Helminths (Worms)

• Helminths


  Parasitic intestinal worms
• Types of helminths

  • Nematodes (roundworms)
  • Cestodes (tapeworms)
  • Trematodes (flukes)
• Helminths

  • Multicellular eukaryotic invertebrates with tube-like or flattened bodies exhibiting bilateral symmetry
  • Triploblastic (with endo-, meso- and ecto-dermal tissues)
  • Flatworms are acoelomate (do not have body cavities)
  • Roundworms are pseudocoelomate (with body cavities not enclosed by mesoderm)
  • Segmented annelids are coelomate (with body cavities enclosed by mesoderm)
  • Many are free-living organisms or parasites in most animals and some plants
• Major assemblages of parasitic helminths

  • Nemathelminthes (roundworms)
  • Platyhelminthes (flatworms)
  • Cestoda (tapeworms)
  • Trematoda (flukes)
• Nematodes (roundworms)


  • Have long thin unsegmented tube-like bodies with anterior mouths and longitudinal digestive tracts
  • Have a fluid-filled internal body cavity (pseudocoelum) which acts as a hydrostatic skeleton
  • Use longitudinal muscles to produce a sideways thrashing motion
  • Form separate sexes with well-developed reproductive systems
• Cestodes (tapeworms)


  • Have long flat ribbon-like bodies with a single anterior holdfast organ (scolex) and numerous segments
  • Do not have a gut and all nutrients are taken up through the integument
  • Do not have a body cavity (acoelomate) and are flattened to facilitate perfusion to all tissues
  • Exhibit slow body flexion produced by longitudinal and transverse muscles
  • Are hermaphroditic and each segment contains both male and female organs
• Trematodes (flukes)


  • Have small flat leaf-like bodies with oral and ventral suckers and a blind sac-like gut
  • Do not have a body cavity (acoelomate) and are dorsoventrally flattened with bilateral symmetry
  • Exhibit elaborate gliding or creeping motion over substrates using compact 3-D arrays of muscles
  • Most species are hermaphroditic (individuals with male and female reproductive systems) although some blood flukes form separate male and female adults
• Helminth life-cycles

  1. Eggs

  2. Larvae

  3. Adults
• Definitive hosts


  Those in which sexual development of helminths occurs
• Intermediate or Paratenic hosts


  Hosts where larval stages of helminths may be found
• Nematode life-cycle

  Egg - larvae (L1-L4) - adult
• Cestode life-cycle

  Egg - metacestode - adult
• Trematode life-cycle

  Egg - miracidium - sporocyst - redia - cercaria - (metacercaria) - adult
• Helminth eggs


  • Have tough resistant walls to protect the embryo while it develops
  • Mature eggs hatch to release larvae either within a host or into the external environment
• Main modes of helminth transmission

  • Faecal-oral
  • Transdermal
  • Vector-borne
  • Predator-prey
• Vector-borne transmission


  Larval stages taken up by blood-sucking arthropods or undergoing amplification in aquatic mollusks
• Predator-prey transmission


  Encysted larvae within prey animals (vertebrate or invertebrate) being eaten by predators where adult worms develop and produce eggs
• Orders of nematodes

  • Trichocephalida (whipworms)
  • Oxyurida (pinworms)
  • Ascaridida (roundworms)
  • Strongylida (hookworms)
  • Rhabditida (threadworms)
  • Camallanida (guinea worms)
  • Spirurida (filarial worms)
• Trichocephalid 'whip-worms'


  • Have long thin anterior ends which they embed in the intestinal mucosa of their hosts
  • Have simple life-cycles where infections are acquired by the ingestion of eggs and emergent larvae molt and mature to adults in the gut
• Oxyurid 'pin-worms'


  • Have small thin bodies with blunt anterior ends
  • Have simple life-cycles, but with an unusual modification where female worms emerge from the anus of their hosts at night and attach eggs to the skin
• Ascarid 'roundworms'


  • Have large bodies with 3 prominent anterior lips
  • Their life-cycles involve a stage of pulmonary migration where larvae released from ingested eggs invade the tissues and migrate through the lungs before returning to the gut to mature as adults
• Strongyle 'hookworms'


  • Have dorsally curved mouths armed with ventral cutting plates or teeth which they embed in host tissues to feed on blood
  • Have complex life-cycles where larvae develop in the external environment (as 'geo-helminths') before infecting hosts by penetrating the skin
• Rhabditid 'threadworms'


  • Have tiny bodies which become embedded in the host mucosa
  • Their life-cycle includes parasitic parthenogenetic females producing eggs which may hatch internally (leading to auto-infection) or externally (leading to transmission of infection or formation of free-living male and female adults)
• Camallanid 'guinea worms'


  • Infect host tissues where the large females cause painful blisters on the feet and legs
  • When hosts seek relief by immersion in water, the blisters rupture releasing live larvae which infect copepods that are subsequently ingested with contaminated drinking water
• Spirurid 'filarial worms'


  • Occur as long thread-like adults in blood vessels or connective tissues of their hosts
  • The large female worms release live larvae (microfilariae) into the blood or tissues which are taken up by blood-sucking mosquitoes or pool-feeding flies and transmitted to new hosts
• Orders of cestodes

  • Cyclophyllidean
  • Pseudophyllidean
• Cyclophyllidean cestodes


  Have terrestrial 2-host life-cycles where adult tapeworms develop in carnivores (scolex with 4 suckers and sometimes hooks) while larval metacestodes form bladder-like cysts in the tissues of herbivores
• Pseudophyllidean cestodes


  Have aquatic 3-host life-cycles, involving the sequential formation of adult tapeworms in fish-eating animals (scolex with 2 longitudinal bothria), procercoid larval stages in aquatic invertebrates (copepods) and then plerocercoid (spargana) stages in fish
• Major groups of trematodes

  • Monogenean trematodes
  • Digenean trematodes
• Monogenean trematodes


  Have complex posterior adhesive organs and direct life-cycles involving larvae called oncomiracidia
• Digenean trematodes


  Have oral and posterior suckers and heteroxenous life-cycles where adult worms infect vertebrates and larval miracidia infect mollusks to proliferate and produce free-swimming cercariae
• Orders of digenean trematodes

  • Echinostomatid fasciolids (liver flukes)
  • Strigeatid schistosomes (blood flukes)
• Echinostomatid fasciolids (liver flukes)


  • Live as adults in hepatic bile ducts of mammals where they cause fibrotic 'pipestem' disease
  • The parasites proliferate in freshwater snails and mammals become infected by ingesting metacercariae attached to aquatic vegetation
• Strigeatid schistosomes (blood flukes)


  • Are unusual in that the adults are not hermaphroditic but form separate sexes which live conjoined in mesenteric veins in mammals
  • Female worms lay eggs which actively penetrate tissues to be excreted in urine/feces or they become trapped in organs where they cause granuloma formation
  • Miracidia released from eggs infect aquatic snails and produce fork-tailed cerceriae which actively penetrate the skin of their hosts
• Flatworms are invertebrates in Phylum Platyhelminthes. Flatworm adaptations include mesoderm, muscle tissues, a head region, and bilateral symmetry. Flatworms are free-living heterotrophs or parasites.
• Roundworms are invertebrates in Phylum Nematoda. Roundworms have a pseudocoelom and complete digestive system. They are free-living heterotrophs or parasites.