Transcript lecture_19_Feb 26_nematodes1

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 contents
Free 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 maturenow releasing infective stages to the environment to coincide with
lambing
Hookworms in India- normally have a prepatent period of 40 dayslengthened 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/insectthese develop into males. If only 1 nematode
is present this becomes a female. This is
partially a function of nutrient availability and
competition.