Platyhelminthes Class: Cestodes Notes

Phylum Platyhelminthes - Class Cestodes

General Features of Cestodes (Tapeworms)

  • Tapeworms have long, segmented bodies.
  • They are flattened and lack a body cavity.
  • Tapeworms are hermaphroditic, possessing both male and female reproductive organs.
  • They lack vascular, respiratory, and digestive systems.
  • The adult worm consists of three distinct parts:
    • Head or Scolex: Equipped with muscular suckers and a crown of hooks (rostellum) for attachment.
    • Unsegmented Neck: Connects the head to the main body.
    • Strobila: The long, segmented body, with each segment known as a proglottid.

Taenia saginata (Beef Tapeworm) and T. solium (Pork Tapeworm)

  • Disease: Taeniasis
  • Habitat: Small intestine
  • Definitive Host: Humans
  • Intermediate Host (IH):
    • Cattle for T. saginata
    • Swine (pigs) for T. solium

Morphology and Life Cycle of Taenia species

  1. Eggs or gravid proglottids are passed in feces and released into the environment.
  2. Cattle (T. saginata) and pigs (T. solium) become infected by ingesting vegetation contaminated with eggs or gravid proglottids.
  3. Oncospheres hatch, penetrate the intestinal wall, and circulate to the musculature.
  4. Oncospheres develop into cysticerci in the muscle tissue.
  5. Humans become infected by ingesting raw or undercooked infected meat.
  6. The scolex attaches to the intestine in the human host.
  7. Adults reside in the small intestine.
  • Infective Stage: Cysticerci in undercooked meat.
  • Diagnostic Stage: Eggs or gravid proglottids in feces.

Pathogenicity of Taenia species

  • Intestinal disturbances, including hunger pain, abdominal discomfort, diarrhea, and constipation.
  • Loss of weight, anxiety, and anaemia.
  • Intestinal obstruction.
  • Mode of infection: Eating imperfectly cooked meat of pork or beef containing cysticercus cellulosae or cysticercus bovis, respectively.
  • Diagnosis: Finding eggs and gravid proglottids in the faeces.

Prevention and Control of Taenia Infections

  • Inspection of beef and pork for cysticerci at slaughterhouses.
  • Thorough cooking of meat to at least 56°C or freezing at -10°C for 5-10 days.
  • Treatment: Praziquantel is the drug of choice.

Cysticercosis (Larval Stage of T. solium Infection)

  • Cysticercosis is an infection of human and pig tissue with the larval stage of T. solium (Taenia cellulosae).
  • Mode of infection:
    • Autoinfection.
    • Ingestion of food or water contaminated with faeces containing T. solium eggs.
  • People living in the same household with someone who has a tapeworm have a much higher risk of getting cysticercosis.
  • Pathogenicity:
    • Subcutaneous tissue nodules under the skin.
    • Larval cysts in the eye, potentially resulting in blindness.
    • Heart lesions, leading to heart failure.
    • Encystment in the CNS (neurocysticercosis), causing seizures, epilepsy, headache, hydrocephalus (due to obstruction of cerebrospinal fluid flow), and mental disturbances.
    • Spinal cord lesions can lead to partial loss of motor control, weakness, and paralysis.

Diagnosis, Prevention, and Control of Cysticercosis

  • Diagnosis:
    • CT scan (Computed tomography) or MRI (Magnetic resonance imaging) of the brain.
    • Antibody test (ELISA) or a biopsy of the affected area.
  • Prevention and control:
    • Human excreta should be treated before use as manure.
    • Environmental sanitation and health education for personnel and food hygiene.
  • Treatment:
    • Surgical removal.
    • Praziquantel and albendazole.
    • Steroid anti-inflammatory drugs to reduce swelling (brain edema).

Hypmenolepis nana (Dwarf Tapeworm)

  • Disease: Hymenolepiasis nana
  • Habitat: Small intestine
  • Definitive Host (DH): Humans
  • Reservoir Host (RH): Rodents

Morphology and Life Cycle of Hymenolepis nana

  1. Embryonated egg in feces
  2. Embryonated egg ingested by humans from contaminated food, water or hands
  3. Humans and rodents are infected when they ingest cysticercoid-infected arthropods
  4. Egg Ingested by insect
  5. Oncosphere hatches
  6. Cysticercoid develops in intestinal villus
  7. Adult in ileal portion of small intestine
  8. Eggs can be released through the genital atrium of the gravid proglottids
  9. Autoinfection can occur if eggs remain in the intestine. the eggs then release the hexacanth embryo, which penetrates the intestinal villus, continuing the cycle
  10. Gravid proglottids can also disintegrate, releasing eggs that are passed in stool
  • Infective Stage: Embryonated egg ingested by humans.
  • Diagnostic Stage: Embryonated egg in feces.

Pathogenicity, Mode of Infection, Diagnosis, and Treatment of Hymenolepis nana

  • Pathogenicity: Enteritis with nausea, vomiting, diarrhea, and abdominal pain.
  • Mode of infection:
    • Ingestion of eggs by autoinfection.
    • Ingestion of fleas or beetles containing cysticercoids.
    • Eating food contaminated with eggs.
  • Diagnosis: Finding eggs in feces.
  • Treatment:
    • Niclosamide and praziquantel.
    • Hygiene is the best control.

Hypmenolepis diminuta (Rat Tapeworm)

  • DH: Rats, mice
  • IH: Insects (fleas, cockroaches)
  • Mode of infection: Humans are accidentally infected by ingesting fleas and cockroaches containing cysticercoids.
  • Pathogenicity, Diagnosis, Treatment: Similar to H. nana.

Echinococcus granulosus

  • Disease: Hydatid disease, hydatidosis, echinococcosis.
  • Habitat: Small intestine
  • DH: Dogs and carnivores.
  • IH: Sheep, cattle, goats…; humans are accidental IH.

Morphology and Life Cycle of Echinococcus granulosus

  1. Embryonated egg in feces
  2. Ingestion of eggs (in feces)
  3. Oncosphere hatches; penetrates intestinal wall
  4. Hydatid cyst in liver, lungs, etc.
  5. Protoscolex from cyst
  6. Ingestion of cyst (in organs)
  7. Scolex attaches to intestine
  8. Adult in small intestine
  • Infective Stage: Embryonated eggs in feces.
  • Diagnostic Stage: Hydatid cyst in liver, lungs, etc.

Pathogenicity of Echinococcus granulosus

  • Jaundice and portal hypertension can result from the pressure effects of a cyst in the liver.
  • Peribronchial cysts cause pulmonary abscesses.
  • Kidney cysts cause renal dysfunction.
  • Acute inflammatory effects may follow brain or spinal cord infection.
  • Ruptured cysts may cause anaphylactic responses.

Mode of Infection and Diagnosis of Echinococcus granulosus

  • Mode of infection:
    • Eating food contaminated with stool of infected dogs.
    • Handling infected dogs.
  • Diagnosis:
    • Serologic tests: ELISA, IHT, Western blot.
    • Ultrasound imaging (US), magnetic resonance (MR), and computed tomography (CT).
    • Casoni skin test (Intradermal reaction).

Treatment and Control of Echinococcus granulosus Infection

  1. Surgical removal of the cyst or inactivation by injecting the cyst with 10% formalin.
  2. Percutaneous Aspiration, Infusion of scolicidal (10% formalin), and Reaspiration (PAIR) can be utilized in lieu of surgery.
  3. It is recommended that a high dose of albendazole be administered prior to and for several weeks after surgery and/or aspiration.
  4. Pet dogs should be dewormed periodically.
  5. Stray dogs should be destroyed.
  6. Preventive measures involve avoiding contact with infected dogs.
  7. Hydatid cysts in slaughtered animals should be destroyed.