Chapter 10

• Cestodes


  Commonly known as tapeworms

  
• Cestodes


  • Classified under the sub-kingdom Metazoa, phylum Platyhelminthes
  • Considered as primitive worms
  • No digestive system nor a nervous system
  • Tegument - outer surface that absorb nutrients and eliminate waste products, host-parasite interface, and metabolically active body covering performing all the vital activities such as protection, absorption and secretion
• Cestode morphology


  • Parasites are flat and consist of three distinct regions: Head, Neck, Body (proglottids)
• Scolex


  Contains an organ of attachment, may consist of hooks, suckers, or sucking grooves
• Rostellum


  Fleshy extension to which hooks may be attached in scolex in some species
• Neck


  Region of growth and connects the head to the body, the worm grows by adding new proglottids from the neck
• Proglottids


  Divided segments that make up the body of the tapeworm, the oldest proglottids are found at the most distal part of the body

  
• Strobila


  Series of proglottids
• Cestode reproduction


  • All cestodes are hermaphroditic (self-fertilizing) with each proglottid containing both male and female reproductive organs, each proglottid is capable of laying eggs (pregnant proglottid or gravid segment)
• Cestode life cycle


  1. Egg

  2. Larva

  3. Adult worm
• Infection in humans


  Usually acquired through ingestion of the undercooked or raw flesh of the intermediate host containing the infective larvae
• Infection process


  1. Ingested larvae are transformed into adult worms in the intestines

  2. Adult worm undergoes self-impregnation with the gravid segment rupturing to release the eggs in the intestines

  3. Eggs are then passed out to the external environment during defecation
• Oncosphere


  Embryo in the egg, which represents the first larval or motile stage for the majority of cestodes
• Eggs


  Equipped with small hooks (hooklets) that eventually enable the parasite to pierce the wall of the intestines, excreted in the feces of infected hosts and are transmitted to the intermediate hosts
• Intermediate hosts


  • Cattle
  • Pig
  • Fish
• Infection with Taenia saginata (Beef Tapeworm) is common in areas where beef is routinely eaten, especially undercooked
• The adult Taenia saginata worm does not produce significant damage in the small intestines
• Symptoms of Taeniasis (Taenia saginata infection)


  • Diarrhea
  • Abdominal pain
  • Loss of appetite with resultant weight loss
  • Body malaise
  • Itchiness in the anal region (pruritus ani)
• Praziquantel


  Drug of choice against the adult Taenia saginata worm
• Preventive measures for Taeniasis


  • Proper waste disposal and sanitation practices
  • Adequate cooking of beef
  • Freezing of beef meat for approximately 10 days to kill encysted larvae
  • Prompt treatment of infected persons
• Taenia solium (Pork Tapeworm)


  Infection acquired through ingestion of improperly cooked or raw pork meat which contains the infective larvae (Cysticercus cellulosae)
• Taenia solium infection can also occur following the ingestion of food or water contaminated with human feces that contain the eggs of the parasite
• Hosts for Taenia solium

  • Pig - intermediate host
  • Human - both intermediate and definitive hosts
• Two ways Taenia solium infective stages enter human host

  1. Ingestion of undercooked or raw pork meat - infective stage is the larval form which transforms into adult worm in the intestines

  2. Ingested worm eggs hatch in the small intestines, burrow through the wall into a blood vessel, and disseminate to various organs
• Taenia solium is more prevalent in underdeveloped communities with poor sanitation and where people eat raw or undercooked pork
• Taeniasis


  Disease produced by the adult Taenia solium worm, most cases are asymptomatic but high worm burden can cause similar symptoms to Taenia saginata infection
• Cysticercosis


  Result of larval Taenia solium encystation in various tissues of the body, most common involvement is the skeletal muscles
• Neurocysticercosis


  Cysticercosis of the brain, most feared and severe involvement, symptoms associated with increased intracranial pressure such as seizures, headache, and vomiting
• Ocular cysticercosis


  Can lead to visual disturbance due to development of inflammation of the uvea (uveitis) and retina (retinitis)
• Diagnosis of Taenia solium infection

  1. Microscopic examination of stool specimen to demonstrate ova or proglottids

  2. Demonstration of the typical morphology of the scolex to differentiate from Taenia saginata

  3. For cysticercosis, diagnostic procedure depends on demonstration of the cyst in tissue, through biopsy or CT scan
• Praziquantel


  Drug of choice for Taenia solium, may also be effective for cysticercosis but usually not recommended for ocular and CNS involvement
• Alternative drugs for Taenia solium

  • Albendazole
  • Paromomycin
  • Quinacrine hydrochloride
• Anticonvulsants may be given in cases of neurocysticercosis
• Surgical removal of the larvae may be necessary in some cases
• Preventive measures for Taenia solium

  • Proper waste disposal and sanitary measures
  • Thorough cooking of pork meat
  • Prompt treatment of infected persons
• Diphyllobothrium latum (Broad Fish Tapeworm)


  Longest of the tapeworms, can reach a length of about 13 meters, its scolex contains a pair of long sucking grooves, gravid segments contain a uterine structure that is centrally located and assumes a rosette formation

  
• Coracidia


  Ciliated larvae in the eggs of Diphyllobothrium latum
• Operculum


  Lid structure occupied in one end of the Diphyllobothrium latum egg
• Plerocercoid


  Infective stage, precursor larval stage of Diphyllobothrium latum
• Diphyllobothrium latum life cycle

  1. Human infection through ingestion of improperly cooked or raw fish containing the plerocercoid (infective stage)

  2. Plerocercoid attaches to the intestinal mucosa and matures into the adult worm

  3. Adult worm self-fertilizes and the eggs are passed out with the stool

  4. Eggs hatch into coracidia which are ingested by a copepod (first intermediate host)

  5. Coracidia develops into procercoid in the copepod

  6. Copepod is eaten by a freshwater fish (second intermediate host) where the procercoid develops into the plerocercoid