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Phylum Nematoda
(with some pics removed)
PJA Shaw
"If all the matter in the universe except the nematodes were
swept away, our world would still be dimly recognizable... we
would find its mountains, hills, valleys, rivers, lakes and oceans
represented by a film of nematodes."
Nathan Augustus Cobb, 1914
General introduction
• The commonest of the “pseudocoelomates” (note that this
taxon is now polyphyletic so invalid) are the roundworms
nematoda - from nema - a thread, eidos - form.
Nematodes are ubiquitous in sediments, soils, and bodies. Most are
c. 1mm long, with a tiny number of giants. The human gut hosts
Ascaris lumbricoides, up to 20cm long in females while the guinea
worm Dracunculus medicensis lives in our flesh and may exceed
1m. Dogs have the misfortune to host the dog kidney worm
Dioctophema renale, 1.2m long and 1cm diameter, while the record
comes from a parasitic nema confined the the placenta of sperm
whales Placentanema gigantissima, recorded up to 7m long!
Basics..
Formally they are bilaterally symmetrical, vermiform. Their body has
well defined tissues and organs. They have a thick complex cuticle
underlain by longitudonal mucles (but no circular muscles), a
pseudocoel, and a single cell-layered gut running from the anterior
mouth to a subterminal anus. The pharynx is strongly muscular, with
a triangular cross section, and sucks in liquid / micro-particulate food.
There is no circulatory system, and the excretory system is very basic
(neither flame cells nor nephridia, but unique structures called renette
cells.).
Almost all have the same body shape - round, pointy at both
ends, with very few exceptions. Some species have hair-like
spines or setea coming off the body wall. A very few plant
parasitic species look like balloons, being immobile and full of
eggs.
Despite high levels of morphological homogeneity, nematodes
are one of the most successful phyla on the planet. They have
colonised every damp habitat yet studied, and their species
richness is still poorly understood.
Approximately 15,000 species are described, but estimates of
species yet to be discoevered run into excess of a million. These
high figures come largely from recent work on the nematodes of
sea-floor sediments at great depths, which have revealed
remarkable levels of species richness.
There is a school of thought that each species of animal has at
least one species-specific nematode parasite, therefore
nematodes must outnumber all other species - this argument
depends crucally on the host-specificity of coleoptera-parasitic
nemas of tropical forests (why?) about which virtually nothing is
known.
Cuticle
Arguably the defining feature of
the nematode body is its thick
cuticle. All nematodes have a thick
collagen body wall retaining a high
internal hydrostatic pressure, up to
half an atmosphere - they are
almost impossible to squash under
normal circumstances. The design
of the body wall is unique to the
phylum. It has up to 9 layers
(typically 4), whose fibres run in
different directions, very much like
a high-pressure tyre.
CO - cortical layer
ME - Medial layer
BA - Basal zone
Implications of cuticle:
The fibres are inelastic, so preventing changes in body
volume while permitting lateral undulations. This high
internal pressure defines many aspects of the nematode’s life:
1: It means that their cross-section is always accurately
circular
2: It accounts for the lack of circular muscle in the body wall.
3: It means that the pharynx has to double as a valve,
preventing the gut contents from exploding, and that food has
to be actively pumped down the pharyx into the gut.
4: When defecating the animal simply opens its anus, and the
resulting jet sqirts out at high speed. Ascaris in the lab squirts up
to 60cm!
5: It means that most species are superficially very similar in
external morphology.
6: Cilia are quite unable to pump against such pressure gradients,
and have been lost in the phylum - all pumping action is muscular.
Interestingly the sperm seem not to be ciliated, and move by
amaeoboid action.
7: Most species are dioeceous - cross fertilisation is the norm, and
when mating the male has to wrap tightly around the female
otherwise she would explode upon penetration!
Nematode eggs
80μm
Trichuris egg
Parallelling the tough cuticle, nematode eggs are also thick walled
and highly resistant, allowing some to build up egg banks in heavily
contaminated areas. This is a real problem both for the plant- and
mammal-parasitic radiations.
The eggs are remarkably uniform in size c.50m diameter, with 7m
whale parasites having eggs the same size as 1mm soil dwellers.
(There is one obscure exception, a 7mm nema from antartica which
produces 1mm diameter eggs)
The outer coating of the egg often has ornate
proteinaceous features, of some taxonomic use.
Most Nemas produce eggs in prodigious quantities. A
typical gut parasite releases 15,000 eggs per day. One
such prolific producer the human hookworm Ancylostoma
duodenale may shed eggs at this rate for its life of 5 - 15
years.
Ancylostoma duodenale and eggs
8: The cuticle’s inflexibility is a problem for growth, and is solved by
the cuticle being shed at regular intervals (much like a snake shedding
its skin).
This is accompanied by the release of fluid which built up between
the old and new cuticles, which is one of the few things nematodes
release to provoke a strong immune response.
There are 5 stages (separated by 4 moults), of which the 1st 4 are
larval and the final is the sexually mature adult. It is common for 1 or
2 moults to take place in the egg before hatching.
Senses
Nematodes have rather limited sensory capabilities, probably
reflecting their relative impregnability. The thick cuticle is also
a problem, in that nerves have to traverse the cuticle without
weakening it unduly.
They have some chemosensors in the head and anal regions,
some specialised sexual sensory organs, and a few species have
occelli for light detection. (The presence of these sensory organs
is of taxonomic significance, with the main division within the
phylum being into 2 classes based on the occurrence of caudal
sensory pits called phasmids.)
Eutely
A curiosity of the phylum is its economy with cell numbers.
After the earliest sytages of embryogenesis, cell number is fixed,
and all subsequent growth occurs by cell expansion. This state
is called Eutely. (What does this mean for the ability to repair
damaged tissues?)
The development is highly deterministic, with each cell in the
body uniquely identifiable. This has made nematodes useful for
studies of development, notably Caenorhabditis elegans whose
entire genome has been mapped and whose development is well
documented.
Feeding
There is also a wide range of feeding habits, despite the constraint
that all food must be pumped in as a liquid or very fine suspension.
The commonest feeding habit for free living species is bacterivory,
ingested as a suspension (such as in soil solution).
Some species have evolved tough plates around their mouths which
act like jaws, and allow the nematode to act as a predator. Prey is
punctured and the contents sucked out under the muscular action of
the pharynx.
Other species are phytophagous or
mycophagous. They puncture the plant / fungal
cell with a hypodermic-like stylet on the anterior
end, and suck out juices.
Xiphynema + stylet
Gut
The gut is a single-celled tube, with no cilia or muscles. Its
transit time is very short, around 3 minutes, with the animals
defecating continuously as fast as is compatible with their
hydrostatic pressure.
It is thought that this is too fast for much enzymic action, and
that they simply absorb useful nutrients without engaging in
digestion per se.
Respiration
These animals are encased in a thick cuticle, and have no
differentiated gills or other O2 exchange surface. How do they
get their O2?
They don’t! Many nematodes lack crucial enzymes in the
Krebs cycle, showing that their metabolism has to function
without oxygen. This is not fatal - it just means that they are
inefficient in extracting energy from foodstuffs.
I wrote this page a few years ago, in the hope of inspiring
undergraduates to think about the constraints imposed by ecdysis..
Similarities to arthropods..
No-one suggests that nematodes have any close evolutionary
connection to arthropods, but both have evolved with a thick cuticle
between them and the outside world, and some convergent traits can
be seen. Both moult the cuticle (obvious and unavoidable), only
becoming sexually mature after the final moult (not so obvious).
Both have specialised thinnings in the cuticle for the sense organs,
but the active and highly mobile arthropods have evolved far more
sensory organs + gas exchange surfaces. Both have lost cilia in the
gut and the sperm (odd - so have higher plants). Any other parallels?
Since then, DNA analysis of highly conserved sequences has led to
the creation of a superphylum ecdysozoa, containing arthropods and
nematodes.
Parasitic forms
The huge majority of nematodes are harmless eaters of bacteria
and fungi, in muds and soils. This should be remembered as we
explore the parasitic forms, because these tend to make the bigger
impression and give the whole phylum a bad reputation.
The phylum has produced many parasitic radiations,
because the fundamental design is pre-adapted to endo
parasitism.
The extent to which they are pre-adapted to parasitism is
reflected in the observation that parasitic species show
almost no modification of body form, in sharp contrast to
most invertebrates (ie flukes in mammals).
1:The thick body wall is an excellent protection against chemical
or immune attack, allowing them to survive gut acids or antibody
attack. (Antibodies certainly do bind to nematode cuticle, but can’t
puncture it. Imagine throwing magnets at a suit of armour!)
2: The vast numbers of resistant eggs allow dispersal in time and
space, maximising the chance of infecting a new host.
3: The microphagous habit pre-adapts to a gut content / body fluid
diet.
4: The body design works well at small body size, which is
advantageous for most parasitic lifestyles.
The basic lifestyle involves sexual fertilisation, release of eggs
whch hatch into larval nematodes, identical in shape form and cell
number to their parents. These undergo 4 moults to attain adult
size. Parasitic species follow this pattern exactly, but still have a
diversity of life styles.
Plant parasites
Fungal feeders
Animal parasites
Plant parasites
Many nemas have evolved to parasitise plants, usually by
attacking their roots. Some species have evolved needle-like
mouthparts to suck sap. These damage the plant directly, and also
are implicated in transmitting plant viruses. They live outside the
root so are known as ectoparasitic forms; Typical species:
Xiphynema, Longidorus. Apart from feeding these have lifestyles
like free-living soil species.
Endoparasitic spp invade the hosts’ tissues, often with high levels of
specificity.
The potato root eel worm Heterodera rostochiensis is an extreme case,
a true cyst-forming nema.
2nd stage larvae are mobile in the soil, and attack potato roots. They
invade the root and feed on cortical cells. Females become fastened to
the root while males retain some mobility, to find and mate with the
stationary females. The female becomes a sac of eggs, fastened to the
root, the potato cyst. Its eggs remain viable for years and are a serious
agricultural pest.
Invertebrate parasites
Some (?the majority?) of nematode spp are invertebrate parasites.
These are mainly of interest as biological control agents.
Mermis spp are active against grasshopppers and have been
studied to control locusts (so far not on a serious field scale).
One nema Phasmarhabditis hermafrodita is marketed as a slug
control agent, though the field trials in the UK have been rather
patchy so far. 300,000 m-2 every 14 days!
Human parasites – the benefits!
Before you get too nauseated by what nematodes do in human bodies,
remember these points:
1: All you ancestors for millennia carried these worms mush of their
lives; we are co-evolved, and our immune system is evolved to attack
worms on a daily basis.
2: There is compelling experimental evidence that having some
nematodes in your gut exerts a helpful dampening down of the immune
system, especially useful against auto-immune diseases. If I had an
auto-immune bowel disease I would certainly make a point of getting a
nematode infection! They may dampen asthma too.
Mammalian parasites
If you think this is
revolting, try doing a
google image search for
Ascaris. If you have a
strong stomach.
A typical example of a mammalian parasitic nema is Ascaris
lumbricoides the human roundworm, which is up to 20cm long.
(A closely related species is found in pigs).
This was recorded in chinese records 4700 years ago (where it
was correctly stated to be caused by eating eggs off raw
vegetables) and egyptian medical records 3000 years BP.
Aristotle stated this species arose spontaneously in the gut,
thereby setting back biology by 2000 years (!).
The circulatory sojourn
The first two larval stages take
place within the egg before
hatching. The egg matures after
being excreted in faeces, and will
not hatch unless eaten by a new
host.
The third stage emerges in the alkaline conditions of the
mid gut, and burrows through the gut wall. It swims in
the blood until the right side of the heart, thence to the
lungs, where they penetrate the lung wall, are coughed up
with mucus and re-swallowed to become adults. This
lung passage was only discovered in 1915.
Ascaris eggs pass out with faeces and stay viable for decades - old
asylums often have heavy infestations. Nowadays one pill sorts it
out (“Ovex”).
Although basically benign, this species sometimes kills humans by
blocking your intestine with hundreds (that emerge from anus,
mouth and nose in heavy infections). The mass of nemas is so
dense that some 20cm adults penetrate the gut wall and wander
around the body cavity.
A few people become intensely allergic to nematodes - there are
stories of technicians who could not enter a lab where Ascaris was
being dissected without starting an allergic reaction. If such a
person gets a gut infection they will probably experience
anaphylactic shock when moulting fluids are released.
Many other parasitic species (such as the facultative parasite
Strongyloides) have this curious lung-penetration sequence for
reasons that are not understood, while other gut parasitic species do
not. The technical term is a circulatory sojourn, and causes minor
damage both to the lung wall and to the gut.
This is dwarfed by the damage caused by a misfunctioning of this
system. If the infestation is high, or species-host adaptation is poor,
some larvae fail to penetrate the lung capillaries but iunstead return
to the heart (left side) and are pumped on to the rest of the body. The
larvae then penetrate capillaries in body organs, causing mechanical
damage and scarring. This is known as visceral larva migrans, and
can cause serious or fatal damage to the host.
Toxocara canis – from dog faeces to human nervous systems;
children have suffered eye and CNS damage.
Enterobius vermicularis
Most of you here will have been infected with nematodes, at least some
of you will have them in your guts today. Luckily the commonest
nematode in western European humans is tiny and harmless - the
pinworm Enterobius vermicularis. The females emerge from your
large colon an hour after bedtime to lay eggs around the anus. (You
almost never see the males, who stay inside the colon as long as they
live). This irritates the skin, so you scratch and get the sticky eggs on
your fingers. These go into your mouth, and being sticky adhere to
door handles (especially toilet doors), bedclothes, mug handles etc.
Nematode eggs are very tough and stay viable for months or years.
Enterobius often affects hotel maids who breathe the eggs in as
dust, and is an occupational hazard in the laundry trade. Kids who
are restless a while after bedtime should be checked for Enterobius.
The worst it usually causes is to enter the vagina, where it causes
really nasty itching that persists long after the infection is cleared
up.
In Holland after the 2nd WW 98% of the population was infected
with Enterobius - this was surveyed by a simple technique in which
cellotape is left stuck across the anus all night, and its surgface
subsequently examined for eggs. Luckily Enterobius does no
harm and is easily cured with a pill from chemists.
Hookworms
Two genera of nematodes cause serious human debiliation: these
are the hookworms, Ancylostoma duodenale (the old world
hookworm) and Necator americana (new world hookworm). Their
larvae live in soil for the 1st 2 instars as harmless bacteria-eaters.
The third instar enters human blood (by being eaten or by forcing
its way into the skin, usually up a hair follicle or wound), then is
coughed up from the lungs and enters the gut.
Unlike Ascaris or Enterobius, these species use their tough toothed
anterior ends to browse on gut villi, causing serious localised
damage. Each worm causes losses of about 0.5ml blood per day,
so a typical infection of 100 worms leads to 50ml blood loss per
day. This causes weakness, anaemia and general debilitude, with
mental retardation developing after years of infection.
These species are commonest among communities where people
walk barefoot in faecally contaminated soil - the dwellings of the
very poor. Not just the third world; hookworm was an endemic
problem in the poor blacks of southern states in the USA. The ill
health resulting only increases poverty and has been seen as a cycle
of deprivation. When the hookworm lifestyle was elucidated it was
hailed as “the worm that causes poverty”.
(Needless to say, curing hookworm with piperizine and proper
sanitation has not ended poverty...)
The Guinea worm Dracunculus medinensis is one of the more notable human
parasites, known since antiquity. It is thought to be the old testament ‘fiery
serpents’ which affected the israelites. It has 2 hosts; a freshwater crustacean
(Cyclops etc), and man. The adult female is unusually large, c. 40cm and lives
below the host’s skin. Eggs may be produced parthenogenetically as males
seem to be scarce.
The eggs hatch in utero and undergo a moult. When ready the female
causes a boil to arise on the hosts skin, and sticks a portion of her
uterus (prolapsed) out of the ulcerated hole. When she senses that the
host is bathing (by a sudden fall in temperature) she ejects larvae by
rupturing her uterus.
The larvae cannot develop further unless eaten by a crustacean
(copepod), where they hatch and moult twice. Typically one copepod
can support one larva - multiple infections will kill the crustacean.
If the crustacean is drunk by a human, the nemas emerges into the gut
lumen, then forces its way into the blood stream. The female alone
punctures the hosts skin, to repeat the cycle. The interval from
drinking the infective Cyclops to release of new larvae is about 12
months.
Note the thread
dangling out of the
open wound. This is
the female nematode,
whose prolapsed
ovary sheds filaria on
immersion into cold
water.
The problem is that this parasite maintains an open wound,
which is a likely cause of infection for the host. Her body can
be seen running below the hosts skin for many cm, but is too
delicate to be pulled out. If she is torn, her body will rot under
the hosts skin and septicemia is certain (probably fatal). The
solution is to pull her out very slowly, winding her around a
stick.
This rather ghastly procedure is ancient, and thought to be the
origin of the aescepulean snake wound around a stick which is
the symbol of medical practice.
Biodiversity paradox:
We can eliminate Dracunculus infection in
humans, and it is on the WHO’s target list
(possibly after polio). In fact it was a UNICEF
goal for children #25 to eliminate it by 2000
(failed). It’s easy to eliminate – educate people to
recognise the boils, not to expose these to water,
to drink filtered water (not even boiled – a
strainer will do to remove the crustaceans).
Then we lose a part of human history – there are
other Dracunculus species: D. insignis attacks a
variety of aquatic predatory mammals (otter,
racoon, fisher, mink). But D. medinensis is
human-endemic, and is said to have given us the
icon of medicine. (I’m not sure that story is true,
but the association is still noteworthy.)
aesculapian snake
Trichinella spiralis is a gut parasite of omnivores and carnivores. It
causes incurable muscle damage. Adults mate in the hosts gut, and the
eggs hatch rapidly producing larvae which penetrate the gut wall.
Unlike many nemas these larvae attack muscle cells (with a lancet and
cytolytic enzymes) wherein they grow and moult 3 times. These stay
encysted in the muscle, waiting for the muscle to be eaten. When
eaten, the larvae excyst, moult and infect a new host.
This is basically a parasite of rats (which are cannibalistic), but infects
pigs when pigs eat a rat. The problem for humans is not the adults, but
the encysted larvae in our muscles. They degrade muscle performance
(there being selection pressure to weaken the intermediate host), and
are presently incurable.
Although Trichinella encephalitis is rare, it is life threatening.
Trichinella
also cycles
in polar
bears,
walrus,
hence any
arctic
scavenger is
a risk.
Wucheria
The one I really wouldn’t want to get is Wucheria bankroftii, causing
filariasis = elephantiasis. This nematode is minute (<0.1mm) and lives
in the circulating blood of its human host. The larvae are small enough
to be sucked up by mosquitos. This allows transmission to a new host.
Inside the host each individual worm does minimal harm, but en masse
the numbers become large enough to clog up lymph ducts. This
prevent fluids draining out of tissues, so the tissues expand, slowly but
inexorable. Filariasis in the leg can make 1 leg swell to 40cm diameter
or more, puffy soft flesh.
The same condition causes hideous facial swelling, and the worst is the
scrotal infection, where the unfortunate male has to go around with his
testes supported by a wheelbarrow. We can now kill the infection, but
not repair the damage caused by a long-standing swelling.
Filariasis, from Wucheria-blocked lymph nodes.