Medical Helminthology: Cestodes

• Medical Helminthology studies the biology, geographic distribution, course, diagnosis, prophylaxis, and control of parasitic worms (helminthes) and helminthic diseases.
• Helminths are invertebrates that develop through egg, larval (juvenile), and adult stages.
• Learning objectives include: epidemiology, morbidity, mortality, morphology, life cycle, hosts, vectors, disease, symptoms, pathogenesis, site, diagnosis, prevention, and control.
• Clinically important cestodes known to be pathogenic to humans:
  • Tenia solium (pork tapeworm)
  • T. saginata (beef tapeworm)
  • Diphyllobothrium latum (fish/broad tapeworm)
  • Hymenolepis nana (dwarf tapeworm)
  • Echinococcus granulosus
  • E. multilocularis (hydatid)
  • Dipylidium caninum

Cestodes

• Cestodes are a class of parasitic worms within the phylum Platyhelminthes (flatworms).
• They are known as tapeworms
• Have a body composed of multiple segments called proglottids
• They lack a digestive system and absorb nutrients directly through their tegument (outer body covering).
• The scolex is the anterior end used for attachment to the host's intestinal wall.
• The neck is the region behind the scolex where new proglottids are generated.
• Mature proglottids contain both male and female reproductive organs.
• Gravid proglottids are filled with eggs and are released from the tapeworm's end.

Tenia solium & T. saginata (Teniasis)

• These cestodes are globally distributed; incidence is higher in developing countries.
• Infection rates are low in North America (1 per 1000) to high in the third world (10%).
• Pork tapeworm has a higher incidence dependent on dietary habits.

Morphology T. saginata

• T. saginata can grow 4-6 meters long and 12 mm wide.
• It has a pear-shaped head (scolex) with 4 suckers but no hooks or neck.
• T. saginata has a long flat body with several hundred segments (proglottids).
• Proglottids are about 18 x 6 mm with a branched uterus (15-30 branches).
• Eggs measure 35 x 45 micrometers, are roundish, yellow-brown, and have peripheral radial striations; contain an embryo with 3 hooklets.

Morphology T. solium

• T. solium is slightly smaller than T. saginata.
• It has a globular scolex with four suckers and a circular row of hooks (rostellum); The hooks give a solar appearance.
• T. solium contains a neck and a long, flat body (0.1 meter in length).
• Proglottids are 5 x 10 mm containing a 7-12 branch uterus.
• The eggs of T. solium and T. saginata are indistinguishable.

Life Cycle Tenia solium & T. saginata

• A tapeworm larval cyst (cysticercus) comes from poorly cooked infected meat.
• The larva avoids the cyst and moves to the small intestine attaching to the mucosa by the scolex suckers.
• Proglottids mature within 3 to 4 months.
• The adult can live in the small intestine for 25 years and pass gravid proglottids with feces.
• Eggs released from proglottids contaminant and persist on vegetation for several days, and are consumed by cattle or pigs and form cysticerci.

Symptoms and Pathology Tenia solium & T. saginata

• Light infections can be asymptomatic.
• Heavier infections may produce abdominal discomfort, epigastric pain, vomiting, and diarrhea.
• Gastrointestinal symptoms may result from being in the presence of the tapeworm.

Cysticercosis

• T. solium eggs can also infect humans and cause cysticercosis (larval cysts in the lung, liver, eye, and brain).
• Cysticercosis causes blindness and neurological disorders.
• Cerebral cysticercosis's incidence can be as high as 1 per 1000 population and accounts for up to 20% of neurological cases in some countries (e.g. Mexico).
• Cysticercosis ocular involvement occurs in about 2.5% of patients; muscular involvement is as high as 10% (India).
• Cysticercosis symptoms result from inflammatory/immune responses.
• Antibodies are produced in cysticercosis and are helpful epidemiological tools.

Tenia solium & T. saginata Eggs

• Taenia saginata and T. solium eggs are morphologically indistinguishable.
• Species identification relies on proglottids or scolices.
• Taeniid eggs are round or subspherical, diameter 31-43 µm, thick, and radially striated with a brown shell.
• Each shell contains an embryonated oncosphere with 6 hooks.
• The egg has the primary membrane surrounding eggs in the proglottids.

Diagnosis

• Diagnosis comes from eggs or proglottids in stool or from the perianal area.
• Cysticercosis is confirmed by antibodies.
• Using India ink allows visualization of primary lateral branches to identify between T. saginata and T. solium
• T. saginata has 15-20 branches, while T. solium has 7-13.
• Genital pores are in the mid-lateral position.

Treatment

• Praziquantel is the drug of choice.
• Expulsion of the scolex is needed for satisfactory treatment.
• Meat inspection
• Thoroughly cook or freeze meat to -10⁰ C and above 50⁰ C to kill cysticerci.

DIPHYLLOBOTHRIUM LATUM (FISH OR BROAD TAPEWORM)

• Fish tapeworm is distributed worldwide, particularly in subarctic and temperate regions, and is associated with eating raw/undercooked freshwater fish.
• The longest tapeworm, it measures 3-10 meters with more than 3000 proglottids.
• The scolex looks like two almond-shaped leaves.
• Proglottids are broader than they are long.
• Eggs are 30 x 50 micrometers and contain an embryo with 3 pairs of hooklets.

DIPHYLLOBOTHRIUM LATUM Life Cycle

• Humans and other animals are infected by eating uncooked fish with plerocercoid larvae (15 x 2 mm), which attach to the small intestinal wall and become adults in 3–5 weeks.
• Eggs released from gravid proglottids in the small intestine pass in the feces.
• Eggs hatch in fresh water, yielding a ciliated coracidium which must be ingested by a water flea (Cyclops) and develop into a procercoid larva.
• When infected Cyclops are ingested by freshwater fish, the procercoid larva penetrates the intestinal wall and turns into a plerocercoid larva.
• The plerocercoid larvae represent the infective stage for humans.

Additional Life Cycle Notes for DIPHYLLOBOTHRIUM LATUM

• The eggs mature in 18 to 20 days.
• After ingestion by a freshwater crustacean (copepod), the coracidia develop into a procercoid larva.
• After the copepod is ingested by a second intermediate host like minnows and small freshwater fish, the procercoid larvae migrate into the fish flesh, turning into a plerocercoid larva (sparganum).
• The plerocercoid develop into immature adults and mature adult tapeworms in small intestine.
• Adults measure more than 10m in length, with more than 3000 proglottids.
• Immature eggs are get released (1,000,000 per day per worm) through the proglottids and into the feces.
• Eggs appear in the feces 5-6 weeks post-infection.
• Other mammals can serve as definitive hosts.
• Proglottids of Diphyllobothrium latum are wider than they are long.
• D. latum adults attach to the intestinal mucosa using two bilateral grooves (bothria) of their scolex.

Diphyllobothrium eggs

• Eggs are oval or ellipsoidal, with an inconspicuous operculum.
• The eggs are passed unembryonated.
• Size range from 58 to 76 μm by 40 to 51 µm.
• The uterus coiled in rosette.
• Genital pore is in the center of the proglottid.

Hymenolepis nana (Dwarf Tapeworm)

• A small tapeworm (20 x 0.7 mm) that infects children.
• Rodents are the reservoir.
• Infection through oro-fecal mode, causing cross and auto infection by normal feces.
• Light infections produce vague abdominal disturbances, but severe infections cause enteritis.
• Diagnosis is based on finding eggs in the feces.
• Praziquantel is the drug of choice.
• Hygiene is the best intervention.

Hymenolepis nana (Dwarf Tapeworm) Life Cycle

• 4 Humans and rodents infected through cysticercoid-infected arthropods.
• Eggs of Hymenolepis nana are immediately infective when passed but cannot survive more than 10 days.
• When eggs are ingested by an arthropod intermediate host, they can infect humans or rodents.
• Oncospheres released when eggs are ingested from food, water or feces penetrate the intestinal villus.
• Cysticeroid larvae penetrate the villus and attach to the intestinal mucosa and resides in the small intestine.
• Eggs pass in the stool released through the proglottids.
• Autoinfection can occur if eggs remain in the intestine.
• The adult worms last 4-6 weeks, but internal autoinfection allows it to persist for years.

Hymenolepis Egg Morphology

• Hymenolepis diminuta eggs are round or slightly oval, 70 - 86 μm X 60 - 80 μm, with a striated outer membrane and a thin inner membrane.
• This space between the membranes is smooth or granular.
• The oncosphere has six hooks.
• H. nana eggs are oval/subspherical and smaller than those of H. diminuta, with the size around 40 - 60 μm X 30 - 50 μm.
• The inner membrane has two poles with 4-8 polar filaments spread.
• The oncosphere has six hooks, appearing as dark lines at 8 o'clock.

Echinococcosis (Hydatid)

• Echinococcus granulosus and E. multilocularis cause hydatid cysts.

Echinococcus granulosus

• Common in Asia, Australia, Eastern Africa, southern Spain, southern parts of South America and northern parts of North America.
• Human infection rate is 1-2 per 1000; can be higher in rural areas.
• Echinococcus granulosus is the smallest tapeworm (3-9 mm long) with only 3 proglottids.
• The adult worm lives in domestic and wild carnivorous animals.
• Eggs are passed by infected animals and are ingested by the grazing farm animals or man and produce embryos.
• Cysts containing many larvae (proto-scolices or hydatid sand) are localized in different organs.
• Proto-scolices escape the cyst, enter the small intestine and develop into adult worms.
• The cyst is round and measures 1-7 cm in diameter and can grow to 30cm.
• The cyst has an outer anuclear hyaline cuticula and an inner nucleated germinal layer with fluid.
• Some cysts, known as brood cysts, may have the larva (hydatid sand).
• Man is a dead end host.

Echinococcus granulosus Life Cycle

• The adult Echinococcus granulosus (3-6 mm long) resides in the small bowel of the definitive hosts, dogs or other canids.
• Gravid proglottids release eggs that pass in the feces.
• After ingestion by an intermediate host (sheep, goat, swine, cattle, horses, camel), the egg hatches and releases an oncosphere.
• The oncosphere penetrates the intestinal wall and migrates to the circulatory system.
• It develops into a cyst that enlarges gradually in various organs, and especially the liver and lungs.
• Protoscolices and daughter cysts fill the cyst interior.
• The definitive host becomes infected by ingesting infected intermediate host organs and the protoscolices evaginate.
• Attach to the intestinal mucosa and turns into adult stages in 32-80 days.

More Echinococcus Species

• The same life cycle occurs with E. multilocularis (1.2-3.7 mm).
• Definitive hosts are foxes and lesser extent dogs, cats, coyotes and wolves; Small rodents are the intermediate host.
• Larval growth (in the liver) remains indefinitely in the proliferative stage, invading the surrounding tissues.
• E. vogeli (up to 5.6 mm long) has definitive hosts/bush dogs and dogs/ the intermediate hosts/rodents; the larval stage (in the liver, lungs and other organs) develops both externally and internally.
• E. oligarthrus (up to 2.9 mm long) has wild felids/definitive hosts and rodents/intermediate hosts.
• Humans get infected by consuming eggs, yielding oncospheres and cause cyst development in various organs.

Echinococcus multilocularis

• Aligns with E. granulosus and causes hydatid cysts in northern Asia and North America.
• Morphology and life cycle are same, but rodents are its intermediate host.
• Humans develop hydatid cysts similar to E. granulosus; however, the cysts are multilocular (many chambers).
• Resistant to praziquantel; high doses of Albendazole can have anti-parasitic effect.
• Surgery is the means of removing the cyst.
• Rodent control is the means of prevention.
• Echinococcus a patient becomes infected after consuming parasite eggs, is unable to transmit it ending the parasite transmission/humans are defined as a "dead end host".

Dipylidium caninum

• Flea larvae consumed egg packets and the oncospheres turn into cysticercoids.
• Flea larvae matures and continues to harbor infective cysticercoids.
• Definitive host consumes fleas with cysticercoids.
• Cats and dogs may harbor infectious fleas to humans.
• Gravid proglottids pass intact or emerge from the perianal region and releases egg packets
• Humans get infected via ingesting infected fleas.