Cestodes: Tapeworm Anatomy and Treatment

• Cestodes, also known as flatworms or tapeworms, have a long, segmented, ribbon-like body that is flat in cross-section.

Important Parts

• The important parts of cestodes include the scolex, rostelium, neck, and strobila.
• SCOLEX: the anterior end (head).
• ROSTELIUM: holds the hook (may or may not have hooks).
• Not all cestodes have hooks.
• NECK: contains germ cells for new proglottids.
• STROBILA: the entire body of the cestode.
  • Proglottids are individual segments that appear in immature, mature, and gravid forms.
    • Immature: near the scolex or head.
    • Mature: middle of the cestode.
    • Gravid: near posterior or tail (typically seen in specimens).
• Cestodes are hermaphroditic.
  • Each cestode contains both male and female reproductive organs.

Treatments and Points to Remember

• Treatments include Praziquantel and Niclosamide.
• Gravid proglottids have fully developed reproductive organs with a uterus filled with eggs.
• Apolysis is the detachment of the gravid segment from the scolex.
• Strobilization is the creation of proglottids in the neck area, pushing others into the strobila.

Cestodes Table

• A patient is considered treated upon the recovery of the scolex (head).

Taenia solium (Pork Tapeworm)

• Also known as the "Pork Tapeworm".
• Shows a cosmopolitan distribution.
• Infection from measly pork can cause autoinfection due to poor hygiene.
• Humans can contract it by ingesting improperly cooked infected meat (measly pork).
• Humans are both the definitive and intermediate host.
• Humans are a dead-end host for cysticercosis.
• Pigs are the intermediate host.
• Both intestinal and tissue infections occur in humans.
• Humans become the intermediate host if they ingest eggs, leading to cysticercosis.
• More pathogenic than T. saginata.
• Shorter than T. saginata
• Contains fewer proglottids than T. saginata.
• Cysticercus cellulosae is the infective stage, and eggs/gravid proglottids are diagnostic.
• Habitat is the upper small intestine.

Scolex Details

• Four acetabula (smaller and more spherical compared to saginata).
• Rostellum: Double crown of 25-30 big and small hooks (absent in saginata).

Proglottids Details

• 7-13 lateral branches in gravid proglottids.
• Relatively less active compared to T. saginata.
• 30,000-50,000 ova contained within gravid proglottid.

Eggs Details

• Thick brown striated embryophore surrounds a hexacanth embryo.
• Ingestion by hogs leads to oncospheres released in intestines.

Oncosphere and Cysticercus

• Penetrates the intestinal mucosa to typically encyst in muscles as cysticercus cellulosae (infective stage).
• It hatches in the duodenum and travels to different organs through the bloodstream.
• The cause of cysticercosis.
• Cysticercus has 4 suckers and 2 rows of hooks, is oval, translucent, and has an opaque invaginated scolex.

Pathogenesis, Diagnosis, and Treatment

• It is found in all tissues, but commonly in muscles, tongue, heart, diaphragm, liver, spleen, and mesentery.
• Intestinal infection causes mild, nonspecific abdominal complaints.
• Cysticercosis often has multiple cysticerci that can develop in any organ/tissue.
• Taeniasis is diagnosed by identifying proglottids, eggs, or the scolex.
• Cysticercosis (NCC) suspected in patients from endemic areas with epileptic seizures, without systemic symptoms.
  • Imaging via computed axial tomography (CAT) scans or nuclear magnetic resonance imaging (MRI).
• Opthalmic cysticercosis diagnosed via visualization through ophthalmoscopy.
• ELISA/DOT-ELISA are very good screening tests for cysticercosis.
• EITB is another diagnostic tool.
• Treatments for taeniasis include praziquantel and niclosamide.
• Treatments for cysticercosis include praziquantel or albendazole, followed by corticosteroids.

Life Cycle and Prevention

• Eggs or gravid proglottids in feces.
• The first pathway is where humans are the intermediate host, by direct ingestion of eggs.
• Oncospheres hatch, penetrate intestinal wall, and circulate to musculature.
• Cysticerci may develop in organs/tissues.
• The second pathway, man is the definitive host: embryonated eggs and/or gravid proglottids are ingested by pigs.
• Oncospheres hatch and penetrate the intestinal wall, and circulate to musculature (pig = intermediate host).
• Oncospheres develop into cysticerci in pig muscle.
• Humans get infected when they ingest raw or undercooked infected meat.
• The scolex attaches to the intestine.
• Adults live in the small intestine (habitat).
• Thorough cooking of meat (primary) and freezing at -20 degrees Celsius for 10 days kills cysticerci.

Additional Notes for Taenia Spp.

• Eggs cannot be differentiated between the two species. When reporting the presence of either T. solium or T. saginata, the medical technology must write “TAENIA SPP. or TAENIA EGG."
• ARMED = with hook (T. solium)
• UNARMED = w/o hook (T. saginata)

Taenia saginata (Beef Tapeworm)

• Also known as “Beef Tapeworm”.
• Humans can only serve as a definitive host and cannot be an intermediate host; thus, cysticercosis does not occur in humans.
• Cattle serve as an intermediate host.
• Habitat is the upper jejunum / small intestine.
• Cysticercus bovis is infective, with eggs/gravid proglottids being diagnostic.
• Usually, only one Tapeworm is present in T. Saginata infections

Features of Adult Worm, Scolex, Mature Proglottids, and Gravid Proglottids

• Adult Worm: 1,000 to 4,000 proglottids; and 4 to 10m but have been reported as long as 25m
• Scolex: cuboidal, has 4 acetabula, no hooks or rostellum, and attaches to the scolex to form a short neck.
• Mature Proglottids: approximately square in shape, has two large lobes of ovaries, a median club-shaped uterus, follicular testes (300-400 testes), a presence of vaginal sphincter, and 15-20 lateral branches.
• Gravid Proglottids: longer than they are wide (16-20mm by 5-7mm), contains 97,000-124,000 ova; and may pass out 594,000,000 ova annually.
• Ova: spherical or subspherical in shape, brownish in color, and has a thick striate embryophore.
• Oncosphere (Embryo): three pairs of hooklets.
• Cysticercus: has 4 suckers with no hook, is ovoidal, milky white, single scolex invaginated into a fluid-filled bladder.

Features and Treatment

• Passage of proglottids in the stool is the most common complaint.
• May also have epigastric pain, vague discomfort, hunger pangs, weakness, weight loss, loss of appetite, and pruritus ani (perianal itching).
• Rare instances of intestinal obstruction caused by entangled proglottids.
• Diagnosis includes flattening gravid proglottids in between two glass slides and examining them against the light.
• Other methods include ink injection through the genital pore to count lateral branches (15-20), and FECT/AECT.

Hymenolepis nana (Dwarf Tapeworm)

• It is the smallest tapeworm and infects humans.
• It is a cyclophyllidean tapeworm, the largest and most diverse order of Cestoda.
• It is the ONLY HUMAN tapeworm that can complete its entire life cycle in a single host, indicating that it does not require an obligatory intermediate host.
• Habitat: Ileum/small intestine.
• Embryonated egg = infective
• Embryonated egg = diagnostic
• Delicate strobila

Features of the Scolex, Proglottids, and Gravid Proglottids

• Scolex: Subglobular with four cup-shaped suckers; and has a retractable rostellum armed with a single row of 20 to 30 Y-shaped hooklets.
• Proglottids: Short anterior proglottids, broad posterior, strobila has 175 to 220 segments, contains three ovoid testes and one ovary in a more or less straight pattern across the segment
• Gravid proglottids: The testes and ovary disappear and the uterus hollows out and becomes filled with eggs.

Pathogenesis

• Spherical or subspherical in shape, colorless or clay-colored.
• Thin outer membrane, thick inner membrane with conspicuous bipolar thickenings, and four to eight hair-like polar filaments embedded in the inner membrane.
• Light worm burden is generally asymptomatic, heavy infections may result in enteritis.
• Headache, dizziness, anorexia, pruritus of nose and anus, diarrhea, vomiting, abdominal pain, pallor (paleness) and weight loss may be shown.
• Demonstration of characteristic eggs in stool samples and concentration techniques are used for diagnosis.
• Praziquantel and Nitazoxanide

Diagnostic and Treatments of H nana

• Treat with dosage that is higher than that for taeniasis.
• Embryonated eggs in feces.
• Egg ingested by insect, and Cysticercoid develops in insect.
• Humans and rodents are infected when they inject cysticercoid-infected arthropods (indirectly).
• Or humans can get infected by ingesting embryonated egg ingested by humans from contaminated food, water, or hands (directly).
• Oncosphere hatches.
• Cysticercoids develops in intestinal villus and creates a scolex.
• It becomes an Adult in the ileal portion of the SI
• Eggs can be released through the genital atrium of the gravid proglottids.
• Gravid proglottids can disintegrate, releasing eggs that are passed in stool.
• Autoinfection can occur if eggs remain in the intestine.
• Eggs release a hexacanth embryo which penetrates the intestinal villus, continuing the life cycle.

Hymenolepis diminuta

• H. Diminuta is a "Rat Tapeworm" that accidentally infects humans, and differs from H. nana as it requires an intermediate host (arthropods).
• The adult tapeworm is larger than H. nana and the cysticcercoid larvae is infective, with eggs being diagnostic.

Habitat of H diminuta

• Small intestine
• The scolex has rudimentary unarmed rostellum but lacks hooks.
• Mature proglottids are broader than long with unilateral genital pores, and there are three ovoid testes and one ovary in a more or less straight pattern across the segment (same as H. nana).
• The gravid proglottids contains a sac-like uterus filled with eggs The eggs are circular, and have bile-stains.
• The oncosphere enclosed in an inner membrane and has bipolar thickenings but lacks the filaments,
• Hooklets have "fan-like” arrangement.
• The worm burden in rodents is relatively low and the highest recorded in humans is 19 worms.

Treatment and Symptoms

• Eggs are identified in the stool samples.
• H. diminuta eggs are identified through circular shape, larger size, and the lack of bipolar filaments.
• It is treated with Praziquantel.

The Life Cycle

• Eggs are passed in feces and ingested by arthropods.
• Oncospheres hatch and penetrate intestinal wall once it is ingested by arthropods.
• Cysticerci in arthropod's body cavity that is ingested by rodents/humans.
• They develop into a scolex and adults in the small intestine and gravid proglottids.