Helminth Parasites
The word
‘worm’ is used loosely to describe organisms with elongated bodies and a
more or less creeping habit.
The word
‘Helminth’ does mean ‘worm’, but in zoological terms it is more
restricted to members of the phyla Platyhelminths , Nematoda and
Acanthocephala.
There
are three groups of medically important helminths; Cestodes (tapeworms),
Nematodes (roundworms) and Trematodes (flukes).
These
parasites live in both the body spaces (gut lumen, bile ducts, lungs, oral
cavity, etc.) and in tissues (blood, muscles and skin).
4. Infections through
the Gastrointestinal Tract
Part Four - The Cestodes
The cestodes (or tapeworms) form a group of worms,
exhibiting two unmistakable morphological features; they all possess flat,
ribbon like bodies and lack an alimentary canal.
Adult tapeworms usually inhabit the alimentary
canal of their hosts (though they occasionally are found in the bile or
pancreatic ducts) and attach themselves to the mucosa by means of a scolex.
Despite the lack of a digestive system they do absorb food from the hosts
intestine; thereby providing the tapeworms a habitat that is associated with
high nutritional levels, feeding the tapeworms high growth rate.
Larvae on the other hand show a wide range of
habitat preferences, being found in almost any organ of both vertebrate and
invertebrate hosts. Though most larval species show a preference for a
particular site.
This lack of an alimentary canal markedly separates
tapeworms from nematodes and trematodes. The outer tegument of the body must
serve not only as a protective coating but also as a metabolically active
layer through which nutritive material can be absorbed, along with secretions
and waste material to be transported out of the body.
The body consists of a chain of segments or
proglottids, which can be immature, mature or gravid; the latter of which
contain a fully developed uterus packed with eggs.
Therefore, each tapeworm is made up of a ‘string of individuals’
having a complete set of reproductive organs in progressive degrees of sexual
maturity and budding off from a body attached to the host tissue by a head or
scolex.
Except for Hymenolepis
nana, which can develop directly in the same host, the lifecycle of
tapeworms involves both an intermediate and definitive host.
Taenia species
Introduction
Taenia species are the most common
cestode parasites of humans. More
than 60 million people are infected with T.
saginata (‘beef’ tapeworm) world wide and about 4 million are infected
with T. solium (‘pork’ tapeworm).
The life cycle of a Taenia species
can be seen in Diag 1. T. saginata has a comsmopolitan distribution, but is more common in
developing countries where hygiene is poor and the inhabitants have a tendency
of eating raw or insufficiently cooked meat. T saginata is the most common adult tapeworm found in man. T
solium is virtually extinct in Europe and the USA.
Diagram
1. Diagram illustrating the life cycle of a Taenia
species.
The
adults of both species live in the small intestine of man (1), the definitive
host. The gravid segments (2) are very active and escape through the anus,
releasing large numbers of eggs (3) in the perianal region or on the ground
where they can survive for long periods. When ingested by pigs or catle (4),
the eggs hatch, each releasing an oncosphere which migrates through the
intestinal wall and blood vessels to reach striated muscle within which it
encysts, forming cysticerci (5). When inadequately cooked meat containing the
cysts is eaten by man, the oncospheres excyst (6), settle in the small
intestine and develop there into adult cestodes (1) over the next 3 months or
so. The segments of T. solium are
somewhat less active than those of the beef tapeworm but its eggs, if released
in the upper intestine, can invade the host (auto-infection) (7), setting up
the potentially dangerous larval infection known as cysticercosis in muscle of
any other site. (Peters & Gilles, 1995)
Both humans and cattle or pigs are necessary to the
complete life cycle of Taenia
species. (In Europe and the USA cattle are the normal intermediate hosts, but
in the tropics several other ruminants, e.g. goat, sheep llama and giraffe,
may serve as the intermediate hosts.) Eggs ingested by the intermediate hosts
usually contain oncospheres. The
oncospheres then hatch out in the duodenum, pass into the intestine where they
penetrate the intestinal wall and are then carried by the circulation to be
deposited in tissues (usually muscle). There
they develop into cysticerci larva which are white and ovoid, measuring
approximately 8 x 5mm. (Fig 5)
Humans become infected by ingesting inadequately,
cooked beef or pork with cysticerci, containing an invaginated protoscolex.
The protoscolexes evaginate and pass into the small intestine where they
attach themselves to the mucosa and develop into adult worms.
Eggs and proglottids are passed out in the faeces,
and are then eaten by the intermediate host, thus, perpetuating the life
cycle.
Morphology
Ova of Taenia
species are spherical, yellowish brown and measure 31 - 34mm in
diameter. The shell is thick and
radially striated. (Fig 1) Within
the shell, the onchosphere has 3 pairs of hooklets. However, the microscopical appearance of the ova of T.
saginata and T. solium are identical. (Table
1 highlights some of the
differences between the two species)
The length of the adult T.
saginata is 4 - 8 meters long and that of T. solium is 3 - 5metres long. (Fig
2) The proglottids of Taenia
species can be identified by the number of uterine branches;
7 - 13 for T. solium and 15 -
20 for T. saginata. (Fig 3 & 4) If the scolex is recovered, the 4 suckers and
rostellum of hooklets of T. solium
will distinguish it from T. saginata,
which has 4 suckers but no hooklets.
Fig
1.Taenia spp.
eggs of the two species are
Fig 2. Segments of an adult
identical, spherical (31 - 34 µm), brown to
tapeworm. Adults of T. solium may
dark-yellow with
a thick shell wall and contain
reach
2 – 8m in length and
an onchosphere. multiple
infections can occur. (Peters &
Gilles,
1995)
Clinical
Disease
The presence of the adult worm rarely causes
symptoms apart from slight abdominal irritation with diarrhoea, constipation
or indigestion. The accidental ingestion of the embryonated ova of T. solium may result in cysticercosis in man. An infection due to an
adult Taenia, in man or animals, is
referred to as taeniasis.
T. saginata (the ‘beef’ tapeworm) does
not cause human cysticercosis.
When the embryonated
eggs are ingested, the embryos hatch out, migrate through the intestinal wall
and are carried around the body in the circulation and deposited in various
tissues. Muscle and subcutaneous
tissues are usually infected, but cysticerci can infect most organs and
tissues. Human cysticercosis is
usually asymptomatic unless the infection is particularly heavy or cysticerci
are formed in some vital area e.g. the brain, resulting in neurological
sequelae.
Fig
3. A gravid proglottid of T.
saginata
Fig 4. Illustrates a gravid proglottid
with more
than 14 lateral uterine branches
of T. solium with less than
14 lateral
(on one
side). (India ink technique)
uterine branches (on one side). (www.medicine.cmu.ac.th)
(India
ink technique)
(www.medicine.cmu.ac.th)
|
Characteristic
|
Taenia Saginata
|
Taenia solium
|
Intermediate Host
|
Cattle, reindeer
|
Pig, wild boar
|
|
Site of Development
|
Muscle, viscera
|
Brain, skin, muscle
|
|
Scolex: adult worm
|
No hooks
|
Hooks
|
|
Scolex: cysticercus
|
No rostellum
|
Rostellum & hooks
|
|
Proglottis: uterine branches
|
23 (14 – 32) *
|
8 (7 –11) *
|
|
Passing of proglottids
|
Single, spontaneous
|
In groups, passively
|
|
Ovary
|
2 lobes
|
3 lobes
|
|
Vagina: sphincter muscle
|
Present
|
Absent
|
*
No universal agreement to the number of uterine branches in these 2 species.
As a rough guide, specimens with more than 16 branches are likely to be those
of T. saginata and those with less
than 10 branches are ikely to be of T.
solium.
Table
1. Some characteristics differentiating T. saginata from T. solium.
(Smyth, 1994)
Fig
5. Cysticercus in pork. The larval stage occurs in the
pig, they are usually white
and ovoid, measuring approximately 8 x 5mm. Man becomes infected by eating
undercooked meat which is infected with cysticerci containing an invaginated
protscolex. (x20)(Peters & Gilles, 1995)
Laboratory
diagnosis
Diagnosis of intestinal taeniasis can be made by
recovery of the characteristic ova in the stool.
However, the ova of T. solium
and T. saginata are identical and
diagnosis is made by the recovery of the segments or scolex.
The
diagnosis of cysticercosis depends upon serology.
MRI scans may reveal the presence of lesions in the brain. Calcified
cysticerci are less often seen in the brain: in about one-third of cases, 10
years or more after infection. Occasionally, the diagnosis is made
histologically on surgical specimens. Calcification in muscles usually appears
3 – 5 years after initial infection, and are most typically seen as
spindle-shaped calcifications, most numerous in the thighs.
Western Blots
Various
Immunodiagnostic tests appear to give good results on serum or CSF.
Diagnosis using an immunodiagnostic test can be
achieved using an in vitro
qualitative assay for the detection of IgG antibodies in serum reactive with T. solium antigens present on a membrane.
Infected individuals develop a predominately IgG
response to the parasite. ELISA has been used as a screening test, but low
sensitivity and frequent artifactual crossreactions, or crossreactions with
antibodies from other parasitic infections, limit its usefulness as a
confirmatory diagnostic test. The Western Blot assay (U.S Patent No.
5,354,660) developed by Tsang et al,
at the U.S. Centers for Disease Control has been shown to provide a reliable
method for evaluation of sera from patients with clinically diagnosed active
cysticercosis. Field studies support a sensitivity of 98% and specificity of
100% for this assay.
This assay is known as the QualiCodeTM
Cysticercosis Kit, the principle behind the test is that it is a qualitative
membrane-based immunoassay manufactured from T.
solium proteins. The T. solium
proteins are fractionated according to molecular weight by electrophoresis on
a polyacrylamide slab gel (PAGE) in the presence of sodium dodecyl sulfate (SDS).
The separated T. solium proteins are
then transferred via electrophoretic blotting from the gel to a nitrocellulose
membrane. This antigen-bearing membrane has been cut into strips for testing
of individual samples. Sera are tested at 100X dilution.
During the procedure,
the strips containing the T. solium
proteins are incubated with serum specimens and washed to remove unbound
antibodies. Visualisation of human immunoglobulins specifically bound to T.
solium proteins is performed by sequential reaction with goat anti-human
immunoglobulin-alkaline phosphatase conjugate and BCIP/NBT substrate. Bands
corresponding to the positions of the resoled T.
solium proteins will be visualised on the strip, indicating the presence
in the serum sample of IgG antibodies direct against Taenia
antigens. Band positions are compared to those on a reference strip
developed using the cysticersosis positive control.
Hymenolepis nana
Introduction
Hymenolepis
nana, the dwarf tapeworm, is the
smallest tapeworm to infect humans. This cestode belongs to a large family
known as Hymenolepididae. The diagnostic features of this family are: scolex
armed with one circlet of five hooks; 1 – 3 large testes and sacciform
uterus. In addition to the H.nana, three other species, H.
diminuta, H. microstoma and H.
citelli have been used extensively for studies on cestodes.
Hymenolepis
nana has a cosmopolitan
distribution and is thought to be the most common tapeworm throughout the
world. The infection is more frequently seen in children although adults are
also infected, causing hymenolepiasis.
Life cycle
The lifecycle of H.
nana does not require an intermediate host, complete development occurring
within the villi of a single host, resulting in a ‘direct’ life cycle.
Though it can also utilise an insect as an intermediate host. (Diag 2)
Diagram
2. Diagram illustrating the life cycle of the
‘dwarf’ tapeworm, Hymenolepis nana.
The eggs
that are released from mature proglottids in the upper ileum are usually
passed out in the faeces. (Fig 6)
If swallowed by another human they develop into hexacanth oncospheres and
burrow into the villi of the small intestine. This is where they develop into
tailless cysticercoids and then migrate towards the ileum and attach to
commence the formation of proglottids. The eggs which are ingested by insects,
such as fleas, beetles or cockroaches hatch to form tailed cysticercoids which
remain unmodified as long as they are inside the insect. If they are
accidentally swallowed by a human they pass down to the ileum and establish
themselves. (Peters & Gilles, 1995)
Morphology
The egg containing the oncosphere bears three pairs
of hooklets and is surrounded by a membrane. (Fig
6 & 7) This membrane has 2 polar thickenings from which arise
threadlike filaments extending into the space between the membrane and the
colourless hyaline shell, unlike those of H.
diminuta which do not possess any filaments.
The adult tapeworm is normally 2.5 - 4cm long.
The scolex is knob like in shape, has a rostellum with hooklets and 4
suckers. (Fig 8)
The segments are wider than they are long.
Ova are spherical or ovoid measuring 30 - 47mm in diameter. This is what
distinguishes it morphologically from H.
diminuta.
Fig 6.
Egg of a Hymenolepis nana found in
an
Fig 7. Oncosphere of a Hymenolepis
Iodine
stained wet mount. Eggs are usually
nana. After humans become
infected the
spherical
, 30 - 47mm
in diameter. They contain
eggs hatch into an oncosphere. (Phase
a
hyaline shell and a 6 hooked oncosphere.
contrast x 1,150) (Peters & Gilles, 1995)
Fig
8. Hymenolepis
nana scolex.
Stained to show the solex with a knob like rostellum bearing a ring of
hookelts. They possess 4 suckers, 2 of which can be seen just below the
protruding rostellum. (www.medicine.cmu.ac.th)
Clinical Disease
Infections due to H.
nana may cause no symptoms even with heavy worm burdens.
However, symptoms of restlessness, irritability, anorexia, abdominal
pain and diarrhoea have been reported. Heavy
worm burdens may be caused by auto-infection which can be a problem in the
immunocompromised.
Laboratory Diagnosis
Diagnosis is based on
recovery and identification of the characteristic ova in a formol-ether
concentrate of faeces. Adult
worms and proglottids are rarely seen in stool samples.
Hymenolepis diminuta
Introduction
Hymenolepis
diminuta is a small tapeworm commonly
found in rats and mice. It has a
world wide distribution in these hosts but is infrequently found in humans,
with only sporadic cases being reported.
Life cycle and transmission
The life cycle of H.
diminuta requires an intermediate arthropod host e.g. earwigs, larval
fleas and various beetles. (Diag 3)
Human infection occurs by the accidental ingestion of an infected arthropod,
which contains the cysticercoids.
Diagram
3.
Diagram illustrating the life cycle of the rat tapeworm, Hymenolepis diminuta.
Morphology
The ova are large, ovoid and
yellowish with a moderately thick shell.
They contain an onchosphere with 6 hooklets and a clear area between
the oncosphere and the shell. They measure 70 - 85mm
by 60 - 80mm.
(Fig 9 & 10)
The adult worm is 20 – 60cm long.
It has a knob like scolex with a rostellum but no hooklets and 4
suckers (in contrast to H. nana). (Fig 11) The
rostellum can be withdrawn into a rostellar sac. The tapeworm contains about 1000 proglottids, each of which
is wider than long.
Clinical
Disease
The symptoms associated with H. diminuta infections are few if any.
Fig
9 & 10. Eggs of H.
diminuta are larger (70 - 85mm by 60 - 80µm) than H.
nana and lack the polar filaments. Typical eggs, measure between 70 x 80µm;
spherical in shape with a yellow-transparent cover. The internal membrane
(that surrounds the oncosphere) has polar salients without filaments and six
hooks on its interior. (www.cdfound.to.it)
Fig
11.
Hymenolepis diminuta scolex
(permanently stained) bearing a knob-like rostellum and 4 suckers, but has no
hooklets. The rostellum can be withdrawn into a rostellar sac.
(www.medicine.cmu.ac.th)
Laboratory Diagnosis
Diagnosis is based on recovery and identification
of the characteristic ova in a formol-ether concentrate of faeces.
Adult worms and proglottids are rarely seen in stool samples.
Diphyllobothrium latum
Introduction
Members
of this order, commonly known as pseudophyllids, are chiefly parasites of
fish-eating mammals, birds and fish. They typically are found with a scolex
which is characterised by two shallow elongated bothria situated with one
dorsally and one ventrally. The proglottids are flattened dorsoventrally.
Diphyllobothrium
latum is
an intestinal tapeworm, known as the human ‘broad’ tapeworm. It is the
largest tapeworm found in man. The term ‘broad’ relates to the fact that
the proglottids are generally wider than they are long. It is an important
human parasite. The adult worms of two other species of the genus, D.
dendriticum and D. ditremum are chiefly parasite of fish-eating birds and mammals.
The tapeworm, D.
latum has a wide distribution, occurring especially in countries bordering
the Baltic Sea (Finland, Sweden etc.): and also in Russia, Switzerland and
North America. It is in these countries where the populations are known to eat
uncooked or partly cooked (i.e. smoked) fish.
Apart from man they are found in many other hosts,
especially the dog, cat and pig. This is due to the host countries allowing
the domestic animals access to the offal from the infected fish.
The life cycle of this tapeworm
requires two intermediate hosts.
Diagram
4.
Diagram illustrating the life cycle of the broad tapeworm, Diphyllobothrium latum.
Morphology
The egg is
usually ovoid and has a small knob at the opercular end and is yellowish-brown
in colour with a smooth shell, of moderate thickness. (Fig 12 & 13) They measure 58 - 75mm by
40 - 50mm in
size.
Adult worms can reach up
to a length of 10 metres or more and may contain up to 3,000 proglottids. (Fig
14) The scolex is spatulate with no rostellum or hooklets.
It has 2 shallow grooves or bothria, which are unlike the typical 4
suckers seen on the Taenia species.
The proglottids measure 3mm long and 11mm wide and have a rosette shaped
central uterus.
Fig 12. Illustrations showing the morphology of the
separate stages of the Diphyllobothrium
latum life cycle. (www.aisr.lib.tju.edu)
Fig
13. Diphyllobothrium
latum egg. They are
Fig 14. The proglottids
measure 3mm
They are usually yellowish brown in colour
rosette shaped central uterus. These
and 58 - 75mm by 40
- 50mm
in size.
proglottids tend
to be passed in
strands
of variable length in the
stool. (www.dpd.cdc.gov)
Clinical Disease
The infection caused by D.
latum is due to the ingestion of raw, poorly cooked or pickled fresh water
fish. The symptoms associated
with D. latum infection may be
absent or minimal with eosinophilia. There may be occasional intestinal
obstruction, diarrhoea, abdominal pain. The most serious symptom is the onset
of pernicious anaemia. This is due to a vitamin B12 deficiency,
caused by excessive absorption of the vitamin by the adult worm and the
absorption of cobalamins from the host intestine (occurring only in a small
percentage of people).
Laboratory diagnosis
Laboratory diagnosis depends on the recovery of
characteristic eggs from a formol ether concentrate of faeces. Proglottids may
also be seen in faecal samples usually in a chain of segments from a few
centimetres to about 0.5 metres in length.
4.1
Identifying Intestinal Helminths
The usual diagnostic stages for identifying
medically important helminths are the eggs and larvae. Occasionally, adult
worms like Ascaris and Enterobius may be seen and segments or proglottids are used for
diagnosing certain tapeworms.
If an egg, is found the following features as
described below and in Fig 15,
should be carefully observed in order to make a specific identification.
1.
Size:
The length and width are measured and are generally within a specific range.
2.
Shape:
Each species has its own particular shape.
3.
Stage
of development when passed: In some species, the eggs consist of a single
cell; in some, there may be several cells; and some species are usually
embryonated (i.e., they contain a larva) when passed in the faeces.
Occasionally, if the stool specimens are several
hours or 1 – 2 days old, eggs may develop to more advanced stages. Ascaris eggs usually have only 1 cell when passed in the faeces;
however, the single cell may divide and, in old specimens, eggs with 2 or 4
cells may be seen. Hookworm eggs in specimens that are several hours old may
contain 16, 32 or more cells. In 12 – 24 hours, the egg may be embryonated
and later still the larvae may hatch. Therefore, when observing the stage of
development of helminth eggs, be sure that the stool specimen is freshly
passed. If it is several hours or a day old, expect to see changes in the
stage of development of some species. Ideally only fresh samples should be
accepted for diagnosis.
4.
Thickness
of the egg shell: Some species, like Ascaris,
have thick shells; others, like hookworm, have thin shells.
5.
Colour:
Some eggs are colourless (e.g., hookworm, Enterobius),
others are yellow or brown (Ascaris,
Trichuris).
6.
Presence
of characteristic like opercula (lids), spines, plugs, hooklets, or
mammillated outer coats.
