Nematodes: Symptoms, Injury to Plants, Characteristics and Life Cycle, Parasites

Nematodes: Symptoms, Injury to Plants, Characteristics and Life Cycle| Parasites

In this article we will discuss about:- 1. Introduction to Nematodes 2. Symptoms Caused by Nematodes 3. Mechanism of Nematode Injury to Plants 4. Interrelationship between Nematodes and Other Plant Pathogens 5. Ecology and Spread 6. Characteristics 7. Life Cycle.

Contents:

  1. Introduction to Nematodes
  2. Symptoms Caused by Nematodes
  3. Mechanism of Nematode Injury to Plants
  4. Interrelationship between Nematodes and Other Plant Pathogens
  5. Ecology and Spread of Nematodes
  6. Characteristics of Plant Pathogenic Nematodes
  7. Life Cycle of Nematodes

1. Introduction to Nematodes:

Nematodes are the most highly developed of the pseudo-coelomates. They belong to the phylum Nemata, previously named Phylum Nemathelminthes or Phylum Aschelminthes. They are widely present in the soil, fresh water or marine water.

Nematodes are the only plant parasites belonging to the animal kingdom which are studied in plant pathology Nematodes, sometimes called eelworms, are worm-like in appearance but quite distinct taxonomically from the true worms- Numerous species of nematodes attack and parasitize man and animals and cause various diseases. Several hundred species are known to feed on living plants as parasites and cause a variety of plant diseases.

A general account of nematodes is given by Crofton (1966). One of the most interesting works on the biology of plant parasitic nematodes was published by Wallace in 1963. Nematology in India was recently reviewed by Swarup and Seshadri (1974). The methods of controlling nematodes have been discussed by Khan. Nematodes as parasites of plants, their ecology and the process of infection are discussed by Dropkin (1977).

All plant parasitic nematodes belong to the phylum Nemathelminthes, class Nematoda. Most of the important parasitic genera belong to the subclass Secementea- Order Tylenchida.

The characteristic features of the subclass Secementea are – Excretory canals (protoplasmic) present, terminal excretory duct cuticularized; caudal glands absent; phasmids usually present; amphids usually minute, pore-like and cephalic in position; sensory organs papilloid, seldom setose; hypodermal glands absent ; male with or without caudal alae.

The taxonomi-problems concerning the phytopathogenic nematodes have been reviewed by Allen and Sher (1967). An extraordinary amount of attention has been given, in the last 20 years, to groups that contain economically important phytopathogenic nematodes.

This has been primarily due to the realization that phytopatho genie nematodes are worldwide in distribution and that they are frequently associated with crop diseases and decreased food production.

Till the end of 1966, a total of 1079 species had been described in the taxa that contain the phytopathogenic nematodes. Up to 1950, 46 genera were recognized in the plant parasitic groups and in the period 1950 to 1966 an additional 65 genera were proposed, making a total of 111 genera with phytoparasitic nematodes.

2. Symptoms Caused by Nematodes:

Nematodes infect the roots as well as the parts of the plant which are above the ground.

Root symptoms may appear as hypertrophy, necrosis or abnormal growth and include the following:

1. Root Knots or Root Galls:

These are enlargements of the roots caused by the feeding of the nematodes which may not necessarily be enclosed within them. The swelling may vary in size from 1 mm to more than 2 cm.

The feeding of nematodes induces the formation of ‘giant cells’ in the host tissue and cell division is stimulated. This leads to the formation of galls of various sorts. Bird (1974) has discussed the response of plants to root knot nematodes under two major headings.

First, the responses of entire plants, and second, the responses of their cells. Whole plants respond to infection by reducing their photosynthetic rate, ‘growth and yield. It is suggested that the nematode influences the physiology of the plant by interfering with the synthesis and translocation of growth hormones produced in the roots. Cells of susceptible plants are changed from normal undifferentiated cells to highly specialized syncytia, also known as giant cells or multinucleate transfer cells.

This process involves a chain of events, starting with nuclear and nucleolar enlargement, followed by cell wall breakdown, synchronous mitoses and incorporation of adjacent cells. This highly specialized syncytium is induced and maintained by, and is completely dependent on a continuous stimulus from the nematodes.

There are discoloured and often collapsed portions of the root, consisting of cells on which nematodes have fed. They vary in size, from being as minute as to be almost invisible to the naked eye to lesions girdling the whole root. Necrotic lesions are probably caused by toxic salivary secretions injected during the feeding of nematodes, for example – Radopholus.

This is caused by the formation of numerous short laterals in the vicinity of nematode injury.

Root rots occur when nematode infections are accompanied by plant pathogenic or saprophytic bacteria and fungi.

This happens when the nematodes feed on or near root tips and cause them to stop growing, enlarging or disintegrating. Nematodes feeding ecto-parasitically at the root tips suppress cell division in the apical meristem and result in short roots, as in Trichodorus, the stubby root nematode.

Nematode root infections are usually accompanied by non- characteristic symptoms in the above-ground parts of plants, appearing primarily as reduced growth, symptoms of nutrient deficiencies, such as yellowing of foliage, excessive wilting in hot or dry weather, reduced yields and poor quality of products.

Nematodes which attack the aerial parts of the plant may cause discolouration, necrosis, blotches, spots, distortion and galls on the leaf, stem and seed. Buds, growing points, or flower primordia are attacked by some nematodes, resulting in the abnormal growth of the affected plant.

These symptoms are thought to result from substances secreted by the eel worm or perhaps by the invaded plant tissue. Krusberg (1963) and Singh (1964) have reviewed the nature of plant reaction to infection by nematodes. Plant response to root knot nematodes has been recently reviewed by Bird (1974). The pathogenesis of nematode-infected plants has been discussed by Endo (1975).

The mechanical injury directly inflicted upon plants by the nematodes during feeding is slight. Most of the damage seems to be caused by a secretion injected into plants while the nematodes are feeding.

This secretion, called saliva, is produced in three glands from which it flows forward into the oesophagus and is ejected through the stylet. Some nematodes are such rapid feeders that in a matter of seconds they pierce a cell, inject saliva, withdraw the cell contents, and move on. Others feed more leisurely and may remain at the same juncture for hours or days.

The sedentary forms (Heterodera and Meloidogyne) remain attached to one point in the tissue throughout their lives. The males of the species may or may not penetrate the roots but the females invariably get established in or on the roots in a fixed position. In such species it is the female which is responsible for the destruction of the host.

Nematode saliva has various functions, depending on the habit of the organism. The saliva of the plant nematodes seems to aid the parasite to penetrate the cell walls and possibly to liquefy the cell contents, making them easier to ingest and assimilate. The saliva, being toxic, proves disastrous to the plant tissues and its effects may reach up to the leaves even if the nematode is present only in the roots.

As a result of the action of this saliva anyone of the following host responses may appear as symptoms of the disease:

(i) Cellular hypertrophy and hyperplasia,

(ii) Suppression of mitosis,

(iii) Cell necrosis, and

(iv) Stimulation of growth.

Dropkin (1969) has discussed the various cellular responses to nematode infection.

The feeding of soine nematodes produces only slight trauma in hot cells. When the nematode Tylenchorhynchus dubius feeds upon the root hairs of Lolium perenne a spherical mass is formed at the tip of the stylet within the host cells.

For about 30 seconds the nematode remains quiet, after which its bulb begins to pulsate and the spherical mass starts diminishing in size and finally disappears after one minute.

In some associations, the stylet acts simply as the organ of penetration and suction tube. The nematode Paraphlenchus acontiodes penetrates the host cells of Pyrenochaeta terrestris and withdraws their contents within 2 to 3 seconds.

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In certain other associations, such as Ditylenchus myceliophagus feeding upon Botrytis cinerea, there is delayed removal of contents. Progressive penetration is shown by the nematode Rotylenchus uniformis when feeding on the root hairs and cortical parenchyma cells of Bolium perenne as well as by the nematode Pratylenchus crenatus when piercing the epidermal cells of Poa annua rootlets.

Many plant parasitic nematodes affect only those cells upon which they feed or a limited number of cells in the immediate vicinity of the feeding site cells of infected tissues separate undergo hypertrophy and lose chloroplasts.

In certain host parasite combinations the pathogen stimulates changes in host cells, bringing about alterations in metabolism vital to the growth and development of the pathogen but do not bring about any cell destruction.

The nematode Tylenchulus semipenetrans brings about an increase in the size of the nuclei and nucleoli, the cytoplasm of the host cells becomes dense, their wan thickens and the vacuoles of the infected cells disappear.

Anguina and Mlltmguina species stimulate gall formation in leaves and flower parts of grasses and other plants. The gads develop here due to hypertrophy and hyperplasia of parenchymatous tissue with a central cavity harbouring the nematode. No syncytia develop here.

Members of the genera belonging to the family Heteroderidae, show the greatest morphological adaptation to parasitism. At the site of the feeding of the nematode, a group of cells develop into characteristic syncytia around the head of the parasite.

The cytoplasm becomes dense and the size of the nuclei and nucleoli elilarges considerably. The cell walls are usually altered. Species of Meloidogyne induce hyperplasia in the pericycle which results in the formation of galls.

Heterodera species do not induce extensive hyperplasia. Nacobbus batatiformis forms galls on the roots of sugar beets and some other plants. Here galls are formed as a result of hypertrophy of cortex and epidermal cells.

The stele is not affected. Syncytial mass is formed by the alteration of cens immediately adjacent to the nematode’s head and extends longitudinally along the root.

In Heterodera infection syncytium is formed as a result of the enlargement of nuclei, incomplete dissolution of cell walls, disintegration of nucleoli, thickening of cell walls and by the cytoplasm becoming dense and granular.

Giant cells of Meloidogyne are formed as a result of the enlargement of nuclei and the cells become polyploid and undergo synchronous mitosis. The cytoplasm of these cells becomes granular and new cells are incorporated by cell wall dissolution. The walls of the syncytium become thickened and the cells of pericycle divide repeatedly.

The role of certain nematodes as vectors of plant viruses has been studied in recent years and there is an increasing awareness that they may also be involved in the transport and inoculation of other pathogens, notably bacteria and fungi attacking roots or other plant organs in the soil.

Several types of nematode-pathogen interaction are suggested by Pitcher (1965), of which the following are examples:

The namatodes transport the pathogen from the soil to the plant or from one part of the plant to another, and the pathogen may fail to develop fully in the absence of the eelworm, as in twist of grasses and cereals caused by the fungus Dilophospora alopecuri in association with Anguina tritici.

Much more important and more common are the interrelationships between nemaflities and viruses. Several plant viruses, such as grapevine fan-leaf virus, arabis mosaic virus, tomato, ring spot virus, tobacco black fing virus, raspberry ring-spot virus and tobacco rattle virus are transmitted by nematodes. Sixteen species of nematodes have so far been thought to be vectors of plant viruses.

The first demonstration that eelworms are capable of transmitting plant viruses was made by Hewitt, et al. (1958). They showed that the fan-leaf disease of grape-vines was spread by the eel worm Xiphinema index.

Several nematode fungus disease complexes are known. Fusarium wilt of several plants increases in incidence and severity when the plants are also infected by nematodes. Similar effects have also been noted in disease complexes involving nematodes ad Verticillium wilt, pythium damping off, Rhizoctonia and Phytophthora root rots.

Relatively few cases of nematode-bacterial disease complexes are known. In most of these the role of the nematode seems to be that of providing the bacteria with an infection court and to assist bacterial infection by wounding the host.

In the yellow ear rot of wheat the role of the nematode is to mechanically carry the bacterium Corynebacterium Iritici inside the ovaries of the wheat flowers. In strawberry infection by the leaf nematode and the bacterium Corynebacterium fascians, a more involved interaction between the two pathogens seems to exist. Plants inoculated with both pathogens produce the so called cauliflower symptom, quite distinct from either of the symptoms produced by each of the pathogens.

Almost all plant pathogenic nematodes lead part of their lives in the soil. Many of these live freely in the soil, feeding superficially on roots and underground stems, and although they may cause injury to plants, they are not strictly parasitic.

Even in the most highly specialized sedentary parasites, the eggs, the pre-parasitic larval stages, and the males are found in the soil for all or part of their lives. Soil temperature, structure (porosity), moisture and aeration affect the survival and movements of nematodes in the soil. Nematodes occur in greatest abundance at a depth of 0-1.5 cm in the soil. The distribution of nematodes in cultivated soils is irregular and is greatest in or around roots of susceptible plants.

The greatest concentration of nematodes is in the region of host roots. This is primarily due to the greater amount of food available and also to the attraction of nematodes to the substances released into the rhizosphere.

To these must be added the so called hatching factor the effect of substances exuded from the root which diffuse into, or are carried to the surrounding soil and markedly stimulate the hatching of eggs of certain species, although most nematode eggs hatch freely in water in the absence of any special stimulus.

Nematodes spread through the soil very slowly and by their own power. The speed of movement in the soil is dependent on pore diameter, particle size, water content of the soil and the diameter and relative activity of nematodes.

In addition to their own movement, however, nematodes can be spread by anything that moves and can carry particles of soil. Farm equipment, irrigation, flood or drainage water, animals and dust storms spread nematodes.

Temperature has a profound effect on the survival and multiplication of nematodes in the soil and also on their parasitism. Some of the soil factors affecting root knot nematodes were reviewed by Kincaid (1946).

Singh (1964, 1965a, 1965b, 1967) has reviewed the literature on the effect of the soil, temperature, moisture , aeration, soil texture, organic matter, rhizosphere, and various cultural operations on the activity of plant parasitic nematodes.

Meloidogyne spp. have been reported to develop most rapidly at about 27 °C. Reproduction in M. halpa is extremely reduced at 35 °C. Ahmedand Khan (1964) found that the optimum temperature for the hatching of larvae in M. Intiognita is 30 °C. The temperature also affects the resistance of resistant varieties and the size of the gans formed by root knot nematodes on the host.

Soil moisture is important for nematodes in many ways. According to Parris (1948), Meloidogyne spp. can enter potato tubers through lenticels which occur on tubers commonly in wet soils.

Galling of tubers is less in dry soils. Other factors which influence the activity of plant parasitic nematodes are soil pH, soil texture, aeration, types of plants grown between the susceptible hosts, duration of cultivation of resistant varieties in the field, type of decomposable organic matter added to the soil, inorganic fertilizers, type of nitrogen and potassium status in the soil.

The phytopathogenic nematodes and their relatives comprise a large number of species placed in 15 families and 111 genera. The gross morphology of these nematodes, with few exceptions, is generally very similar. The body is covered by a cuticle. Internally there are digestive, excretory and reproductive systems.

The nematode body is more or less transparent. It is covered by a colourless cuticle which is usually marked by striations, or may show bristles, punctuations, warts, or other markings.

The cuticle molts when the nematodes go through their successive larval stages. The cuticle is produced by the hypodermis which consists of living cells and extends into the body cavity as four chords separating four bands of longitudinal muscles. These muscles enable the nematode to move. Additional specialized muscles exist at the mouth and along the digestive tract and the reproductive structures.

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The oesophagus (pharynx) which is the anterior end of the digestive system consists in the Tylenchida, of stylet (stomatostyle), corpus, isthmus and glandplar posterior enlargement. Modifications in the morphology of the oesophagus are widely used at all levels of nematode taxonomy.

Many of the groupings of higher taxa in the Tylenchida, as well as in those groups of Dorylaimida containing phytoparasitic nematodes, are based largely upon modifications of gross shape and internal morphology of the oesophagus. The digestive system is a hollow tube extending from the mouth through the buccal cavity, oesophagus, intestine, rectum and anus. Lips, usually six numbers, surround the mouth.

All plant parasitic nematodes have a protrusible hollow stylet or spear. This is the main characteristic that suggests that the nematode is a plant parasite. This structure has been homologized in Tylenchida with the stoma of other nematodes and is used as a piercing organ through which nematodes acquire food. It is actually a three-part structure taxonomically; the stylet is used at all levels of classification.

The body cavity is rudimentary in its development and contains a fluid through which circulation and respiration take place.

The excretory system is not well developed in nematodes. However, the nervous system is well developed and consists of many nerves, ganglia and sensory structures.

The reproductive systems are well developed. The female nematodes have one or two ovaries followed by an oviduct and uterus terminating in a slit-like vulva.

The male’s reproductive structure is similar to the females but has a testis, seminal vesicle and ejaculatory duct, and terminates in a common cloaca with the intestine. A pair of protrusible, copulatory spicules are also present in the male. Reproduction in the nematodes is through the eggs and may be sexual, hermaphrodite or parthenogenetic. Many species lack males.

The life histories of most plant parasitic nematodes are, in general, quite similar. There are six stages in the life cycle of a nematoded Egg, L1 (larval stage), L2, L3, L4 and Adult. Eggs hatch into larvae, whose appearance and structure are usually similar to those of the adult nematodes except for the development of the reproductive systems.

Larvae grow in size and each larval stage is terminated by a molt. After the final molt the nematodes differentiate into adult males and females. The females can then produce fertile eggs either after mating with a male or in the absence of males, parthenogenetically, or they can produce sperm themselves.

The time for the completion of the life cycle s about 20 -40 days under optimum environmental conditions but longer in cooler temperatures.

In some species of nematodes the first or second larval stages cannot infect plants and they depend for their metabolic functions on the energy stored in the egg. When the infective stages are produced, however, they must feed on a susceptible host or else starve to death. If suitable hosts are not available, all the individuals of certain nematode species may die but in other species the eggs may remain dormant in the soil for years.

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Next-Generation Molecular-Diagnostic Tools for Gastrointestinal Nematodes of Livestock, with an Emphasis on Small Ruminants

Florian Roeber, . Robin B. Gasser, in Advances in Parasitology , 2013

Abstract

Parasitic nematodes of livestock have major economic impact worldwide. Despite the diseases caused by these nematodes, some advances towards the development of new therapeutic agents and attempts to develop effective vaccines against some of them, there has been limited progress in the development of practical diagnostic methods. The specific and sensitive diagnosis of parasitic nematode infections of livestock underpins effective disease control, which is now particularly important given the problems associated with anthelmintic resistance in parasite populations. Traditional diagnostic methods have major limitations, in terms of sensitivity and specificity. This chapter provides an account of the significance of parasitic nematodes (order Strongylida), reviews conventional diagnostic techniques that are presently used routinely and describes advances in polymerase chain reaction (PCR)-based methods for the specific diagnosis of nematode infections. A particular emphasis is placed on the recent development of a robotic PCR-based platform for high-throughput diagnosis, and its significance and implications for epidemiological investigations and for use in control programmes.

Etiologic Agents of Infectious Diseases

Parasitic nematodes are found in a variety of ecological settings that include human habitation. Many are transmitted to children due to pica, since the long-lived infectious eggs are contained in feces that contaminate most outdoor environments. Some nematode infections have zoonotic importance as disease-producing agents resulting in potentially serious childhood infections. A few of these can permanently affect a child’s ability to learn. This chapter covers clinical information relevant to nematodes that take up residence in the deep tissues: Toxocara canis and T. cati, Baylisascaris procyonis, Anasakis spp., Angiostrongylus spp., Ancylostoma braziliense, Gnathostoma spinigerum, and Trichinella spp.

Amphibian Therapy

Parasitic nematodes are ubiquitous in amphibian collections. It is important that a discussion take place between the veterinarian and the client to understand the purpose of captive amphibians and to develop a preventive health program based on an assessment of the health risk posed by nematodes and the potential impact their presence may have on management (e.g., exchanging specimens with other collections). A particularly thorny issue is the presence of nematodes in amphibians destined for assurance colonies. These amphibians are genetic reservoirs against extinctions in the wild, and they arrive with a spectrum of parasites that they shared with their natural environments. The host–parasite–environment triangle shifts in captivity for a variety of reasons, many of which are incompletely understood. Is it the best management choice to eliminate these parasites in captive specimens, thereby possibly artificially selecting for amphibians that have immune systems that have never encountered these nematodes when their offspring may one day be reintroduced into the wild and have to face these same parasites?

Haemonchus contortus and Haemonchosis – Past, Present and Future Trends

6 Nucleic Acid Metabolism

Parasitic nematodes , like other eukaryotic parasites, are characterized by substantial cellular multiplication rates associated with high nucleic acid synthesis. One adult female of H. contortus can produce up to 10,000 eggs per day ( Veglia, 1916 ). In comparison, A. lumbricoides can produce 2 × 10 5 eggs per day ( Wehner and Gehring, 1995a,b ).

6.1 Purine metabolism

Nematodes including H. contortus are not able to synthesize purines de novo and, therefore, are dependent on the supply of suitable purine precursors from the host ( Köhler, 2006 ). Purine bases are converted to the different purine nucleotides, either by phosphoribosyltranserases (PRTases) or by reactions involving nucleoside phosphorylases and nucleoside kinases. The pattern of salvage pathways for purines can vary considerably, depending on nematode species and developmental stage ( Barrett, 1981; Köhler, 2006 ). Nematodes are able to synthesize pyrimidine nucleotides de novo, but can also produce these compounds via salvage pathways. There are substantial differences among nematode species in synthesis capacities ( Köhler, 2006 ).

Haemonchus contortus and Haemonchosis – Past, Present and Future Trends

R.B. Gasser, . N.D. Young, in Advances in Parasitology , 2016

1 Introduction

Parasitic nematodes (roundworms) of humans and other animals are of major significance as pathogens ( Anderson, 2000 ). In particular, parasitic nematodes of livestock, including species of Haemonchus, Ostertagia and Trichostrongylus (Strongylida: Trichostrongyloidea) cause substantial economic losses due to reduced growth, poor productivity, costs of anthelmintic treatment and deaths ( Kaplan and Vidyashankar, 2012; Wolstenholme et al., 2004 ). In addition to their economic impact, anthelmintic resistance in nematodes of livestock ( Gilleard, 2006; Wolstenholme and Kaplan, 2012 ) has stimulated research to develop alternative intervention and control strategies against these parasites. Despite current knowledge of many aspects concerning Haemonchus contortus (see Anderson, 2000 ; chapter: The pathophysiology, ecology and epidemiology of Haemonchus contortus infection in small ruminants by Besier et al., 2016 – in this issue; Sutherland and Scott, 2010 ), little has been understood about the molecular biology and genetics of this worm, the interaction that it has with its host animals and the disease (haemonchosis) that it causes at the molecular and biochemical levels. Gaining an improved understanding of these areas could offer a possible pathway to discover new methods of diagnosis, treatment and control. Advanced genomic and bioinformatic technologies provide some opportunities to explore, for example, basic developmental and reproductive processes in nematodes. In particular, genomic and transcriptomic studies of parasites have become instrumental in various areas, such as gene discovery and characterization, and for gaining insights into aspects of gene expression, regulation and function ( Bird et al., 2015; Blaxter and Koutsovoulos, 2015; Lv et al., 2015; Weiberg et al., 2015; Zarowiecki and Berriman, 2014 ). The purpose of this chapter is to (1) give a brief background on H. contortus and haemonchosis, immune responses, vaccine research, anthelmintics and problems associated with drug resistance; (2) describe progress in transcriptomics prior to the availability of genomic information for H. contortus and the challenges associated with this work; (3) review the characterization of two draft genomes of H. contortus and provide an account of the insights that annotation and analyses of these genomes have given into the biology of this worm, its relationship with its host and disease as well as identify prospects for designing new approaches to combat haemonchosis.

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Decoding the Ascaris suum Genome using Massively Parallel Sequencing and Advanced Bioinformatic Methods – Unprecedented Prospects for Fundamental and Applied Research

Aaron R. Jex, . Robin B. Gasser, in Ascaris: The Neglected Parasite , 2013

Abstract

Parasitic nematodes cause substantial morbidity and mortality in animals and people globally and major losses to food production annually. Ascaris is among the commonest geohelminths of swine and people worldwide, and causes major disease and socioeconomic losses, particularly in developing countries. The control of ascariasis has become a global health and welfare priority, but current treatment programs carry a significant risk of inducing anthelmintic resistance. Therefore, there is a need to work toward the sustainable control of Ascaris/ascariasis, built on a solid understanding of its molecular biology and genetics. Recently, we reported the 273 megabase (Mb) draft genome of Ascaris suum (sequenced from the reproductive tract of a single adult female worm) and explored transcription in different organs, stages, and both sexes of this nematode using advanced sequencing and computer technologies. We characterized key genes and biological pathways linked to the parasite’s migration in the host, and its immunobiology, reproduction, and development. We also predicted and prioritized drug targets in A. suum, providing a basis for discovering new groups of nematocides. The present chapter provides an account of these recent advances, describes new methodologies established, and emphasizes prospects for profound investigations into the comparative genomics, genetics, evolution, immunobiology, epidemiology, and ecology of Ascaris from both pig and human hosts as well as for the development of new interventions against ascariasis and other helminthiases.

General Characteristics of the Nematoda

Burton J. Bogitsh, . Thomas N. Oeltmann, in Human Parasitology (Fifth Edition) , 2019

Physiology

Parasitic nematodes derive much of their energy from the metabolism of glucose. Glycogen reserve is stored primarily in the hypodermis, the intestine, the noncontractile parts of the muscles, and parts of the reproductive system. It is difficult to establish with certainty whether adult intestinal nematodes are exclusively aerobic or anaerobic in their usage of carbohydrate or whether free-living larvae are invariably aerobic. Experimental data strongly suggest the importance of determining not only whether each species uses oxygen but also the manner in which it is utilized. For instance, intestinal nematodes belonging to the genus Ascaris can use oxygen if available; however, it serves only as a terminal electron acceptor in a system independent of an electron transport system or a functional Krebs’ cycle. On the other hand, both larvae and adults of Trichinella spiralis have a functional TCA cycle and an electron transport system with oxygen being the final electron acceptor. Thus, the former organism generates ATP by means of substrate-level phosphorylation, while the latter organisms do so primarily through oxidative phosphorylation. The eggs and first- and second-stage larvae of Ascaris spp. exhibit aerobic metabolism with a functional Krebs’ cycle. Indeed, optimal larval development in most species that have been studied is dependent upon relatively high concentrations of oxygen.

Certain relatively large nematodes possess some sort of oxygen-binding system, including utilization of the respiratory pigments myoglobin and hemoglobin. Such pigments are usually found in the pseudocoelomic fluid and the hypodermis.

Haemonchus contortus and Haemonchosis – Past, Present and Future Trends

C.E. Lanusse, . A.L. Lifschitz, in Advances in Parasitology , 2016

1 Introduction

Parasitic nematodes of ruminants represent one of the greatest infectious disease problems in grazing livestock systems worldwide. Despite promising research results, nonchemical parasite control strategies are not yet available for routine, commercial use. Thus, nematode control in livestock still relies on the use of antiparasitic drugs, which comprise the largest sector of the animal pharmaceutical industry. The integration of available information on the host-parasite-environment relationship with an understanding of the pharmacological properties of traditional antiparasitic molecules has contributed to more efficient parasite control. However, due to the magnitude of the investment in drug medications, it is necessary to acquire scientific information on how to improve the use of available molecules, and particularly, to avoid/delay the development of anthelmintic resistance. Most fields of chemotherapy benefit from in vitro test systems that can be used to accurately predict drug concentrations required for efficacy in vivo. However, it has been difficult to develop a culture system for nematodes to assess the potency of anthelmintics in vitro. This inconvenience has been a major limitation to estimating the active drug concentration required to achieve optimal in vivo activity and has delayed advances in drug development. However, progress made in understanding pharmacokinetic behaviour and pharmacodynamic mechanisms of drug action/resistance has allowed deep insights into the pharmacology of the main chemical classes including some of the few recently discovered anthelmintics. A remarkable amount of pharmacology-based knowledge has been acquired using the sheep abomasal nematode Haemonchus contortus as model. The scientific contributions to the pharmacology of anthelmintic drugs based on the use of H. contortus as a model nematode are summarized in the present chapter.

Harnessing the Toxocara Genome to Underpin Toxocariasis Research and New Interventions

Robin B. Gasser, . Neil D. Young, in Advances in Parasitology , 2016

2 Significance of Toxocara and Diagnostic Considerations

Parasitic nematodes of the superfamily Ascaridoidea infect all major groups of vertebrates (cf. Hartwich, 1974; Sprent, 1956, 1958, 1959, 1983, 1992; Anderson, 2000; Bowman, 2014 ), and some taxa are of particular socioeconomic importance. Toxocara is a significant genus because some species cause clinical disease and are transmissible from animals to humans (i.e. are zoonotic) (e.g. Pawlowski, 2001; Despommier, 2003; Magnaval and Glickman, 2006; Macpherson, 2013; Bowman, 2014 ). In particular, larvae of T. canis are capable of invading human tissues and causing various forms of disease (including VLM, OLM, CT and NT). Larvae of other ascaridoids, including Toxocara cati, Toxocara vitulorum and Toxascaris leonina, can also invade the tissues of laboratory animals, but there has been some uncertainty as to what extent they are implicated in human disease ( Holland and Smith, 2006; Bowman, 2014 ). Toxocara cati appears to be underestimated as a zoonotic agent ( Fisher, 2003 ), and there is a possibility that Toxocara malaysiensis ( Gibbons et al., 2001 ) might also be zoonotic in endemic countries, but it is unclear to what extent.

Extending early studies ( Rohde, 1962; Lee et al., 1993 ), employing molecular methods, Zhu et al. (1998a) showed that ‘T. canis’ from Malaysian cats was genetically more similar to T. cati than T. canis. In a subsequent morphological study, Gibbons et al. (2001) named this parasite T. malaysiensis n. sp. and found three key morphological features (shape of cervical alae in cross section; spicule length; lip structure) that allow its differentiation from T. canis, T. cati and other congeners, such as Toxocara apodemi, Toxocara mackerrasae (from rodents), Toxocara paradoxura and Toxocara sprenti (viverrids), Toxocara pteropodis (bats), Toxocara tanuki (canids) and Toxocara vajrasthirae (mustelids). The discovery of this new species of Toxocara in cats raised doubts about the identity of ‘T. canis’ found in cats in other parts of the world (cf. Calero et al., 1951; Hitchcock, 1953; Ash, 1962; Sprent and Barrett, 1964; Parsons, 1987; Baker et al., 1989; Scholz et al., 2003 ) and encourages genetic comparisons of Toxocara spp. from different hosts and geographical origins. Also a study from Switzerland raises questions as to the epidemiological significance of detecting T. cati eggs in dog faeces ( Fahrion et al., 2011 ), warranting future investigations.

Clearly, the accurate identification and differentiation of Toxocara species are central to the specific diagnosis of toxocariasis in any vertebrate host and to studying their life cycles, epidemiology, and population biology. Traditionally, ascaridoids have been identified using morphological characters of different life cycle stages and their predilection site(s) in their host(s) ( Bowman, 2014 ). Sprent (1983) assessed the taxonomic relevance of structures such as the caecum, excretory system, labia, male tail and oesophagus. Other authors described features for specific identification ( Nichols, 1956; Bowman, 1987; Averbeck et al., 1995 ). For instance, Nichols (1956) described the diagnostic characters of T. canis and T. cati larvae, and Averbeck et al. (1995) reported criteria for the differentiation of T. canis from T. leonina and Baylisascaris procyonis based on the morphology of the adult and egg stages. Nonetheless, despite existing descriptions and keys, there are challenges in specifically identifying and differentiating some developmental stages of ascaridoids, or parts thereof, using microscopic approaches.

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