Lecture notes
nematodes
- Module
- Mechanisms of pathology
- Institution
- Aston University, Birmingham (Aston)
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Nematodes.General features and tissue nematodes.
Parasitic infections.
The defining feature of disease-causing parasitic organisms is that they depend on
their infected host for survival.
They can be divided into 2 groups: protozoans, which are microscopic, single-celled
eukaryotes. And helminths, which are macroscopic, multicellular worms.
Helminths can be further divided into 3 groups that can infect humans: trematodes
(flukes), cestodes (tapeworms) and nematodes (roundworms).
Nematodes can be further divided into gastrointestinal form; these nematodes use
only one host to complete their life cycle. And blood and tissue form, these
nematodes usually use multiple hosts to complete their life cycle.
Nematodes:
Extremely abundant
Can be small predators or saprophytes not all are parasites.
Inhabit diverse habitats, can live in freshwater, free-living marine and soil species.
They can be plant and animal parasites.
They are eukaryotic
They are non-segmented and cylindrical. They are tapering at both ends.
They are either diecious, which means separate sex in separate animals, or they can
be hermaphroditic, which means both sexes are combined in one animal.
They are 1mm to 3 feet.
If we cut up the nematode worm, outside the worm is covered in a tough cuticle
which are a protective function. This will be on top of the epidermis layer. Inside
there will be a body cavity with several distinctive tissues, such as muscle tissues
because worms need to move around. They have a primitive nerve system, so they
have a dorsal nerve cord and a ventral nerve cord. They also have a primitive
excretory system. They have a highly developed reproductive system and a
gastrointestinal system.
General development of nematodes.
The general development of nematodes is that they undergo 4 stages or 4 molts and
4 larval stages to reach the adult stage.
Their life cycle usually starts when they are adults, and they will mate. Then they will
lay eggs. Then the embryo will start to develop and when it hatches out this will be
the first larval stage.
It will then progress through the development of another 3 larval stages. In between
these larval stages the animals molt, they go into an inactive state, they shed their
old cuticle and draw a new cuticle and this is how they progress through these larval
stages.
In parasitic nematodes the L3 and L4 stages are the ones which are parasitic.
Modes of infection
, People can get infected by nematodes by ingestion, such as food and drink which is
contaminated by embryonated eggs. Or it can also be growing embryos in
intermediate hosts. Sometimes the intermediate host will be infected cyclops and
by ingesting these infected intermediate hosts, this is how the infection can be
acquired. It is also for some worms such as trichinella its encysted embryos are
actually eating uncooked meat, specifically pig meat.
Certain embryonated eggs because they are very small and light they can be inhaled
by dust, especially if it is very dry. They can become air borne and they can be
inhaled.
They can also penetrate skin, this is true for filariform larvae which can bore through
the skin. This is a typical way how people get infected by hookworms.
There is also possible transmission via blood sucking insects.
Anthelminthic
There are ways and there are agents to get rid of parasitic worm infections, there are
several groups depending on their mode of action.
The 3 major groups:
Piperazine: this acts as a GABA mimetic, which means that this will act on GABA
gated chloride channels and by doing that it will affect membrane properties of cells
and will cause body wall muscle paralysis. In one way or another these agents will
paralyse parasitic worms.
The next class is benzimidazoles, these act by interacting with the cytoskeleton,
specifically the beta-tubulin. By interfering with the cytoskeleton, it will cause the
paralysis of animals.
Imidazothiazole, these act as nicotinic acetylcholine receptor agonists, by action of
this it will cause paralysis of worm muscle. So the worms cannot move, they cannot
conduct peristalsis of the intestines and they can’t resist that and they will be
excreted out.
Classification- tissue nematodes.
There are 2 groups: intestinal nematodes and there are tissue nematodes.
Tissue nematodes can inhabit different tissues in the human body such as: the
lymphatic system, subcutaneous or even in the eye (conjunctiva).
Most of the tissue nematode infections are happening in tropical and subtropical
areas.
Tissue nematodes can induce disease in tissues, blood and the lymph system.
The 4 major types of tissue nematodes use humans as definitive hosts.
Once they are infected when they are inside the lymphatic system or subcutaneous
tissue, they reside there for years.
The adult worms will mate, and they will discharge live offspring which is called
microfilariae.
This will circulate through blood or tissue and these microfilariae can be ingested by
blood sucking insects. And this is how they will transmit this disease to the next host.
Loiasis (Loa loa)- specific example.
Loiasis is the disease, and the worm is the Loa loa worm.
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