6.2 Tissue Nematodes, Filarial Worms, Cestodes, and Trematodes

Tissue Nematodes, Filarial Worms, Cestodes, and Trematodes

This section continues medical helminthology beyond the intestinal soil-transmitted nematodes. It covers nematodes whose diagnostic stage lives in tissue or blood, the tapeworms, and the flukes, with emphasis on species endemic to the Philippines (Schistosoma japonicum, Paragonimus westermani, and the filarial worms). Parasitology carries about 30% of the Microbiology and Parasitology subject in the board blueprint, and life cycles, morphology, and diagnostic specimen choice are the most heavily tested parts. Egg measurements for every helminth are collected in the comparison matrix of the stool examination section.

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1. Tissue Nematodes and Filarial Worms

Trichinella spiralis

• Transmission & Biology: Ingestion of raw or undercooked pork containing encysted infective larvae. Larvae mature into adults in the small intestine, mate, and viviparous females release newborn larvae directly into mesenteric lymphatics and blood. Larvae penetrate striated muscle fibers, transforming them into collagen-encapsulated nurse cells.
• Predilection Sites: Highly oxygenated skeletal muscles with high metabolic rates: diaphragm, tongue, masseter, extraocular muscles, and intercostal muscles.
• Clinical Manifestations & Diagnostics: Cardinal symptoms include marked bilateral periorbital edema, splinter hemorrhages beneath fingernails, severe myalgia, fever, and extreme peripheral blood eosinophilia ($20\%\text{ to }>50\%$). Diagnosis is confirmed by skeletal muscle biopsy demonstrating coiled encysted larvae, serological testing (ELISA), and elevated serum muscle enzymes (creatine kinase [CK], lactate dehydrogenase [LD], and aldolase).

Filarial Nematodes: Wuchereria bancrofti vs. Brugia malayi

Lymphatic filariasis affects coastal and rural regions of the Philippines (Bicol peninsula, Samar, Leyte, Palawan). Speciation relies on microscopic examination of Giemsa-stained thick blood smears:

  Wuchereria bancrofti Microfilaria:
  ┌────────────────────────────────────────────────────────┐
  │ [Sheath] === ( Smooth, graceful body curves ) === [Tail]─────► (Tip clear of nuclei)
  └────────────────────────────────────────────────────────┘

  Brugia malayi Microfilaria:
  ┌────────────────────────────────────────────────────────┐
  │ [Sheath] === ( Angular, stiff, kinked curves ) === [Tail]────► (2 distinct terminal nuclei)
  └────────────────────────────────────────────────────────┘

Other Tissue-Invasive Nematodes Seen in the Philippines

• Angiostrongylus cantonensis (rat lungworm): Humans acquire infective larvae by eating raw or undercooked snails (such as the golden apple snail and other freshwater snails), slugs, or contaminated vegetables. The larvae migrate to the central nervous system and cause eosinophilic meningitis. The key laboratory clue is a CSF eosinophil count above 10% of leukocytes; larvae are rarely seen.
• Toxocara canis and T. cati: Ingested dog or cat roundworm eggs hatch in humans, but the larvae cannot mature and wander through tissues (visceral and ocular larva migrans). Stool examination is negative because no adult worms develop; diagnosis relies on eosinophilia and serology (ELISA).

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2. Medical Cestodology: Tapeworms of Clinical Significance

Cestodes (tapeworms) are dorsoventrally flattened, ribbon-like hermaphroditic platyhelminths lacking a digestive tract. The adult worm consists of a scolex (attachment organ), an unsegmented neck (germinative zone), and a strobila comprised of chains of proglottids (immature, mature, and gravid).

Taenia saginata (Beef Tapeworm) vs. Taenia solium (Pork Tapeworm)

Both species shed identical spherical eggs measuring $31\text{--}43\ \mu\text{m}$ featuring a thick, brown, radially striated embryophore containing a 6-hooked hexacanth embryo (oncosphere). Speciation requires examination of the scolex or gravid proglottid injected with India ink:

Clinical Warning: Neurocysticercosis

Infection with Taenia solium eggs (via fecal-oral auto-infection or contaminated food/water) produces neurocysticercosis, the leading cause of acquired adult epilepsy in developing nations. Oncospheres encyst in brain parenchyma, ventricles, and subarachnoid spaces, presenting with intractable seizures, focal neurological deficits, and intracranial hypertension.

Diphyllobothrium latum (Broad Fish Tapeworm)

• Scolex: Almond-shaped / spatulate, lacking suckers or hooklets; attaches via two longitudinal, dorsal and ventral sucking grooves (bothria).
• Gravid Proglottid: Distinctly wider than long ($3\text{ mm long} \times 10\text{--}20\text{ mm wide}$); contains a central, rosette-shaped (coiled) dark uterine structure that releases eggs continuously through a uterine pore (tocostome).
• Egg Morphology: $58\text{--}75\ \mu\text{m} \times 40\text{--}50\ \mu\text{m}$; broadly oval, yellow-brown; possesses an operculum at one pole with an inconspicuous seated opercular rim, and a small, distinct abopercular knob at the opposite pole.
• Transmission & Megaloblastic Anemia: Ingestion of raw or pickled freshwater fish (salmon, trout, pike) containing plerocercoid larvae (sparganum). The worm has an intense avidity for cyanocobalamin, absorbing up to $80\%\text{ to }100\%$ of dietary vitamin B12 in the proximal jejunum, resulting in hypersegmented neutrophils, macrocytic megaloblastic anemia, and subacute combined degeneration of the spinal cord.

Hymenolepis nana (Dwarf Tapeworm) vs. Hymenolepis diminuta (Rat Tapeworm)

• Hymenolepis nana:
  • Size: Smallest cestode infecting humans ($25\text{--}40\text{ mm}$ long).
  • Scolex: 4 suckers, retractile rostellum armed with a single circle of 20 to 30 hooks.
  • Egg: $30\text{--}47\ \mu\text{m}$, spherical to subspherical, thin outer shell; inner embryophore possesses two distinct polar thickenings from which 4 to 8 wavy polar filaments arise, radiating into the space between the shell and the hexacanth embryo.
  • Direct Life Cycle & Autoinfection: H. nana is the only tapeworm capable of completing its entire life cycle in humans without requiring an intermediate host. Ingested eggs hatch in the duodenum, penetrate villi to become cysticercoids, and re-emerge into the lumen as mature adult worms. Internal autoinfection produces heavy parasite loads in institutionalized pediatric populations.
• Hymenolepis diminuta:
  • Egg: $60\text{--}80\ \mu\text{m}$, larger, yellowish-brown; polar thickenings are present on the inner membrane, but polar filaments are completely absent. Requires an arthropod intermediate host (grain beetle, mealworm).

Echinococcus granulosus (Hydatid Tapeworm)

• Definitive Host: Dogs and other canids (adult worm has only 3 proglottids: immature, mature, gravid). Humans are accidental intermediate hosts through ingesting eggs shed in dog feces.
• Hydatid Cyst Anatomy: Slow-growing unilocular fluid-filled cysts in the liver ($70\%$) and lungs ($20\%$). The cyst wall consists of an outer cellular adventitial layer (host fibrous reaction), an intermediate thick, acellular, laminated, elastic cuticle, and an inner nucleated germinal layer.
• Hydatid Sand: The germinal layer buds internally to produce brood capsules and thousands of invaginated protoscolices, which detach to form free "hydatid sand". Diagnostic needle aspiration has traditionally been avoided because cyst leakage can cause anaphylactic shock and secondary seeding; percutaneous treatment, when used, is done under scolicidal and antiparasitic drug cover.

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3. Medical Trematodology: Blood, Liver, Intestinal, and Lung Flukes

Trematodes (flukes) are unsegmented, dorsoventrally flattened, leaf-shaped platyhelminths possessing oral and ventral suckers (acetabulum). All medically important flukes are hermaphroditic (monoecious) except Schistosoma species, which are dioecious (unisexual) with cylindrical bodies. All flukes require a freshwater snail as their mandatory first intermediate host.

Schistosoma Species (Blood Flukes)

Schistosomes reside within the venous plexuses of humans. Infection occurs via cutaneous penetration by free-swimming, fork-tailed cercariae released from infected aquatic snails.

  Schistosoma japonicum Egg:               Schistosoma mansoni Egg:                 Schistosoma haematobium Egg:
  ┌─────────────────────────┐              ┌─────────────────────────┐              ┌─────────────────────────┐
  │      ( Rounded )        │              │       ( Elongated )     │              │      ( Elongated )      │
  │   • Small lateral knob  │              │   • Large, prominent,   │              │   • Distinct, sharp     │
  │     or rudimentary hook │              │     sharp lateral spine │              │     terminal spine      │
  └─────────────────────────┘              └─────────────────────────┘              └─────────────────────────┘
     Venous Habitat: Superior                 Venous Habitat: Inferior                 Venous Habitat: Vesical
     mesenteric veins (feces)                 mesenteric veins (feces)                 venous plexus (urine)

Intestinal and Liver Flukes

• Fasciolopsis buski (Giant Intestinal Fluke):
  • Largest trematode parasitizing humans ($20\text{--}75\text{ mm} \times 8\text{--}20\text{ mm}$). Resides in duodenum and jejunum.
  • Egg: $130\text{--}140\ \mu\text{m} \times 80\text{--}85\ \mu\text{m}$; large, ellipsoidal, golden-yellow, with an indistinct operculum and unsegmented internal ovum. Morphologically indistinguishable from Fasciola hepatica.
  • Transmission: Ingestion of freshwater plants (water chestnuts, water caltrop, morning glory) carrying encysted metacercariae.
• Fasciola hepatica (Sheep Liver Fluke):
  • Resides in the intrahepatic bile ducts of sheep, cattle, and humans. Acquired by ingesting contaminated freshwater watercress. Causes mechanical bile duct obstruction, hepatic parenchymal necrosis during larval tunneling, biliary colic, and intense eosinophilia.
• Clonorchis sinensis (Chinese Liver Fluke):
  • Adult worm resides in distal biliary capillaries.
  • Egg Morphology: $27\text{--}35\ \mu\text{m} \times 12\text{--}19\ \mu\text{m}$; flask-shaped or pitcher-shaped, yellowish-brown; features prominent opercular shoulders (a thickened circum-opercular collar) and a small, comma-shaped abopercular knob at the opposite end. Shell exhibits a delicate wrinkled "melon-skin" surface texture.
  • Transmission & Carcinogenesis: Ingestion of raw, salted, or pickled freshwater cyprinoid fish containing metacercariae. Chronic infection induces biliary duct hyperplasia, recurrent pyogenic cholangitis, and cholangiocarcinoma (bile duct cancer).

Paragonimus westermani (Oriental Lung Fluke)

• Endemicity in the Philippines: Highly endemic in Sorsogon, Camarines Sur, Eastern Samar, Leyte, and Mindoro.
• Egg Morphology: $80\text{--}120\ \mu\text{m} \times 45\text{--}65\ \mu\text{m}$; golden-brown, broadly oval, asymmetrical (one side slightly flattened); possesses an operculum surrounded by a distinct thickened opercular rim (collar); the abopercular shell end is characteristically thickened and wavy.
• Diagnostic Specimen: Recovered in viscous, rusty-brown sputum (often described as flecked with "iron filings") or in feces following swallowing of respiratory secretions.
• Transmission & Intermediate Hosts: First intermediate host is a freshwater snail (Brotia asperata). Second intermediate host is a freshwater mountain crab (Sundathelphusa philippina) or freshwater crayfish. Infection follows ingestion of raw or pickled crab preparations.
• Clinical Mimicry: Encysted adults in lung parenchyma produce cavitary fibrous cysts that rupture into bronchioles. Patients present with chronic hemoptysis, pleuritic chest pain, productive cough, and cavitary lesions on chest radiographs that frequently lead to misdiagnosis as pulmonary tuberculosis (PTB).

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4. Diagnostic Specimen and Infection-Route Summary