Nematodes (Nemathelminthes) – Comprehensive Study Notes

General characteristics of Nematodes (Phylum Nemathelminthes)

  • Adult nematodes are elongated, cylindrical worms with bilateral symmetry.
  • Anterior end may bear hooks, teeth, plates, and papillae for abrasion, attachment, and sensory response.
  • Body wall structure: outer hyaline, noncellular cuticle; subcuticular epithelium; a layer of muscle cells.
  • Alimentary tract is a simple tube from mouth to anus; no circulatory system.
  • Digestive system is generally complete with a muscular pharynx that is characteristically triradiate.
  • Life cycle: Egg → larva → adult.
  • Reproduction: separate sexes (dioecious); some may be parthenogenetic.
    • Female: larger; pointed posterior; no spicule.
    • Male: small; curved posterior; with spicule.
  • Modes of egg production in females:
    • A. Oviparous
    • B. Oviviparous / Ovolviparous
    • C. Viviparous / Larviparous
  • Sensory organs:
    • Amphids: cephalic chemoreceptors located at anterior end.
    • Phasmids: caudal chemoreceptors.
    • Worms with phasmids are called phasmid nematodes; those without are aphasmid.

Classification by presence of phasmids

  • Among medically important nematodes, only 3 are aphasmid (Adenophorea).
    • Aphasmid (without caudal chemoreceptors) = Adenophorea.
    • Phasmid (with caudal chemoreceptors) = Secernentia; the rest.

Class Secernentia (Phasmidia) – examples of medical/public health importance

  • Enterobius vermicularis
  • Ascaris lumbricoides
  • Ancylostoma duodenale
  • Necator americanus
  • Strongyloides stercoralis
  • Trichostrongylus spp.
  • Anisakis spp.
  • Wuchereria bancrofti
  • Brugia malayi
  • Loa loa
  • Onchocerca volvulus
  • Mansonella perstans
  • M. ozzardi
  • M. streptocerca
  • Dracunculus medinensis
  • Angiostrongylus cantonensis

Classification by habitat of the adult worms

  • Small intestine: duodenum, jejunum, ileum; includes Capillaria spp.; Ascaris lumbricoides; Strongyloides stercoralis; hookworms; Trichinella spiralis (adult in small intestine) among others.
  • Large intestine / colon: Trichuris trichiura; Enterobius vermicularis (adult colon).
  • Extraintestinal habitats include:
    • Lymph nodes and lymph vessels (filarial worms such as Wuchereria, Brugia).
    • Eyes and meninges (rare filariae).
    • Encysted in host muscle (e.g., Trichinella spiralis).
    • Encysted larva in muscle: Trichinella spiralis.
  • Filarial worms inhabit the lymphatic system; adult worms often reside in lymphatics or subcutaneous tissues; microfilariae circulate in blood.

Ascaris lumbricoides – Giant intestinal roundworm

  • Common name: Giant intestinal roundworm.
  • Epidemiology: Most common intestinal nematode of humans; frequent in the tropics.
  • Vector: Flies and cockroaches.
  • Group: Soil-transmitted helminths (STH); part of the HAT (Heart-Lung) migratory group.
  • Habitat: Small intestine.
  • Final host: Man.
  • Diagnostic stage: Eggs found in stool (fertilized or unfertilized).
  • Infective stage: Embryonated egg.
  • Life cycle (brief):
    • Adults inhabit the small intestine.
    • Eggs pass in feces in undeveloped form; in warm, moist soil they embryonate.
    • Ingested by humans; larvae hatch in the small intestine and migrate (larval migration) before maturing into adults.
  • Pathogenesis and clinical features:
    • Heart-Lung migration (Loeffler’s syndrome): cough, pneumonia-like symptoms during larval migration; eosinophilia; Charcoal Leyden crystals (eosinophil degradation products).
    • Adult worms can form a bolus and cause intestinal obstruction.
  • Egg morphology (fertile vs infertile):
    • Eggs can be fertilized or unfertilized.
    • Fertilized eggs are corticated; unfertilized eggs are decorticated (shell lacking cortication).
    • Unfertilized eggs contain lecithin granules; fertilized eggs lack lecithin granules.
    • Egg structure (three layers):
    • Inner vitelline/lipoidal/lecithin layer (present in fertilized eggs; absent in unfertilized eggs).
    • Middle glycogen layer.
    • Outer layer: albuminous/mammillated/corticated protective coat.
  • Egg morphometrics (example references):
    • Fertilized egg: corticated; presence of inner vitelline membrane (absent in unfertilized).
    • Unfertilized egg: lacks vitelline membrane; lecithin granules present; decorticated or corticated status differs by fertilization state.
  • Life cycle highlights: adult in small intestine; eggs in feces; embryonation in soil; embryonated egg ingested; larval hatch and migration; maturation to adult.
  • Diagnostics: stool examination for eggs.
  • Laboratory/clinical notes: part of the diagnostic framework for helminths.
  • Treatment: Albendazole or Mebendazole.

Enterobius vermicularis – Pinworm (Oxyuris vermicularis)

  • Common name: Pinworm or social worm.
  • Habitat: Large intestine (colon).
  • Final host: Man.
  • Diagnostic stage: Embryonated egg.
  • Infective stage: Embryonated egg.
  • Modes of transmission (MOT): Inhalation, ingestion, autoinfection (external autoinfection possible).
  • Adult morphology: Small, whitish or brownish worm; anterior end with lateral wings (cephalic alae).
  • Female behavior: Gravid female migrates to perianal region to deposit eggs; then dies; autoinfection possible.
  • Egg morphology: Elongated, flattened on one side; D-shaped and embryonated; double-layered (albuminous layer and lipoidal layer); no glycogen layer.
  • Life cycle:
    • Adult in colon; gravid female migrates to perianal region to deposit eggs.
    • Egg ingestion or autoinfection leads to hatching in small intestine and development into adults.
  • Clinical symptoms: Pruritus ani, especially nocturnal itching; sleep disturbance; possible extraintestinal autoinfection (reproductive tract involvement).
  • Laboratory diagnosis: Stool exam for eggs; perianal swab; Scotch tape test (cellulose tape swab).
  • Treatment: Mebendazole or Albendazole.
  • Additional notes: This is the most common helminth infection in humans in some settings; high emphasis on personal hygiene and cleanliness to prevent autoinfection.

Trichuris trichiura – Whipworm

  • Common name: Whipworm.
  • Habitat: Large intestine/colon.
  • Final host: Man.
  • Diagnostic stage: Ova found in stool.
  • Infective stage: Embryonated egg.
  • MOT: Ingestion of embryonated egg.
  • Adult morphology: Freshly colored or pinkish-gray; slender or attenuated anterior.
  • Egg morphology: Barrel-shaped with bipolar mucus plugs; football-shaped; resembles a Japanese lantern.
  • Life cycle:
    • Adults in colon.
    • Eggs pass in feces and embryonate in warm, moist soil.
    • Embryonated eggs ingested by humans; larvae hatch in the small intestine; penetrate and develop in villi; return to lumen and migrate to cecum to mature.
  • Pathology and diseases: Trichuriasis and trichocephaliasis; whipworm infection.
  • Symptoms: Bloody or mucoid diarrhea; weight loss; abdominal pain; rectal prolapse in heavy infection.
  • Laboratory: Stool examination for eggs (DS).
  • Treatment: Albendazole or Mebendazole.

Capillaria philippinensis – Pudoc worm / Pudoc disease

  • Common name: Pudoc worm / Pudoc disease (Capillariasis).
  • Habitat: Small intestine.
  • Final host: Man.
  • Natural host: Migratory birds.
  • Intermediate host: Fresh water fish / brackish water fish.
  • Diagnostic stage: Larva and/or ova found in stool.
  • Infective stage: Larval stages in fish meat.
  • MOT: Ingestion of raw or undercooked fish containing infective larvae.
  • Adult morphology: Delicate tiny worms; male with chitinized spicule; females typically have eggs in utero.
  • Egg morphology: Bipolar mucus plugs; striated; smaller than Trichuris egg; guitar- or peanut-shaped.
  • Viewing notes: Egg measurements cited include size around 45 × 21 μm (peanut shape).
  • Pathology: Malabsorption; borborygmi; altered throughput of bowel.
  • Laboratory: Stool exam for eggs/larvae (DS).
  • Treatment: Albendazole or Mebendazole.

Hookworms – Ancylostoma and Necator spp.

  • Common hookworms:
    • Ancylostoma duodenale (Old World hookworm)
    • Necator americanus (New World hookworm; “American murderer”)
    • Ancylostoma braziliense (Cat hookworm)
    • Ancylostoma caninum (Dog hookworm)
  • Habitat: Small intestine.
  • Final host: Man.
  • Diagnostic stage: Egg or larva.
  • Infective stage: L3 filariform larva (sheathed).
  • Mode of transmission (MOT): Skin penetration by filariform larva.
  • Adult morphology: Variable; buccal cavity shapes differ; notable features include teeth (Necator) vs cutting plates (Ancylostoma duodenale).
  • Eggs: Ovoidal, thin-shelled, colorless; contain a germ cell with 2–8 blastomeres.
  • Species-specific morphology (selected notes):
    • Necator americanus: S-shaped (hook) body; two dentate cutting plates?; copulatory bursa bipartite.
    • Ancylostoma duodenale: C-shaped; two pairs of teeth; copulatory bursa tripartite; biting mouthparts more elaborate.
  • Life cycle:
    1) Adults in small intestine produce eggs in feces.
    2) Eggs hatch in soil to rhabditiform larvae.
    3) After two molts, develop into infective filariform larvae (L3).
    4) Filariform larvae penetrate human skin, enter lymphatics and blood, migrate to lungs, ascend to respiratory tract, are swallowed, and mature in the small intestine.
  • Disease and symptoms:
    • Larval penetration: “ground itch” (allergic reaction at penetration site) in humans; in animals may cause creeping eruption.
    • Larval migration: heart–lung migration.
    • Adult infection: enteritis and abdominal pain; chronic blood loss leading to iron-deficiency anemia.
  • Laboratory diagnosis: Stool exam for eggs; Harada Mori/Laboratory culture techniques (e.g., Harada-Mori culture).
  • Treatment: Albendazole or Mebendazole.
  • Visual notes: Morphology of adult and larval stages depicted in figures (scale and structure vary by species).

Strongyloides stercoralis – Threadworm

  • Common name: Threadworm.
  • Notable features: Smallest nematode infecting humans; can be free-living or parasitic (facultative).
  • Habitat: Small intestine (parasitic female).
  • Diagnostic stages: Rhabditiform larva or filariform larva; infective larval stage is L3 (filariform) unsheathed.
  • Lifecycle traits: Free-living form can reproduce in the environment; autoinfection possible in humans.
  • Mode of transmission: Skin penetration.
  • Developmental notes: Rhabditiform larvae in stool; L3 larvae are infective.
  • Laboratory/diagnosis: Stool examination for rhabditiform/filari-form larvae; Baermann technique; Harada-Mori culture; treatment with ivermectin (drug of choice) or albendazole.
  • Morphology: Distinctive larval forms and hypodermis; adults reside in the small intestine.

Trichinella spiralis – Trichina worm

  • Common name: Muscle worm / Trichina worm.
  • Habitat:
    • Adult: small intestine.
    • Larva: encysted in striated muscle.
  • Final host: Pigs (natural host).
  • Diagnostic stage: Encysted larva in muscle biopsy.
  • Infective stage: Encysted larva within muscle tissue of infected meat.
  • MOT: Ingestion of undercooked or raw meat containing encysted larvae.
  • Adult morphology: Male with conical papillae aiding copulation; female with club-shaped uterus.
  • Life cycle:
    1) Adult resides in small intestine.
    2) Viviparous female releases larvae that penetrate intestinal mucosa and disseminate via bloodstream.
    3) Larvae encyst in striated muscle of host.
    4) In meat-eating hosts (humans, pigs), infection occurs when encysted larvae are ingested.
    5) Larvae freed in the intestine of the new host mature to adults.
  • Pathology and symptoms: Larval migration can cause fever, facial edema, eosinophilia; muscle pain at encystation sites (believed in limbs and facial regions); difficulty swallowing and breathing when muscles are affected.
  • Laboratory: Muscle biopsy to detect encysted larvae; serologic tests (immunotests).
  • Treatment: Removal of muscle with encysted larvae; thiabendazole; prednisone.

Filarial worms

  • Major species covered include Wuchereria bancrofti and Brugia malayi; Dirofilaria immitis (dog heartworm) is also discussed in this group.
  • General features:
    • Habitat: Adults reside in lymphatics (Wuchereria, Brugia) or subcutaneous tissues (some filariae).
    • Microfilariae circulate in blood; final host is human; intermediate host typically mosquitoes or other arthropods; infective larvae transmitted by arthropod bites.
    • Diagnostic stage: Microfilaria in blood; some species have tissue-dwelling microfilariae; others may be found in lymphatics or skin.
    • Disease manifestations include lymphatic damage leading to elephantiasis, hydrocele, and occult filariasis (pulmonary or systemic symptoms with no demonstrable microfilariae in blood).
  • Wuchereria bancrofti vs Brugia malayi – distinguishing features (morphology and staining):
    • W. bancrofti: larger, with a continuous nuclei pattern; sheath present; two terminal nuclei; microfilaria often in nocturnal blood.
    • Brugia malayi: shorter; nuclei do not extend to the tail tip; two terminal nuclei present; microfilaria often nocturnal in blood.
  • Diagnostic and treatment notes:
    • Knott’s technique for concentrating microfilariae in blood.
    • Capillary blood sampling; diethylcarbamazine (DEC) and ivermectin as treatments.
  • Dirofilaria immitis – Dog heartworm:
    • Humans infected unintentionally typically present with solitary nodules or coin lesions in the lung (coin lesions) or subcutaneous nodules rather than full adult heartworm disease.

Dirofilaria immitis – Dog heartworm

  • Common name: Dog heartworm.
  • Human infections typically present as solitary nodules in the lung or subcutaneous nodules; rarely cause full systemic heartworm disease in humans.

Dracunculus medinensis – Guinea worm

  • Common name: Guinea worm, fiery serpent.
  • Habitat: Subcutaneous tissue.
  • Intermediate host: Aquatic crustacean (copepods).
  • Life cycle:
    1) Intermediate host is an aquatic copepod containing larvae.
    2) Ingestion of contaminated water introduces larvae into human intestine.
    3) Larvae emerge and migrate to subcutaneous tissues.
    4) Gravid female migrates to surface and forms a painful ulcer; when in contact with water, female releases larvae to continue cycle.
  • Notable historic/public health relevance: eradication campaigns have targeted Dracunculus transmission via safe drinking water and copepod control.

Angiostrongylus cantonensis – Rat lungworm (Parastrongylus cantonensis)

  • Common name: Rat lungworm.
  • Natural definitive host: Rats.
  • Intermediate host: Mollusks (snails/slugs).
  • Transmission to humans: Ingestion of mollusks or foods containing infective larvae (often via contaminated produce or improperly washed mollusks).
  • Pathology: Larvae migrate to brain and spinal cord causing eosinophilic meningitis.
  • Adult morphology: Female worm described as Barber’s pole appearance.

Anisakis spp. – Herring’s worm

  • Common name: Herring’s worm; causes anisakiasis (Herring’s disease).
  • Epidemiology: Common in Japan; associated with raw or undercooked fish.
  • Life cycle:
    • Final hosts: whales or dolphins.
    • First intermediate host: copepods.
    • Second intermediate host: smaller fish.
    • Third intermediate host: larger fish (e.g., salmon).
    • Humans become accidental hosts by consuming raw/undercooked fish containing larvae.
  • Pathology: Abdominal pain and granulomatous reactions around migrating larvae in the intestinal wall.

Laboratory, prevention, and treatment highlights

  • Laboratory diagnosis (general for intestinal nematodes): stool examination for eggs or larvae; perianal swabs and tape tests for Enterobius; scotch tape methods are widely used for pinworm eggs.
  • Common treatment options across nematodes: Albendazole and Mebendazole are widely used across species; ivermectin is particularly noted for Strongyloides and general nematode infections.
  • Public health notes: Emphasize safe water, environmental sanitation, hygiene education, and regular deworming as part of WASH (Water, Sanitation, Hygiene) initiatives to prevent soil-transmitted helminth infections.

Quick reference: key diagnostic and life-cycle highlights by genus

  • Ascaris lumbricoides
    • Diagnostic stage: Egg in stool; Fertile or infertile eggs.
    • Infective stage: Embryonated egg.
    • Life cycle emphasis: Heart-Lung migration during larval part of life cycle.
  • Enterobius vermicularis
    • Diagnostic stage: Embryonated egg (perianal region sampling).
    • Typical diagnostic test: Scotch tape swab.
    • Unique feature: Autoinfection capability via perianal region exposure.
  • Trichuris trichiura
    • Diagnostic stage: Ova in stool.
    • Egg morphology: Barrel-shaped with bipolar plugs.
  • Capillaria philippinensis
    • Infective stage: Larvae in raw/undercooked fish; diagnostic stage: larvae or eggs in stool.
    • Egg morphology: Bipolar plugs; guitar/peanut shape.
  • Hookworms (Ancylostoma/Necator)
    • Infective stage: L3 filariform larva (skin penetration).
    • Pathology: Ground itch; heart-lung migration; iron-deficiency anemia in chronic infection.
  • Strongyloides stercoralis
    • Diagnostic stages: Rhabditiform and filariform larvae; L3 infective.
    • Treatment: Ivermectin (drug of choice).
  • Trichinella spiralis
    • Infective stage: Encysted larvae in muscle meat.
    • Diagnostic stage: Encysted larvae in muscle biopsy.
  • Filarial worms (Wuchereria bancrofti, Brugia malayi)
    • Habitat: Lymphatics (adult worms); microfilariae in blood.
    • Diagnostics: Blood smears; Knott’s technique; capillary blood sampling.
    • Treatments: Diethylcarbamazine (DEC), ivermectin.
  • Dirofilaria immitis
    • Human presentation: Solitary nodules or coin lesions in lungs.
  • Dracunculus medinensis
    • Key feature: Ulcer formation with emergence of adult female; transmission through drinking water containing copepods.
  • Angiostrongylus cantonensis
    • Human disease: Eosinophilic meningitis; migration of larvae to brain/spinal cord.
  • Anisakis spp.
    • Disease: Anisakiasis from ingestion of raw fish; larvae cause granulomas.

Connections to foundational principles and practical relevance

  • Nematodes illustrate fundamental parasitology concepts: direct life cycles vs complex life cycles with multiple hosts; site-specific localization (intestinal vs tissue-dwelling).
  • Public health relevance: Soil-transmitted helminths (e.g., Ascaris, Trichuris, hookworms) are deeply tied to sanitation, hygiene, and education; interventions like WASH reduce transmission.
  • Diagnostic strategies reflect life-cycle stages: stool egg exams detect eggs/larvae; perianal swabs detect Enterobius eggs; blood-based microfilariae require special concentration techniques (Knott).
  • Treatment choices across nematodes emphasize broad-spectrum anthelmintics (albendazole, mebendazole) and targeted therapies (ivermectin, diethylcarbamazine) depending on species and life stage.
  • Ethical and practical implications: Control programs must balance food safety (Capillaria philippinensis with fish-borne transmission), water quality (Dracunculus and Angiostrongylus), and food handling practices to minimize infection risks.

Formulas and numerical notes (examples in the transcript)

  • Larval sizes and dimensions cited for specific stages:
    • Immature newly hatched rhabditiform larvae (hookworm lineage): approximately 270  μm×15  μm270\ \,\mu m\times 15\ \,\mu m
    • Size range at 5 days old for some rhabditiform larvae: 540 – 700 μm540\,\text{--}\,700\ \mu m long.
  • Typical adult lengths (examples):
    • Necator americanus: about 9–12 mm9\text{--}12\ \text{mm} long; width 0.25−−0.5 mm0.25--0.5\ \text{mm}.
    • Ancylostoma duodenale: about 5−−10 mm5--10\ \text{mm} long; width 0.2−−0.4 mm0.2--0.4\ \text{mm}.
  • Egg morphology descriptors (as described):
    • Ascaris eggs: ovoid; fertilized vs unfertilized differences in shell and membranes; layers: inner vitelline/lipoidal/lecithin layer; middle glycogen layer; outer corticated shell.
    • Capillaria and Trichuris eggs: shapes (peanut/guitar for Capillaria; barrel-shaped with bipolar plugs for Trichuris).
  • These measurements are for reference in identifying species during microscopic examination.

Summary and exam-ready takeaways

  • Nematodes are a diverse phylum with a range of life cycles, host specificities, and disease manifestations.
  • Key diagnostic approaches rely on stage-specific specimens: eggs in stool (Ascaris, Trichuris, hookworms), embryonated eggs in perianal swabs (Enterobius), and microfilariae in blood (filarial worms).
  • Public health control of intestinal nematodes hinges on improving water, sanitation, hygiene, and regular deworming programs.
  • Treatment regimens often involve broad-spectrum benzimidazoles (albendazole, mebendazole) with adjuncts like ivermectin for certain infections (e.g., Strongyloides, filarial infections).
  • Understanding morphology (e.g., egg shape, buccal cavity structure, asexual vs sexual reproduction features) is essential for accurate species identification and appropriate treatment.