Helminth

Aka: Helminth, Parasitic Worm, Worms, Helminthiasis, Tapeworm, Cestode, Fluke, Trematode, Platyhelminth, Flatworm, Nematode, Roundworm

advertisement

II. Definitions

  1. Helminth (Parasitic Worm)
    1. Parasitic invertebrates, divided into Flatworms (Tapeworms and Flukes) and Roundworms
  2. Platyhelminth (Flatworms)
    1. Bilaterally symmetrical Flatworms divided into Tapeworms (Cestodes) and Flukes (Trematodes)
  3. Nematode (Roundworm)
    1. Round-bodied, unsegmented, Parasitic Worms

III. Epidemiology

  1. Helminths are the most common infectious cause in the developing world
  2. In contrast, less common in returning travelers than other forms of infectious disease (Bacteria, virus or Parasite)
    1. Helminth infections are unusual if travel less than 1 month
    2. Infection may be associated with decreased Eosinophils

IV. Pathophysiology

  1. Helminths generate no significant immune response while alive
    1. Immune response is typically robust to eggs and dead Helminths
  2. Helminths stimulate an immune response when they migrate out of the intestinal tract
    1. Extraintestinal migration to other human tissue results in Eosinophilia
  3. Organisms that remain in the intestinal tract (without migrating to other human tissue) do NOT cause Eosinophilia
    1. Trichuris trichiura (Whipworm)
    2. Enterobius Vermicularis (Pinworm)

V. Causes: Flatworms (Platyhelminths)

  1. Background
    1. Flatworms lack their own intestinal tract, and must obtain metabolized molecules from their environment
      1. Trematodes (Blood Flukes) reside in host veins and ingest nutrients from the blood
      2. Cestodes (Tapeworms) reside in the host intestinal tract and take in digested nutrients
    2. Sexual reproduction differs between organsim classes
      1. Trematodes undergo sexual reproduction between male and female adult worms
      2. Cestodes are hermaphrodites, having both male and female organs within the same worm
        1. Single worm can produce fertilized eggs
  2. Blood Flukes (Trematodes)
    1. Exposed skin penetration
      1. Schistosomiasis
      2. Trichobilharzia schistosomes (Swimmer's Itch)
  3. Tapeworms (Cestodes)
    1. Background
      1. Tapeworms are long and flat
      2. Tapewoms are composed in a chain of proglotitids (box-like segments)
      3. Tapeworm head (scolex) contains suckers, and for some species hooks
    2. Ingestion of Tapeworm larvae in undercooked or raw fish or meat
      1. Taenia Solium (Pork Tapeworm, Cysticercosis)
        1. Primary infection with Pork Tapeworm is via ingestion of raw pork with encysted larvae
        2. However, Cysticercosis arises from ingestion of Tapeworm eggs
      2. Taenia Saginata (Beef Tapeworm)
      3. Diphyllobothrium Latum (Freshwater Fish Tapeworm)
    3. Ingestion of fertilized Tapeworm eggs
      1. Echinococcosis (Hydatid Disease)
        1. Hydatid Disease (Echinococcus Granulosus)
        2. Alveolar Hydatid Disease (Echinococcus Multilocularis)
      2. Hymenolepis nana (Dwarf Tapeworm)
        1. Shortest of the Tapeworms that infect humans (1.5 to 5 cm)
        2. Oral-fecal transmission between humans (without intermediate hosts)
        3. May present with Abdominal Pain, and in some cases Nausea, Vomiting

VI. Causes: Roundworms (Nematodes)

  1. Intestinal Nematodes (worms mature in human intestinal tract)
    1. Ingestion of Nematode eggs
      1. Ascaris Lumbricoides
        1. Larvae emerge from ingested eggs in Small Intestine and penetrate intestinal wall
        2. Larvae migrate to lung, are coughed, swallowed and mature in the intestinal tract
      2. Trichuris trichiura (Whipworm)
        1. Remain in intestinal tract without Eosinophil response
      3. Enterobius Vermicularis (Pinworm)
        1. Remain in intestinal tract without Eosinophil response
    2. Ingestion of encysted Nematode larvae (in pig Muscle)
      1. Trichinella spiralis (Trichinosis)
    3. Exposed Skin penetration by Nematode larvae
      1. Ancylostoma duodenale or Necator americanus (Hookworm)
        1. Eggs hatch in soil, develop into filariform larvae, penetrate between barefoot toes
        2. Larvae migrate to lung, are coughed, swallowed and mature in the intestinal tract
      2. Strongyloides Stercoralis
        1. Filariform larvae, penetrate between barefoot toes OR ingested larvae penetrate bowel wall
        2. Larvae migrate to lung, are coughed, swallowed and mature in the intestinal tract
  2. Blood and tissue Nematodes
    1. Vector Borne (Filarial Infections)
      1. Background: Filarial Infections (Filariae, Family Filarioidea)
        1. Blood and tissue Nematodes transmitted by Arthropod Bite
        2. Adult filariae, living in the Lymphatic System, birth prelarval microfilariae
        3. Microfilariae invade tissue and circulate in blood and lymph
          1. Microfilariae are transmitted between human hosts via Arthropod Bite
        4. Filariae are hosts to an endosymbiotic, Rickettsial-like Bacteria, Wolbachia
          1. Killing Wolbachia (e.g. doxycyline) decreases microfilaria and renders adult worms sterile
      2. Onchocerca Volvulus (River Blindness)
        1. Via Black Fly bite (near rivers and streams in Africa, Central America and South America)
      3. Lymphatic Filariasis (Wuchereria Bancrofti or Brugia Malayi)
        1. Wuchereria Bancrofti (via Mosquito Bite in pacific islands or Africa)
        2. Brugia Malayi (via Mosquito Bite in southeast asia)
    2. Ingestion
      1. Dracunculus Medinensis
        1. Via ingestion of Nematode larvae in contaminated fresh water (Africa, Middle East or India)
      2. Visceral Larva Migrans (Toxocariasis)
        1. Via egg ingestion of Toxocara canis from dogs or Toxocara cati from cats
    3. Skin Penetration
      1. Cutaneous Larva Migrans (Ancylostoma braziliense, Ancylostoma caninum)
        1. Via skin penetration by Tapeworm larvae from dog or cat feces
        2. Occurs in tropical and subtropical regions (Southeast Asia, Africa, South America, Caribbean)
        3. Occurs in southeastern U.S.

VII. Causes: Presentations

  1. Soil Transmitted Helminths (STH)
    1. Worldwide the most common intestinal Parasites worldwide (2.6 billion infections/year)
    2. Ascaris Lumbricoides (Roundworm)
    3. Ancylostoma duodenale or Necator americanus (Hookworm)
    4. Trichuris trichiura (Whipworm)
  2. Transmission from Dogs and Cats
    1. Visceral Larva Migrans
    2. Ocular Larva Migrans
    3. Cutaneous Larva Migrans
  3. Transmission by Ingestion
    1. Ingestion of larvae (e.g. undercooked or raw fish or meat, contaminated water)
      1. Taenia Solium (Pork Tapeworm, Cysticercosis)
      2. Taenia Saginata (Beef Tapeworm)
      3. Diphyllobothrium Latum (Freshwater Fish Tapeworm)
      4. Trichinella spiralis (Trichinosis, pork)
      5. Dracunculus Medinensis
    2. Ingestion of fertilized eggs
      1. Echinococcosis (Hydatid Disease)
      2. Hymenolepis nana (Dwarf Tapeworm)
      3. Ascaris Lumbricoides
      4. Trichuris trichiura (Whipworm)
      5. Enterobius Vermicularis (Pinworm)
      6. Visceral Larva Migrans (Toxocariasis)
  4. Exposed skin penetration
    1. Schistosomiasis
    2. Trichobilharzia schistosomes (Swimmer's Itch)
    3. Cutaneous Larva Migrans (Ancylostoma braziliense, Ancylostoma caninum)
    4. Ancylostoma duodenale or Necator americanus (Hookworm)
    5. Strongyloides Stercoralis
  5. Vector-Borne Helminths
    1. Onchocerca Volvulus (River Blindness, via Black Fly bite)
    2. Lymphatic Filariasis (Wuchereria Bancrofti or Brugia Malayi via Mosquito Bite)
  6. Nematode Larvae that migrate to the lungs (and are coughed, swallowed and mature in the intestinal tract)
    1. Ascaris Lumbricoides
    2. Ancylostoma duodenale or Necator americanus (Hookworm)
    3. Strongyloides Stercoralis

VIII. Management

  1. See specific organism management
  2. Synthetic benzimidazole derivatives (e.g. Mebendazole, Albendazole) are effective against most Roundworms
    1. Ascaris Lumbricoides (Roundworm)
    2. Trichuris trichiura (Whipworm)
    3. Enterobius Vermicularis (Pinworm)
    4. Trichinella spiralis (Trichinosis)
    5. Ancylostoma duodenale or Necator americanus (Hookworm)
    6. Strongyloides Stercoralis
  3. Pyrantel Pamoate (alternative agent, safe in pregnancy)
    1. Ascaris Lumbricoides (Roundworm)
    2. Enterobius Vermicularis (Pinworm)
    3. Ancylostoma duodenale or Necator americanus (Hookworm)

IX. Prevention

  1. See Prevention of Foodborne Illness

X. References

  1. Gladwin, Trattler and Mahan (2014) Clinical Microbiology, Medmaster, Fl, p. 362-81
  2. Alshami (2023) Helminthiasis, StatPearls, Treasure Island, Fl +PMID: 32809360 [PubMed]
  3. Hotez (2008) J Clin Invest 118(4):1311-21 +PMID: 18382743 [PubMed]

Images: Related links to external sites (from Bing)

These images are a random sampling from a Bing search on the term "Helminth." Click on the image (or right click) to open the source website in a new browser window. Search Bing for all related images

Related Topics in Helminth Infestation

advertisement