General Nematode Characteristics

• Adult nematodes are elongated, cylindrical worms with bilateral symmetry.
• Anterior end may have hooks, teeth, plates, and papillae for abrasion, attachment, and sensory response.
• Body wall organization: (1) outer hyaline, non-cellular cuticle, (2) subcuticular epithelium, (3) a layer of muscle cells.
• Alimentary tract is a simple tube from mouth to anus; no circulatory system.
• Digestive system: generally a complete digestive tract with a muscular pharynx that is characteristically triradiate; no circulatory system.
• Life cycle: Egg stage → Larval stage → Adult stage.
• Reproduction: separate sexes (dioecious); some may be parthenogenetic.
  • Female: larger, pointed posterior, no spicules.
  • Male: smaller, curved posterior, with spicule.
• Female worms produce eggs by:
  • A. Oviparous
  • B. Oviviparous / Ovoliviparous
  • C. Viviparous / Larviparous

Amphids and Phasmids (Chemoreceptors)

• Amphids: cephalic chemoreceptors located at the anterior end.
• Phasmids: caudal chemoreceptors located at the posterior end.
• Phasmids presence defines phasmid nematodes; absence defines aphasmid nematodes.

Aphasmid vs Phasmid Classification

• Among medically important nematodes, only a few are aphasmid (without caudal chemoreceptors)namely
  • Aphasmid (Adenophorea): examples include TTC (text indicates a shorthand/abbreviation in slides; specific taxa may vary in other sources).
• Phasmid (with caudal chemoreceptors) = Secernentia (the rest of the nematodes).

Classification by Habitat of the Adult Worms

• Small intestine: Ascaris lumbricoides, Strongyloides stercoralis, Hookworms, Trichinella spiralis (adult stage in small intestine for some, larval forms are migratory), Trichuris trichiura (in large intestine though), Enterobius vermicularis (large intestine).
• Duodenum / Jejunum / Ileum: sites for several intestinal nematodes.
• Large intestine (colon): Trichuris trichiura, Enterobius vermicularis, others.
• Extraintestinal nematodes: tissues such as lymph nodes, lymph vessels, eyes and meninges; encysted in host muscle; filarial worms in blood; etc.
• Filarial worms (Wuchereria bancrofti, Brugia spp.) inhabit lymphatic system; loiasis and other tissue sites vary.
• Encysted in host muscle: Trichinella spiralis (larvae encyst in striated muscle).
• Encysted in host tissues (muscle) and migratory larvae account for extraintestinal disease.

Class Secernentia (Phasmidia) – Notable Members

• 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

A. Ascaris lumbricoides (Giant intestinal roundworm)

• Common name: Giant intestinal roundworm.
• Most common intestinal nematode in humans; frequent in the tropics.
• Vector: Flies and cockroaches.
• Member of soil-transmitted helminths (STH; HATS).
• Part of the group with heart-lung migration (ASH).
• Habitat: Small intestine.
• Final host: Man.
• Diagnostic stage: Ova found in stool (fertilized or unfertilized).
• Infective stage: Embryonated egg.
• Life-cycle highlights:
  1) Adults reside in small intestine.
  2) Undeveloped eggs in feces embryonate in warm, moist soil.
  3) Embryonated egg ingested by man.
  4) Larva hatch in small intestine and begin larval migration, then mature into adults.
• Symptoms/pathology: Pneumonia, cough, fever with eosinophilia during larval migration; vomiting and abdominal pain; potential intestinal perforation.
• Prevention/control (WASHED framework): safe water, environmental sanitation, hygiene education, regular deworming.
• Heart–Lung migration (Loeffler’s syndrome) features: eosinophilia; Charcoal Leyden crystals (eosinophil degradation products).
• Adult worms can form a bolus when grouped.

Egg characteristics (fertile vs infertile)

• Eggs can be fertile or infertile.
• Unfertilized eggs contain lecithin granules; fertilized eggs develop respective layers.
• Egg walls consist of three layers:
  • Inner vitelline / lipoidal / lecithin layer (not found in unfertilized eggs)
  • Middle glycogen layer
  • Outer corticated / albumin coat (protection)
• Fertilized vs unfertilized eggs can be distinguished by layers and cortication status.

Egg illustrations (fertilized vs unfertilized)

• Fertilized eggs: corticated (+), decorticated (-) for unfertilized eggs; inner vitelline membrane present.
• Unfertilized eggs: no vitelline membrane; lecithin granules present; glyocen layer present; non-corticated.

Egg dimensions and morphology examples

• Egg dimensions (e.g., with measurements as seen in slides):
  • Fertilized egg: included in descriptions of cortication and vitelline membrane.
  • Unfertilized egg: lacks vitelline membrane and may show lecithin granules.
• Size-related notes are provided in slide figures (not in standard text): common references include-scale images for egg features.

Ascaris life cycle (summary)

• Lifecyle steps:
  1) Adults in small intestine.
  2) Embryonated eggs in feces contaminate soil.
  3) Humans ingest embryonated eggs.
  4) Larvae hatch in the intestine and migrate (through liver and lungs) before returning to the small intestine to mature.

Symptoms, pathology, and prevention

• Pulmonary symptoms during larval migration: pneumonia-like symptoms, cough, fever; eosinophilia.
• Abdominal symptoms: vomiting, abdominal pain.
• Severe complications: intestinal perforation (rare but possible).
• Prevention: safe water, sanitation, hygiene, deworming; integrated in WASH initiatives.

Laboratory diagnosis and treatment

• Diagnostic method: Stool examination for eggs (fertilized or unfertilized).
• Treatment: Albendazole and Mebendazole.

B. Enterobius vermicularis (Pinworm)

• Also known as Oxyuris vermicularis.
• Habitat: large intestine.
• Final host: Man.
• Diagnostic stage: embryonated egg.
• Infective stage: embryonated egg.
• Mode of transmission (MOT): inhalation, ingestion; autoinfection (external).

Adult nematode and egg morphology

• Adult worm: small, whitish or brownish; anterior end with lateral wings or cephalic alae.
• Male: dies after copulation.
• Female: migrates to perianal region to oviposit; dies; may autoinfect.
• Egg: elongated, flattened on one side, D-shaped; embryonated; double-layered: albuminous and lipoidal; no glycogen layer.
• Size/appearance for reference: typical eggs are small and easily detected by perianal swab or scotch tape test.

Life cycle

1) Adult in colon; gravid female migrates to perianal region to deposit eggs.
2) Eggs ingested; hatch in small intestine; larvae released and mature into adults.
3) Eggs deposited in perianal region lead to autoinfection.

Symptoms and pathology

• Pruritus ani (nocturnal pruritus ani).
• Sleep disturbance due to itching.
• Extraintestinal autoinfection: vagina, uterus, fallopian tubes (rare).
• Laboratory diagnosis: stool exam for eggs (low yield); perianal swab; Scotch tape or cellulose tape swab during morning.

Treatment and notes

• Most common helminth infection in some settings; treatment options include Mebendazole or Albendazole.
• Transmission is strongly linked to personal hygiene and perianal contamination.

C. 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: slender anterior end, thicker posterior; the anterior portion is threadlike and the posterior is thicker.
• Egg: barrel-shaped with bipolar plugs; football-shaped; resembles a Japanese lantern.

Life cycle

1) Adult in the colon.
2) Undeveloped eggs shed in feces; eggs embryonate in warm, moist soil.
3) Embryonated eggs ingested by humans.
4) Larvae hatch in the small intestine, penetrate villi, return to lumen, migrate to cecum, mature to adults.

Symptoms and pathology

• Trichuriasis and related conditions (trichocephaliasis).
• Bloody or mucoid diarrhea; abdominal pain; weight loss.
• Rectal prolapse with heavy infections.
• Laboratory diagnosis: stool exam for eggs.
• Treatment: Albendazole or Mebendazole.

D. Capillaria philippinensis (Pudoc worm)

• Common name: Pudoc worm; Pudoc disease / Mystery disease.
• Close relative of Trichuris trichiura.
• Habitat: small intestine.
• Final host: Man; Natural host: migratory birds.
• Intermediate host: freshwater or brackish water fish.
• Diagnostic stage: larva and/or ova in stool.
• Infective stage: larval stages in infected fish.
• MOT: ingestion of raw/undercooked fish bearing the infective larva.

Adult and egg morphology

• Adult: delicate tiny worms; male with chitinized spicule; females usually have eggs in utero.
• Egg: bipolar mucus plugs; striated; smaller than Trichuris egg; guitar- or peanut-shaped.
• Size reference: approximately 45 imes 21 \noldsymbol{oldsymbol{\mu m}} for Capillaria philippinensis eggs.

Symptoms and pathology

• Malabsorption, borborygmi, alternating bowel movements (LBM).
• Laboratory diagnosis: stool exam for eggs/larvae.
• Treatment: Albendazole or Mebendazole.

E. Hookworm

• Includes:
  1) Ancylostoma duodenale (Old World Hookworm)
  2) Necator americanus (New World Hookworm)
  3) Ancylostoma braziliense
  4) Ancylostoma caninum
• Habitat: small intestine.
• Final host: Man.
• Diagnostic stage: Egg/Larva.
• Infective stage: L3 / filariform larva (sheathed).
• MOT: skin penetration.

Adult, eggs, and basic morphology

• Eggs: ovoidal, thin-shelled, colorless; inside a germ cell with 2–8 blastomeres.
• Species differences include buccal cavity shape, presence/absence of teeth, and copulatory bursa structure:
  • Necator americanus: S-shaped buccal cavity; simple copulatory bursa with two digits.
  • Ancylostoma duodenale: C-shaped buccal cavity; tripartite copulatory structures; 2 digits (bipartite).
• Typical sizes of adult worms:
  • Necator americanus: $9-12 ext{ mm} imes 0.25-0.5 ext{ mm}$ (length × width).
  • Ancylostoma duodenale: $5-10 ext{ mm} imes 0.2-0.4 ext{ mm}$ (length × width).

Life cycle

1) Adults in small intestine.
2) Eggs shed in feces; eggs hatch into rhabditiform larvae in soil.
3) After 2 molts become filariform larvae.
4) Filariform larvae penetrate skin; enter lymphatics and bloodstream; migrate to lungs; ascend to pharynx; swallowed; reach small intestine and mature.

Symptoms and pathology

• Larval penetration: skin/site reaction (ground itch); animal hookworms can cause creeping eruption.
• Larval migration: heart–lung migration.
• Adults: enteritis and abdominal pain; chronic blood loss leading to iron-deficiency anemia.

Laboratory and treatment

• Stool examination for eggs (DS).
• Harada Mori or culture techniques can be used for larval growth.
• Treatment: Albendazole or Mebendazole.

F. Strongyloides stercoralis (Threadworm)

• Common name: Threadworm; smallest nematode infecting humans.
• Can be free-living or parasitic (facultative).
• Habitat: small intestine (parasitic form).
• Diagnostic stages: rhabditiform larvae and filariform larvae; lifecycle involves unsheathed L3 in human infection.
• MOT: skin penetration.

Morphology and life stages

• Rhabditiform larva: non-sheathed; shorter buccal cavity; genital primordium not prominent.
• Filariform larva: sheath present; long esophagus; pointed tail; infective larva (L3).
• Egg: rhabditiform-like eggs in stool; sometimes misidentified as hookworm eggs.
• Key comparative features: Strongyloides has a distinctive buccal capsule and very prominent genital primordium in larvae.

Life cycle (summary)

• Free-living and parasitic cycles; autoinfection possible.
• In parasitic cycle: larvae penetrate skin, migrate via bloodstream to lungs, ascend to pharynx, swallowed, mature in small intestine.

Laboratory diagnosis and treatment

• Stool exam: presence of rhabditiform or filariform larvae.
• Baermann technique for larval recovery.
• Harada Mori culture technique.
• Treatment: Albendazole; drug of choice: Ivermectin.

G. Trichinella spiralis (Muscle worm / Trichina worm)

• Common name: Muscle worm / Trichina worm.
• Habitat: adult in small intestine; larvae encysted in striated muscle.
• Final host: Pigs (classically).
• Diagnostic stage: encysted larva in muscle biopsy.
• Infective stage: encysted larva in undercooked meat; humans infected by ingestion of infected meat.
• Adult: males have conical papillae that aid copulation; females have club-shaped uterus.
• Histology: nurse cell development surrounding larva (stichosome structure) in muscle tissue.

Life cycle (summary)

1) Adult in small intestine.
2) Viviparous female releases larvae that penetrate intestinal mucosa and disseminate via bloodstream.
3) Larvae encyst in striated muscle.
4) Infection occurs when humans ingest encysted larvae in undercooked meat.
5) Larvae released from muscle during digestion mature to adults in the intestine.

Symptoms and pathology

• Larval migration: fever, facial edema, eosinophilia.
• Muscle pain and edema at encystation sites (often eyelids, arms, legs).
• Difficulty swallowing and breathing when muscles involved.
• Adult worms in small intestine cause diarrhea and abdominal pain.

Laboratory and treatment

• Diagnostic: muscle biopsy showing encysted larvae; immunotests.
• Treatment: removal of muscle with encapsulated larvae; thiabendazole or prednisone.

H. Filarial Worms (Lymphatic and Subcutaneous Filariases)

• Includes Wuchereria bancrofti and Brugia malayi.

General features

• Wuchereria bancrofti: Smooth movement; typical microfilaria in blood; size around 240–300 μm.
• Brugia malayi: Slightly different morphology; microfilariae around 200–280 μm.
• Habitat: adults live in lymphatics; microfilariae circulate in blood.
• Final host: Man. Intermediate host: mosquitoes (vector-transmitted) or other arthropods depending on species.
• Diagnostic stage: microfilaria found in blood; infective stage: microfilaria in vector; filariform larvae in tissue after transmission.
• MOT: bite of infected mosquito/arthropod.

Life cycle (summary)

1) Adults in lymphatics and lymphoid tissues; microfilariae circulate in blood.
2) Microfilariae ingested by intermediate host (IH) during blood meal.
3) Microfilariae develop into infectious larvae (IS) in IH.
4) Infective larvae enter the human host when IH transmits during a mosquito bite.
5) Larvae migrate and mature to adults in human tissue.

Symptoms and pathology

• General: lymphatic obstruction leading to elephantiasis; hydrocele; occult filariasis (pulmonary or asthmatic symptoms with no detectable microfilariae in blood).
• Tropic pulmonary eosinophilia can occur during larval migration.

Laboratory and treatment

• Diagnosis: Knott’s technique (to detect microfilariae in blood); capillary blood tests.
• Treatment: Diethylcarbamazine (DEC); ivermectin for some filarial infections.

I. Dirofilaria immitis (Dog Heartworm)

• Common name: Dog heartworm.
• Human infection: often presents as solitary peripheral nodules in the lung (coin lesions) or subcutaneous nodules.
• In humans, infections are typically not patent and do not produce adult worms in most cases; presentation is usually nodules in the lungs.

J. Dracunculus medinensis (Guinea Worm)

• Common name: Guinea worm, Fiery Serpent.
• Habitat: subcutaneous tissue.
• Intermediate host: aquatic crustacean (copepod).
• Life cycle highlights:
  1) Infectious copepod is ingested with drinking water.
  2) Larvae released in the intestine; migrate to subcutaneous tissue.
  3) Gravid female migrates to surface skin; forms an ulcer.
  4) Contact with water causes rupture of the worm and release of larvae to continue the cycle.

K. Parastrongylus (Angiostrongylus) cantonensis (Rat Lungworm)

• Common name: Rat lungworm.
• Natural/definitive host: Rats.
• Intermediate host (IH): Mollusks (snails, slugs).
• Transmission: ingestion of IH containing infective larvae by humans.
• Pathology: larvae migrate to brain and spinal cord causing eosinophilic meningitis.
• Adult worm: female with barber’s pole appearance.

L. Anisakis spp. (Herring’s worm)

• Common name: Herring’s worm; associated with Herring disease.
• Geography: common in Japan.
• Definitive hosts: whales and dolphins.
• First IH: copepods; second IH: smaller fishes; third IH: larger fishes (e.g., salmon).
• MOT: ingestion of raw fish infected with larva.
• Symptoms and pathology: abdominal pain and granuloma around migrating larva in the intestinal wall.

Practical notes and identifiers across nematodes

• Phasmids presence helps distinguish phasmid nematodes from aphasmid nematodes.
• Heart–lung migration is a hallmark of Ascaris and other helminths during larval travel to the lungs.
• The “unholy three” (historical/trade-off term) and other mnemonic references appear in lecture slides; practical lab focus remains on stool examination, perianal swabs, tissue biopsies, and serology where appropriate.
• Common diagnostic tools across listed nematodes include stool ova analysis, perianal swabs, Baermann or Harada Mori culture methods, muscle biopsy for Trichinella, and specific serological/immunologic tests for filarial infections.
• Standard anthelmintic treatments mentioned include Albendazole, Mebendazole, and Ivermectin, with DEC specifically noted for filarial infections.

Note: All measurements and sizes are cited from the included slides; key numerical references include:

• Hookworms: adult lengths approximately $9-12\,\mathrm{mm}$ (Necator americanus) and $5-10\,\mathrm{mm}$ (Ancylostoma duodenale); widths $0.25-0.5\,\mathrm{mm}$ and $0.2-0.4\,\mathrm{mm}$ respectively.
• Ascaris rhabditiform larvae initial size: $270\,\mu\mathrm{m} \times 15\,\mu\mathrm{m}$; length at 5 days: $540-700\,\mu\mathrm{m}$.
• Filarial worms: Wuchereria bancrofti microfilaria length: $240-300\,\mu\mathrm{m}$; Brugia malayi microfilaria length: $200-280\,\mu\mathrm{m}$.
• Capillaria philippinensis eggs: $45 \times 21\,\mu\mathrm{m}$.
• Capillaria eggs and Trichuris eggs are morphologically distinct with characteristic shapes and shell structures (bipolar plugs for Trichuris; bipolar plugs with striation for Capillaria).

• Connections to foundational principles:

  • Nematodes show diverse life cycles, including direct infection via ingestion, skin penetration, or intermediate hosts, reflecting ecological adaptation and transmission strategies.
  • The presence of a cuticle and pseudocoelom supports their classification within Ecdysozoa and pseudocoelomate body plan.
  • Helminth infections illustrate host–parasite coevolution and public health implications, including the role of sanitation, vector control, and education in disease prevention (WASH framework).

• Ethical, philosophical, and practical implications:

  • Emphasizes the importance of sanitation and personal hygiene as ethical and public health responsibilities.
  • Highlights global health disparities in tropical regions where parasitic infections are most prevalent.
  • Demonstrates the need for integrated control programs (deworming, education, clean water) as sustainable approaches to reduce disease burden.

• Real-world relevance:

  • The organisms covered are classic models in medical parasitology and remain relevant for understanding STH transmission, vector-borne filariasis, and tissue-invasive nematodes.

• Quick reference summary (selected highlights):

  • Ascaris lumbricoides: embryonated eggs in soil → ingestion → larval migration → pulmonary symptoms → intestinal disease; heavy infections can cause intestinal blockage; eggs are morphologically eggs with lecithin layers.
  • Enterobius vermicularis: perianal egg deposition with autoinfection; scotch tape test is commonly used; nocturnal pruritus.
  • Trichuris trichiura: whipworm; barrel-shaped eggs with bipolar plugs; rectal prolapse in heavy infections.
  • Capillaria philippinensis: similar to Trichuris but with lineage through raw fish; pesudo-tissue migration causing malabsorption.
  • Hookworms: skin penetration causing ground itch; heart–lung migration; iron-deficiency anemia due to chronic blood loss.
  • Strongyloides stercoralis: autoinfection can persist; diagnosis via Baermann or culture; ivermectin is effective.
  • Trichinella spiralis: encysted larvae in muscle; ingestion of undercooked meat; muscle pain and edema.
  • Filariases: lymphatic filariasis and related diseases; vector control essential; DEC and ivermectin as treatments.
  • Dirofilaria immitis: dog heartworm; human exposure leads to nodules in lungs.
  • Dracunculus medinensis: waterborne transmission via copepods; classic ulcer and release of worm when in contact with water.
  • Angiostrongylus cantonensis: rat lungworm; eosinophilic meningitis.
  • Anisakis spp.: foodborne zoonosis from raw fish; migratory larvae cause abdominal symptoms.