nematodes a class of unsegmented helminths with fundamental bilateral symmetry. many species are...
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Nematodes
A class of unsegmented helminths with fundamental bilateral symmetry. Many species are parasites
Nematodes:
Ascaridida: Ascaris: adults wander
Trichuris : larval migrans
Pinworms: no significant problems
Filarial worms- all use intermediate hosts:
Brugia sp. adults are problem
Onchocerca sp: juveniles are the problem
Dracunculus medinensis: secondary infections
Trichinella- Adults and larvae cause problem
Mermithidae- kill their insect hosts
Nematodes occupy almost every possible niche: As parasites they infect vertebrates, insects, and plants. As free-living organisms they are found in soil, fresh water and sea water. They have been isolated from high in the Andean mountains and swimming in hot springs. In short they are a highly successful group of organisms, second only to the insects.
Despite their diversity of habitat, the nematodes have a remarkable consistency of shape, normally being vermiform, long and slender with pointed ends.
This constancy of form is brought about by the constraints of their morphology. They are bilaterally symmetrical, non-segmented pseudocoelomates (embryologically the pseudocoelum is derived from the blastocoel).
In general they are diecious, females > males. The female reproductive system opens to the exterior via the vulva on the ventral surface of the worm, in many species about half way along the length of the worm. Males have a cloaca, which is a common urogenital opening also located on the ventral surface at the posterior end of the worm and usually marks the start of the tail. In addition, they have accessory copulatory organs called the spicules. In some species of nematodes females can be parthenogenic or hermaphroditic.
Unlike the digeneans and monogeneans, the nematodes have a continuous gut with a separate mouth at one end and an anus at the other. Because of the hydrostatic pressure of the pseudocoelum which acts on the intestine, the nematodes require a muscular pharynx or oesophagus in order to feed.
Their nervous system is very simple and consists of a central ganglion or circumpharyngeal nerve ring or commissure from which projects the dorsal, ventral and lateral nerve chords which run to the posterior of the worm. Arising from the same nerve ring and running anteriorly are a variety of cephalic nerves, which ennervate a number of sensory receptors located in the head region.
Body Plan
Nematodes are important parasites of animals and plants and are of major medical and agricultural importance. Nematodes are parasites of vertebrates and invertebrates and it has been estimated that each vertebrate group has its own specialist group of nematode parasites. Over 100,000 species of nematodes have been described from vertebrates alone. Approximately a quarter of the world's population suffers from a nematode infection of some sort (1 person in 4).
As well as infecting animals, nematodes are important parasites of plants, species like Globodera, the potato root eelworm and Meloidogyne, the root knot nematode.
Plant parasitic nematodes cause much damage to plants and, most importantly, to agricultural crops. Some sources indicate that nematode plant parasites can ruin as much as 15 % of each year’s agricultural crop in the United States, a figure that is comparable with the destructive effects of insects
Nematodes Characteristics - round in cross section - bilaterally symmetrical - variable size - 1 mm to 1 metre - organs - digestive, nervous, excretory, cuticle, muscle, sexual - develops by molting (shedding cuticle - separate sexes - reproduction and development: egg egg fertilization embryo in egg larva 4 molts adult
Despite the tremendous medical and economic importance of nematodes, the great majority of them are free-living and they occur in every conceivable environment. The majority of nematodes are small, usually 1mm or less in length. The outstanding feature of the nematodes, both free-living and parasitic is the stylized body plan and the stylized life cycle.
Nematodes all look alike, the smallest being scaled down versions of the largest. Nematodes are typically spindle shaped, bilaterally symmetrical, unsegmented worms. There are no cilia or flagella, although what appear to be non-motile cilia occur in the sense organs. There are no respiratory or circulatory organs.
Nematodes have been known for ages:
Aristotle described Ascaris lumbricoides
Ebers papyrus (1550 BC) describes Hookworms
Eggs commonly found in preserved bodies
The body wall is composed of a thick cuticle, a hypodermis and a layer of longitudinal muscle only, there is no circular or diagonal muscle. When the longitudinal muscles contract, the restoring force is provided by elastic components in the cuticle, and the two are linked hydrostatically via the pseudocoel. When nematodes are moving internal hydrostatic pressures can be as high as 100 to 200mmHg.
The cuticle of the nematode plays an important part in its physiology. The cuticle is a complex structure that varies from genus to genus and between larva and adult. Most of the detailed work has been done on the cuticle of the large animal parasitic nematode Ascaris lumbricoides. In Ascaris there are 4 main layers in the cuticle (cortical layer, fibrillar layer, homogenous layer and fibre layer) and these four regions are usually present in most nematodes, although the number of layers in the cuticle can vary, and in particular the fibre layer is often reduced especially in larvae.
Cuticle
Complex structure- great functional significance
3 regions: epicuticle, middle layer, inner fibrous layers
Fibrous layers- parallel strands of collagen at different angles, forming a latticework arrangement 75 and 135 degrees to axis
Fibrous layers important in the hydrostatic skeleton- strands are not extensible but they allow longitudinal stretching and compression of overlaying cuticle by changes in the angles between the layers.
Pseudocoel and Hydrostatic skeleton
The musculature and body wall enclose a fluid-filled area- pseudocoelom
Pseudocoel functions as hydrostatic skeleton- common in invertebrates
-depends on enclosure of a volume of fluid, their ability to apply pressure to the fluid, and the transmission of the pressure in all directions.
The contractions of circular muscles and relaxation of longitudinal muscles make many animals long and thin and vice versa
However in nematodes the somatic musculature is composed of longitudinal fibers and the muscles act not against antagonistic muscles but against stretching and compression of the cuticle.
As muscles on ventral or dorsal side of body contract they compress the cuticle- force transmitted to the other side of the nematode- stretching the cuticle there- the stretching/compression of cuticle are the forces that return body to resting position.
Alternation of contraction and relaxation in dorsal and ventral muscles moves the body in a series of curves in a single dorsoventral plane- characteristic of nematode movement
Hydrostatic pressure is very high- up to 200 mm Hg
This internal pressure influence many functions- ingestion, defecation, copulation, laying eggs.
The liquid is called hemolymph- few cells, complex solution, carries electrolytes, proteins, fats, sugars
The nervous system of nematodes is fairly simple. There is a nerve ring around the pharynx and associated with this are a number of small ganglia. Six or occasionally 8 longitudinal nerves run posteriorly from these ganglia, the ventral nerve chord being the largest.
As in many invertebrates, the muscle sheet shows spontaneous activity which is modulated by the nervous system. Each muscle receives a dual innervation from an excitatory and an inhibitory nerve fibre. The excitatory fibres release acetylcholine that stimulates muscle contraction, the inhibitory nerve fibres release 4-aminobutyric acid which inhibits muscle contraction. Other nerve fibres may release catecholamines (adrenalin, nor-adrenalin, octopamine).
A number of neuropeptides have now been isolated from nematodes. The ones so far purified are similar to ones already described in mammals and invertebrates. The neuropeptides may function as neurohormones, co-ordinating maturation, reproduction and moulting. The production by animal parasitic nematodes of mammalian type neuropeptides has given rise to suggestions that these peptides may in fact exert a physiological effect on the host, reducing gut mobility and modulating the immune system.
Digestion
Digestive system is complete- mouth, gut, anus
Cuticle lines the buccal cavity and esophagus, nematodes shed cuticular lining during moults
Mouth- opening surrounded by 6 lips- sometimes modified
Buccal cavity between mouth and esophagus
Ingested food enters muscular region- esophagus/pharynx
This is pumping organ that sucks/pumps food into the alimentary canal and into intestine
Interspersed in the muscles of the esophagus are glands- digestive enzymes- amylase, proteases, pectinases, chitinases, anticoagulants
Rapid contractions of buccal muscles and anterior esophagus- opens mouth, dilates esophagus- sucks in food. High hydrostatic pressure in the pseudocoel closes the mouth and lumen of esophagus- food passes via posteriorly progressing waves of muscle contractions until it reaches the intestine- a kind of peristalsis
Digestion:
Intestine: simple tube from esophagus, single layer of cells
Non-muscular- food moves by pressure of more food being added
Movements of body can put pressure in intestine or open anus for defecation
Several digestive enzymes identified- intestinal digestion is probably minimal
Food in parasitic nematodes is blood, tissue cells, intestinal contentsFree living nematodes feed on bacteria etc.Plant parasitic nematodes plug into plant tissues
Secretion-Excretion
No flame cells or nephridia
Several types of tubular excretory systems- two long canals in the lateral hypodermis connect to each other and the transverse canal opens to the exterior- excretory pore
The major nitrogenous waste product is ammonia- also some amino acids, peptides and amines, carbon dioxide and fatty acids
Nematodes are sexual animals, some are monoecious, some dioecious, and some are parthenogenetic. Sexual dimorphism can exist and the male is generally slightly smaller than the female. Nematode reproduction in free-living specimens involves six stages including an egg stage, four larval stages (L1, L2, L3, L4), and an adult stage. Females give rise to eggs that are then fertilized and laid. Once the embryos in these eggs are mature they will hatch into the L1 larval stage. The juvenile nematode will then undergo four molts before it becomes an adult and is capable of reproduction. During molting, a nematode will shed its skin in order to facilitate growth. The third L3 larval stage is normally the infectious stage for parasitic nematodes.
Arrested Development:
Some nematodes will not mature if adults are present – in horses treat to get rid of adults- immatures will mature
Haemonchus contortus in sheep: fecal counts are low in winter- rise in spring. Larvae acquired during the last growing season did not mature- now releasing infective stages to the environment to coincide with lambing
Hookworms in India- normally have a prepatent period of 40 days- lengthened to 6 months to avoid releasing eggs into very dry season
The Mermithidae are an unusual order of entomophilic nematodes that parasitize a wide range of insects.
They are unusual because the adult stage is the one which is free-living and the larval stages are parasitic. These worms gain entry into the host either by the insect eating the nematode eggs, which have been laid on vegetation, or in some species the larval stage penetrates into the insect.
Once inside the insect, the larvae pass through the normal developmental phases of nematodes, except that by the L4 stage they are often very large, occupying much of the insects' body. They frequently result in sterilization of the insect. In most cases, however, the act of penetrating or bursting out of the insect results in the insects' death.
These combined features of the parasites' life cycle have made them of prime interest as possible bio-control agents. For instance, one species, Romanomermis culicivorax, which infects mosquitoes, is one of several species of mermithids that have been investigated as a potential bio-control agent for mosquito control.
Romanomermis culicivorax
The gender of these nematodes is not determined until they develop in the insect host. If there are several nematodes/insect- these develop into males. If only 1 nematode is present this becomes a female. This is partially a function of nutrient availability and competition.
