Cestodes (Tapeworms) Overview

Cestodes (Tapeworms)

• Cestodes are tapeworms.
• Genera: Taenia spp. and Echinococcus spp.

Taenia spp.

• T. saginata

  • Humans are the definitive host.
  • Worldwide distribution.
  • Adult length: 5-10 meters
  • Intermediate host: Cattle
  • Larval stage: Cysticercus bovis
  • Larva site in intermediate host: Muscles
  • Larva morphology: Small (1 cm), fluid-filled cyst with a single invaginated scolex (4 suckers).

• T. solium

  • Humans are the definitive host.
  • Worldwide distribution, except for Latin America, Africa, and Asia.
  • Adult length: 5 meters
  • Intermediate host: Pigs and humans.
  • Larval stage: Cysticercus cellulosae
  • Larva site in intermediate host: Muscles
  • Larva morphology: Similar to T. saginata in size and shape, but with an armed rostellum.

• T. ovis

  • Dogs are the definitive host
  • Worldwide distribution.
  • Adult length: 2 meters.
  • Intermediate host: Sheep
  • Larval stage: Cysticercus ovis
  • Larva site in intermediate host: Muscles
  • Larva morphology: Small (0.5 cm), fluid-filled cyst with a single invaginated scolex and armed rostellum

• Hydatigena

  • Dogs are the definitive host.

  • Worldwide distribution.

  • Adult length: 5+ meters

  • Intermediate host: Sheep, goat, buffalo, camel, pig, rabbit

  • Larval stage: Cysticercus tenuicollis

  • Larva site in intermediate host: Liver, peritoneum, mesentery

  • Larva morphology: Large (up to 8 cm in diameter). Fluid filled cyst with single invaginated scolex.

  • Other Taenia spp.: T. multiceps, T. serialis, T. pisiformis, T. taeniformis

• Macroscopic Morphology

  • T. saginata: 5-10 meters long, large segmented tapeworm.
  • T. solium: 3-7 meters long, large segmented tapeworm.
  • T. multiceps, T. serialis, T. hydatigena, T. ovis, T. pisiformis, T. taeniformis: Varying lengths

• *Scolex Morphology for Taenia saginata and Taenia solium

• T. saginata: Relatively large (1.5 mm in diameter), 4 cup-shaped muscular suckers, and no rostellum or hooks.

• T. solium: Relatively small (0.6-1 mm in diameter), 4 cup-shaped muscular suckers, rostellum armed with double rows of hooks.

Taenia spp. Mature Segments

• Nearly squarish or slightly broader segments.
• One set of genital organs.
• Irregularly alternating common genital pores.
• Numerous testes (300-400).
• Bilobed ovaries and Mehlis gland between the lobes.
• Vitelline glands behind the ovaries forming a compact mass.
• Uterus is a simple tube extending anteriorly in the median plane.
• Gravid Segments:
  • T. saginata have longer, broader segments with 15-20 lateral uterine branches.
  • T. solium have longer than broad segments, with 7-12 lateral uterine branches.

Taenia spp. Life Cycle

• Humans can become infected by eating undercooked or raw infected meat.
• Oncospheres hatch, penetrate intestinal wall, and migrate to muscles.
• Cysticerci develop in muscles of intermediate hosts.
• Humans become infected by ingesting raw or undercooked infected meat.
• Eggs or gravid proglottids are passed into the environment.

Taenia spp. Eggs

• Small in size (35-45 µm)
• Globular or spherical in shape.
• Brownish in color.
• Embryophore (double contoured, radially striated) containing a hexacanth embryo.

Cysticercus spp. (Larval Forms)

• Cysticercus bovis: Spindle-shaped, cream-white cysts in cattle muscles.

• Cysticercus cellulosae: Spindle-shaped, cream-white cysts in pig muscle and human tissues potentially leading to neurocysticercosis.

• Cysticercus tenuicollis: Large bladder-like cyst in ruminants (sheep, goats, cattle).

• Strobilocercus fasciolaris: Small vesicle or cyst in rat and mice liver containing a segmented neck with invaginated scolex.

• Coenurus cerebralis: Large fluid-filled cyst in sheep and other ruminants' brain & spinal cord.

Echinococcus spp.

• Echinococcus granulosus:

  • Definitive host: Dogs
  • Intermediate host: Ruminants (e.g., sheep, cattle) and humans
  • Larval stage: Hydatid cyst (in lungs, liver)
  • Site in definitive host: Small intestine
  • Site in intermediate host: Any organ of the host body (often the lung and liver).

• *Adult E. granulosus (Scolex and strobila)

• Scolex is small with 4 suckers and a rostellum with 2 crowns of hooks, strobila has 3-4 (usually 3) segments; one immature, one mature, one gravid.

• Mature segment is longer than broad and has a horse-shoe shaped ovary found posteriorly and has Vitelline glands in a compact mass behind it.

• Gravid segment is longer than broad and has a longitudinal uterus with 12-15 main lateral pouches containing eggs.

• Life Cycle (of E. granulosus):

  • Dogs defecate eggs into environment.
  • Intermediate hosts (e.g., sheep, cattle) ingest eggs.
  • Oncospheres develop into hydatid cysts in organs (e.g., lungs, liver).

• Echinococcus multilocularis:

  • Definitive host: Dogs,
  • Intermediate host: Rodents and possible humans
  • Larval stage: Alveolar cyst
  • Location: Lungs and liver. Fatal disease in humans, similar to cancer

Echinococcus granulosus

• Infective to humans through ingestion of eggs or gravid proglottids.
• Infections in humans are asymptomatic in the majority of cases (in dogs and pigs)
• Development of large hydatid cysts in organs (lungs, liver) causes problems.
• Can cause anaphylactic shock in susceptible individuals due to cyst rupture.

Pathology, Epidemiology, and Control of Echinococcus spp.

• Pathogenesis (various species)
• Epidemiology (various species)
• Diagnosis (various species)
• Control (for species and infections)