ovine parasitic nematodes. These effects may be one of the ways in which these forages may affect parasitic infections in grazing livestock.

Structure and expression of a novel filarial gene for glia maturation factor

Factors that influence neural growth and development in parasitic nematodes have not yet been identified. We have isolated and sequenced a Brugia malayi nematode 3.4-kb genomic DNA fragment and its corresponding cDNA, which encode a predicted protein of 138 amino acids with 52% identity and 75% similarity to the mammalian neuroglial growth factor, glia maturation factor-β (GMF). GMF promotes the differentiation of mammalian glia and neurons and stimulates axonal regeneration. The filarial nematode gene Bmgmf for Brugia malayi glia maturation factor contains six predicted exons, with the first exon encoding only the initiation methionine. Brugia malayi GMF (BmGMF) is also related to a large family of eukaryotic actin depolymerizing factors (ADFs). Although BmGMF does not contain the consensus actin-depolymerizing motif of ADFs, it does share a similar intron–exon structure, including the unusual first exon, with the unc-60 ADF gene of the nematode Caenorhabditis elegans. RT-PCR experiments reveal that BmGMF is trans-spliced with the nematode spliced leader sequence SL1 and is expressed in microfilariae but not in third-stage larvae or adult worms. We speculate that BmGMF may function as a stage-specific neuroglial growth factor.

Effects of Horn Fly and House Fly (Diptera: Muscidae) Larvae on the Development of Parasitic Nematodes in Bovine Dung

Laboratory studies were conducted to determine the effects of horn fly, Haematobia irritans (L.), and house fly, Musca domestica L., larvae on the development of a mixed population of parasitic nematodes in compressed and crumbled bovine dung. Fresh dung (100 g per sample) from a single calf passing trichostrongyle type eggs was infested with 150 horn fly or 150 house fly eggs. After 14-15 d, more horn flies and house flies had emerged from the compressed dung than from the crumbled dung, but more third stage parasitic nematode larvae were recovered from the crumbled dung containing either fly species than from dung containing no flies.

Ascaris suum: Role of ecdysteroids in molting

Three studies were conducted to examine the function of ecdysteroids in the development of parasitic nematodes. Ecdysone and 20-hydroxyecdysone were extracted, separated chromatographically, and measured in the reproductive tracts of adult female Ascaris suum. Perienteric fluid and the body wall did not contain measurable levels of these steroids. Levels of 20-hydroxyecdysone were correlated with the third and fourth molts of larvae grown in vitro from the third stage. In a bioassay, addition of ecdysteroid extracted from the female reproductive tract or synthetic ecdysteroid increased the proportion of third-stage larvae that molted after 4 days in culture. This evidence supports the role of ecdysteroids in molting in A. suum, as well as suggesting a function in gametogenesis and embryogenesis.

Thyroid Condition of Chickens and Development of Parasitic Nematodes

Considerable recent literature has been devoted to effects of a thyroactive iodocasein (Protamone) and thiouracil on the growth, fattening, and feathering of broilers, vide Parker (1943), Irwin, Reineke, and Turner (1943), Turner, Irwin, and Reineke (1944), Kempster and Turner (1945), and Andrews and Schnetzler (1946). One immediate effect of these substances in the diet is exerted upon the activity of the thyroid and when the substances are fed at proper levels it is possible to produce mild hypothyrosis or mild hyperthyrosis in chickens. The above literature suggested that a further effect of these substances in the diets of chickens might be tested, namely, their effect on resistance to parasitism. It has been reported, Larsh (1947), that the daily addition of 3 mg of thyroid to the diets of mice resulted in a much higher percentage development of Hymenolepis than occurred in control animals. The present writer (1948a, in press) will report significantly superior growth of parasitized chicks with a mild hyperthyrosis when compared with growth of parasitized normal controls, and, when compared with growth of parasitized chicks with a mild hypothyrosis. Wheeler et al (1948) reported that 0.1 percent thiouracil in the diet of 2-week old chicks was not prejudidicial to the survival of birds artificially infected with cecal coccidiosis, while their data on 0.02 percent Protamone in the diet suggested that birds on such a diet were somewhat resistant to the same infection.