Animal Parasitic Nematodes - an overview | ScienceDirect Topics

Animal and Human Parasitic Nematodes
Animal parasitic nematodes infect animals from all habitats – there are about 342 species which infect humans. Humans worldwide are infected by nematodes each year, mostly in tropical areas and developing countries, in the order of 3.5 billion. Nematodes parasitic on humans include ascaris, filarial nematodes, hookworms, pinworms, and whipworms. Species include Ancylostoma duodenale, Necator americanus, Trichinella spiralis, Wuchereria bancrofti, Onchocerca volvulus, etc. They can attack the muscles, alimentary canal, eyes, and other body tissues. Entry can be through skin, ingestion of eggs in food, or through bites by an infected vector (e.g., mosquitoes that carry filarial worms). In some cases, animals may be intermediate hosts where the nematodes enter and grow for a period of time as larvae and then become dormant cysts. If a human eats the infected meat, the juveniles become active again and grow into adult worms. Ascaris and Trichuris spp. can survive dry conditions or unfavorable temperatures as eggs. Tapeworms also infect cows, fish, or pigs and then latch onto the intestinal wall of a human that consumes them. Heartworms causing heartworm disease infest hearts, arteries, and lungs of dogs and some cats.
How Many Species of Vertebrate Parasitic Nematodes Exist?
All species of vertebrates examined thus far serve as hosts for at least one species of parasitic nematode. Some mammalian hosts harbor many species of nematodes that are distributed through several orders and families. Some of these nematodes are highly host-specific, surviving and reproducing successfully only in host individuals comprising a single species or perhaps a closely related group of species. Other nematodes show less specificity, being much more likely to jump from one suitable vertebrate host to another during opportune times during their life-history (Brant and Gardner, 2000).
Within a host, many different types of habitats may be occupied by nematodes. As a species, Homo sapiens harbors around 35 species of host-specific parasitic nematodes (Chitwood and Chitwood, 1977). To illustrate the diversity of habitats in a single animal host, humans will be used as an example. In a human, nematodes can occur as juveniles in muscle tissues, usually smooth muscle like the diaphram or the tongue (Trichinella), in the mucosa of the intestine (Strongyloides), as migrating forms in blood, and lungs (Ascaris), in blood or lymph as microfilariae (Filarioids of humans), as adults in the small and large intestines (Ancylostoma, Necator, Ascaris), as adults in mesentaeries and subcutaneous tissues (Onchocerca, Loa, Wuchereria) and Enterobius in the large intestine and cecum.
Cobb was well acquainted with animal parasitic nematodes, and his familiarity with host-specificity in the Nemata led him to estimate that as early as 1914, well over 80,000 species of nematodes would eventually be found parasitizing vertebrates alone. Similar estimates could be derived for parasites of invertebrates and plants resulting in totals of free-living and parasitic species numbers far in excess of the approximately 20,000 species of nematodes known today.
Cobb (1915) stated “There must be hundreds of thousands of species of nematodes. Of this vast number only a very few thousand have been investigated, and of these, comparatively few with any degree of thoroughness.”
Present estimates are that only about 20,000 species of parasitic nematodes have been described from all taxa of slightly more than 48,000 currently recognized species of vertebrates. In addition, many species of wild mammals each harbor more than 2 host-specific species of nematode so the above estimate of 8900 species of nematodes only from mammals can be considered very low.
As of this writing, the natural history, development, and transmission parameters of only around 561 species are known (Anderson, 1992). As parasites of only vertebrates, Anderson (1992) estimated that there were about 2300 described genera distributed among 256 families comprising about 33% of all nematode genera known. This proportion is about equal to the percentage of genera of Nemata presently known in marine and freshwater habitats. It is agreed by most workers that this bias is due to the larger number of parasitologists working on selected groups of Nemata relative to the number of specialists working on the free-living marine and freshwater forms.
If each of the approximately 4450 known species of mammals were infected with only 2 species of host-specific nematodes, we would expect to find a minimum of 8900 species of parasitic nematodes only in the Class Mammalia.
Pinworms, nematodes of the Order Rhabditida Superfamily Oxyuroidea (Fig. 1) show high levels of host specificity and it is well-known that almost all species of rodents and primates have one or more species-specific pinworm. Both recent and historical studies have shown that pinworm nematodes exhibit high levels of host specificity, and they have co-speciated with their primate and rodent hosts (Hugot, 1999). Currently, slightly more than 800 species of Oxyuroidea have been described with the vertebrates hosting about 500 species and invertebrates species with about 300. The greatest diversity in the Oxyuroidea to be found in the future is expected to come from examination of the catfishes of the Amazon basin (Rodrigues et al., 2020) and the Arthropoda, especially beetles, cockroaches (with around 4000 species of blattoidea described and more than 20–30,000 expected to be found) and millipedes (17,000 species described and more than 60,000 species expected to be found). This gives huge numbers of pinworms occurring just in the cockroaches and the millipedes if each species harbors its own species of pinworm. At least 2 species of pinworms (genus Thylastoma) occur in laboratory colonies of the American Cockroach (Periplaneta americana) and more are expected to be found in free-living populations (Hugot, Personal Communication).
When adequate surveys are completed, large numbers of species of Oxyuroidea are also expected to be described from Neotropical rodents of the family Muridae. Up to the present time, only around 7 species of pinworms have been described from Neotropical murids and these rodents potentially host from 400 to 800 undescribed species of Oxyuroidea, (given that only one to two new species of Oxyuroid nematode nematode is found in each species of rodent examined) (Hugot, Personal Communication). Another comparative study in the Oxyuroidea of rodents show that the larger the body-size of the rodent, the larger the body size of oxyurid nematodes that it harbors (Morand et al., 1996).
I.D. The Reproductive System
The reproductive system of most animal parasitic nematodes is adapted to produce extremely large numbers of eggs (e.g., Ascaris). Some characteristics of larger animal parasitic nematodes include very large body, very two large ovaries, and an equally large uterus. Free-living nematodes generally have much smaller bodies and are therefore individually less prolific. However, the reproductive systems of most nematodes have the following basic structural similarities: Most females have two ovaries, one anterior (as in Pratylenchus, Fig. 21b) and one posterior, each connected to an oviduct and uterus with the uteri connecting to the vagina, terminating in the vulva. Males usually have one testes as in the Secernentea or two as in the Adenophorea, a seminal vesicle, and a vas deferens (see Maggenti, 1991a) connecting to the outside via the cloaca (Fig. 13) which is a joining of the reproductive system and the rectum. Secondary sexual organs in male nematodes are usually much more pronounced and variable than that of the female (Figs. 2, 6, 11, 12, 13, 22, 23, 24, 25) and most males have one or two spicules (Fig. 22), a spicular pouch, sometimes with a spicular sheath (Fig. 24), and a gubernaculum (Fig. 23). Some males of the O. Strongylida have a well-developed copulatory bursa (Fig. 22).
Figure 22. Posterior end of a male trichostrongyloid showing the well-developed copulatory bursa, bursal rays, and the long thin paired spicules.
Figure 24. Posterior end of Trichuris from a Bolivian species of Ctenomys. The long spicule and everted spicular sheath can be seen.
Figure 25. Posterior end of the filaroid nematode Litomosoides from Ctenomys opimus from high altitude western Bolivia. Note the dimorphic nature of the spucules in this species: one being long and filamentous, the other short and stubby.
Some plant parasitic nemas produce eggs directly into the plant where the nematode lives (in the case of species of the genus Pratylenchus, the female produces eggs that hatch within the tissues of the plant and the juveniles begin feeding), whereas in others such as Heterodera (which are ectoparasites) the body of the female fills with eggs, forming a sac that eventually dries and transforms into a resistant cyst, with the juveniles within capable of resisting environmental extremes.
6 Concluding remarks
Recent studies have identified, for the first time, some natural product scaffolds with anthelmintic activity against an animal-parasitic nematode, thus contributing to anthelmintic drug discovery and opening up avenues for future development of selected candidates as anthelmintics (Herath et al., 2017, 2018, 2019a,b, 2020). This article emphasises the significance of considering natural products in anthelmintic drug discovery, and discussed the strengths, challenges and key areas deserving of improvement to enable anti-parasite drug discovery. The prospects of developing anthelmintic ‘lead’ candidates from identified natural product scaffolds via future chemical optimisation, efficacy and safety assessments, activity spectrum assessments, and target identification have also been identified. Thus, the future optimisation of a ‘lead’, and subsequent pre-clinical and clinical assessments should enable the development of anthelmintic drugs from natural products.
Sex pheromones
Greet (1984) was the first to report sex attraction in a free-living nematode, P. rigidus. Later, Green (1966) reported sex attraction in cyst nematodes, e.g., Globodera rostochiensis and Heterodera schachtii.Roche (1996) reported sex attractants from animal-parasitic nematodes, Ancylostoma caninum. Since 1966, more than 50 species of microbivorus, plant-parasitic, and animal-parasitic nematodes were reported to be influenced by the pheromones. Aumann et al. (1998) characterized female sex pheromones in H. schachtii and showed that volatile components of female sex pheromones can be analyzed directly by gas chromatography. Novel control methods may be developed by identifying and formulating various pheromones obtained from H. schachtii and other cyst nematode species; such techniques would lead to disorient males and reducing female fertilization in the field.
Sex pheromones recognize species, identify gender, and evaluate sex partners. Both females and males can act as an emitter or a receiver. The pheromones are volatile substances, which reinforce the selection of the right partner. Many species of nematodes use glycosides to regulate sex attraction, mate aggregation, and dispersal behaviors as well as dauer formation. Panagrellus redivivus is one such example, which emits attractants specific to the opposite sex (Duggal, 1978).
6 Final considerations
The fungus Duddingtonia flagrans is an efficient agent for the biological control of nematodes and has shown to be a plant-growth promoter. The large production of chlamydospores by D. flagrans has made its commercialization in some countries possible, but so far, only for the control of animal-parasitic nematodes. The efficiency of D. flagrans in controlling the parasitic nematodes of domestic animals encouraged research on plant-parasitic nematodes. The studies on the association of this fungus with plants are recent, but present promising results for the agriculture, since the fungus preys on plant-parasitic nematodes andcauses improvements in the nutritional status of the plants. This control alternative can contribute to the reduction in the use of anthelmintics and chemical nematicides in the veterinary and agriculture, respectively, leading to a reduction in the environmental impacts.
1.7 Parallels with Plant Parasites
Like animals, plants offer concentrated sources of nutrients and energy for parasites, but they also present distinct challenges and, as hosts, they are a completely different proposition. From an energetic perspective, plants are very different kinds of resources than multicellular animals. For any given body mass, the metabolic rate of plants is lower than that of animals, even after correction for temperature effects (Brown et al., 2004; Gillooly et al., 2001). This should mean that less energy can be extracted per gram of host tissue and per unit time from a plant than from an animal. Also, plants are sessile, with tough cell walls as well as thick outer surfaces, and they have no easy point of entry like a mouth for internal parasites. Nevertheless, if the parasitic strategies defined above are indeed adaptive peaks reached by convergent evolutionary trajectories, we should expect to find roughly equivalent strategies among plant parasites.
Excluding a few carnivorous taxa, plants are autotrophs that do not feed on other organisms, which rules out parasites using trophic transmission (unless the plant is used as intermediate host). However, other strategies are well represented among parasites of plants. Many large groups within the insect order Hemiptera, such as aphids and leafhoppers, qualify as micropredators. These phytophagous insects feed on small amounts of sap or other tissue on one individual plant, and then move on to other plants (Kuris and Lafferty, 2000); their virulence is clearly intensity dependent, as one would expect from micropredators. They also serve as carriers of several vector-transmitted parasites including fungi, viruses and bacteria (Agrios, 1997; Dixon, 1998); these can multiply on a plant host, and have the same kind of intensity-independent effects on their host as unicellular parasites of animals transmitted by vectors. Other hemipterans behave as directly transmitted parasites. Scale insects, for instance, feed on sap drawn from a plant's vascular system, inducing intensity-dependent effects on the plant; females are permanently attached to the plant host, while the young they produce disperse to other plants (Edwards and Wratten, 1980). Other insects plaguing plants are reminiscent of parasitoids or castrators. Consider lepidopterans: an adult female lays eggs on a plant, these hatch into caterpillars that feed on leaf tissue while increasing several-fold in body size, and then pupate as they metamorphose into adults. This resembles the behaviour of several parasitoid wasps and flies that attack insects, except that in the case of lepidopterans, the host plant survives. However, in cases involving small plants, the host's survival is greatly at risk when many caterpillars feed on its tissues (Edwards and Wratten, 1980), a very parasitoid-like outcome. Certain fungi can also castrate their plant hosts and usurp the latter's reproductive structures for their own purposes (Roy, 1993), as the castrators described earlier do. These few examples show that the same parasitic strategies emerge, whether the host is a plant or an animal. Plants are not only parasitised by insects or fungi; the rest of this section examines two different groups of plant parasites to determine whether they also adopt the previously described strategies.
1.7.1 Plant–parasitic nematodes
Transitions to parasitism on plants have occurred at least three times in the phylogenetic history of nematodes, if not more (Blaxter et al., 1998). Plant–parasitic nematodes display a few morphological and biochemical adaptations necessary for the exploitation of plant hosts. For instance, all nematodes parasitic on plants have an oral stylet, a sort of hollow protrusible spear that serves to perforate plant cell walls, to inject secretions into the plant cytoplasm and to suck in the cytoplasm (Davis et al., 2004; Jasmer et al., 2003). Also, they possess cellulases, the digestive enzymes needed to break down plant cellulose; these were possibly acquired via lateral gene transfer from bacteria in the distant past (Dieterich and Sommer, 2009; Smant et al., 1998). After they enter the plant, often in the root tissues, some nematodes remain migratory, spending their lives moving through or between plant cells within the host (Moens and Perry, 2009); others, like the root-knot nematodes Meloidogyne spp., quickly become sedentary, inducing the drastic transformation of a host cell into an enlarged, multinucleate feeding cell in which the parasite remains for life (Davis et al., 2004; Dieterich and Sommer, 2009).
Recently, Dieterich and Sommer (2009) commented that plant–parasitic nematodes “have almost nothing in common with animal parasites in terms of their parasitic lifestyle”. Jasmer et al. (2003), in contrast, found numerous similarities between the two groups of parasitic nematodes. I agree with the latter authors and feel that any differences lie mostly at a superficial, mechanistic level, and that at a strategic level, plant–parasitic nematodes are essentially directly transmitted parasites not really different from those exploiting animals. An individual plant–parasitic nematode invades one plant individual, stays there for life, reproduces and its progeny leave to infect other plants. Parasite-to-host size ratios for plant–parasitic nematodes clearly fall within the range seen for animal parasites (Fig. 1.5), since nematodes are invariably small whereas their plant hosts range from grasses to trees. The pathology caused to the host plant is clearly intensity dependent. These are clearly all attributes of directly transmitted parasites. Even several infection processes are reminiscent of what is observed in animal–parasitic nematodes. For instance, the penetration of the epidermal cell layer of plants by a combination of thrusting of the stylet combined with enzymatic secretions is not different from what hookworms do to penetrate mammalian skin (Jasmer et al., 2003). Also, the major cell modification induced by plant–parasitic nematodes to create both a home and a feeding structure for themselves is strikingly similar to the cellular rearrangement induced by animal–parasitic nematodes like Trichiuris spp. and especially Trichinella spiralis, the latter invading a muscle cell in its host and proceeding to restructure it completely (Davis et al., 2004; Despommier, 1993; Jasmer et al., 2003). Both the animal–parasitic nematodes and their plant counterparts achieve this by manipulating gene expression in their host's cells (Davis et al., 2004; Jasmer et al., 2003; Niebel et al., 1994). Finally, not unlike many animal parasites facing the problem of encountering mates, plant–parasitic nematodes have evolved asexual forms of reproduction like parthenogenesis (Dieterich and Sommer, 2009). Thus, at proximate and ultimate levels, it can be argued that the adaptive peak corresponding to the direct-transmission strategy defined earlier and which was an attractor in the evolutionary landscape of animal parasites has been one too in the evolution of plant parasites.
1.7.2 Parasitic plants
There are over 4000 species of parasitic angiosperms (flowering plants) that derive sustenance from other plants, and these are spread across 19 families (Heide-Jørgensen, 2008; Press and Graves, 1995). Here again, parasitism has evolved repeatedly and independently. Parasitic plants range along a continuum from hemiparasites, like mistletoes, to holoparasites such as dodder, Cuscuta spp. Hemiparasites may be facultatively parasitic only; they derive water and minerals from their hosts, as well as nutrients, but have retained some of their photosynthetic ability. Holoparasites are obligate parasites that have lost all chlorophyll, and that cannot assimilate carbon and inorganic nitrogen on their own. Parasitic plants can also be divided based on their site of attachment to the host, which is either the stem or the roots. The distinguishing feature possessed by all parasitic plants is a modified root, the haustorium, which penetrates the host plant and connects to its vascular system; it serves for both attachment and feeding (Heide-Jørgensen, 2008; Stewart and Press, 1990).
Parasitic plants differ from animal parasites in some interesting ways. In both the present parasite classification scheme and in earlier ones (Kuris and Lafferty, 2000), the number of host individuals and host species attacked by an individual parasite during a particular life stage or throughout its life played a key role in the division of parasites into distinct strategies. Many parasitic plants, however, can do something that animal parasites cannot: one individual parasite can simultaneously parasitise several host individuals, not necessarily all of the same species (Pennings and Callaway, 2002). While at first this may appear like a major departure from the strategies outlined earlier, it is in fact a relatively trivial difference. As hosts, plants are sessile and, at canopy level, often in close proximity if not actual physical contact with their neighbours. If similar opportunities existed for animal parasites, surely they too would exploit more than one host at a time, without necessarily using a parasitic strategy different from the six described earlier.
On the whole, parasitic angiosperms fit well within the definition of directly transmitted parasites. An individual attaches to one plant host for life (though it may simultaneously draw nutrients from adjacent hosts starting later in its life). The parasite can manipulate the physiology and morphology of the host plant by interfering with growth hormone production (Pennings and Callaway, 2002; Stewart and Press, 1990), possibly to the parasite's benefit. Its seeds then proceed to infect new hosts, either by breaking their dormancy only in response to chemical cues from suitable hosts (not unlike the eggs of many animal–parasitic nematodes), or following germination, by growing towards a host (Pennings and Callaway, 2002). The impact on host fitness is often minimal when the parasite is small, but increases in an intensity-dependent fashion, except that here ‘intensity’ means the mass of the parasitic plant instead of the number of parasites (Marvier, 1996; Pennings and Callaway, 2002). Eventually, after it reaches a certain size relative to that of its host, the parasitic plant causes severe reductions in host growth and reproduction, and even occasionally host death (Pennings and Callaway, 2002; Press and Phoenix, 2005; Stewart and Press, 1990). Although these effects are reminiscent of those induced by parasitoids and parasitic castrators, they are truly those of directly transmitted parasites at high infection intensity. Host death is neither necessary for the success of the parasitic plant nor an inevitable outcome (as it is for parasitoids), and the reproductive tissues of the host plants are not specifically targeted (as they would be by a parasitic castrator). The indeterminate growth of parasitic plants just means that eventually they reach a biomass at which the amounts of resources diverted from the host are unsustainable for the latter. Parasitic angiosperms range in size from small herbaceous species to large trees, and so do their host plants (Heide-Jørgensen, 2008; Press and Graves, 1995; Stewart and Press, 1990). Various parasite–host combinations thus span the whole range of parasite-to-host size ratios seen for directly transmitted parasites (Fig. 1.5). The main difference between directly transmitted parasites of animals and parasitic plants is that, in the latter, growth of the parasite will almost invariably cause the severe fitness losses in the host that are only rarely achieved by the former.
There are, however, true parasitoids among plants that parasitise other plants. In many tropical forests, strangler figs (Ficus spp.) use a host tree for physical support; although they never draw nutrients from their host, the support they get from the host is an essential resource allowing them to germinate in the canopy, where they receive ample sunlight (Putz and Holbrook, 1989). They extend a network of roots down to the ground from the canopy, encircling the trunk of the host tree. As these roots grow thicker, they squeeze, or ‘strangle’, the host, cutting off nutrient flow along its vascular system, and eventually killing it. The figs do not benefit from host death, just as parasitoid wasps or nematomorphs do not actually gain from death of their host; however, host death is an inevitable consequence of the exploitation strategy of all these parasites, uniting them as parasitoids. In the case of the fig-tree interaction, it takes years for the parasite to kill the host (Putz and Holbrook, 1989). This is simply due to the lower growth and metabolic rates of host and parasite compared to their animal counterparts: even though all happens on a different time scale, strangler figs are still parasitoids.
The result that acylthiourea as linker contributed to the increase of the nematicidal activity of 1,2,3-benzotriazin-4-one derivatives promoted us to explore further potential linkers. We selected piperazine as linker. The use of piperazine as linker in drugs is a well-known approach as demonstrated by seroquel, viagra, ziprasidone hydrochloride, ranolazine which often showed significant biological functions, including antimicrobial,64 antidepressant,65 antiproliferative,66 and antihistaminic activity.67 Most importantly, diethylcarbamazine is an anthelmintic drug of livestock to prevent animal parasitic nematode.68 The strategy of molecular design is shown in Fig. 21.10. The idea of R-group selection is to search for some heterocycles or active fragment from pesticides, fungicides, nematicides, and natural products.
Figure 21.10. The molecular design of series E.
At first, benzoyl as R-group was introduced into 1,2,3-benzotriazin-4-one containing piperazine. Compared with the compound in which R is hydrogen, it displayed 28.6% inhibition activity at the concentration of 20 mg/L. The preliminary bioassay results inspired us to synthesize more compounds bearing substituents on phenyl ring. Strong electron-donating CH3O-groups and electron-withdrawing NO2-groups were introduced to examine the influence of substituent electronic effect. It was found that, when 1,2,3-benzotriazin-4-one ring had no substituent, the compound bearing NO2 at ortho or para position of phenyl ring exhibited better bioactivity than the compound bearing OCH3 with the inhibition reaching 71.4% against M. incognita in both cases. Except NO2, the other electron-withdrawing groups F, Cl, CN, OCF3 did not bring surprise to us. It suggested that electron-withdrawing groups are not beneficial for the nematicidal activity. However, when OCH3 and NO2 substituents were introduced into different position of 1,2,3-benzotriazin-4-one ring, it was disappointing that compound bearing NO2 at ortho position of phenyl ring almost lost the bioactivity totally. This indicated that the activities varied significantly depending on the types and positions of the substituents on 1,2,3-benzotriazin-4-one ring. The attempt to introduce heterocycles failed except for benzothiadiazole, which showed 56.3% inhibitory activity at the concentration of 20 mg/L. In order to explore the importance of carbonyl phenyl group, methyl, cyclopropyl methylene were introduced into piperazine ring. The result is consistent with our expectations with the nematicidal bioactivity decreasing sharply without carbonyl phenyl group, which further confirmed the key role of carbonyl phenyl group in the structure.69
Introduction
After the Second World War, the colonial rules gradually came to an end, and a large number of countries got freedom. The freedom also brought along the food crisis in the newly liberated countries, because by enlarge their economy as well as indigenous food production systems were based on the colonial rule. However, with gradual planning and regular FAO support coupled with the green revolution, the indigenous crop production systems in the independent countries got established, and the cereal production gradually and steadily increased to present day 2996 million tons, which is around 3.5 times greater than the food produced globally in 1961 (FAOSTAT, 2022). At present, the annual global production of cereals, oilseeds, pulses, vegetables, and fruits accounts for 2996, 247, 89, 1148, and 887 million tons, respectively (Fig. 1; FAOSTAT, 2022). The production of fiber and beverage crops in 1961 was 0.114 and 5.48 million tons, which has increased to 0.515 and 19.2 million tons in 2020, respectively (FAOSTST, 2022).
Fig. 1. Global annual food production.
Despite stupendous increase in the global production of food and other commodities, the problem of hunger and the difference between rich and poor have gradually deepened, and the socioeconomic status of farmers in several Asian and African countries, in particular, is on a continuous decline. The crop growers have not been able to get adequate value of their produce, and often, the total earnings are less than the value of inputs given to grow the crop because of poor productivity as well as a lower market value. Among important factors, pest and disease attack is considered a major biotic factor responsible for poor economic returns from crop production. Plant nematodes constitute an important group of biotic pathogens, and attack all kinds of agricultural crops, significantly affecting their productivity (Khan & Jairajpuri, 2010a, 2010b, 2012; Sikora et al., 2018). Nematode infestation in agricultural crops becomes more significant because of the hidden nature of the causal agent as the nematodes generally inhabit in soil and attack underground parts of plants (Khan, 2008). As a result, farmers are not able to recognize the nematode infestations in the crops and for this reason do not take any management measures.
Nematodes belong to a highly diverse group of pseudocoelomate animals and are the most abundant and ubiquitous multicellular microorganisms that are found in all types of environments, be it freshwater, marine, or terrestrial habitat (Khan, 2008). They may be free-living or parasitic. Nematodes parasitize a large number of organisms including plants, animals, fungi, and even human beings. Animal parasitic nematodes were known to human beings as early as 1550–1530 BC, but the history of plant parasitic nematodes is of much recent origin, as the first plant parasitic nematode, Anguina tritici was discovered in 1743 by T. Needham (Needham, 1743), and the second plant nematode, Meloidogyne sp. was recorded around 112 years later in 1855 by M. J. Berkeley (Berkeley, 1855). Plant parasitic nematodes are of great agricultural significance as they reduce the quantity and quality of the crop produce (Khan, 2016; Sikora et al., 2018). Plant nematodes generally attack roots, and feed on the root hairs, rot epidermis, cortex, and/or stele, and on the basis of parasitism are grouped as ectoparasites, semiendoparasites, and endoparasites (Khan & Jairajpuri, 2010b). The ectoparasitic nematodes such as Tylenchorhynchus, Rotylenchus, Helicotylenchus, Belonolaimus, Hoplolaimus, Trichodorus, Longidorus, and Xiphinema feed on the root hairs and root surface (Khan & Jairajpuri, 2012). A few nematodes, namely Tylenchulus semipenetrans (citrus nematode) and Rotylenchulus reniformis (reniform nematode), insert only their anterior half into the host and are called semiendoparasitic nematodes (Khan et al., 2019). Some nematodes fully penetrate into the host and are called endoparasites. The root-knot nematodes, cyst-forming nematodes, root-lesion nematodes, burrowing nematodes, etc., are endoparasites, as their whole body lies inside the host tissue (Haque & Khan, 2021). Some nematodes such as Anguina, Aphelenchoides, and Ditylenchus, which attack stems, leaves, flowers, seeds, etc., are called foliar nematodes (Hunt, 1993).
The nematode infestation commonly debilitates the plant even without any specific symptoms, which usually appear as sparse foliage, stunted growth, and pale green leaves (Khan & Jairajpuri, 2010a, 2010b, 2012). Due to lack of knowledge about plant nematodes, farmers consider these symptoms to be of nutrition deficiency and apply fertilizers rather than a nematicide, which gives no control over the problem (Khan et al., 2019). However, the nematode infestation in the field can be distinguished on the basis of the pattern of symptoms, which usually appear in patches of plants, distributed irregularly in the field. The infested plants may also show wilting during the active period of transpiration despite having adequate soil moisture in the field. The nematode-infested plants become highly susceptible to the infection by soil-borne bacteria and fungi leading to the development of disease complexes (Khan, 1993). Nematodes often enhance the severity of wilts caused by fungi and bacteria, and may also break host resistance (Khan & Sharma, 2020). Some plant nematodes transmit certain plant pathogenic bacteria, fungi, and viruses. Seed gall nematode, Anguina tritici, acts as a vector to carry Rathayibacter (=Clavibacter) tritici and Dilophospora alopecuri to the apical meristem of wheat (Khan & Dasgupta, 1993). Some dorylaim nematodes, Xiphinema, Trichodorous, etc. transmit NEPO and tobra viruses (Weischer, 1993).
On average, plant nematodes cause about 7%–12% yield loss, which varies greatly depending on environmental conditions, inoculum level, host and nematode species (Khan et al., 2019). However, on a specific crop, the yield loss may be higher, and sometime, a complete crop failure may take place. The nematode-induced crop losses are considerably high in the developing countries, where farmers are not aware of plant nematodes. Exceptionally small size of farms, nonavailability of effective nematicides, and poor socioeconomic status of the farming community are responsible for farmer's negligence toward nematodes, which lead to greater nematode damage to the crops (Khan & Jairajpuri, 2012). This does not mean that the developed countries do not face nematode problems; despite having planned and modern agriculture, plant nematodes cause considerable crop losses in the developed countries. Nematodes attack agricultural crops in every region and country, but the yield losses vary considerably depending on the management practices adopted by the growers. The nematode infestation in different crop groups and the associated yield losses are summarized in Table 1, and are described concisely, under the following headings:
Biological Aspects
Nutrition
Family C1 cysteine proteases are ubiquitously expressed in the digestive tract of larval and adult trematodes (for example, Caffrey et al.[52] and Chapter 444) and nematodes [36,53]. Together with Family A1 aspartic proteases, cysteine proteases form part of a network or cascade of hydrolytic activities that generates absorbable peptides and amino acids from ingested host proteins (e.g. hemoglobin and albumin in hematophagous parasites [54,55]). Interestingly, the same orthologs of Family C1 (cathepsins B, C, F and L) and A1 (cathepsin D) digestive proteases are employed by divergent protozoan and invertebrate parasites ranging from plasmodia through trematodes and nematodes to ticks [54,56]. This type of digestive proteolytic network pre-dates the appearance of and eventual predominance by Clan PA serine proteases in insects and higher metazoa [55]. Gut-associated helminth Family C1 proteases are developmentally regulated with either novel or increased expression at the onset of blood-feeding [22,57].
In some cases, gene suppression via RNA interference (RNAi) has helped demonstrate the importance of gut proteases to worm growth, such as for plant nematodes [58] and parasitic trematodes [59]. However, the technique has proven less reliable in establishing a similar importance for proteases from a number of animal parasitic nematodes, including H. contortus and Ostertagia ostertagi[60]. In other studies, for example, with C. elegans, no phenotypes have been recorded even after combinatorial RNAi of five gut-associated Family C1 proteases, suggestive of further functional redundancy with other uncharacterized C1 proteases and/or proteases of different mechanistic classes [61].
A conservation of promoter elements driving the expression of digestive cysteine proteases in nematodes has been demonstrated. Transformation of C. elegans with the promoter of the H. contortus AC-2 cysteine protease gene linked to the reporter lacZ led to the selective expression of the reporter in gut cells [62] and extension of this approach to other H. contortus cathepsin B gene promoters has shown similar gut-specific expression (Johnston and Britton, unpublished). With the increased availability of parasitic helminth genome data, the regulatory motifs controlling spatial expression can be investigated and promoter activity tested in C. elegans or by direct transient transfection of the relevant nematode [63,64] or flatworm parasites [65].
Development, Moulting and Excystment
RNAi or genetic mutant analysis has demonstrated the essential nature of a C. elegans cathepsin L-like protease, Ce-CPL-1, for processing of egg-yolk proteins and embryogenesis [66,67]. Conservation of function in related parasitic nematode species was demonstrated by gene rescue studies [25,50]. The importance of cathepsin L activity for development of filarial nematodes was also indicated by RNAi of Bm-cpl-1 and Bm-cpl-5, members of a cathepsin L gene family in B. malayi[20], that decreased both embryonic survival and the release of microfilaria from treated adult worms [68]. Recently, the application of in vivo RNAi in the mosquito intermediate host demonstrated essential roles for Bm-cpl-1 in larval development and migration, identifying this as a potential transmission blocking target [69]. For agriculturally important nematodes, transgenic expression of plant-derived cysteine protease inhibitors has been employed as a control strategy, presumably by interrupting digestive processes in the gut of the parasite (Marra et al.[70] and references therein). Likewise, the heterologous expression of the pro-domain of an Heterodera glycines (soybean cyst nematode) cysteine protease in soybean decreased both the number of H. glycines females recovered from roots and their fecundity [70]. Finally, the contribution of cysteine proteases to the development of H. glycines and Globodera pallida (potato cyst nematode) was demonstrated by RNAi that altered the gender ratio [71].
Family C1 proteases are closely linked with the process of cuticle moulting in nematodes, e.g. in C. elegans[67] and the filarial nematodes, Dirofilaria immitis (heartworm) [72] and Onchocerca volvulus (causes ‘river blindmess’) [73]. With the latter parasite, dipeptidyl fluoromethane inhibitors decreased the number of third-stage larvae moulting to the fourth stage in a time- and dose-dependent manner. The inhibitors apparently prevented separation of the old cuticle from the new. Following on, RNAi of either O. volvulus cathepsin L (Ov-CPL) or cathepsin Z (Ov-CPZ) proteases severely inhibited moulting of third-stage larvae [17]. Cysteine proteases are also associated with the excystment of larval metacercariae of the lung fluke, Paragonimus westermani[74], and the Chinese live fluke, Clonorchis sinensis[75] that occurs in the lumen of the host gut.
Tissue Migration
For the larval stages of many helminth parasites the ability to migrate through host tissue is vital. Although most of the published research deals with trematode parasites (see Chapter 444), cysteine proteases secreted by cestodes and nematodes have been reported to degrade the relevant connective tissue proteins such as collagen [34,49] fibronectin and laminin [76,77].
Immune Modulation, Therapy and Diagnosis
Because they are often present at the helminth’s interface with the host (surface, gut and ES products), Family C1 proteases have been investigated for their immune-modulatory effects [78–80] and their practical utility as vaccine [6] and diagnostic candidates [7,81,82].
Of obvious advantage to parasitism by helminths is the ability to hydrolyse host molecules relevant to both innate and acquired immune function. Examples include IgG [49,83] and Toll-like receptors, which in the case of F. hepatica cathepsin L, involves the specific cleavage of endosomal TLR3, thus diminishing the macrophage’s capacity to release pro-inflammatory molecules upon stimulation of both TLR3 and TLR4 [80]. Cysteine protease activity released from metacestodes of the pork tapeworm, Taenia solium, significantly depleted co-cultured CD4+ T-cells [84]; subsequently, it was shown that apoptosis was involved in that depletion [85]. It is still unclear exactly how the protease mediates this apoptosis, but if demonstrated in vivo, activity would be a powerful method by which cell-mediated immune responses are dampened.
It is also appropriate to note here that cystatins (cysteine protease inhibitors) released by helminths (particularly filarial parasites) are potent immune-modulators. They interfere with host Family C1 and C13 (legumain) protease activities, including those associated with antigen-processing by antigen-presenting cells (McKerrow et al.[86] and Gregory & Maizels [87] for reviews). In addition, nematode parasite cystatins (but not the C. elegans ortholog [88]) exert significant effects on host cytokine production, in particular the upregulation of IL-10 by macrophages that contributes to T cell hypo-responsiveness and an anti-inflammatory environment, thereby favoring parasite survival (reviewed in Hartmann & Lucius [89]). Recent data suggest that the cystatin-induced modulation of macrophage cytokine levels operates via the phosphorylation of the mitogen-activated protein kinases ERK1/2 and p38 [90].
Cysteine proteases are potential or validated targets for chemotherapy of many parasitic protozoa [7,91]. Apart from S. mansoni and F. hepatica (see Abdulla et al.[5] and Alcala-Canto et al.[92] and references therein) similar drug development research with helminths is lacking, reflecting in part a greater effort placed on developing vaccines [6].