Medical Parasitology: Cestoda (Tapeworms)

• Medical parasitology notes covering Platyhelminths (Cestoda)
• These notes discuss general features, life cycle, modes of infection, prevention and control, classification according to habitat, and morphological differences.

General Features of Cestoda (Tapeworms)

• Shape: Long, flat, segmented, and tape-like.
• Size: From few millimeters to several meters.
• Tegument: Smooth and protective.
• Adult Body Division: Scolex (head), neck, and strobila (segments or proglottid).
• Scolex: Equipped with organs of attachment, typically 4 suckers (with or without rostellum and hooks).
• Neck: A thin region responsible for forming new segments.
• Strobila: Composed of immature, mature, and gravid segments (proglottids).
• Immature Segments: Located near the neck, small, and lack developed genitalia.
• Mature Segments: The majority of segments, characterized by well-developed genitalia.
• Gravid Segments: Lowermost segments filled with a uterus containing eggs.
• Digestive System: Absent; nutrients are absorbed through the body wall.
• Circulatory System: Absent.
• Excretory System: Features multiple flame cells and collecting tubules, opening posteriorly on a bladder in the last segment.
• Reproductive System: All cestodes are hermaphroditic.
• Each Segment: Contains multiple testes and a bilobed ovary, which open together through their ducts in a common genital pore.

Lifecycle of Cestodes

• Definitive Host: Typically humans for most cestodes.
• Intermediate Host: Mainly animals or arthropods.
• Adult Habitat: Small intestine of the definitive (D.H.) or reservoir host (R.H.).
• Egg Release: Gravid segments or eggs are passed in the feces of the D.H. or R.H.
• Larval Stages: Reside in the tissues of the intermediate host (vertebrate or invertebrate).
• Lifecycle Completion: Typically requires two different hosts, except for Hymenolepis nana, where humans can serve as both definitive and reservoir hosts.
• Egg Stage Characteristics: Most have mature, non-operculated eggs.
• Egg Ingestion: Eggs are typically ingested by the intermediate host for further development.
• Onchosphere: Hexacanth embryo liberated inside the intermediate host after egg ingestion.
• Cystic Larval Stage: The onchosphere develops into a cystic larva in the intermediate host's tissue.
• Infective Stage: It is the infective stage for the definitive and reservoir hosts for most cestodes.
• Larval Cysts: Contain one or many scolices of future adult worms.

Key Larval Forms

• Cysticercus:
  • Fluid-filled cyst.
  • Contains an attached single invaginated scolex.
  • Associated with Taenia solium (pork tapeworm).
• Cysticercoid:
  • Small solid cyst
  • Contains an attached single evaginated scolex
  • Associated with Hymenolepis
• Coenurus:
  • Similar to cysticercus
  • Has multiple scolices from its germinal wall
  • Associated with Taenia multiceps.
• Hydatid Cyst:
  • Large, fluid-filled cyst
  • Lined with germinal epithelium
  • Has multiple scolices that may lie free inside
  • Associated with Echinococcus.

Modes of Infection

• Ingestion of eggs via contaminated food and drink: Includes Taenia solium (cysticercosis), Echinococcus granulosus (hydatidosis), and Hymenolepis nana.
• Ingestion of larva in:
  • Undercooked meat, specifically Taenia saginata.
  • Undercooked pork, specifically Taenia solium.
• Ingestion of insect vector containing larva: Primarily Hymenolepis nana.
• Autoinfection: Can occur with Taenia solium (cysticercosis) and Hymenolepis nana.

Prevention and Control of Cestodes

• Promote health education to avoid infections
• Improve sanitation and hygiene
• Ensure sanitary treatment of human waste
• Conduct regular deworming of infected individuals and animal hosts (dogs, cats, etc.)
• Control intermediate hosts like rodents, fleas, and grain beetles
• Inspect meat for Taenia (measly meat)
• Properly cook pork, beef, and lamb
• Prolonged freezing of meat to eliminate some tapeworms

Classification of Cestodes by Habitat

• Intestinal Cestodes (adult in small intestine):
  • Taenia saginata
  • Taenia solium
  • Hymenolepis nana
• Tissue Cestodes (larva in tissue/organs):
  • Echinococcus granulosus
  • Taenia solium
  • Hymenolepis nana

Differences Between Intestinal & Tissue Cestodes

• Intestinal Cestodes:
  • Humans serve as the definitive host.
  • Adult worms reside in the small intestine.
  • Humans typically ingest the larval stages.
  • Intestinal irritation is the primary complaint.
  • Diagnosis relies on stool examination.
• Tissue Cestodes:
  • Humans serve as the intermediate host.
  • Larval stages reside in tissues such as the liver and brain.
  • Humans typically ingest the mature eggs.
  • Space-occupying lesions within organs are typical.
  • Diagnosis typically involves biopsy, radiological, or serological methods.

Morphological Differences Between Trematodes & Cestodes