Cestodes: Structure and Characteristics

Cestodes (Tapeworms)

• Characterized by segmented, ribbon-like morphology
• All are hermaphroditic
• Lack nervous and digestive systems
• Morphology consists of:
  • Scolex: main organ of attachment to definitive host
  • Neck: region of growth
  • Segments or proglottids: become more mature distally, with most distal segments being gravid

Taenia Saginata (Beef Tapeworm)

• Intermediate host: cattle
• Definitive host: human
• Life cycle:
  • Eggs go into blood vessels within cattle's intestines and are transported to skeletal muscles, where they develop as larvae (cysticerci)
  • Infection occurs through ingestion of improperly cooked or raw beef containing infective larva (cysticercus)
  • Maturation occurs in small intestine, taking approximately 3 months
• Can grow up to 10 meters in length
• Adult tapeworms are irritated by alcohol, leading to passage of proglottids after consumption
• Causes Taeniasis, leading to diarrhea, abdominal pain, loss of appetite, and weight loss
• Laboratory diagnosis: examination of fecal specimens
• Treatment: Praziquantel
• Prevention and control: proper waste disposal, sanitation practices, and adequate cooking of beef; freezing of beef meat for 10 days can kill encysted larvae

Taenia Solium (Pork Tapeworm)

• Humans are both intermediate and definitive hosts
• Life cycle:
  • Ingested worm eggs hatch in small intestine, burrow to intestinal wall, and disseminate to various organs
  • Infection occurs through ingestion of improperly cooked or raw pork meat containing infective larva (cysticercus cellulosae)
• Diagnostic methods:
  • Stool examination
  • Perianal swabs
  • Examination of gravid proglottids flattened between two glass slides
  • Injection of India ink through genital pore to count lateral branches of uterus
  • CT scan, MRI for neurocysticercosis
  • Ophthalmoscopy for ophthalmic cysticercosis
• Causes Cysticercosis, a parasitic infection caused by larvae of tapeworm

Pathogenesis of Taenia Solium

• Minor intestinal damage from adult tapeworms
• Irritation at site of attachment
• Entangled proglottids may cause intestinal obstruction
• Cysticerci can develop in various tissues, including striated muscle, brain, subcutaneous tissues, eye, heart, lung, and peritoneum
• Living cysticerci can cause inflammation or death, leading to inflammation and calcification

Diphyllobothrium Latum (Broad Fish Tapeworm)

• 1st intermediate host: copepods (Cyclops and Diaptomus)
• 2nd intermediate host: fish
• Longest medically important tapeworm
• Has two elongated sucking grooves for attachment
• Has operculated eggs (unique among cestodes)
• Causes Diphyllobothriasis, characterized by:
  • Abdominal pain and diarrhea
  • Hyperchromic, megaloblastic anemia (due to vitamin B12 deficiency)
  • Thrombocytopenia and leukopenia
• Treatment: Praziquantel
• Criteria for cure: recovery of scolex

Hymenolepis Nana (Dwarf Tapeworm)

• Smallest tapeworm
• Most frequently found tapeworm in developed countries
• Common in orphanages, day care centers, and mental institutions
• Can complete its entire life cycle in a single host (no intermediate host required)
• Ingestion of infected rice or flour beetles with cysticercoid larvae can cause infection
• Eggs are directly infectious for humans and can pass in stool or reinfect small intestine (autoinfection)
• Eggs have 8-10 polar filaments and six-hooked larva
• Treatment: Praziquantel