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The
word ‘helminth’ is a general term meaning ‘worm’,
but there are many different types of worms. Prefixes are
therefore used to designate types: platy-helminths for flat-worms
and nemat-helminths for round-worms. All helminths are multicellular
eukaryotic invertebrates with tube-like or flattened bodies
exhibiting bilateral symmetry. They are triploblastic (with
endo-, meso- and ecto-dermal tissues) but the flatworms are
acoelomate (do not have body cavities) while the roundworms
are pseudocoelomate (with body cavities not enclosed by mesoderm).
In contrast, segmented annelids (such as earthworms) are coelomate
(with body cavities enclosed by mesoderm).
Many helminths
are free-living organisms in aquatic and terrestrial environments
whereas others occur as parasites in most animals and some
plants. Parasitic helminths are an almost universal feature
of vertebrate animals; most species have worms in them somewhere.
Biodiversity
Three
major assemblages of parasitic helminths are recognized: the
Nemathelminthes (nematodes) and the Platyhelminthes (flatworms),
the latter being subdivided into the Cestoda (tapeworms) and
the Trematoda (flukes): |
nematode |
cestode |
trematode |
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Nematodes
(roundworms) have long thin unsegmented tube-like bodies
with anterior mouths and longitudinal digestive tracts.
They have a fluid-filled internal body cavity (pseudocoelum)
which acts as a hydrostatic skeleton providing rigidity
(so-called ‘tubes under pressure’). Worms
use longitudinal muscles to produce a sideways thrashing
motion. Adult worms form separate sexes with well-developed
reproductive systems. |
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Cestodes
(tapeworms)
have long flat ribbon-like bodies with a single anterior
holdfast organ (scolex) and numerous segments. They
do not have a gut and all nutrients are taken up through
the tegument. They do not have a body cavity (acoelomate)
and are flattened to facilitate perfusion to all tissues.
Segments exhibit slow body flexion produced by longitudinal
and transverse muscles. All tapeworms are hermaphroditic
and each segment contains both male and female organs. |
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Trematodes
(flukes) have small flat leaf-like bodies with oral
and ventral suckers and a blind sac-like gut. They do
not have a body cavity (acoelomate) and are dorsoventrally
flattened with bilateral symmetry. They exhibit elaborate
gliding or creeping motion over substrates using compact
3-D arrays of muscles. Most species are hermaphroditic
(individuals with male and female reproductive systems)
although some blood flukes form separate male and female
adults. |
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Unlike
other pathogens (viruses, bacteria, protozoa and fungi), helminths
do not proliferate within their hosts. Worms grow, moult,
mature and then produce offspring which are voided from the
host to infect new hosts. Worm burdens in individual hosts
(and often the severity of infection) are therefore dependent
on intake (number of infective stages taken up). Worms develop
slowly compared to other infectious pathogens so any resultant
diseases are slow in onset and chronic in nature. Although
most helminth infections are well tolerated by their hosts
and are often asymptomatic, subclinical infections have been
associated with significant loss of condition in infected
hosts. Other helminths cause serious clinical diseases characterized
by high morbidity and mortality. Clinical signs of infection
vary considerably depending on the site and duration of infection.
Larval and adult nematodes lodge, migrate or encyst within
tissues resulting in obstruction, inflammation, oedema, anaemia,
lesions and granuloma formation. Infections by adult cestodes
are generally benign as they are not invasive, but the larval
stages penetrate and encyst within tissues leading to inflammation,
space-occupying lesions and organ malfunction. Adult flukes
usually cause obstruction, inflammation and fibrosis in tubular
organs, but the eggs of blood flukes can lodge in tissues
causing extensive granulomatous reactions and hypertension.
Life-cycles
Helminths
form three main life-cycle stages: eggs, larvae and adults.
Adult worms infect definitive hosts (those in which sexual
development occurs) whereas larval stages may be free-living
or parasitize invertebrate vectors, intermediate or paratenic
hosts. Nematodes produce eggs that embryonate in utero or
outside the host. The emergent larvae undergo 4 metamorphoses
(moults) before they mature as adult male or female worms.
Cestode eggs released from gravid segments embryonate to produce
6-hooked embryos (hexacanth oncospheres) which are ingested
by intermediate hosts. The oncospheres penetrate host tissues
and become metacestodes (encysted larvae). When eaten by definitive
hosts, they excyst and form adult tapeworms. Trematodes have
more complex life-cycles where ‘larval’ stages
undergo asexual amplification in snail intermediate hosts.
Eggs hatch to release free-swimming miracidia which actively
infect snails and multiply in sac-like sporocysts to produce
numerous rediae. These stages mature to cercariae which are
released from the snails and either actively infect new definitive
hosts or form encysted metacercariae on aquatic vegetation
which is eaten by definitive hosts. |
nematode
cycle
egg - larvae (L1-L4) - adult |
cestode
cycle
egg - metacestode - adult |
trematode
cycle
egg-miracidium-sporocyst-redia-cercaria-(metacercaria)-adult |
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Helminth
eggs have tough resistant walls to protect the embryo while
it develops. Mature eggs hatch to release larvae either within
a host or into the external environment. The four main modes
of transmission by which the larvae infect new hosts are faecal-oral,
transdermal, vector-borne and predator-prey transmission: |
faecal-oral
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trasdermal
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vector-borne
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predator-prey
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faecal-oral
transmission
of
eggs or larvae passed in the faeces of one host and
ingested with food/water by another (e.g. ingestion
of Trichuris eggs leads directly to gut infections
in humans, while the ingestion of Ascaris eggs
and Strongyloides larvae leads to a pulmonary
migration phase before gut infection in humans). |
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transdermal
transmission
of
infective larvae in the soil (geo-helminths) actively
penetrating the skin and migrating through the tissues
to the gut where adults develop and produce eggs that
are voided in host faeces (e.g. larval hookworms penetrating
the skin, undergoing pulmonary migration and infecting
the gut where they feed on blood causing iron-deficient
anaemia in humans). |
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vector-borne
transmission
of
larval stages taken up by blood-sucking arthropods or
undergoing amplification in aquatic molluscs (e.g. Onchocerca
microfilariae ingested by blackflies and injected into
new human hosts, Schistosoma eggs release miracidia
to infect snails where they multiply and form cercariae
which are released to infect new hosts). |
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predator-prey
transmission
of
encysted larvae within prey animals (vertebrate or invertebrate)
being eaten by predators where adult worms develop and
produce eggs (e.g. Dracunculus larvae in copepods
ingested by humans leading to guinea worm infection,
Taenia cysticerci in beef and pork being eaten
by humans, Echinococcus hydatid cysts in offal
being eaten by dogs). |
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Taxonomic
overview
Two
classes of nematodes are recognized on the basis of
the presence or absence of special chemoreceptors known
as phasmids: Secernentea (Phasmidea) and Adenophorea
(Aphasmidea). While many different orders are recognized
within these classes, the main parasitic assemblages
infecting humans and domestic animals include one aphasmid
order (Trichocephalida) and 6 phasmid orders (Oxyurida,
Ascaridida, Strongylida, Rhabditida, Camallanida, and
Spirurida). |
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trichocephalid
‘whip-worms’
have long thin anterior ends which they embed in the
intestinal mucosa of their hosts. They have simple life-cycles
where infections are acquired by the ingestion of eggs
and emergent larvae moult and mature to adults in the
gut. Trichuris infections in humans may cause
inflammation, tenesmus, straining and rectal prolapse. |
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oxyurid
‘pin-worms’
have small thin bodies with blunt anterior ends. They
have simple life-cycles, but with an unusual modification.
Female worms emerge from the anus of their hosts at
night and attach eggs to the skin. This causes peri-anal
itching and eggs are transferred by hand to mouth. Infections
by Enterobius cause irritability and sleeplessness
in humans, especially children. |
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ascarid
‘roundworms’
have large bodies with 3 prominent anterior lips. Their
life-cycles involve a stage of pulmonary migration where
larvae released from ingested eggs invade the tissues
and migrate through the lungs before returning to the
gut to mature as adults. Ascaris infections
in humans cause gastroenteritis, protein depletion and
malnutrition and heavy infections can cause gut obstruction. |
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strongyle
‘hookworms’
have
dorsally curved mouths armed with ventral cutting plates
or teeth which they embed in host tissues to feed on
blood. They have complex life-cycles where larvae develop
in the external environment (as ‘geo-helminths’)
before infecting hosts by penetrating the skin. Once
inside, they undergo pulmonary migration before settling
in the gut to feed. Heavy infections by Ancylostoma
and Necator cause severe iron-deficient anaemia
in humans, especially children. |
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rhabditid
‘threadworms’
have tiny bodies which become embedded in the host mucosa.
Their life-cycle includes parasitic parthenogenetic
females producing eggs which may hatch internally (leading
to auto-infection) or externally (leading to transmission
of infection or formation of free-living male and female
adults). Super-infections by Strongyloides
may cause severe haemorrhagic enteritis in humans. |
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camallanid
‘guinea worms’
infect
host tissues where the large females cause painful blisters
on the feet and legs. When hosts seek relief by immersion
in water, the blisters rupture releasing live larvae
which infect copepods that are subsequently ingested
with contaminated drinking water. The ‘fiery serpents’
mentioned in historical texts are thought to refer to
Dracunculus infections. |
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spirurid
‘filarial worms’occur
as long thread-like adults in blood vessels or connective
tissues of their hosts. The large female worms release
live larvae (microfilariae) into the blood or tissues
which are taken up by blood-sucking mosquitoes or pool-feeding
flies and transmitted to new hosts. Onchocerca
infections cause nodules, skin lesions and blindness
in humans, while those of Wuchereria cause
elephantitis. |
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| whipworm |
pinworm |
hookworm |
threadworm |
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roundworm |
filarial
worm |
guinea
worm |
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Two
subclasses of cestodes are differentiated on the basis
of the numbers of larval hooks, the Cestodaria being
decacanth (10 hooks) and the Eucestoda being hexacanth
(6 hooks). Collectively, 14 orders of cestodes have
been identified according to differences in parasite
morphology and developmental cycles. Two orders have
particular significance as parasites of medical and
veterinary importance.
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Cyclophyllidean
cestodes
have terrestrial 2-host life-cycles where adult tapeworms
develop in carnivores (scolex with 4 suckers and sometimes
hooks) while larval metacestodes form bladder-like cysts
in the tissues of herbivores. The larvae of Taenia
spp. cause cysticercosis in cattle, pigs and humans,
while those of Echinococcus cause hydatid disease
in humans, domestic and wild animals. |
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Pseudophyllidean
cestodes
have
aquatic 3-host life-cycles, involving the sequential
formation of adult tapeworms in fish-eating animals
(scolex with 2 longitudinal bothria), procercoid larval
stages in aquatic invertebrates (copepods) and then
plerocercoid (spargana) stages in fish e.g. Diphyllobothrium
in humans, dogs and cats being transmitted through copepods
and fish. |
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| Cyclophyllidea |
Pseudophyllidea |
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| Two
major groups of trematodes are recognized on the basis
of their structure and development: monogenean trematodes
with complex posterior adhesive organs and direct life-cycles
involving larvae called oncomiracidia; and digenean
trematodes with oral and posterior suckers and heteroxenous
life-cycles where adult worms infect vertebrates and
larval miracidia infect molluscs to proliferate and
produce free-swimming cercariae. Monogenea are almost
exclusively ectoparasites of fishes while Digenea are
endoparasites in many vertebrate hosts and have snails
as vectors. Some 10 digenean orders are recognized on
the basis of morphologic and biologic differences, two
orders are of particular medical and veterinary significance. |
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echinostomatid
fasciolids
(liver flukes) live as adults in hepatic bile ducts
of mammals where they cause fibrotic ‘pipestem’
disease. The parasites proliferate in freshwater snails
and mammals become infected by ingesting metacercariae
attached to aquatic vegetation. Several Fasciola
spp. cause hepatic disease in domestic ruminants and
occasionally in humans. |
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strigeatid
schistosomes
(blood
flukes) are unusual in that the adults are not hermaphroditic
but form separate sexes which live conjoined in mesenteric
veins in mammals. Female worms lay eggs which actively
penetrate tissues to be excreted in urine/faeces or
they become trapped in organs where they cause granuloma
formation. Miracidia released from eggs infect aquatic
snails and produce fork-tailed cerceriae which actively
penetrate the skin of their hosts. Several Schistosoma
spp. cause schistosomiasis/bilharzia in humans. |
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