STH OVERVIEW

• Class Nematoda - nema means threadlike
• nematodes are the most abundant and widespread animal group
• Considered as parasites of both animals and plants
• Nematodes are elongated, cylindrical, bilaterally symetrical, unsegmented with tapering ends
• body is covered with tough cuticle which may be smooth, striated, bosses or spiny
• Nematodes are generally termed as roundworms
• Adults may vary in size

  1. < 5m - T. spiralis, S. stercoralis
  2. 1m - D. medinensis
• Males are smaller than females
• the posterior end of males are curved or coiled ventrally
• sex organs are located at the posterior part
• adult forms are unsegmented (no proglotids)
• nematodes possesses a mouth, esophagus, and anus
• reproduction of nematodes - diecious
• fertilization of nematodes - oviparous, ovoviviparous, larviparous
• mode of transmission

  1. ingestion
  2. penetration of larvae to surfaces
  3. anthropod vector
• Oviparous

  1. Laying unsegmented eggs - Ascaris lumbricoides and Trichuris trichiura
  2. Laying segmented eggs - Ancylostoma duodenale (30%) and Necator americanus (70%)
• Viviparous

  • Dracunculus medinensis
  • Wuchureria bancrofti
  • Brugia malayi
  • Trichinella spiralis
• Ovoviviparous

  • Strongyloides stercoralis
• Daily output of eggs per female worm is:

  1. Ascaris lumbricoides - > 200,00
  2. Strongyloides stercoralis - few eggs
• Infective stage is usually third-stage lavae (L3)
• Man is the optimum host, except for Trichinella spiralis (pig is the optimum host, man is alternative host)
• it only requires one host (except Dracunculoidea and Filarioidea)
• STH reside in:

  Small intestine

  1. Ascaris lumbricoides
  2. Ancylostoma duodenale
  3. Necator americanus
  4. Strongyloides stercoralis

  Large intestine

  1. Trichuris trichiura

  Tissue dwelling

  1. Trichinella spiralis

  Blood and tissue

  1. Lymphatic and Non lymphatic filaria
• fecal debris/ artifacts/ confusers/ pseudo parasites can be confused as parasite
• larvae in eggs hatch in soil into infective larvae that penetrates human skin
• Infective stage is embryonated egg
• When hookworm is found, it is not required to state the scientific name especially when DFS (direct fecal smear) is used
• Geohelminths (common names)

  1. Ascaris lumbricoides - giant intestinal roundworm
  2. Ancylostoma duodenale - old world hookworm
  3. Necator americanus - new world hookworm
  4. Strongyloides stercoralis - pin worn
  5. Trichuris trichiura - whipworm