19.2 Gastrointestinal Infections II: Amebiasis, Giardiasis & Cryptosporidiosis, Soil-Transmitted Helminths, Pinworm, Capillariasis and Tapeworms

Why this area matters

The TOS gastrointestinal area names:

• ascariasis;
• hookworm infection;
• tapeworms;
• pinworm;
• amebic dysentery.

Soil-transmitted helminths remain common in Filipino children, contributing to anemia, undernutrition and poor school performance, which is why the DOH runs mass deworming. Questions describe a child with anemia and barefoot exposure, a patient with dysentery or a liver abscess, or a seizure in a pork-eating community. You then name the parasite, the infective and diagnostic stages, the egg or trophozoite morphology and the drug of choice.

Stool examination and diagnostic stages

• Direct fecal smear (saline and iodine): shows motile trophozoites in fresh liquid stool, and cysts and eggs.
• Formalin-ether concentration: increases sensitivity for cysts and eggs.
• Kato-Katz thick smear: counts eggs per gram of stool to grade infection intensity. It is used in surveys for soil-transmitted helminths and schistosomiasis.
• Cellophane (Scotch) tape test: used for Enterobius eggs on perianal skin.
• Special tests:
  • modified acid-fast stain for Cryptosporidium;
  • stool antigen tests for Giardia and E. histolytica;
  • agar plate culture or Baermann technique for Strongyloides larvae.
• Infective versus diagnostic stage:
  • the cyst is infective for the intestinal protozoa;
  • the embryonated egg is infective for Ascaris, Trichuris and Enterobius;
  • the filariform larva is infective for hookworm and Strongyloides;
  • the cysticercus is infective for intestinal taeniasis;
  • the egg of T. solium is infective for cysticercosis.

Intestinal Protozoan Infections

Entamoeba histolytica (Amebiasis)

Entamoeba histolytica is an invasive pseudopod-forming protozoan transmitted via the fecal-oral route through ingestion of mature, quadrinucleate cysts from fecally contaminated food or drinking water. Cysts resist gastric acidity; excystation in the terminal ileum yields eight motile trophozoites (15 to 20 um in diameter, containing a single spherical nucleus with a small, central karyosome and fine, evenly distributed peripheral chromatin).

Pathogenesis & Tissue Invasion

Trophozoites adhere to colonic mucosal enterocytes via the galactose/N-acetylgalactosamine (Gal/GalNAc) lectin. Upon contact, they release amoebapores (pore-forming peptides) and cysteine proteases that lyse host enterocytes, neutrophils, and extracellular matrix. Trophozoites breach the mucosa and spread laterally in the submucosa, producing characteristic flask-shaped ulcers marked by a narrow mucosal neck and broad submucosal base.

Diagnostic Hallmark: Erythrophagocytosis

During tissue invasion, trophozoites actively engulf host erythrocytes. The microscopic visualization of trophozoites containing ingested red blood cells (erythrophagocytosis) within fresh stool or tissue aspirates is the pathognomonic diagnostic hallmark of pathogenic E. histolytica. This feature definitively distinguishes E. histolytica from the morphologically identical, non-pathogenic commensal amoebae Entamoeba dispar and Entamoeba moshkovskii (which never exhibit erythrophagocytosis and cannot produce tissue destruction).

Amebic Liver Abscess

Invasive trophozoites can erode submucosal mesenteric venules, traveling via the portal circulation to the liver. This causes liquefactive necrosis of hepatic parenchyma, resulting in an amebic liver abscess. Amebic liver abscesses are classically solitary, located in the posterosuperior aspect of the right hepatic lobe. Percutaneous needle aspiration yields thick, odorless, reddish-brown fluid resembling "anchovy paste" or chocolate sauce (composed of necrotic, liquefied hepatocytes and degraded cellular debris; live trophozoites reside in the advancing periphery of the abscess wall, not in the necrotic center).

Pharmacotherapy

Curative management requires sequential dual-agent therapy:

1. Tissue Amebicide: Metronidazole (750 mg orally TID for 7 to 10 days) or Tinidazole (2 g orally once daily for 3 to 5 days) to destroy invasive trophozoites in the bowel wall and liver parenchyma.
2. Luminal Amebicide: Immediately followed by a non-absorbable luminal amebicide—such as Diloxanide furoate (500 mg TID for 10 days), Paromomycin, or Iodoquinol—to eradicate surviving cysts within the intestinal lumen and prevent asymptomatic carrier transmission.

Giardia duodenalis (G. lamblia / G. intestinalis)

Giardia duodenalis is a flagellated, non-invasive intestinal protozoan that colonizes the human duodenum and upper jejunum. Ingestion of oval, quadrinucleate cysts from contaminated water sources (frequent among campers, day-care centers, and flood-prone communities) initiates infection.

Morphology & Motility

• Trophozoite: Pear-shaped (12 to 15 um), bilaterally symmetrical, possessing two prominent ovoid nuclei each containing a large central karyosome, giving the characteristic appearance of a "face with glasses" or "old man looking at you". It carries four pairs of flagella and a concave, rigid ventral sucking disk (composed of microtubules, alpha-tubulin, and giardin).
• Motility: Demonstrates a distinctive, erratic, wobbling "falling-leaf" motility on direct saline wet mount microscopy of diarrheal feces.

Pathophysiology & Clinical Disease

Unlike E. histolytica, Giardia does not invade tissue, does not enter the bloodstream, and never produces bloody stools. Trophozoites attach firmly to the brush border of duodenal enterocytes via suction generated by the ventral disk. This mechanical barrier and associated host inflammatory responses lead to shortening and blunting of intestinal microvilli, loss of disaccharidase enzymes (causing acquired lactose intolerance), and impaired micellar solubilization of dietary lipids.

Patients experience explosive, non-bloody, voluminous, foul-smelling, greasy diarrhea (steatorrhea), marked abdominal distension, sulfurous eructations (burps smelling of rotten eggs), and significant malabsorption with weight loss. Diagnosis is confirmed via stool antigen ELISA, direct fluorescent antibody (DFA) testing, or wet mount microscopy. Treatment of choice is Metronidazole (250 mg TID for 5 to 7 days) or single-dose Tinidazole (2 g).

Cryptosporidium hominis & Cryptosporidium parvum

Cryptosporidium is an obligate intracellular, extracytoplasmic coccidian protozoan that resides within the microvillar brush border of intestinal enterocytes. It is transmitted via thick-walled, environmentally stable oocysts that are highly resistant to standard municipal water chlorination.

Diagnostic Staining: Modified Acid-Fast Stain

On standard fecal wet mount preparations, Cryptosporidium oocysts are easily overlooked because they are small (4 to 6 um in diameter) and non-pigmented. They are definitively identified using the modified Kinyoun acid-fast stain, which stains the spherical oocysts bright carmine red against a blue-green background. Each mature oocyst contains four motile, banana-shaped sporozoites.

Clinical Spectrum & Management

In immunocompetent hosts, cryptosporidiosis presents as an acute, self-limited, watery secretory diarrhea resolving spontaneously within 10 to 14 days. In severely immunocompromised individuals—most notably patients with advanced HIV/AIDS with CD4 counts < 100/uL—Cryptosporidium causes intractable, voluminous, cholera-like watery diarrhea (losing up to 10 to 15 liters of fluid daily), complicated by severe electrolyte wasting, malabsorption, and ascending sclerosing cholangitis. In immunocompetent patients, Nitazoxanide shortens symptom duration; in patients with HIV/AIDS, the only definitive, curative intervention is prompt immune reconstitution via Antiretroviral Therapy (ART).

Soil-Transmitted Helminths (STHs)

Soil-Transmitted Helminthiases (STHs) are caused by nematodes whose eggs or larvae require an incubation period in warm, moist soil to become infective. In the Philippines, the Department of Health (DOH) oversees the Integrated Helminth Control Program (IHCP), administering school-based Mass Drug Administration (MDA) twice a year using single-dose Albendazole (400 mg) or Mebendazole (500 mg) for children.

Ascaris lumbricoides (Giant Intestinal Roundworm)

Ascaris lumbricoides is the largest intestinal nematode infecting humans (females measure 20 to 35 cm in length). Infection occurs via ingestion of fully embryonated eggs from fecally contaminated soil, water, or raw vegetables.

• Pulmonary Migration Phase: Ingested eggs hatch in the duodenum, liberating rhabditiform larvae that penetrate the intestinal wall. Larvae migrate via mesenteric venules to the liver, then travel via the vena cava and right heart into the pulmonary circulation. They break into alveolar airspaces, ascend the bronchial tree and trachea, are coughed up, and swallowed back into the esophagus to mature into adult worms in the jejunum (a 10- to 14-day transpulmonary journey). This phase can incite Löffler syndrome: transient eosinophilic pneumonitis presenting with cough, dyspnea, wheezing, transient migratory pulmonary infiltrates, and marked peripheral blood eosinophilia.
• Intestinal Pathology: Adult worms feed on luminal chyme. Heavy worm burdens in young children can form large, tangled boluses that cause mechanical intestinal obstruction at the ileocecal valve. Adult worms exhibit high wandering propensity in response to stress, fever, or general anesthesia, migrating into the biliary tree to cause biliary colic, ascending suppurative cholangitis, acalculous cholecystitis, and acute pancreatitis, or perforating through the appendix.
• Egg Morphology:
  • Fertilized Egg: Golden-brown, broadly rounded or oval ($45-75 \times 35-50\text{ um}$), possessing a thick shell surrounded by an irregular, coarsely mammillated, tuberculated albuminous coat; decorticated eggs lack this bumpy outer coat.
  • Unfertilized Egg: Elongated and larger (up to about 90 um), with a thinner shell and disorganized, amorphous internal granular yolk.

Trichuris trichiura (Whipworm)

Trichuris trichiura is an intestinal nematode whose adult worm (3 to 5 cm) resembles a whip: the anterior three-fifths is attenuated, slender, and thread-like, while the posterior two-fifths is thickened and stout. Infection is acquired by ingesting embryonated eggs; there is NO heart-to-lung pulmonary migration phase—larvae hatch directly in the small intestine and migrate straight to the cecum and ascending colon, where the slender anterior body embeds deep within the glandular crypts of the colonic mucosa.

• Egg Morphology: Highly characteristic barrel-shaped or football-shaped eggs ($50-54 \times 22\text{ um}$) with a smooth, golden-brown triple shell and prominent, transparent, mucoid bipolar plugs at each end ("lemon-like" appearance).
• Pathology & Clinical Disease: Heavy worm burdens in malnourished children induce Trichuris dysentery syndrome: chronic, recurrent bloody mucoid stools, abdominal pain, severe tenesmus, failure to thrive, and profound microcytic hypochromic iron deficiency anemia. Chronic intense straining during defecation combined with mucosal inflammation and loss of tone in the pelvic musculature leads to the classic clinical hallmark of pediatric rectal prolapse (visible adult whipworms attached to the exteriorized, hyperemic rectal mucosa). Treatment requires a 3-day course of Albendazole (400 mg daily) or Mebendazole (100 mg BID), as single-dose regimens exhibit suboptimal efficacy against Trichuris.

Hookworms: Necator americanus & Ancylostoma duodenale

Hookworms are blood-feeding nematodes of the upper small intestine. Humans acquire infection when infective filariform larvae in moist, warm, shaded soil actively penetrate unbroken skin (typically barefoot agricultural laborers):

• Species Differentiation:
  • Necator americanus (New World Hookworm): The predominant hookworm across the Philippines. Buccal capsule features a pair of ventral and dorsal semilunar cutting plates.
  • Ancylostoma duodenale (Old World Hookworm): Buccal capsule features two pairs of curved ventral cutting teeth; consumes substantially more blood per worm per day ($0.15-0.25\text{ mL/day}$) than Necator ($0.03\text{ mL/day}$).
• Life Cycle & Clinical Course: Larval skin penetration produces a pruritic, maculopapular, erythematous eruption known as "ground itch" or "dew itch". Larvae undergo transpulmonary migration (alveoli $\rightarrow$ bronchi $\rightarrow$ pharynx $\rightarrow$ swallowed) to reach the jejunum, where adults anchor to mucosal villi, secreting anticoagulants and lacerating mucosal capillaries. The chronic hallmark of infection is progressive microcytic, hypochromic iron deficiency anemia and hypoalbuminemia, causing chlorosis, lethargy, exertional dyspnea, and stunted physical and neurocognitive development in children.
• Egg Morphology: Thin-shelled, colorless, transparent, oval eggs ($60-70 \times 40\text{ um}$) containing an unsegmented or 4- to 8-cell morula stage in freshly passed feces (eggs of Necator and Ancylostoma are indistinguishable on standard microscopy).
• Cutaneous Larva Migrans: Caused by non-human dog and cat hookworms (such as Ancylostoma braziliense). Infective larvae penetrate human skin but cannot breach the basal membrane to enter circulation; they wander through the epidermis, producing intensely pruritic, creeping, serpiginous, erythematous linear tracks. Treated with topical or oral albendazole.

Strongyloides stercoralis (Threadworm)

Strongyloides stercoralis is an intestinal nematode with unique biological properties: it exhibits both free-living and parasitic cycles, and is capable of autoinfection.

• Autoinfection: Non-infective rhabditiform larvae can transform directly into infective filariform larvae within the intestinal lumen or on the perianal skin. These filariform larvae immediately re-penetrate the bowel mucosa or perianal skin, re-entering circulation without ever passing into the soil. This allows Strongyloides to persist indefinitely within a human host for decades.
• Diagnostic Hallmark: Larvae, Not Eggs: In standard stool microscopy, one finds rhabditiform larvae, NOT eggs! Eggs hatch inside the intestinal mucosa before feces are passed. The rhabditiform larva is recognized by its short buccal cavity and prominent, genital primordium (distinguishing it from hookworm rhabditiform larvae, which have a long buccal cavity).
• Larva Currens: Rapidly advancing, intensely pruritic, serpiginous, urticarial eruption (migrating 5 to 10 cm/hour) across the buttocks, perineum, and thighs caused by wandering filariform larvae during perianal autoinfection.
• Hyperinfection Syndrome & Disseminated Disease: Occurs when cell-mediated immunity is suppressed—especially by systemic corticosteroid therapy, HTLV-1 infection or malnutrition. Massive numbers of filariform larvae invade the bowel wall and migrate throughout the lungs, liver, brain, and kidneys. As larvae breach the intestinal mucosa, they carry enteric bacteria into the bloodstream, triggering recurrent polymicrobial Gram-negative bacteremia, sepsis, and meningitis. Drug of choice is Ivermectin (200 ug/kg/day for 2 days for uncomplicated disease; daily until larvae clear in hyperinfection).

Enterobius vermicularis (Pinworm / Seatworm)

Enterobius vermicularis is an intestinal nematode transmitted via direct ingestion of infective embryonated eggs through contaminated fingers (scratching), clothing, bedsheets, or dust.

• Pathogenesis & Diagnostics: Adult female worms (approx. 1 cm long) migrate out of the anus nocturnally to oviposit thousands of sticky eggs on the perianal and perineal skin, causing intense pruritus ani, restlessness, and sleep disturbances. Standard fecal microscopy is insensitive (< 5% yield) because eggs are deposited externally. The diagnostic standard of choice is the cellulose tape test (Scotch tape test), performed in the early morning before bathing or defecation by pressing transparent adhesive tape to the perianal folds.
• Egg Morphology: Distinctive planoconvex or D-shaped eggs ($50-60 \times 20-30\text{ um}$) featuring one flattened side and one rounded side, containing an embryonic larva.
• Management: Single dose of Albendazole (400 mg), Mebendazole (100 mg), or Pyrantel pamoate, repeated in 2 weeks. Crucially, the entire household must be treated simultaneously to prevent mutual reinfection.

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Endemic Tissue Nematodes: Capillaria philippinensis ("Pudoc Disease")

Capillaria philippinensis is a minute intestinal nematode (adult females measure 3 to 5 mm, males 2 to 3 mm) first described in a patient from Ilocos Norte in the early 1960s. It became known as "Pudoc disease" after a deadly epidemic of diarrheal illness in the 1960s in Pudoc, Tagudin, Ilocos Sur, Northern Luzon. It remains endemic across Northern and Central Luzon (Ilocos, Cagayan Valley, Pangasinan) and specific riverine provinces in Mindanao (Agusan del Norte, Davao del Sur, Zamboanga).

Life Cycle & Transmission

Humans acquire infection by ingesting raw, salted, or undercooked small freshwater or brackish fish—most notably bagsit (Hypseleotris bipartita), ipon, or irok—harboring infective larvae (often consumed as kinilaw). Adult worms embed in the mucosa of the jejunum and ileum. Female worms can produce unsegmented eggs as well as viviparous free larvae that reinfect the host (internal autoinfection), causing worm burdens to multiply into tens of thousands within a single patient.

Clinical Pathology & The Diagnostic Triad

Massive worm burdens flatten and destroy jejunal villi, infiltrate the lamina propria, and cause marked loss of mucosal disaccharidases. Clinically, patients present with:

1. Audible, continuous borborygmi ("gurgling intestines") accompanied by colicky abdominal pain
2. Intractable, chronic, voluminous watery diarrhea
3. Profound protein-losing enteropathy (causing severe hypoalbuminemia, anasarca, and profound cachexia) and severe electrolyte wasting

If untreated, patients die of refractory hypokalemia (triggering lethal cardiac dysrhythmias) or secondary bacterial sepsis. Early epidemics carried a high case-fatality rate before effective therapy was established.

Diagnostic Egg Morphology & Therapy

Stool microscopy reveals characteristic peanut-shaped or guitar-shaped eggs ($35-45 \times 20\text{ um}$) featuring a moderately thick, striated shell and flattened, non-protruding bipolar mucus plugs (distinguishing them from Trichuris trichiura eggs, which are larger, barrel-shaped, and possess prominent, protruding bipolar plugs). Treatment requires Mebendazole (200 mg orally BID for 20 days) or Albendazole (400 mg orally once daily for 10 days), combined with vigorous fluid, potassium, and albumin repletion.

Cestodes: Taenia saginata, Taenia solium & Tissue Tapeworms

Cestodes (tapeworms) are dorsoventrally flattened, segmented hermaphroditic flatworms consisting of a head (scolex), a neck, and a chain of reproductive segments (proglottids):

The Critical Pathological Distinction: Neurocysticercosis

A critical parasitological rule tested on licensure examinations is that humans can serve as intermediate hosts ONLY for Taenia solium, NEVER for Taenia saginata!

• Intestinal Taeniasis: Acquired by eating raw or undercooked pork containing larval cysticerci (cysticercus cellulosae); larvae mature into adult tapeworms in the human jejunum, causing mild abdominal discomfort or passage of proglottids in stool.
• Cysticercosis & Neurocysticercosis (NCC): Acquired when humans ingest Taenia solium EGGS through fecal-oral contamination of food/water or autoinfection in an individual carrying an adult tapeworm. Oncospheres hatch in the small intestine, penetrate mesenteric venules, and disseminate hematogenously to subcutaneous tissues, skeletal muscle, eyes, and the central nervous system.

In the brain, larvae form cystic lesions. On contrast-enhanced CT or MRI, parenchymal cysts with a visible mural nodule representing the invaginated scolex produce the pathognomonic "hole-with-a-dot" sign. When cysts degenerate, the breakdown of immune tolerance triggers intense local edema and inflammation, presenting with new-onset generalized or focal seizures, elevated intracranial pressure, and hydrocephalus. Treatment involves Albendazole or Praziquantel administered under mandatory co-administration of high-dose systemic corticosteroids (Dexamethasone) to suppress life-threatening neuroinflammation and cerebral edema.

Other Medically Important Cestodes

• Diphyllobothrium latum (Fish / Broad Tapeworm): Largest human tapeworm (up to 15 meters long). Possesses two longitudinal sucking grooves (bothria) on its scolex; proglottids are broader than long with a central rosette-shaped uterus. Acquired by eating raw freshwater fish containing plerocercoid larvae. Resides in the ileum, where it selectively and avidly absorbs dietary vitamin B12, causing host Vitamin B12 deficiency manifested as macrocytic megaloblastic anemia and subacute combined degeneration (SCD) of the spinal cord.
• Echinococcus granulosus (Hydatid Tapeworm): Minute tapeworm (3 to 6 mm) whose definitive hosts are canines (dogs). Humans become accidental intermediate hosts by ingesting eggs from canine feces. Oncospheres penetrate the bowel wall and migrate via the portal vein to the liver (70%) and lungs (20%), forming slow-growing hydatid cysts. Cysts consist of an outer acellular laminated layer and an inner germinal layer producing brood capsules with protoscolices ("hydatid sand"). Cyst rupture releases highly antigenic fluid that can trigger life-threatening anaphylactic shock and secondary peritoneal seeding. Diagnosed via serology and imaging (showing the "water-lily sign" of detached endocyst membrane); managed via surgical resection or the PAIR technique (Puncture, Aspiration, Injection of scolicidal agent [hypertonic saline], Re-aspiration) under albendazole coverage.

Prevention and control

• Mass deworming: the DOH Integrated Helminth Control Program targets preschool and school-aged children. It gives single-dose albendazole or mebendazole twice a year and combines this with water, sanitation and hygiene programs (WASH): toilets, hand washing and footwear.
• Food safety:
  • cook pork and beef thoroughly to prevent taeniasis;
  • avoid raw freshwater fish, the source of Capillaria;
  • prevent fecal contamination of food and water, the route for amebiasis, giardiasis and cysticercosis.
• Treat carriers: treating people who harbor an adult T. solium worm protects their household from cysticercosis.