Cestodes & Trematodes

• Source of infection


  How a parasite is acquired
• Mode of transmission


  Method of transfer to the host
• Infective stage


  How a parasite is able to enter the host
• Pathogenic stage


  A stage where the parasite is capable of causing a disease
• Diagnostic stage


  A stage which provides evidence for making a specific diagnosis
• Intermediate host


  Harbors the larval stage (after birth before adult)
• Definitive host


  Harbors the adult stage of the parasite
• Cestodes (Tapeworms)


  • Majority are long, segmented and tape-like
  • Dorso-ventrally flattened
  • Living worms are white or yellowish
  • Hermaphrodite - self-fertilizing, each proglottid contains both male and female reproductive organs

  
• Cestodes (Tapeworms)


  • Size varies from a few mm to several meters
  • Adult worms are found in the intestinal canal of man and animals
  • Tegument - outer surface for absorption of nutrients and elimination of waste products

  
• Cestodes (Tapeworms)


  • 3 regions in an adult worm: Head (scolex) - provided with suckers and sometimes hooks for attachment, Neck - region of growth, Strobila (body or trunk) - consist of a series of segments called proglottids

  
• Life cycle of Cestodes


  1. Egg - contains an embryo called oncosphere (1st larval or motile stage)

  2. Larva - infective stage, transformed into adult worm in intestine of infected host

  3. Adult - pathogenic stage, undergoes self-impregnation with gravid segment rupturing to release eggs in the intestines
• Intestinal Cestodes


  • Taenia saginata (Beef tapeworm)
  • Taenia solium (Pork tapeworm)
  • Diphyllobothrium latum (Fish tapeworm)
  • Hymenolepsis nana (Dwarf tapeworm)

  
• Extraintestinal Cestodes


  • Echinococcus granulosus (Dog tapeworm)

  
• Taenia saginata (Beef Tapeworm)


  Intermediate host: cattle, Infective stage: larva (cysticerci), Definitive host: human, Scolex: has no rostellum, Proglottid: rectangular (T.solium: square), Uterine branches: 15-30 (T.solium: 7-15)

  
• Transmission of Taenia saginata (Beef Tapeworm)


  Ingestion of improperly cooked or raw beef containing the cysticercus, Matures into adult worms in small intestine within 3 months
• Taenia solium (Pork Tapeworm)


  Infective stages: egg, larva (cysticercus cellulosae), Autoinfection, Intermediate host: human, Definitive host: human

  
• Transmission of Taenia solium (Pork Tapeworm)


  Ingestion of improperly cooked or raw pork meat, food or water containing the cysticercus cellulosae, Transforms into adult worm in intestines
• Diseases caused by Taenia solium


  • Taeniasis
  • Cysticercosis (result of larval encystation in tissues, most common in skeletal muscles, Neurocysticercosis - most feared, most severe involvement, Ocular cysticercosis - inflammation of uvea and retina)
• Diphyllobothrium latum (Fish Tapeworm)


  The longest tapeworm (13 meters), Operculum - a lid structure at the end of the egg, Scolex has a pair of long sucking grooves, Infective stage: plerocercoid larval stage

  
• Life cycle of Diphyllobothrium latum


  1st intermediate host: copepods, 2nd intermediate host: freshwater fish, Definitive host: human, Ingestion of improperly cooked or raw fish containing the plerocercoid
• Diphyllobothriasis


  Diarrhea, abdominal discomfort, Asymptomatic (most common), Megaloblastic anemia - Vitamin B12 deficiency, parasite competes with the host for vitamin B12 in jejunum and ileum
• Hymenolepsis nana (Dwarf Tapeworm)


  Most common tapeworm recovered in the US, Does not require an obligatory intermediate animal host, Eggs are directly infectious - accidental ingestion of eggs, Hyperinfection syndrome due to autoinfection leading to secondary bacterial infection

  
• Transmission of Hymenolepsis nana


  Ingestion of fecally-contaminated food or water, Touching one's mouth with contaminated fingers, Contaminated soil
• Proper waste disposal & sanitation practices

  Alternative measure
• Adequate cooking of fish

  Main measure
• Freezing of fish for 24-48 hours at -18° C


  May kill the encysted larvae
• Prompt treatment of infected persons

  Helps prevent the spread
• Cestodes (Dwarf Tapeworms)


  • Phylum: Platyhelminthes
  • Parasite: Hymenolepis nana
  • Important Properties: Most common tapeworm recovered in the US, Does not require an obligatory intermediate animal host, Eggs are directly infectious

  
• Mode of Transmission/Life cycle

  1. Ingestion of fecally-contaminated food or water

  2. Touching one's mouth with contaminated fingers

  3. Contaminated soil

  4. Rice or flour beetles containing infective larvae

  5. Rodents

  6. Eggs gains access to human host

  7. Transform into cysticercoid larvae

  8. Mature into adult worms

  9. Self-reproduction

  10. Autoinfection
• Hymenolepiasis


  • Asymptomatic
  • High worm burden: nausea, weakness, loss of appetite, diarrhea, abdominal pain
  • Children with high worm burden: pruritus ani, Headache
• Laboratory Diagnosis


  Fecalysis (+) eggs
• Treatment


  • Praziquantel
  • Niclosamide
• Prevention


  • Proper waste disposal & sanitation practices
  • Control of transport host population and rodent control
  • Proper storage of grains and flour must be observed to prevent infestation with flour and grain beetles
  • Prompt treatment of infected individuals to prevent spread of parasite
• Echinococcus granulosus (Dog Tapeworm or Hydatid Tapeworm)


  Zoonotic type of infection, Definitive host: dogs, Intermediate host: sheep, Accidental or dead-end host: human, Infective stage: eggs, Pathogenic stage: hydatid cyst
• Mode of Transmission/Life cycle


  1. Ingestion of eggs from food & water contaminated by dog feces

  2. Contact with contaminated dog feces

  3. Transform into larva in intestine

  4. Penetrate intestine

  5. Migrate through bloodstream

  6. Other tissues (liver & lungs)

  7. Embryo develops into large, fluid-filled hydatid cysts

  8. Act as space-occupying lesions

  9. Rupture of cyst

  10. Release of antigens

  11. Anaphylaxis

  12. Widespread dissemination of parasite
• Disease


  • Echinococcosis
  • Hydatid Cyst Disease
  • Hydatid disease
  • Hydatidosis
• Laboratory Diagnosis


  Examination of biopsy specimen, Serologic test (ELISA), Radiography to demonstrate the hydatid cyst (CT scan or ultrasound)
• Treatment


  Surgery: removal of cyst, Mebendazole, Albendazole, Praziquantel
• Prevention


  • Improvement of personal hygiene practices
  • Prevention of contamination of food and water with dog feces
  • Avoidance of feeding pet dogs with contaminated viscera
  • Prompt treatment of infected canines and humans to prevent spread of the parasite
  • Chemoprophylaxis should be given to dogs in endemic areas
  • Health education
• Trematodes (Flukes)