Understanding the Intricate World of Cestodes: Tapeworms and their Fascinating Life Cycles, Morphology, and Influence on Hosts

Understanding the Intricate World of Cestodes: Tapeworms and their Fascinating Life Cycles, Morphology, and Influence on

 

HostsINTRODUCTION

Platyhelminthes (platy means flat; helminth means worm) are

divided into two courses: Cestoda (tapeworms) and Trematoda

(flukes). The trematodes are described in Chapter 55.

Tapeworms include two most important components: a rounded head referred to as

a scolex and a flat physique consisting of a number of segments. Every

section is known as a proglottid. The scolex has specialised means

of attaching to the intestinal wall, specifically, suckers, hooks, or

sucking grooves. The worm grows by including new proglottids

from its germinal middle subsequent to the scolex. The oldest proglot-

tids on the distal finish are gravid and produce many eggs, which

are excreted within the feces and transmitted to numerous intermediate

hosts reminiscent of cattle, pigs, and fish.

People often purchase the an infection when undercooked

meat or fish containing the larvae is ingested. Nevertheless, in two

essential human ailments, cysticercosis and hydatid illness, it

is the eggs which might be ingested and the ensuing larvae trigger the

illness.

There are 4 medically essential cestodes: Taenia solium,

Taenia saginata, Diphyllobothrium latum, and Echinococcus

granulosus. Their options are summarized in Desk 54–1, and

the medically essential phases within the life cycle of those organ-

isms are described in Desk 54–2. Three cestodes of lesser

significance, Echinococcus multilocularis, Hymenolepis nana, and

Dipylidium caninum, are described on the finish of this chapter.

TAENIA

There are two essential human pathogens within the genus Taenia:

T. solium (the pork tapeworm) and T. saginata (the meat tapeworm).

Taenia solium
Illness
The grownup type of T. solium causes taeniasis. Taenia solium lar-
vae trigger cysticercosis.
Vital Properties
The life cycle of T. solium is proven in Determine 54–1. Taenia solium
will be recognized by its scolex, which has 4 suckers and a
circle of hooks, and by its gravid proglottids, which have 5 to
10 major uterine branches (Figures 54–2A, B and 54–3). The
eggs seem the identical microscopically as these of T. saginata and
Echinococcus species (Determine 54–4A).
In taeniasis, the grownup tapeworm is situated within the human
gut (see Determine 54–1). This happens when people are
contaminated by consuming uncooked or undercooked pork containing the
larvae, referred to as cysticerci. (A cysticercus consists of a pea-sized
fluid-filled bladder with an invaginated scolex.) Within the small
gut, the larvae connect to the intestine wall and take about
3 months to develop into grownup worms measuring as much as 5 m. The
gravid terminal proglottids containing many eggs detach every day,
are handed within the feces, and are by accident eaten by pigs. Be aware
that pigs are contaminated by the worm eggs; due to this fact, it’s the lar-
vae (cysticerci) which might be discovered within the pig. A six-hooked embryo
(oncosphere) emerges from every egg within the pig’s gut. The
embryos burrow right into a blood vessel and are carried to skeletal
muscle. They turn into cysticerci within the muscle, the place they
stay till eaten by a human. People are the definitive hosts,
and pigs are the intermediate hosts.
In cysticercosis, a extra harmful sequence happens when a
individual ingests the worm eggs in meals or water that has been
contaminated with human feces (Determine 54–5). Be aware that in cys-
ticercosis, people are contaminated by eggs excreted in human feces,
not by ingesting undercooked pork. Additionally, pigs shouldn’t have the
grownup worm of their gut, so they aren’t the supply of the
eggs that trigger human cysticercosis. The eggs hatch within the small
gut, and the oncospheres burrow by the wall right into a
blood vessel. They will disseminate to many organs, particularly
the eyes, pores and skin, and mind, the place they encyst to type cysticerci
(Determine 54–6). Every cysticercus accommodates a larva.
Pathogenesis & Epidemiology
The grownup tapeworm connected to the intestinal wall causes little
harm. The cysticerci, however, can change into very
massive, particularly within the mind, the place they manifest as a space-
occupying lesion (see Determine 54–6). Dwelling cysticerci don’t
trigger irritation, however once they die, they will launch sub-
stances that provoke an inflammatory response. Finally, the
cysticerci calcify.
The epidemiology of taeniasis and cysticercosis is expounded
to the entry of pigs to human feces and to consumption of
uncooked or undercooked pork. The illness happens worldwide however is
endemic in areas of Asia, South America, and Jap Europe.
Most circumstances in the USA are imported.
Scientific Findings
Most sufferers with grownup tapeworms are asymptomatic, however
anorexia and diarrhea can happen. Some might discover proglottids in
the stools. Cysticercosis within the mind causes headache, vomiting,
and seizures. Cysticercosis within the eyes can seem as uveitis or
retinitis, or the larvae will be visualized floating within the vitreous.
Subcutaneous nodules containing cysticerci generally happen.
Cysts are also generally present in skeletal muscle.
Laboratory Prognosis
Identification of T. solium consists of discovering gravid proglot-
tids with 5 to 10 major uterine branches within the stools. In
distinction, T. saginata proglottids have 15 to twenty major uterine
branches. Eggs are discovered within the stools much less typically than are pro-
glottids. Prognosis of cysticercosis depends upon demonstrating
the presence of the cyst in tissue, often by surgical removing or
computed tomography (CT) scan. Serologic assessments (e.g., enzyme-
linked immunosorbent assay [ELISA]) that detect antibodies
to T. solium antigens can be found, however they could be unfavourable in
neurocysticercosis.
Remedy
The therapy of alternative for the intestinal worms is praziquantel.
The therapy for cysticercosis is both praziquantel or alben-
dazole, however surgical excision could also be essential.

Prevention

Prevention of taeniasis includes cooking pork adequately and

disposing waste correctly in order that pigs can’t ingest human feces.

Prevention of cysticercosis consists of therapy of sufferers to

stop autoinfection plus statement of correct hygiene, includ-

ing handwashing, to stop contamination of meals with the eggs.

2. Taenia saginata

Illness

Taenia saginata causes taeniasis. Taenia saginata larvae don’t

trigger cysticercosis.

Vital Properties

Taenia saginata has a scolex with 4 suckers however, in distinction

to T. solium, no hooklets. Its gravid proglottids have 15 to 25

major uterine branches, in distinction to T. solium proglottids,

which have 5 to 10 (see Determine 54–2C and D). The eggs are mor-

phologically indistinguishable from these of T. solium.

The life cycle of T. saginata is proven in Determine 54–1. People

are contaminated by consuming uncooked or undercooked beef containing

larvae (cysticerci). Within the small gut, the larvae connect

to the intestine wall and take about 3 months to develop into grownup

worms measuring as much as 10 m (Determine 54–7). The gravid pro-

glottids detach, are handed within the feces, and are eaten by cattle.

Pathogenesis & Epidemiology

Little harm outcomes from the presence of the grownup worm in

the small gut. The epidemiology of taeniasis attributable to

T. saginata is expounded to the entry of cattle to human feces and

to the consumption of uncooked or undercooked beef. The illness

happens worldwide however is endemic in Africa, South America, and

Jap Europe. In the USA, most circumstances are imported.

Scientific Findings

Most sufferers with grownup tapeworms are asymptomatic, however

malaise and gentle cramps can happen. In some, proglottids seem

within the stools and should even protrude from the anus. The proglot-

tids are motile and should trigger pruritus ani as they transfer on the

pores and skin adjoining to the anus.

Laboratory Prognosis

Identification of T. saginata consists of discovering gravid proglot-

tids with 15 to twenty uterine branches within the stools. Eggs are discovered

within the stools much less typically than are the proglottids.

Remedy

The therapy of alternative is praziquantel.

 

 

Prevention

Prevention includes cooking beef adequately and disposing

waste correctly in order that cattle can’t devour human feces.

DIPHYLLOBOTHRIUM

Illness

Diphyllobothrium latum, the fish tapeworm, causes

diphyllobothriasis.

Vital Properties

In distinction to the opposite cestodes, which have suckers, the scolex of

D. latum has two elongated sucking grooves by which the worm

attaches to the intestinal wall (see Determine 54–2E). The scolex has

no hooks, in contrast to T. solium and Echinococcus. The proglottids are

wider than they’re lengthy, and the gravid uterus is within the type of a

rosette. In contrast to different tapeworm eggs, that are spherical, D. latum

eggs are oval and have a lidlike opening (operculum) at one finish

(see Determine 54–4B). Diphyllobothrium latum is the longest of the

tapeworms, measuring as much as 13 m.

People are contaminated by ingesting uncooked or undercooked fish

containing larvae (referred to as plerocercoid or sparganum larvae). In

the small gut, the larvae connect to the intestine wall and develop

into grownup worms. Gravid proglottids launch fertilized eggs by

a genital pore, and the eggs are then handed within the stools. The

immature eggs have to be deposited in contemporary water for the life cycle to

proceed. The embryos emerge from the eggs and are eaten by tiny

copepod crustacea (first intermediate hosts). There, the embryos

differentiate and type procercoid larvae within the physique cavity. When

the copepod is eaten by freshwater fish (e.g., pike, trout, and

perch), the larvae differentiate into plerocercoids within the muscle of

the fish (second intermediate host). The cycle is accomplished when

uncooked or undercooked fish is eaten by people (definitive hosts).

Pathogenesis & Epidemiology

An infection by D. latum causes little harm within the small gut.

In some people, megaloblastic anemia happens in consequence

of vitamin B12 deficiency attributable to preferential uptake of the

vitamin by the worm.

The epidemiology of D. latum an infection is expounded to the

ingestion of uncooked or inadequately cooked fish and to contamina-

tion of our bodies of contemporary water with human feces. The illness is

discovered worldwide however is endemic in areas the place consuming uncooked fish

is the customized, reminiscent of Scandinavia, northern Russia, Japan,

Canada, and sure north-central states of the USA.

Scientific Findings

Most sufferers are asymptomatic, however belly discomfort and

diarrhea can happen.

Laboratory Prognosis

Prognosis depends upon discovering the everyday eggs (i.e., oval, yellow-

brown eggs with an operculum at one finish) within the stools. There

isn’t any serologic take a look at.

Remedy

The therapy of alternative is praziquantel.

Prevention

Prevention includes sufficient cooking of fish and correct

disposal of human feces.

ECHINOCOCCUS

Illness

Echinococcus granulosus (canine tapeworm) causes echinococcosis.

The larva of E. granulosus causes unilocular hydatid cyst illness.

Multilocular hydatid illness is attributable to E. multilocularis, which

is a minor pathogen and is mentioned later.

Vital Properties

Echinococcus granulosus consists of a scolex and solely three

proglottids, making it one of many smallest tapeworms (see

Figures 54–2F and G and Determine 54–8). The scolex has a circle of

hooks and 4 suckers just like T. solium. Canine are probably the most

essential definitive hosts. The intermediate hosts are often

sheep. People are virtually at all times dead-end intermediate hosts.

The life cycle of E. granulosus is proven in Determine 54–9. In

the everyday life cycle, worms within the canine’s gut liberate thou-

sands of eggs, that are ingested by sheep (or people) (see

Determine 54–4). The oncosphere embryos emerge within the small

gut and migrate primarily to the liver but in addition to the lungs,

bones, and mind. The embryos turn into massive fluid-filled

hydatid cysts, the interior germinal layer of which generates

many protoscoleces (larval type) (Determine 54–10) inside “brood

capsules.” The life cycle is accomplished when the entrails (e.g., liver

containing hydatid cysts) of slaughtered sheep are eaten by canines.

Pathogenesis & Epidemiology

Echinococcus granulosus often types one massive fluid-filled

cyst (unilocular) that accommodates 1000’s of particular person proto-

scoleces in addition to many daughter cysts throughout the massive cyst.

Particular person protoscoleces mendacity on the backside of the big cyst

are referred to as “hydatid sand.” The cyst acts as a space-occupying

lesion, placing strain on adjoining tissue. The outer layer of

the cyst is thick, fibrous tissue produced by the host. The cyst

fluid accommodates parasite antigens, which might sensitize the host.

Later, if the cyst ruptures spontaneously or throughout trauma

or surgical removing, life-threatening anaphylactic shock can

happen. Rupture of a cyst also can unfold protoscoleces extensively.

The illness is discovered primarily in shepherds residing within the

Mediterranean area, the Center East, and Australia. Within the

United States, the western states report the most important variety of circumstances.

Scientific Findings

Many people with hydatid cysts are asymptomatic, however

liver cysts might trigger hepatic dysfunction. Cysts within the lungs

can erode right into a bronchus, inflicting bloody sputum, and cerebral

cysts could cause headache and focal neurologic indicators. Rupture of

the cyst could cause deadly anaphylactic shock.

Laboratory Prognosis

Prognosis is predicated both on microscopic examination dem-

onstrating the presence of brood capsules containing a number of

protoscoleces or on serologic assessments (e.g., the oblique hemag-

glutination take a look at).

Remedy

Remedy includes albendazole with or with out surgical

removing of the cyst. Excessive care have to be exercised to stop

launch of the protoscoleces throughout surgical procedure. A protoscolicidal

agent (e.g., hypertonic saline) ought to be injected into the cyst to

kill the organisms and forestall unintended dissemination.

Prevention

Prevention of human illness includes not feeding the entrails of

slaughtered sheep to canines.

CESTODES OF MINOR IMPORTANCE

1. Echinococcus multilocularis

Lots of the options of this organism are the identical as these of

E. granulosus, however the definitive hosts are primarily foxes, and the

intermediate hosts are varied rodents. People are contaminated

by unintended ingestion of meals contaminated with fox feces.

The illness happens primarily in hunters and trappers and is

endemic in northern Europe, Siberia, and the western prov-

inces of Canada. In the USA, it happens in North and

South Dakota, Minnesota, and Alaska.

Throughout the human liver, the larvae type multiloculated cysts

with few protoscoleces. No outer fibrous capsule types, so the cysts

proceed to proliferate, producing a honey-comb impact of hun-

dreds of small vesicles. The scientific image often includes jaun-

cube and weight reduction. The prognosis is poor. Albendazole therapy

could also be profitable in some circumstances. Surgical removing could also be possible.

2. Hymenolepis nana

Hymenolepis nana (dwarf tapeworm) is probably the most ceaselessly

discovered tapeworm in the USA. It’s only 3 to five cm lengthy and

is completely different from different tapeworms as a result of its eggs are immediately

infectious for people (i.e., ingested eggs can turn into grownup

worms with out an intermediate host). Throughout the duodenum, the

eggs hatch and differentiate into cysticercoid larvae after which into

grownup worms. Gravid proglottids detach, disintegrate, and launch

fertilized eggs. The eggs both go within the stool or can reinfect the

small gut (autoinfection). In distinction to an infection by different

tapeworms, the place just one grownup worm is current, many H. nana

worms (generally lots of) are discovered.

An infection causes little harm, and most sufferers are

asymptomatic. The organism is discovered worldwide, generally

within the tropics. In the USA, it’s most prevalent within the

southeastern states, often in youngsters. Prognosis is predicated on

discovering eggs in stools. The attribute characteristic of H. nana

eggs is the 8 to 10 polar filaments mendacity between the membrane

of the six-hooked larva and the outer shell. The therapy is

praziquantel. Prevention consists of excellent private hygiene and

avoidance of fecal contamination of meals and water.

3. Dipylidium caninum

Dipylidium caninum is the commonest tapeworm of canines

and cats. It often infects people, often younger youngsters,

whereas enjoying with their pets. Human an infection happens when

canine or cat fleas carrying cysticerci are ingested. The cysticerci

turn into grownup tapeworms within the small gut. Most

human infections are asymptomatic, however diarrhea and pruritus

ani can happen. The analysis in animals and people is made by

observing the everyday “barrel-shaped” proglottids within the stool or

diapers. Niclosamide is the drug of alternative.