Helminths: Common Organisms & Life Cycles (80/20)

Big Picture: 3 Groups

NEMATODES (Roundworms)

Universal life cycle principle

1. Ascaris lumbricoides (Giant roundworm)

• Size: Up to 40 cm - largest intestinal nematode
• Route: Fecal-oral (ingested egg)
• Soil phase required: Yes (eggs embryonate in warm moist soil; viable for years)
• Life cycle:
  1. Eggs ingested → gastric acid dissolves outer coat → larvae hatch in small intestine
  2. Larvae penetrate gut mucosa → portal blood → liver → right heart → lungs
  3. Penetrate alveoli → ascend bronchial tree → swallowed back down
  4. Mature into adults in small intestine
  5. Female produces >200,000 eggs/day → shed in feces
• Patency (egg to detectable adult): 2-3 months
• Key feature: Loeffler syndrome (pulmonary phase eosinophilia), intestinal obstruction at high worm load
• Harrison's, p. 1858 (22E)

2. Hookworms (Necator americanus, Ancylostoma duodenale)

• Route: Skin penetration (filariform larvae in soil)
• Life cycle:
  1. Eggs shed in feces → hatch into rhabditiform larvae in soil → molt to filariform (infective) larvae
  2. Larvae penetrate skin (feet) → bloodstream → lungs → alveoli → bronchial tree → swallowed
  3. Mature in small intestine → adults attach, suck blood
  4. Eggs shed in feces
• Key feature: Iron-deficiency anemia, Loeffler syndrome (lung phase), cutaneous larva migrans (Ancylostoma especially)
• Infective form: Filariform larvae (NOT egg)

3. Strongyloides stercoralis

• Route: Skin penetration (same as hookworm)
• Unique feature: Autoinfection - larvae can develop to infective form WITHIN the intestine and re-penetrate the gut wall without leaving the host
• Life cycle:
  1. Filariform larvae penetrate skin → lungs → swallowed → adults in small intestine
  2. Females lay eggs parthenogenetically → hatch into rhabditiform larvae in gut
  3. Rhabditiform larvae shed in feces (diagnostic stage) OR
  4. In autoinfection: rhabditiform → filariform → penetrate colon/perianal skin → back into circulation
  5. Has a free-living cycle in soil (can reproduce outside human host)
• Clinical: Hyperinfection syndrome in immunosuppressed (especially steroids, HTLV-1) - fatal
• Diagnostic form: Rhabditiform larvae in stool (not eggs)
• Sherris & Ryan, p. 1824

4. Enterobius vermicularis (Pinworm)

• Route: Fecal-oral; direct person-to-person
• No soil phase required
• Life cycle:
  1. Eggs ingested → hatch in small intestine → adults mature in cecum/colon
  2. Gravid female migrates to perianal region at night → deposits eggs
  3. Eggs become infective within hours; retroinfection (eggs hatch at anus, larvae migrate back in) is possible
  4. Common in children; household clusters
• Diagnosis: Scotch tape test (perianal, morning) - NOT stool O&P (eggs not in feces)
• Infective form: Egg (embryonates on perineum, not soil)

5. Trichuris trichiura (Whipworm)

• Route: Fecal-oral (embryonated egg in soil)
• Life cycle: Ingested egg → hatch in small intestine → larvae migrate to cecum/colon → adults (whip-shaped: thick posterior, thin anterior) embed in mucosa
• No lung migration
• Key feature: Rectal prolapse in heavy infection; "barrel-shaped" eggs with polar plugs

6. Tissue/Blood Nematodes

TREMATODES (Flukes)

Universal life cycle principle

• Encyst as metacercaria on aquatic vegetation or in fish/crustaceans (ingested), OR
• Free-swimming cercariae penetrate skin directly (schistosomes only)

1. Schistosoma spp. (Blood flukes)

• Unique: NOT hermaphroditic; male and female are separate
• Route: Cercariae penetrate intact skin in freshwater
• Life cycle:
  1. Eggs shed in urine (S. haematobium) or feces (S. mansoni, S. japonicum)
  2. Eggs hatch in water → miracidia → penetrate snail
  3. In snail: develop into cercariae
  4. Cercariae released → penetrate human skin → schistosomulae
  5. Migrate via bloodstream to portal system → adult pairs reside in:
     • S. mansoni: inferior mesenteric veins → hepatosplenic disease, portal hypertension
     • S. haematobium: vesical plexus → hematuria, bladder carcinoma
     • S. japonicum: superior mesenteric veins → more eggs, severe liver disease
  6. Eggs lodge in tissues → granuloma formation (immune-mediated pathology)
• No intermediate host for human infection (cercariae directly penetrate skin)
• Robbins Pathology, block4

2. Liver Flukes

• Clinical: Biliary obstruction, cholangitis, cholangiocarcinoma (especially Clonorchis/Opisthorchis)

3. Lung Fluke (Paragonimus westermani)

• Intermediate hosts: Snail → freshwater crab/crayfish
• Route: Ingested undercooked crustaceans
• Clinical: Hemoptysis, pulmonary cysts; mimics TB

CESTODES (Tapeworms)

Universal life cycle principle

1. Taenia saginata (Beef tapeworm)

• Definitive host: Humans (adult worm in small intestine)
• Intermediate host: Cattle
• Route: Ingested undercooked beef containing cysticerci
• Human is only definitive host - no cysticercosis from T. saginata
• Diagnosis: Eggs and proglottids in stool (eggs identical to T. solium)

2. Taenia solium (Pork tapeworm) - HIGH YIELD

• Taeniasis (adult worm): Humans eat undercooked pork → adult in small intestine
• Cysticercosis (larval stage): Humans ingest T. solium eggs (fecal-oral, often from a human tapeworm carrier) → larvae hatch → migrate to tissues (brain, muscle, eye, subcutaneous)
• Neurocysticercosis: Most common cause of acquired epilepsy worldwide in endemic areas
• Life cycle:
  1. Adult worm in human intestine sheds gravid proglottids/eggs in feces
  2. Pig ingests eggs → larvae (cysticerci) form in pig muscle
  3. Human eats undercooked pork → cysticerci develop into adult worm (taeniasis)
  4. Human accidentally ingests eggs → cysticercosis (same as pig)
• Tietz, block38; Robbins, block4

3. Echinococcus granulosus (Hydatid disease)

• Definitive host: Dogs (adult worm in intestine)
• Intermediate host: Sheep, cattle, humans (larvae form hydatid cysts)
• Route: Humans ingest eggs from dog feces (fecal-oral, often petting dogs)
• Cysts: Liver (most common, ~65%), lung, bone; slow-growing
• Anaphylaxis risk if cyst ruptures
• Alveolar hydatid disease: E. multilocularis (fox tapeworm); more invasive, liver

4. Diphyllobothrium latum (Fish tapeworm)

• Intermediate hosts: Copepods (first) → freshwater fish (second)
• Route: Ingested undercooked freshwater fish
• Clinical: Longest human tapeworm (up to 10 m); competes for vitamin B12 → megaloblastic anemia
• Diagnosis: Operculated eggs in stool (unlike Taenia)

5. Hymenolepis nana (Dwarf tapeworm)

• Unique: Can complete entire life cycle within one human host (no intermediate host required)
• Autoinfection possible; most common tapeworm in the US

Life Cycle Summary Table (80/20 Exam Focus)

High-Yield Exam Mnemonics

• T. solium → Neurocysticercosis (ingest eggs)
• Echinococcus → Hydatid cysts (ingest eggs from dog)
• Toxocara → Visceral larva migrans (ingest eggs from cat/dog)

Helminth MCQs

• A) Stool ova and parasite examination
• B) Perianal Scotch tape test in the morning
• C) Duodenal aspirate
• D) Serology (ELISA)
• E) Blood film at night

• A) Loa loa
• B) Wuchereria bancrofti
• C) Onchocerca volvulus
• D) Brugia timori
• E) Mansonella perstans

• A) Ascaris lumbricoides
• B) Necator americanus
• C) Strongyloides stercoralis
• D) Trichuris trichiura
• E) Enterobius vermicularis

• A) Skin → lymphatics → lungs → intestine
• B) Intestine → portal vein → liver → lungs → intestine
• C) Skin → blood → lungs → intestine
• D) Intestine → peritoneum → liver → lung
• E) Lymphatics → bloodstream → eye → intestine

• A) Ascaris lumbricoides
• B) Trichuris trichiura
• C) Necator americanus
• D) Strongyloides stercoralis
• E) Enterobius vermicularis

• A) Fasciola hepatica — hepatocellular carcinoma
• B) Clonorchis sinensis — cholangiocarcinoma
• C) Schistosoma mansoni — hepatocellular carcinoma
• D) Paragonimus westermani — lung carcinoma
• E) Echinococcus granulosus — biliary carcinoma

• A) S. mansoni — portal hypertension
• B) S. japonicum — Katayama fever
• C) S. haematobium — squamous cell carcinoma of the bladder
• D) S. mansoni — pulmonary hypertension
• E) S. haematobium — adenocarcinoma of the bladder

• A) Taenia saginata — ingestion of undercooked beef
• B) Taenia solium — ingestion of T. solium eggs (fecal-oral)
• C) Echinococcus granulosus — ingestion of dog tapeworm eggs
• D) Toxoplasma gondii — reactivation in immunosuppression
• E) T. solium — ingestion of undercooked pork containing cysticerci

• A) Taenia solium — pig
• B) Echinococcus granulosus — dog
• C) Echinococcus multilocularis — fox
• D) Fasciola hepatica — sheep
• E) Clonorchis sinensis — fish

• A) Trichinella spiralis
• B) Anisakis simplex
• C) Diphyllobothrium latum
• D) Toxocara canis
• E) Gnathostoma spinigerum

• A) Taenia saginata
• B) Taenia solium
• C) Diphyllobothrium latum
• D) Hymenolepis nana
• E) Echinococcus granulosus

• A) Blood loss from adult worm attachment
• B) Hemolysis by parasite toxins
• C) Competition for intrinsic factor
• D) Competition for vitamin B12 in the ileum
• E) Folate malabsorption

• A) Ingested embryonated egg
• B) Microfilariae
• C) Filariform (L3) larvae
• D) Rhabditiform (L1) larvae
• E) Adult worm

• A) S. mansoni — terminal spine — vesical plexus
• B) S. haematobium — lateral spine — portal vein
• C) S. japonicum — large egg, lateral spine — inferior mesenteric vein
• D) S. mansoni — lateral spine — inferior mesenteric vein
• E) S. haematobium — lateral spine — superior mesenteric vein

• A) Ascaris lumbricoides
• B) Necator americanus
• C) Strongyloides stercoralis
• D) Trichuris trichiura
• E) Toxocara canis

Quick Scoring Guide

Obstetrics MCQs - Based on Your Learning Guide

SECTION 1: Signs, Symptoms & Diagnosis of Pregnancy (Q6 & Q7)

• A) Fetal heartbeat
• B) Gestational sac
• C) Fetal pole
• D) Crown-rump length measurement
• E) Biparietal diameter

• A) Fetal heartbeat on Doppler
• B) Fetal movements felt by the examiner
• C) Visualization of fetus on ultrasound
• D) Positive urine pregnancy test (beta-hCG)
• E) Fetal parts palpated on X-ray

• A) 100 mIU/mL
• B) 500 mIU/mL
• C) 1,000-2,000 mIU/mL
• D) 5,000 mIU/mL
• E) 10,000 mIU/mL

• A) Positive serum beta-hCG
• B) Amenorrhea for 8 weeks
• C) Uterine enlargement on pelvic examination
• D) Transvaginal ultrasound showing a gestational sac with a yolk sac
• E) Presence of morning sickness

SECTION 2: Gestational Age & EDD Calculation (Q8)

• A) 1st January of the following year
• B) 8th January of the following year
• C) 8th February of the following year
• D) 1st February of the following year
• E) 15th January of the following year

• A) 6 weeks
• B) 8 weeks
• C) 9 weeks
• D) 10 weeks
• E) 12 weeks

• A) 28 days; add 7 extra days to EDD
• B) 30 days; no adjustment needed
• C) 28 days; no adjustment needed
• D) 28 days; subtract 7 days from EDD
• E) 30 days; add 5 extra days to EDD

SECTION 3: Physiological Changes in Pregnancy (Q9 & Q10)

• A) Cardiac output decreases by 20%
• B) Systolic blood pressure increases by 15%
• C) Heart rate increases by approximately 20%
• D) Peripheral vascular resistance increases
• E) Central venous pressure increases significantly

• A) Aortic compression by the gravid uterus reducing cardiac output
• B) Compression of the inferior vena cava reducing venous return
• C) Increased progesterone causing vasodilation
• D) Reduced plasma volume in the third trimester
• E) Cardiac arrhythmia due to displacement of the heart

• A) Functional Residual Capacity (FRC) increases by 20%
• B) Tidal volume decreases
• C) Minute ventilation increases by 50%
• D) Respiratory rate increases by 30%
• E) PaCO₂ increases due to increased CO₂ production

• A) GFR decreases; creatinine increases
• B) GFR increases by 50%; creatinine decreases
• C) GFR unchanged; creatinine unchanged
• D) GFR decreases by 20%; creatinine decreases
• E) GFR increases; creatinine increases proportionally

SECTION 4: Uterine/Placental Blood Supply (Q10)

• A) Increases 5-fold; 50% to placenta
• B) Increases 20-fold; 80% to placenta
• C) Increases 10-fold; 60% to placenta
• D) Increases 3-fold; 90% to placenta
• E) Increases 20-fold; 50% to placenta

• A) Spiral arteries constrict; in preeclampsia they dilate excessively
• B) Endothelium is replaced by trophoblast, walls lose smooth muscle; in preeclampsia this remodeling fails in the myometrial segment
• C) Spiral arteries develop atherosclerotic plaques normally; preeclampsia reverses this
• D) Spiral arteries become varicose; preeclampsia prevents this
• E) No change normally; in preeclampsia they dilate due to high estrogen

SECTION 5: Obstetric Investigations (Q5)

• A) Amniocentesis for karyotype
• B) Maternal serum alpha-fetoprotein (MSAFP) / Quadruple screen
• C) Group B Streptococcus (GBS) swab
• D) Glucose challenge test
• E) Non-stress test (NST)

• A) Biparietal diameter (BPD)
• B) Femur length (FL)
• C) Abdominal circumference (AC)
• D) Crown-rump length (CRL)
• E) Head circumference (HC)

SECTION 6: Antenatal Care & Counselling (Q11-Q13)

• A) Iron 60 mg/day - prevents maternal anemia
• B) Folic acid 400-800 mcg/day - prevents neural tube defects
• C) Calcium 1000 mg/day - prevents gestational hypertension
• D) Vitamin D 1000 IU/day - prevents preterm birth
• E) Iodine 150 mcg/day - prevents hypothyroidism

• A) Diagnostic ultrasound
• B) MRI without contrast
• C) Ionizing radiation above 50 mGy (5 rad)
• D) Low-level occupational radiation (<1 mGy/year)
• E) Therapeutic ultrasound physiotherapy

SECTION 7: Common Pregnancy Complaints Related to Physiological Changes (Q14)

• A) Increased gastric acid secretion in pregnancy
• B) Reduced lower esophageal sphincter pressure + upward displacement of stomach by the uterus
• C) Accelerated gastric emptying displacing food upward
• D) Increased GFR causing electrolyte imbalance
• E) Progesterone increasing GI motility

• A) Increased plasma albumin reducing oncotic pressure
• B) Decreased aldosterone causing sodium retention
• C) Chronic partial compression of the inferior vena cava by the gravid uterus
• D) Cardiac failure due to increased cardiac output
• E) Increased GFR causing fluid overload

• A) Plasma volume +20%, RBC mass +20% → no anemia
• B) Plasma volume +55%, RBC mass +20-30% → dilutional anemia
• C) Plasma volume +10%, RBC mass +40% → polycythemia
• D) Plasma volume +40%, RBC mass +40% → no change in Hb
• E) Plasma volume +55%, RBC mass +55% → no change in Hb

Score Guide