Moulting of parasitic nematodes

Nematodes are usually divided into two major groups, the Adenophora which are common in water and the Secernentea largely from soil. Most research on moulting has been done with the Secernentea, which include the majority of the parasitic forms. Cuticle composition and morphology is discussed in relation to the physiology and possible reasons for moulting in the Nematoda.     

The FAR protein family of parasitic nematodes

Fatty acid–and retinol-binding proteins (FARs) belong to a unique family of excreted/secreted proteins (ESPs) found exclusively in nematodes. Much of our understanding of these proteins, however, is limited to their in vitro binding characteristics toward various fatty acids and retinol and has provided little insight into their in vivo functions or mechanisms. Recent research, however, has shown that FARs elicit an immunomodulatory role in plant and animal model systems, likely by sequestering lipids involved in immune signaling. This alludes to the intricate relationship between parasitic nematode effectors and their hosts.     

Neurobiology of plant parasitic nematodes

The regulatory constraints imposed on use of chemical control agents in agriculture are rendering crops increasingly vulnerable to plant parasitic nematodes. Thus, it is important that new control strategies which meet requirements for low toxicity to non-target species, vertebrates and the environment are pursued. This would be greatly facilitated by an improved understanding of the physiology and pharmacology of these nematodes, but to date, these microscopic species of the Phylum Nematoda have attracted little attention in this regard. In this review, the current information available for neurotransmitters and neuromodulator in the plant parasitic nematodes is discussed in the context of the more extensive literature for other species in the phylum, most notably Caenorhabditis elegans and Ascaris suum. Areas of commonality and distinctiveness in terms of neurotransmitter profile and function between these species are highlighted with a view to improving understanding of to what extent, and with what level of confidence, this information may be extrapolated to the plant parasitic nematodes.     

The systematic position of some nippostrongyline nematodes (Trichostrongylina: Heligmosomoidea) parasitic in Argentinean sigmodontine rodents

The systematic position of two nippostrongyline nematodes described from Argentinean sigmodontine rodents is clarified. The first species, Hassalstrongylus multiovatus Suriano & Navone, 1992, parasitic in Akodon simulator Thomas from the province of Tucumán, was studied on the basis of type and voucher material. H. multiovatus is proposed as a junior synonym of Trichofreitasia lenti Sutton & Durette-Desset, 1991, a parasite described from Oligoryzomys flavescens (Waterhouse) in the province of Buenos Aires. The holotype and three of seven paratypes deposited as H. multiovatus were identified as T. lenti. One male paratype was identified as Guerrerostrongylus uruguayensis Sutton & Durette-Desset, 1991, a parasite described from O. flavescens in Uruguay. Three female paratypes were identified as Guerrerostrongylus sp. The second species, Stilestrongylus scapteromys Suriano & Navone, 1996, parasitic in Scapteromys aquaticus Thomas from the province of Buenos Aires, was studied on voucher material. Stilestrongylus scapteromys and Malvinema frederici Digiani, Sutton & Durette-Desset, 2003, the type-species of Malvinema Digiani, Sutton & Durette-Desset, 2003, were described from the same host and geographical region. As they are considered to refer to one and the same taxon, the new combination Malvinema scapteromys n. comb. is proposed for this species.     

The effects of tannin-rich plants on parasitic nematodes in ruminants

Apart from the obvious role of plants in herbivore nutrition, they are also a rich source of bioactive products that can operate either to the benefit or the detriment of grazing animals. Here, we review the available evidence for the potential beneficial effects that plant-derived bioactive substances can have on gastrointestinal parasites. Tannin-rich plants have attracted most attention for their effect on internal nematodes in ruminants. These plants could act through direct antiparasitic activity but might also act indirectly by increasing host resistance. The effects vary with the species of plant, parasite and host. More research is required to understand better the mechanisms of action, and therefore make more pertinent use of these bioactive plants in livestock systems.     

Genes and genomes of parasitic nematodes

Our knowledge of gene and genome organization in nematodes is growing rapidly, partly as a result of the Caenorhabditis elegans genome project. Here Martin Hammond and Ted Bianco review what is known about the organization of genes and genomes in parasitic nematode species, using information gained from molecular and cytological approaches. They suggest that there are implications not only for a wide range of problems in parasitology but also for our understanding of genome evolution in eukaryotes.     

Sexual size dimorphism in parasitic oxyurid nematodes

As in many invertebrates, female oxyurids are larger than male. Sexual size dimorphism (SSD) of oxyurid nematodes (the hosts of which are both invertebrate and vertebrate), is investigated regarding body size of both host and parasite. SSD of parasites appeared to be weakly, but not significandy, correlated with invertebrate and vertebrate host body size. However, this study reveals a different pattern for SSD with respect to either type of host. SSD does not increase in tandem with body size in vertebrate parasites either at the level of species or genus. SSD is much more pronounced in Syphaciidae than in Heteroxynematidae, two families of vertebrate parasites exhibiting different modes of transmission (members of the Syphaciidae are transmitted through perianal contamination). SSD is investigated in one monophyletic group of parasites of primates, for which a phylogeny is known. Independent comparisons method is used and we find that the body size of female parasite is strongly correlated with that of the male. The hypoallometry (slope<1) of the relationship suggests that the SSD is not linked to an increase of parasite body size. Moreover, there is no influence of host body size on parasite SSD. The pattern in parasites of invertebrates is different. First, SSD has been found to increase with parasite body size in two groups of invertebrate parasites: the oxyurids of Dictyoptera and Coleoptera. Second, female body size of invertebrate parasites is not correlated with male body size either at genus or species level. Finally, the evolution of SSD is discussed in relation to the demographic patterns of invertebrate parasites and the haplodiploid mode of reproduction of these parasitic nematodes.     

The soil as an environment for plant parasitic nematodes

British arable soils are uniform in texture to plough depth but vary in structure and the content of new organic matter according to recent cultivations. Subsoils are less uniform and little influenced by cultivations. Although remarkably uniform in structure and outward appearance nematodes vary greatly in length and girth. Root ectoparasites live wholly in the soil but root endoparasites may spend only brief periods there. Nematodes that spend much time in the soil surface are subject to a harsher microclimate than those inhabiting deeper layers. The movement and activity of nematodes in soil is influenced by the thickness of water films, the amount of pore space with dimensions the nematodes can traverse, the stability and packing of aggregates, the oxygen consumption of competing organisms and the rate of supply. Other factors of which little is known are the degree of continuity of the usable spaces and their tortuosity. The moisture characteristic curve which relates water withdrawn to the suction pressure (matric potential) applied is a useful tool which enables the space available to a given nematode to be related to its cross-sectional diameter. Although total pore space and usable pore space are correlated in a general way with each other and with texture (e.g.% sand), and bulk density is correlated with pore space within textural classes, none of these measures can be substituted for an estimate of the usable pore space appropriate to the size of the nematode being studied. For most purposes the solid matrix of arable soils can be regarded as an inert skeleton supporting the pore space. The stability of the matrix is therefore an important parameter, determining changes of space and time. Little is known about the distribution of macro-voids in soils and rapid methods of assessing them are needed. These may be more important for and more easily used by long nematodes than short ones. For nematodes living near the soil surface diurnal temperature fluctuations may be large. Their movement and activity should be related to temperature fluctuations via the Q₁₀-curve. For nematodes living deeper in the soil diurnal fluctuations are unimportant. So development can be mirrored by curves of accumulated temperature in day degrees above basal development temperature. Unlike the pattern of rainfall accumulation, which varies from place to place and year to year, and must greatly affect nematode multiplication, crop damage and activity, the pattern of temperature accumulation is remarkably stable. Differences from place to place and year to year are moderated by the changes in sowing date they impose. Consequently a nematode species and the host plant it infests usually develop together under similar soil climatic conditions from planting date onwards. In real soils the spaces in which nematodes live are partly filled by water in winter but are progressively drained as the season advances and become air filled. Then water films are too thin to permit movement except while the soil is draining after rainfall. Therefore the duration of activity is proportional to rainfall, and total activity throughout development is proportional to accumulated rainfall after discounting amounts too small to penetrate the soil. The activity of root ectoparasitic nematodes could be modelled by multiplying the duration of activity by temperature, using the Q₁₀ curve if necessary (i.e. by the rate of activity). The soil environment imposes constraints on the animals that live in it. Populations are relatively static and inbred. The pore space also limits the properties of substances that link parasite with parasite (pheromones) and parasite with host (phytomones). The many additional complications that arise when plants are grown are depicted diagrammatically.     

Organic and inorganic acids as the stimulus for exsheathment of infective juveniles of nematodes

Petronijevic T., Rogers W. P. and Sommerville R. I. 1986. Organic and inorganic acids as the stimulus for exsheathment of infective juveniles of nematodes. International Journal for Parasitology 16: 163–168. A variety of organic and inorganic acids stimulated exsheathment of infective juveniles of Nematospiroides dubius over the range pH 2–7. Activity, in relation to pK values, suggests that the undissociated form of the acid was the active agent. Under normal conditions of exposure, exsheathment of infective juveniles of Haemonchus contortus was not obtained below pH 6, and at higher pH values activity was low except in the presence of CO₂ at pressures >9-5 kPa in the gas phase. Brief exposure to HCl at concentrations up to 2N did not induce exsheathment but in the presence of CO₂ in the gas phase, 95.3 kPa, activity was obtained in 0-O1N HCl when the times of exposure were 3–25 min.The differences in the responses of N. dubius and H. contortus to a stimulus at low pH values may be attributed to the stability of the exsheathing enzyme of N. dubius in acid media as well as to the relative sensitivity of the two species to stimuli generally.     

Nematode-trapping fungi against parasitic cattle nematodes

Interactions between larvae of bovine gastrointestinal nematode parasites and nematode-trapping fungi, such as Arthrobotrys and Duddingtonia species and strains have been studied in Denmark. In this article J?rn Gr?nvold, Jens Wolstrup, Peter Nansen and Svend Aage Henriksen discuss how these fungi are able to grow, trap and kill parasitic nematode larvae, both on agar in the laboratory and in cattle faeces in the field.     

RNA interference and plant parasitic nematodes

RNA interference (RNAi) has recently been demonstrated in plant parasitic nematodes. It is a potentially powerful investigative tool for the genome-wide identification of gene function that should help improve our understanding of plant parasitic nematodes. RNAi should help identify gene and, hence, protein targets for nematode control strategies. Prospects for novel resistance depend on the plant generating an effective form of double-stranded RNA in the absence of an endogenous target gene without detriment to itself. These RNA molecules must then become available to the nematode and be capable of ingestion via its feeding tube. If these requirements can be met, crop resistance could be achieved by a plant delivering a dsRNA that targets a nematode gene and induces a lethal or highly damaging RNAi effect on the parasite.     

Mucosal immunity against parasitic gastrointestinal nematodes

The last two decades witnessed significant advances in the efforts of immunoparasitologists to elucidate the nature and role of the host mucosal defence mechanisms against intestinal nematode parasites. Aided by recent advances in basic immunology and biotechnology with the concomitant development of well defined laboratory models of infection, immunoparasitologists have more precisely analyzed and defined the different immune effector mechanisms during the infection; resulting in great improvement in our current knowledge and understanding of protective immunity against gastrointestinal (GI) nematode parasites. Much of this current understanding comes from experimental studies in laboratory rodents, which have been used as models of livestock and human GI nematode infections. These rodent studies, which have concentrated on Heligmosomoides polygyrus, Nippostrongylus brasiliensis, Strongyloides ratti/S. venezuelensis, Trichinella spiralis and Trichuris muris infections in mice and rats, have helped in defining the types of T cell responses that regulate effector mechanisms and the effector mechanisms responsible for worm expulsion. In addition, these studies bear indications that traditionally accepted mechanisms of resistance such as eosinophilia and IgE responses may not play as important roles in protection as were previously conceived. In this review, we shall, from these rodent studies, attempt an overview of the mucosal and other effector responses against intestinal nematode parasites beginning with the indices of immune protection as a model of the protective immune responses that may occur in animals and man.     

Optimal timing of first reproduction in parasitic nematodes

The time between infection and the onset of reproduction (maturation time) is a key determinant of body size, fecundity and generation time in parasitic nematodes. An optimality model for maturation time is developed centred on prematurational growth, the duration of which has opposing consequences for fecundity and for survival to reproductive age. The maturation time favoured by natural selection is found to be inversely proportional to prematurational mortality rate. The product of maturation time and mortality rate is predicted to be an invariant number equal to the allometric slope linking daily fecundity to maturation time. Predictions are tested using comparative data on mammalian gastrointestinal nematode taxa. Half the cross-species variation in prepatent period (the time from infection until propagules are shed from the host) is accounted for by this adaptive trade-off hypothesis, even though many biological details are not explicitly modelled. These results are discussed in the light of previous studies and in the context of general models of life history evolution.     

Possible utilization of weeds for the management of plant parasitic nematodes infesting some vegetable crops

Leaf extracts of noxious weeds such as Solanum xanthocarpum and Argemone maxicana were used as bare-root dip treatment for the management of three important plant-parasitic nematodes, Meloidogyne incognita, Rotylenchulus reniformis and Tylenchorhynchus brassicae infesting tomato (Lycopersicon esculantum ) and chilli (Capsicum annuum) plants. Significant reduction was observed in the root-knot development caused by M. incognita, multiplication of nematode populations of R. reniformis and T. brassicae on both the test plants. Larval penetration of second stage juveniles of M. incognita was also inhibited at various concentrations of leaf extracts and dip durations. Leaf extract of S. xanthocarpum caused relatively more inhibition in root-knot development in case of root-knot nematode, nematode multiplication of reniform and stunt nematodes than that of A. maxicana. Because of dip treatment in leaf extracts of Argemone maxicana and Solanum xanthocarpum, the plants show better growth and at the same time the populations of nematodes such as M. incognita, R. reniformis and T. brassicae significantly decreased, which naturally improved plant growth. The efficacy of root-dip treatment with respect to improvement in plant weight and reduction in root-knot development and nematode populations, increased with increasing the concentration of leaf extracts and dip durations.