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).
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.
Tapeworms (Taenia species)
The tapeworms are hermaphroditic and require
an intermediate host. The adult tapeworms found in humans have flat body, white
or grayish in color. They consist of an anterior attachment organ or scolex
(suckers or grooves and has rosetellum,
which has 1 or 2 rows of hooks situated on the center of the scolex) and a
chain of segments (proglottids) also called strobilla. 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.
Adult tapeworms inhabit the small intestine,
where they live attached to the mucosa. Tapeworms do not have a digestive
system. Their food is absorbed from the host’s intestine.
Taenia species
v Taenia solium (pork tapeworm)
v Taenia saginata (beef tapeworm)
v Hymonolepis nana (dwarf tapeworm)
v Echinococcus granulosis (Dog tapeworm)
v Taenia saginata and Taenia solium
Genus Taenia has at least 32
recognized species, of which Taenia saginata
and Taenia solium are the two species
of medical importance, which cause infections in man. T.
saginata has a cosmopolitan 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. A new third species, Taenia asiatica (also known
as Asian Taenia) has been identified to cause human infection in China.
Taenia saginata
•
Taenia saginata is known as the beef tape worm.
•
It is
the commonest large tapeworm of human causing intestinal taeniasis.
•
The
infection is acquired orally by ingesting beef infected with the larvae
(Cysticercus bovis) of the parasite.
The disease is relatively common in Africa, some parts of Eastern
Europe, the Philippines, and Latin America. This parasite is found anywhere where beef
is eaten, even in countries such as the United States, with strict federal
sanitation policies. In the US, the incidence of infection is low, but 25% of
infected cattle are still sold. The total global infection is estimated to be
between 40 and 60 billion. It is most prevalent in Sub-Saharan Africa and Middle East.
Habitat and Morphology
Adult
It is a white, ribbon like, flattened and segmented worm
measuring 4 meter to 10 meter, even 15 meter in length. An adult worm consists
of a head (scolex), a
neck and body (strobila), consisting a chain of segments.
The scolex is quadrate in shape and measures less than 2 mm in diameter. It
bears 4 cup shaped muscular suckers, which may be pigmented. A short and
fragile neck measuring 3 mm to 7mm in length follows the scolex. The gravid
segments are rectangular, measure 20 mm by 5mm to 7mm and are present in the
posterior part of the worm. Typically these gravid segments break off from the
strobila and are passed in the faeces singly and often simultaneously. In the gravid proglottid, the uterus contains up to 15 side branches
filled with eggs.
Egg
The eggs are round or oval and measure 33micrometer-43micrometer in
diameter. They are bile stained.
Fig: Eggs of Taenia saginata
Life cycle
The life cycle of Taenia saginata
The life
cycle of T.
saginata is indirect and digenetic, involving cattle and humans with an
interim of living in the environment. Humans as the definitive host harbour adult worms which release
infective eggs into the environment. Cattle as the intermediate host pick up the viable eggs from contaminated
vegetation.
Intermediate host
Cattle acquire the embryonated eggs called the
oncospheres when they eat contaminated food. Oncospheres enter duodenum, the anterior portion of small intestine.
The oncospheres hatch in the duodenum under the influence of gastric juices. The embryonic membranes are removed,
liberating free hexacanth ("six hooked") larvae. With
their hooks they get attached to the intestinal wall and penetrate the intestinal mucosa into the blood vessels. The larvae
can move to all parts of the body via general circulatory
system, and finally
settle in skeletal
muscles within 70
days. Inside the tissue they cast off their hooks and instead develop a
protective cuticular shell, called cyst. Thus it becomes a fluid-filled cysticercus. Cystercus can also form in
lungs and liver. The inner membrane of the cysticercus soon develops numerous
protoscolices (small scolices) that are invertedly attached to the inner
surface. The cysticercus of T. saginata is specifically named cysticercus
bovis to differentiate from that of T, solium, cysticercus
cellulosae.
Definitive host
Humans contract infective cysticercus by eating raw or
undercooked meat. Once reaching the jejunum the inverted scolex becomes
evaginated to the exterior under stimuli from the digestive enzymes of the host. Using the scolex it gets
attached on the intestinal wall. It takes about 5 to 12 weeks for the larva to
mature into adulthood. Adult worm can live to about 25 years in the host.
Usually only a single worm is present at time. However, multiple worms are also
reported. In each mature proglottid self-fertilisation produces zygotes, which divide and differentiate into
embryonated eggs called oncospheres. With thousands of oncospheres, the oldest
gravid proglottid detach. Unlike in other Taenia gravid proglottids are
shed individually. In some cases the proglottid ruptures inside the intestine,
and the eggs are released. The free proglottids and liberated eggs are removed
by peristalsis into the environment. On the ground
the proglottids are motile and will shed eggs as they move. These oncospheres
in external environment can remain viable for several days to weeks in sewage, rivers, and pastures.
Pathogenesis
Humans develop a tapeworm infection by
eating raw or undercooked beef. The cysticercus becomes activated, attaches to
the wall of the small intestine by the scolex, and becomes a mature tapeworm.
This maturation process takes 10-12 weeks after that adult tapeworms live in
the human small intestine causing intestinal disturbances and intestinal obstruction. Humans pass gravid eggs in
feces; a single tapeworm produces an average of 50,000 eggs per day and may
live 25 years. There
is no known secretory products as its virulence determinants. Survival in the
intestine is the key to virulence. It survives on host’s nutrition. As it has no
digestive system, nutrition is host’s nutrition . Another aspect of virulence
is the long survival period, that creates persistent and unresolved intestinal
motility disorder of the host sufficiently linked to develop psychological outburst. Segments
creeping out of anus cause: irritation, itching & anexiety and may cause
appendicitis or cholangitis. These
mature eggs contaminate pastures and barnyards, where cattle and pigs ingest them.
Upon reaching the alimentary canal of infected animals, the embryos are
released, penetrate the gut wall, and enter the circulation.
.
Clinical Symptoms
T. saginata infection is usually asymptomatic. However heavy infection often
results in weight loss, dizziness, abdominal pain, diarrhoea, headaches, nausea, constipation, or chronic indigestion, and loss of appetite. There can be intestinal obstruction
in humans and this can be alleviated by surgery. The tapeworm can also expel antigens that can cause an allergic reaction
in the individual. It is an also rare cause of ileus, pancreatitis, cholecystitis and cholangitis.
Laboratory Diagnosis
Microscopy
(Detection of eggs in faeces )
The basic diagnosis is done from a stool sample. Feces
are examined microscopically to find
parasite eggs. The eggs look like other eggs from the family Taeniidae, so it is only possible to identify the eggs to the
family, not to the species level. Since it is difficult to diagnose using eggs
alone, looking at the scolex or the gravid proglottids can help identify it as Taenia
saginata. Differentiation of the species from other species of Taenia, such as T. solium and T. asiatica, is
notoriously difficult because of their close morphological resemblance, and their eggs are more or less
identical. Detection of eggs can be
done in perianal region by a swab.
Proglottids
identification (Segments may
creep out of anus)
Proglottids
sometimes trickle down the thighs of infected humans and are visible with
unaided eye, so can aid with identification. Observation of scolex help
distinguish between T. saginata, T. solium and T. asiatica.
When
the uterus is injected with India ink, its branches become visible.
Counting the uterine branches enables some identification (Taenia saginata
uteri have 12 or more branches on each side, while other species such as Taenia
solium only have five to 10).
Antibody
detection
May
prove useful especially in the early invasive stages, when the eggs and
proglottids are not yet apparent in the stools.
Other
methods
PCR detection of ribosomal 5.8S
gene. T.
saginata’s uterus stems out from its center to form 12
to 20 branches, but in contrast to its closely related Taenia species,
the branches are much less in number and comparatively thicker; in addition,
the ovaries are bilobed and testes are twice as many.
Eosinophilia and elevated IgE levels are chief hematological findings. Also Ziehl–Neelsen
stain can be used to
differentiate between mature Taenia saginata and Taenia soium, in
most cases Taenia saginata will stain while Taenia solium will
not, however the method is not strict.
Treatment
Taenaisis
is easily treated with praziquantel (5–10 mg/kg,
single-administration) or niclosamide (adults and children over 6 years: 2
g, single-administration after a light breakfast, followed after 2 hours by a
laxative; children aged 2–6 years: 1 g; children under 2 years: 500 mg).
Albendazole is also highly effective for
treatment of cattle infection.
Prevention
- Adequate cooking (56°C for 5 minutes)
of beef viscera destroys cysticerci.
- Refrigeration, freezing (-10°C for 9
days) or long period salting is lethal to cysticerci.
- Inspection of beef and proper disposal
of human excreta are also important measures.
Taenia solium
•
Taenia solium is the pork tape worm, which causes
intestinal taeniasis, an infection similar to that caused by T. saginata.
•
It is
an intestinal zoonotic parasite found throughout the world, and is most
prevalent in countries where pork is eaten.
•
In
addition to this, the larva (Cysticercus cellulosae) of the worm also causes a
serious disease in man known as cysticercosis.
•
T. solium is the only cestode for which man acts as both the
definitive host (harbouring the adult worm) and the intermediate host (harbouring
the larva of the parasite).
•
The
tape worm causes cysticercosis,especially neurocysticercosis, a serious disease
in humans.
Taenia solium
is found worldwide. Because pigs are intermediate hosts of the parasite,
completion of the life cycle occurs in regions where humans live in close
contact with pigs and eat undercooked pork. Taeniasis and cysticercosis is most common in Latin America, Southeast
Asia and Africa. It is particularly prevalent in rural areas where
domestic pigs are allowed to roam freely. It is diminishing in eastern and southern Europe, and is very rare in Muslim
countries.
Morphology
The
characterstics of eggs are Spherical and brown in colour, measures 31-43μm in
diameter. The thin, outer transparent shell, when present, represents the remnant
of the yolk mass; it causes the eggs to clump together.The inner embryophore is
brownthick walled and radially striated. It contains an oncosphere(14 to20μm in
diameter) with 3 pairs of
hooklets.
•
The
eggs of T. solium and T. saginata are morphologically similar.
Adult
Adult tapeworms develop, (up to 2
to 7 m in length and produce less than 1000 proglottids, each with
approximately 50,000 eggs) and reside in the small intestine for years as
much as 25 years
. The adult worm has a scolex, neck and
strobila consisting of segments. Scolex is round,measures 1mm in diameter, has
four suckers and is armed with a conspicuous rostellum. The neck is short,
about 5mm to 10mm in length. The strobila or body consists of 800-1000 segments
or proglottids which are i) immature ii) mature and iii) gravid. The immature
segments are broader than long. The mature segments are wider than long. The
gravid proglottids are longer than broad (12mm by 6mm). The gravid segments
look grayish-black and transparent when fully developed.
Life cycle
T. solium is a digenetic helminth and its life cycle is indirect. It passes
through pigs, as intermediate hosts, into humans, as definive hosts. From
humans the eggs are released in the environment where they ingestion by another
host. Humans as the definitive hosts are directly infected from contaminated
meat.
Humans are infected by the larval stage called
cysticercus (cysticercus cellulosae) from a measly pork. A cysticercus
is oval-shaped containing inverted scolex (specifically
"protoscolex"), which pops out externally once inside the small
intestine. This process of evagination is stimulated by bile juice and digestive enzymes of the host. Using the scolex it
anchors to the intestinal wall. It grows in size using nutrients from the
surrounding. Its strobila lengthens as new proglottids are formed at the neck.
In 10–12 weeks after initial infection, it becomes adult worm. As hermaphrodite
it reproduces by self-fertilisation, or cross-fertilisation. A single gravid proglottid can
contain more than 50,000 embryonated eggs. Gravid proglottids often rupture in
the intestine liberating the eggs in faeces. The intact gravid proglottids are
shed off in groups of 4 or 5. The free eggs and detached proglottids are
released into the environment through peristalsis. Eggs can survive in the environment
for up to two months.
Intermediate host
Pigs ingest the eggs from human faeces or vegetation
contaminated with human excreta. The embryonated eggs enter the intestine where
they hatch into motile oncospheres. The
embryonic and basement membranes are removed by the host's digestive enzymes
(particularly pepsin). Then the free oncospheres get
attached on the intestinal wall using their hooks. With the help of digestive
enzymes from the penetration glands, they penetrate the intestinal mucosa to enter blood and lymphatic vessels. They move along the general circulatory
system to various
organs, and large number are cleared in the liver. The surviving oncospheres preferentially migrate to striated muscles, as well as the brain, liver, and other tissues, where they settle to form cysts called cysticerci. A single cysticercus is spherical measuring
1–2 cm in diameter and contains invaginated protoscolex. The central space
is filled with fluid like a bladder, and hence it is also called bladder
worm. Cysticerci are usually formed within 70 days and may continue to grow for
a year.
Humans are also accidental primary hosts when they are
infected by embryonated eggs, either by autoinfection or ingestion of
contaminated food. As in pigs, the oncospheres hatch, enter blood circulation,
and have predilection for brain tissue and other soft muscle tissues. When they
settle to form cysts, clinical symptoms of cysticercosis appears. The cysticercus is often
called metacestode. If they localize in the brain, serious neurocysticercosis follows.
Pathogenesis
Ingestion of T.
solium eggs or proglottid rupture within the host intestine can
cause larvae to migrate into host tissue to cause
cysticercosis. The severity of cysticercosis depends on location, size and
number of parasite larvae in tissues, as well as the host immune response. Viable cysticerci (larval
form) in most organs cause minimal or no tissue reaction, but death of the
cysts in the CNS can elicit an intense tissue response. Infection in the brain
(cerebral cysticercosis) may result in severe symptoms due to mass effect and
inflammation induced by degeneration of cysticerci and release of antigens.
Any increase in intracranial pressure
will result in a corresponding increase in arterial
blood pressure, as
the body seeks to maintain circulation to the brain. Due to this, dementia or hypertension can occur due to perturbation of the
normal circulation of cerebrospinal
fluid.
The neurocysticercosis may
present with seizures, signs of increased intracranial pressure, hydrocephalus,
focal neurologic signs, altered mental status, or aseptic meningitis. Cysticerci
may also infect the spinal cord
Clinical manifestation (taeniasis)
Most individuals with taeniasis are either asymptomatic
or have mild-to-moderate complaints. Abdominal pain and nausea are reportedly
more common in the morning and are characteristically relieved by eating small
amounts of food. Children are more symptomatic than adults and often manifest
change of appetite, both increased and decreased. Symptoms in infants are more
pronounced and consist of vomiting, diarrhea, fever, weight loss, and
irritability.
The most common serious complication of adult tapeworm
infection is appendicitis. Other reported complications include
obstruction of bile ducts, pancreatic duct and tapeworm growth in ectopic
locations (eg, middle ear, adenoid tissue, uterine cavity). A mild eosinophilia
of 5-15% may occur in 5-45% of patients; higher levels are rare.
Cysticercosis and neurocysticercosis (NCC)
In cysticercosis, the cysticerci are most often located
in subcutaneous and intermuscular tissues, followed by the eye and then the
brain. The CNS is involved in 60-90% of patients (ie, NCC). Most patients have
more than one cyst; as many as 200 cysts have been reported.
NCC symptoms include 3 characteristic syndromes: convulsions
and/or seizures, intracranial hypertension, and psychiatric disturbances, which
may occur separately or simultaneously.Onset can be insidious (eg, elevated
intracranial pressure) or abrupt (eg, floating cysticerci suddenly block
cerebrospinal fluid [CSF]).
Other possible infections
- Ocular
cysticercosis: The subretinal space, vitreous, and conjunctiva are the
most frequent sites of infection. Common manifestations of infection
include severe pain and blurred or lost vision.
- Muscular and
dermatologic cysticercosis: Cysticerci in muscles often are associated
with NCC. Any muscle mass may be involved and appear as acute myositis.
However, most patients are asymptomatic. Subcutaneous nodules may be
evident.
- Coenurosis: Patients
with cerebral coenurosis can present with seizure or intracranial
hypertension. Ocular coenurosis manifests as a red and painful.
Laboratory
Diagnosis
Microscopy Examination
Microscopic identification of eggs and proglottids in
feces is diagnostic for taeniasis, but is not possible during the first 3
months following infection, prior to development of adult tapeworms. Repeated
examination and concentration techniques will increase the likelihood of
detecting light infections.
Nevertheless,
identification of Taenia is not possible if solely based on
microscopic examination of eggs, because all Taenia species produce
eggs that are morphologically identical. Eggs of Taenia spp. are also
indistinguishable from those produced by cestodes of the genus Echinococcus
(tapeworms of dogs and other canid hosts).
Microscopic identification of gravid proglottids (or,
more rarely, examination of the scolex) allows species determination.
Figure : Mature proglottid of T. saginata, stained with India
ink. Note the number of primary uterine branches (>12)
Separation
of T. saginata and T. solium is best accomplished by
examination of mature proglottids. Taenia saginata has 12-30
primary lateral uterine branches, while T. solium has 7-13 primary
lateral uterine branches. Visualization of the branches can be improved by
clearing the specimen in lactophenol followed by India ink injection into the
lateral genital pore.
Antibody
detection
May
prove useful especially in the early invasive stages, when the eggs and
proglottids are not yet apparent in the stools.
Other methods
- Biopsy from suspected nodule .
- Ziehl- Neelsen
stain differentiates between eggs of T. solium & T. saginata.
-
Gravid segments should be inspected.
-
Cysticercosis, is diagnosed radiologically.
- CT
and/or MRI testing for patients with CNS symptoms
Treatment
Praziquantel is
the medication most often used to treat active taeniasis, given at 5-10 mg/kg
orally once for adults and 5-10 mg/kg orally once for children. Praziquantel is
cysticidal and can cause inflammation around dying cysts in those with
cysticercosis, which may lead to seizures or other symptoms.
Niclosamide
is an alternative, given at 2 g orally once for adults and 50 mg/kg orally once
for children. After treatment, stools should be collected for 3 days to search
for tapeworm proglottids for species identification
Prevention and
control
The best way to avoid getting tapeworms is to not eat
undercooked pork. Moreover, a high level of sanitation and prevention of faecal
contamination of pig foods also plays a major role in prevention. Infection can
be prevented with proper disposal of human faeces around pigs, cooking meat
thoroughly and/or freezing the meat at −10°C for 5 days.
For human cysticercosis, dirty hands are attributed to be
the primary cause, and especially common among food handlers.Therefore,
personal hygiene such as washing one's hands before eating is an effective
measure.
Taenia asiatica
v Taenia
asiatica, commonly
known as Asian taenia or Asian tapeworm, is a parasitic tapeworm of humans and pigs.
v It is one of the three species of Taenia infecting humans and causes taeniasis.
v T. asiatica causes intestinal taenisis in humans
and cysticercosis in pigs. There is a suspicion that it
may also cause cysticercosis in human.
v Like other taenids, humans are the definitive hosts, but in contrast, pigs, wild boars, as well as cattle can serve as intermediate hosts.
T.
asiatica is
generally neglected in the global elimination context of human taeniasis
cysticercosis. T. asiatica remains ignored mainly for 2 reasons; first, its
supposedly non-cosmopolitan character, being restricted to Asian countries, and
second, its close molecular similarities to T.
saginata suggesting that T. asiatica
probably does not cause human cysticercosis since T. saginata eggs do not infect
humans.
Based on the appearance of eggs and
proglottids, the diagnosis was first believed to be Taenia saginata, the beef
tapeworm. However, on further investigation including patient history,the worm
probably is T. asiatica (also known
as the Asian Taenia species). Discovered only in 1980s from Taiwan and other East Asian countries as an unusual species. The rate of
a prevalence is estimated to be up to 21% in Asian taeniasis which is
documented in nine countries in Asia,
including Taiwan, South Korea, Indonesia, the Philippines, Thailand, south-central China, Vietnam, Japan and Nepal.
Morphological structures
Adult
The body of Taenia asiatica is yellowish-white in
colour, about 350 cm long and 1 cm broad, divided into the anterior scolex, followed by a short neck and a highly extended body
proper called strobila. It is an acoelomate animal with no body cavity or digestive system. The scolex bears four simple suckers as
attachment organs to the intestinal wall of the host. The strobila is composed
of a chain of ribbon-like segments called proglottids. There are more than 700 proglottids in the
strobila, but less than 1000 (~900), while T. saginata in comparison
have more than 1000 proglottids. The proglottids are distinguishable into
mature and gravid proglottids. The gravid proglottids are full of fertilised
eggs.
Egg
Eggs of Taenia asiatica
is similar to the egg of Taenia solium. The eggs are round or oval and measure
33micrometer-43micrometer in diameter. They are bile stained.
Fig: Egg of Taenia asiatica
Life cycle
The life
cycle of T.
asiatica is indirect and digenetic, and is completed in humans as the
definitive host, and the intermediate host is mostly pigs (including wild boar
in Taiwan), and possibly cattle on rare occasion. The complete life cycle is
shortest among human taenids. Humans ingest the infective larvae called
cysticercus from raw or undercooked meat, or viscera of pigs. The adult worm inhabits
the small
intestine where it
gets attached to the mucosa using its suckers and rostellar hooklets. Upon
sexual maturity it undergoes self-fertilisation. Fertilized eggs are released through
the faeces along with the gravid proglottid which gets detached
from the strobila. The number of proglottids released per day may vary from 0
to 35. Cysticercus grows into adult in about 2.5 to 4 months, by the time
gravid proglottids are found in faeces.
T. solium
eggs infect only omnivorous intermediate hosts (pigs, humans, and dogs). In
nature,
T. asiatica eggs, just as
T.
solium, do not infect herbivorous hosts since they only hatch in
omnivorous intermediate hosts (pigs and wild boars).
Pigs and wild boars ingest the infective embryo while grazing. The digestive enzymes will break the thick shell of the egg
and allow formation of the zygotes called "oncospheres". These
oncospheres then penetrate the mucous layer
of the digestive
tract and enter the circulation of the host. This is where the young larval stages
form a pea-sized, fluid filled cyst, also known as “cysticercus”, which migrate to visceral organs like liver,
serosa and lungs in pigs, and liver in cattle. In contrast to T. saginata,
the larval development is short, taking about four weeks. Cystecerci have a
predilection for liver.
Pathogenesis
The adult worms live in
the lumen of the intestine and may
cause demage in it and migrant to duodenum. Due to which,
multiple erosions and active bleeding
from ulcers in the stomach and duodenum. They acquire
nutrients from the intestine, leading to malnutrition of the host.
T. asiatica rarely cause
neurocysticercosis in humans.
Clinical symptoms
T. asiatica causes
intestinal taenisis in humans and cysticercosis
in pigs. In pigs cysticercus has
a tendency to produce in vital organs such as liver and lungs. In intestinal
taenisis, infection may lead to cause ulcer with bleeding.
In neurocysticercosis symptoms include severe
headache, blindness, convulsions, and epileptic seizures, and can be fatal.
Neurocysticercosis is the most frequent preventable cause of epilepsy
worldwide.
Laboratory Diagnosis
Microscopy Examination
The basic diagnosis is examination of a stool sample to find the parasite eggs. However,
there is a serious limitation as to the identification of the species because
the eggs of all human taenids look the same. Even with the
proglottids it is extremely difficult to identify T. asiatica from other
taenids because of their striking resemblances. The species and T. saginata
are frequently confused due to their morphological similarities and sympatric distribution.
serology
To date the most relevant diagnosis of taeniasis due to T.
asiatica is by enzyme-linked immunoelectrotransfer blot (EITB). EITB can
effectively identify it from other taenid infections.
others
Identification
often requires histological observation of the uterine branches and PCR detection
of ribosomal 5.8S gene. The presence of rostellum on the
scolex, a large number of uretine branches (more than 57) and prominent
posterior protuberances in gravid proglottids, and wart-like formation on the
surface of the larvae are the distinguishing structures.
Treatment
Niclosamide (2 mg) is very effective against
experimental infection in human. In general cestode infections are treated with praziquantel and albendazole. Atrabine is quite effective but indicates adverse effects in humans. The commonly used drugs
for tapeworms, benzimidazoles are relatively ineffective.
Praziquantel at a single dose of 150 mg is the most effective medication
against T. asiatica without causing side effects.
Differences between taenia species T. saginata, T. asiatica, T. solium
Hymenolepis nana
Dwarf tapeworm (Hymenolepis
nana, previously known as Vampirolepis nana, Hymenolepis fraterna,
and Taenia nana) is a cosmopolitan species though most common in
temperate zones, and is one of the most common cestodes of humans, especially
children.
Hymenolepis
nana has been called
the dwarf tapeworm and has a worldwide distribution. The fact that an
intermediate host is not required in the life cycle was determined in the late
1800s. In 1921, Saeki demonstrated direct
cycle of transmission of H. nana in
humans; transmission without an intermediate host. H. nana is the most common cestode in
humans with infection prevalence highest
among children encountered worldwide and
in warm arid climates with poor sanitation facilities. Although
the parasite has a wide distribution, it is more prevalent in warm areas. It
exists in Egypt, Sudan Portugal, Spain, Sicily, India, Japan, South America,
Cuba and parts of Eastern Europe. The
infection is most commonly seen in children, although adults are also infected.
Morphology
Adult
The entire worm is small, measuring up to
40mm in length by a maximum of 1mm in diameter. The size of the strobila is
usually inversely proportional to the number of worms present in the host. The
minute scolex (0.32mm in diameter) is rhomboidal, has four suckers (80μm in cross section) and a short rostellum,
armed with 20-30 spines in one ring, capable of invagination into the apex of
the organ. Beginning with short, narrow, immature proglottids, those more
distal in position become increasingly broader.
Eggs of Hymenolepis nana which measure 30 to 47 µm
in diameter. The oncosphere is covered with a thin, hyaline, outer membrane and
an inner, thick membrane with polar thickenings that bear several filaments. The
rostellum remains invaginated in the apex of the organ.are immediately infective when passed with
the stool and cannot survive more than 10 days in the external
environment .
LifeCycle
Infection is usually acquired by the ingestion of H.
nana eggs, primarily from human stool. The eggs hatch in the stomach or
small intestine, and the liberated larvae, or oncospheres, penetrate the villi
in the upper small intestine. The larvae develop into the cysticercoid stage in
the tissue and migrate back into the lumen of the small intestine, where they
attach to the mucosa. The adult worms mature within several weeks. They are
very small compared with eggs of the Taenia species and measure up to
40 mm long. The more worms present, the shorter the total length of each
worm. Under heavy infections, autoinfection may occur when
some of the eggs regurgitated into stomach hatch in the upper small intestine. H. nana can also develop in beetles,
fleas or other insects to the cysticercoid stage. If the host accidentally
ingests any of these intermediate hosts, the liberated cysticercoid larva
attaches to the intestine and develops into an adult worm, this
mode of infection is probably not common.
Fig: Life cycle of H. nana
Pathogenesis
Hymenolepis nana lodges itself in the intestines and absorbs nutrients
from the intestinal lumen. In human adults, the tapeworm is more of a nuisance
than a health problem, but in small children, many H. nana can be
dangerous. Usually it is the larva of this tapeworm that causes the most
problem in children. The larva will burrow into the walls of the intestine, if
there are enough tapeworms in the child, severe damage can be inflicted. This
is done by absorbing all the nutrition from the food the child eats. Usually a single
tapeworm will not cause health issues.
Hymenolepis nana usually will not cause deaths unless in extreme
circumstances and usually in young children or in people who have weakened
immune systems. In some parts of the world, individuals that are heavily
infected are a result of internal autoinfection. H. nana's larval stage
occurs either inside an auto infected host's intestinal villus or an
intermediate rat host
Clinical manifestations
Symptoms in humans are due to allergic responses or
systematic toxaemia caused by waste products of the
tapeworm. Light infections are usually symptomless, whereas infection with more
than 2000 worms can cause enteritis, abdominal pain, diarrhea, loss of appetite, restlessness,
irritability, restless sleep, anal and nasal pruritus. Rare symptoms include
anorexia, increased appetite, vomiting, nausea, bloody diarrhea, hives,
extremity pain, headache, dizziness and behavioral disturbances. Occasionally epileptic
seizures occur in
infected children.
An infection with H.
nana may cause no symptoms even with a heavy worm burden. Heavy
human infection can be attributed to internal autoinfection in which the eggs
hatch in the intestine and follow the normal life cycle to the adult worm. This
autoinfection feature of the life cycle can lead to complications in the
compromised patient. Heavy human infection can be attributed to internal
autoinfection in which the eggs hatch in the intestine and follow the normal life
cycle to the adult worm. This autoinfection feature of the life cycle can lead
to complications in the compromised patient.
Laboratory Diagnosis
Microscopy examination
Egg: The identification
of H. nana eggs in stool specimens. The eggs are most easily seen on a
direct wet smear or a wet preparation of the concentration sediment. H.nana
eggs are frequently spherical or ovoid with a thin hyaline shell and
measure 30-47 μm in diameter. The oncosphere with its 3 pairs of hooklets lies
in the center of the egg and is separated from the outer shell by sizeable
space.
Since the eggs of H. nana and
H. diminuta look very much alike, identification to the visual
identification of the polar filaments seen in H. nana eggs.
Adult worms:
The adult worm is rarely seen in the stool. The scolex has four suckers and a
short rostellum with hooks. The worms are very small compared with worms of the
Taenia species and measure up to 40 mm long. The more worms present,
the shorter the total length of each worm. The adult worm is rarely seen
in the stool.
Fig: Egg and adult worm of Hymenolepis nana
Fig: Egg and adult worm of Hymenolepis nana
Serology
Different
serology tests like Enzyme-linked immunosorbent assay (ELISA), immunodiffusion
(ID), immunoelectrophoresis (IEP) (10,11), double diffusion (DD),
immunoprecipitation (IP) and indirect immunofluorescent antibody test
(IFAT) are used in the diagnosis of Hymenolepis spp. infections. This test is
used to differetiate the species
of Hymenolepis tapeworm.
Treatment
The 2 drugs that have been described for the treatment of
hymenolepiasis are praziquantel and niclosamide. Praziquantel, which is parasiticidal
in a single dose for all the stages of the parasite, is the drug of choice
because it acts very rapidly against H. nana. Although structurally
unrelated to other anthelminthics, it kills both adult worms and larvae. In
vitro the drug produces vacuolization and disruption of the tegument in the
neck of the worms, but not in more posterior portions of the strobila.
Praziquantel is well absorbed while taken orally, it undergoes first-pass
metabolism and 80% of the dose is excreted as metabolites in urine within 24
hours.
Prevention
v Preventing fecal contamination of food
and water in institutions and crowded areas is of primary importance.
v General sanitation and rodent and
insect control (especially control of fleas and grain insects) are also
essential for prevention of H. nana infection.
v Repeated treatment is required for Hymenolepis nana at an interval of
7-10days.
Hymenolepis diminuta is a small
tapeworm commonly found in rats and mice. Hymenolepis diminuta, also known as rat tapeworm, is a species of Hymenolepis tapeworm that causes hymenolepiasis. H. diminuta is prevalent
worldwide, but only a few hundred human cases have been reported. Few cases have
ever been reported in Australia, United States, Spain, and Italy. In countries
such as Malaysia, Thailand, Jamaica, Indonesia, the prevalence is higher. It has a world wide distribution in these
hosts but is infrequently found in humans, with only sporadic cases being
reported.
Morphology
It has slightly bigger eggs and
proglottids than H. nana and infects mammals using insects as intermediate hosts. The adult structure is 20 to
60 cm long and the mature proglottid is similar to that of H. nana,
except it is larger. The ova are large, ovoid and yellowish with a moderately
thick shell. They contain an oncosphere with six hooklets and a clear
area between the oncosphere and the shell. They measure 70-85µm by 60-80µm.
Life
cycle and Transmission
The life cycle of H. diminuta requires an
intermediate arthropod host e.g. earwigs, larval fleas and various
beetles. Human infection occurs by the accidental ingestion of an
infected arthropod, which contains the cysticercoids.
The cycle begins as arthropods
ingest the eggs. Arthropods are then able to act as the intermediate host. When
ingested, the eggs develop into cysticercoids. Rodents can become infected when
they eat arthropods. Humans, especially children, can ingest the arthropods as
well and therefore become infected via the same mechanism. As the definitive
host (rats) eats an infected arthropod, cysticercoids present in the body
cavity transform into the adult worm. The resulting eggs are then passed through
the stool.
Clinical manifestation
Human infection results from
eating such foods as dried fruits and
precooked breakfast cereals in which the infected grain insects larvae are
present. Eating an intermediate host of H. diminuta. H. diminuta is often asymptomatic. However, abdominal pain, irritability,
itching, and eosinophilia are among the existing symptoms in a few of the
reported cases.The symptoms
associated with H. diminuta infections are few if any.
Laboratory
Diagnosis
Diagnosis is based on
recovery and identification of the characteristic ova in a formol-ether
concentrate of feces. Adult worms and proglottids are rarely seen in
stool samples.
Treatment
The treatment for this condition is
a single dose of praziquantel, repeated in 10 days.
Prevention
Good hygiene, public health and
sanitation programs, and elimination of rats help prevent the spread of
hymenolepiasis.
Echinococcus granulosus
· Echinococcus granulosus, also called the Hydatid worm
or Hyper Tape-worm or Dog Tapeworm.
· it causes cystic echinococcosis, also known as hydatid disease.
·
Hydatid
disease’ and
‘hydatidosis’ are terms that should be restricted to infection with the
metacestode, and ‘echinococcosis’ to infection with the adult stage.
·
Infection
with Echinococcus may be naturally transmitted between humans and other
animals and therefore claims membership of the most significant group of
communicable diseases, the zoonoses. ‘
Cystic
echinococcosis (CE) is
caused by infection with the larval stage of Echinococcus granulosus.
CE is found in Africa, Europe, Asia, the Middle East, Central and South
America, and in rare cases, North America. The disease is most commonly found
in people involved in raising sheep, as a result of the sheep's role as an
intermediate host of the parasite and the presence of working dogs that are
allowed to eat the offal of infected sheep.
Morphology
The adult tapeworm ranges in length from 2 mm to 7 mm and has
three proglottids ("segments") when intact an immature
proglottid, mature proglottid and a gravid proglottid. Like all
cyclophyllideans, E. granulosus has four suckers on its scolex ("head"), and E. granulosus also has
a rostellum with hooks.
Characteristics of Hydatid cyst
Size: Measures about 22-25μm in thickness, gives rise to brood capsules with scolices, secretes hydatid fluid and forms the outer layer.
•
Outer
layer-ectocyst
•
Inner
germinal layer-endocyst
Characteristic
features of hydatid fluid
•
Clear
colorless fluid
•
Specific
gravity(1.005-1.010)
•
Slightly
acidic
•
Contains
sodium chloride, sodium sulfate, sodium phosphate and sodium and calcium salts
of succinic acid.
•
Antigenic
•
Highly
toxic
•
Hydatid
sand.
Life
cycle
E. granulosus requires two host types, a
definitive host and an intermediate host. The definitive host of this parasite
are dogs and the intermediate host are most commonly sheep, however, cattle,
horses, pigs, goats, and camels are also potential intermediate hosts. Humans
can also be an intermediate host for E. granulosus, however this is
uncommon and therefore humans are considered an aberrant intermediate host. The
parasite is transmitted to dogs when they ingest the organs of other animals
that contain hydatid cysts. The cysts develop into adult tapeworms in the dog.
The adult Echinococcus granulosus
(3 to 6 mm long) 
resides in the small bowel
of the definitive hosts, dogs or other canids. Gravid proglottids release eggs
that are passed in the feces. Infected dogs shed tapeworm eggs in their feces
which contaminate the ground. Sheep,
cattle, goats, and pigs ingest tapeworm eggs in the contaminated ground; once
ingested, the eggs hatch and develop into cysts in the internal organs. After
ingestion by a suitable intermediate host (under natural conditions: sheep,
goat, swine, cattle, horses, camel), the egg hatches in the small bowel and
releases an oncosphere that penetrates the intestinal wall and migrates through
the circulatory system into various organs, especially the liver and lungs. In
these organs, the oncosphere develops into a cyst that enlarges gradually,
producing protoscolices and daughter cysts that fill the cyst interior. The
definitive host becomes infected by ingesting the cyst-containing organs of the
infected intermediate host. After ingestion, the protoscolices evaginate,
attach to the intestinal mucosa, and develop into adult stages in 32 to 80 days.
The most common mode of transmission to humans is by the accidental consumption
of soil, water, or food that has been contaminated by the fecal matter of an
infected dog. Echinococcus eggs that have been deposited in
soil can stay viable for up to a year.
Pathogenesis
The damage produced is mainly mechanical. The young cyst that develops from embryos lodged in vital centers
may soon interfere with functions of the organ with damaging, even fatal
results. Benign cyst may be asymptomatic
or it may produce physical burden to the patient. The severity depends on the type of tumor and organ or tissue
where it first becomes implanted. Anaphylactic
reactions develop.
Clinical manifestations
Many hydatid
cysts remain asymptomatic, even into advanced age. The parasite load, the site,
and the size of the cysts determine the degree of symptoms. Theoretically,
echinococcosis can involve any organ. The liver is the most common organ
involved, followed by the lungs. These 2 organs account for 90% of cases of
echinococcosis. Organs affected by E
granulosus are the liver (63%), lungs (25%),
muscles (5%), bones (3%), kidneys (2%), brain (1%), and spleen (1%).
- In the liver, the pressure effect of the
cyst can produce symptoms of obstructive jaundice and abdominal pain. With
biliary rupture, the classic triad of biliary colic, jaundice, and
urticaria is observed. Passage of hydatid membranes in the emesis (hydatid
emesia) and passage of membranes in the stools (hydatid enterica) may
occur rarely.
- Involvement of the lungs produces chronic
cough, dyspnea, pleuritic chest pain, and hemoptysis. Expectoration of
cyst membranes and fluid is observed with intrabronchial rupture.
- Headache, dizziness, and a decreased level
of consciousness may signify cerebral involvement. Specific neurologic
deficits may occur depending on the location of the cyst in the brain.
Secondary
complications may occur as a result of infection of the cyst or leakage of the
cyst. Minor leaks lead to increased pain and a mild allergic reaction
characterized by flushing and urticaria. Major rupture leads to a full-blown
anaphylactic reaction, which is fatal if not treated promptly. A rupture into the biliary tree can lead to
obstruction by the daughter cysts, producing cholangitis. Rupture into the bronchi can
lead to expectoration of cyst fluid.
Laboratory diagnosis
Clinically, Slowly growing cystic tumor with
history of close contact to dogs which can be indentified by
X-ray imaging: (esp. in the
lungs, & calcified cysts)
-
Rounded solitary, or multiple, sharply contoured cysts of 1-15 cm. in diameter.
-
Internal daughter systs, give a car wheel appearance.
-
Thin crescent or ring shape calcifications.
Computarized tomography (CT) &
ultrasonography
-
Detects uncalcified cysts.
- special value in the follow up of treated
cases.
Finding hydatid material: After surgery, in sputum or urine.
Serological tests:
-
IHA, or better by ELISA
Molecular diagnosis:
-
DNA analysis or PCR.
Intradermal test of Casoni:
- May give false +ve
results in 18% of cases.
-
May give allergic reaction
High eosinophilia
Treatment
Humans should avoid handling fecal
matter of canines and avoid consuming infected animals and home slaughtering
animals. If a human, becomes infected, there are a variety of methods for
treatment. The most common treatment in the past years has been surgical
removal of the hydatid cysts . However in recent years, less invasive treatments
have been developed such as cyst puncture, aspiration of the liquids, the
injection of chemicals, and then re-aspiration.
Benzimidazole-based chemotherapy is also a new
treatment option for humans.
Prevention
— In order to prevent
transmission to dogs from intermediate hosts, dogs can be given anthelminthic
vaccinations. In the case of intermediate hosts, especially sheep, these
anthelminthic vaccinations do cause an antigenic response meaning the body produces
antibodi avinash response however it does not prevent infection in the host.
— Clean slaughter and high
surveillance of potential intermediate host during slaughter is key in
preventing the spread this cestode to its definitive host. It is vital to keep
dogs and potential intermediate host as separated as possible to avoid
perpetuating infection.
— According to mathematical
modeling, vaccination of intermediate hosts, coupled with dosing definitive
hosts with anthelminths is the most effect method for intervening with
infection rates.
