PARASITOLOGY

Cestodes

Hermea Nicole P. Ancuna

 Cestodes
• Classified under the subkingdom Metazoa, phylum Platyhelminthes.
Primitive worms, do not possess a digestive system and nervous system.
• They absorb nutrients and eliminate waste products through their outer
surface called tegument.
• a.k.a. tapeworms, flat and consists of the head, neck & body (proglottids)
• Head - contains an organ of attachment called scolex (hooks, suckers or
sucking grooves). In some species, the scolex has a fleshy extension
called rostellum to which hooks may be attached.
Body- divided into multiple segments called proglottids.
Strobila- a series of proglottids (plural strobili)
• All cestodes are hermaphroditic (self-fertilizing) with each proglottid
containing both male and female reproductive organs and capable
of laying eggs (pregnant proglottid or gravid segment).
Neck- region of growth and connects the head to the body of the worm. It
grows by adding new proglottids from neck. The oldest proglottids are found
in the most distal part of the body.
Life Cycle: egg, larva and adult worm
Egg contains embryo called the oncosphere which is the first larval or motile
stage. It is equipped with small hooks called hooklets that enable the
parasite to pierce the intestinal walls.
Eggs are excreted in the feces of infected hosts and transmitted to the
intermediate hosts (cattle, pig or fish).
Human infection occurs through ingestion of the undercooked or raw flesh of
the intermediate host containing the infective larvae.
After ingestion, the larvae will transform into adult worms in intestines of the
infected host.
The adult worm undergoes self-impregnation with gravid segment rapturing
to release the eggs in the intestines.
Defecation, eggs are passed out to the external environment.
 Intestinal Cestodes
Taenia saginata (Beef Tapeworm)
Properties and Life Cycle
• The intermediate host is the cattle, eggs enter the blood vessels within the
cattle’s intestines. The eggs are transported to the skeletal muscles where
they develop into cysticerci (larvae)
• Infection is acquired by ingestion of improperly cooked or raw beef
containing infective larva (cysticercus). Then matures into adult worm
(pathogenic stage) in the small intestines within three months. They can
grow in length as much as 10 meters. Humans are definitive hosts.
The eggs of Taenia saginata are indistinguishable from the eggs of pork
tapeworms Taenia solium.
The scolex of Taenia solium contains a rostellum while Taenia saginata
does not.
Taenia saginata proglottid is rectangular and contains more uterine
branches (15-30) while Taenia solium is square containing about 7-15
uterine branches.
Taenia saginata is common in areas of the world where beef is routinely
eaten especially undercooked beef.
Endemic in Eastern Europe, Russia, Eastern Africa and Latin America.
Adult worms do not produce significant damage in the small intestines.

Disease: Taeniasis
Majority are asymptomatic.
Those with high worm burden may complain diarrhea, abdominal pain, loss
of appetite with resultant weight loss and body malaise. The gravid prolottids
may reach the anus where egg-laying may occur resulting in itchiness in the
anal region (pruritus ani).
Laboratory Diagnosis
• Examination of fecal specimen from infected patients.
• Eggs or gravid proglottids may be recovered from the stool although eggs
are less often found than the proglottids.
Treatment
Adult worm is praziquantel
Prevention and Control
Proper waste disposal and sanitation practices as well as the adequate
cooking of beef. Freezing of beef meat for approximately 10 days may kill the
encysted larvae. Prompt treatment of infected persons help prevent the
spread of the disease.
Taenia solium (Pork tapeworm)
Infection is acquired through ingestion of improperly cooked or raw pork
meat which contain infective larva called cysticercus cellulosae.
Infection can also occur by ingestion of food or water contaminated with
human feces that contain the eggs of the parasite, unlike beef tapeworm,
Taenia solium has two infective stages: eggs and larvae.
Pigs serve as intermediate hosts while humans serve as both intermediate
and definitive hosts.
Humans serve as definitive host, when infection is acquired through ingestion
of undercooked or raw pork meat, the infective stage is the larval form which
transforms into adult worm in the intestines of infected individuals.
Humas serve as intermediate host, when ingested worm eggs hatch in the
small intestines, burrow through the intestinal wall into the blood vessel, and
disseminate to various organs.
Comparison of scolex and gravid segments of Taenia solium (Pork
tapeworm) and Taenia saginate (Beef tapeworm)
Characteristics Taenia saginate (Beef Taenia solium (Pork
tapeworm) tapeworm)

Scolex
Number of suckers Four Four
Rostellum Absent Present
Hooks Absent Present
Gravid Proglottid
Appearance , shape Rectangular Square
Number of uterine 15-30 7-15
branches on each side
of uterus
Taenia solium infection is more prevalent in underdeveloped communities
with poor sanitation and where people eat raw or undercooked pork.
Like in Latin America, Eastern Europe, sub-Sharan Africa, and Asia.
Adult worms produce little damage in the intestines. Encysted larvae
produce damage in the tissues where they disseminates.
In the brain, they manifest as space-occupying lesions and they evoke little
inflammatory response. When the encysted larvae die, they release
substances what induce an allergic reaction in the host and may be fatal
due to the development of anaphylactic shock.
Diseases:
1. Taeniasis- disease produced by adult worm. Mostly are asymptomatic
but in the presence of high worm burden, manifestations may be similar
to beef tapeworm infection.
2. Cysticercosis- result of larval encystation in various tissues of the body.
Involvement of the skeletal muscles where patients complain of muscle
pain. Neurocysticercosis which is Cysticercosis of the brain is the most
feared and most severe. Symptoms are seizures, headache and vomiting.
Laboratory diagnosis
• Microscopic examination of stool specimen.
• Demonstration of ova or proglottids, typical morphology of the scolex can
differentiate pork tapeworm from beef tapeworm.
• For cysticercosis, biopsy or CT scan.
Treatment – proper waste disposal and sanitary measures
Drugs: albendazole, paromomycin, and quinacrine hydrochloride.
Surgery may be necessary to remove larvae and anti-convulsants in
case of neurocysticercosis.
Diphylluobothrium latum (Broad Fish Tapeworm)
*The longest tapeworm that can reach about 13 meters.
• Its eggs consist of ciliated larvae called coracidia (s. coracidium).
• One end of the egg is occupied by a lid structure called an operculum
• Its scolex contains a pair of long sucking grooves.
• The gravid segment contain a uterine structure that is centrally located.
Human infection with D. latum is through ingestion of improperly cooked or
raw fish containing the plerocercoid (infective stage), the precursor larval
stage.
After ingestion, the plerocercoid attaches to the intestinal mucosa and
matures into adult worm.
The adult form, self-fertilizes and the eggs are passed out with the stool.
If the eggs come in contact with fresh water, the coracidium hatches and is
ingested by the first intermediate host, the tiny crustaceans called copepod
(Cyclops sp.). After ingestion, coracidium develops into larval stage called
procercoid .
The copepod is then eaten by a freshwater fish(second intermediate host)
where procercoid develops into the plerocercoid.
Definitive hosts are humans and other fish-eating mammals such as dogs,
cats, bears and seals
D. Latum occurs in countries where raw freshwater fish is consumed. In some
instances, the parasite may compete for vitamin B12 causing deficiency of
this vitamin.
Disease: Diphyllobothriasis
1. Asymptomatic disease- most common
2. Diphyllobothriasis- gastrointestinal involvement including diarrhea and
abdominal discomfort. When adult worm attaches itself to the jejunum
and ileum, the patient may develop deficiency of vitamin B12 leading to
anemia.
Laboratory Diagnosis
Stool specimen
Treatment
Praziquantel. Alternative is niclosamide.
Prevention and Control
Proper sanitary procedures, through cooking or fish prior to consumption.