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Introduction
1. Anatomy
2. Microbiology
2.1 General bacteriology
2.2 Introduction to systemic bacteriology
2.3 Gram positive cocci
2.4 Gram negative cocci
2.5 Gram positive bacilli
2.6 Gram negative bacilli
2.7 Other important bacteria
2.8 Virology
2.9 Parasitology
2.9.1 Entamoeba histolytica
2.9.2 Acanthamoeba
2.9.3 Naegleria fowleri
2.9.4 Giardia lamblia
2.9.5 Cryptosporidium parvum
2.9.6 Trichomonas
2.9.7 Plasmodium spp.
2.9.8 Babesia
2.9.9 Toxoplasma gondii
2.9.10 Taenia
2.9.11 Hymenolepis nana
2.9.12 Enterobius vermicularis
2.9.13 Trichuris
2.9.14 Trichinella
2.9.15 Ascaris
2.9.16 Hookworms
2.9.17 Strongyloides
2.9.18 Additional information
2.10 Mycology
3. Physiology
4. Pathology
5. Pharmacology
6. Immunology
7. Biochemistry
8. Cell and molecular biology
9. Biostatistics and epidemiology
10. Genetics
11. Behavioral science
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2.9.15 Ascaris
Achievable USMLE/1
2. Microbiology
2.9. Parasitology

Ascaris

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Ascaris

Infection spreads when you ingest fertile eggs from contaminated hands, food, or water. The common pathogens are Ascaris lumbricoides and Ascaris suum. A. suum is transmitted from pigs.

After you ingest fertile eggs, they hatch in the small intestine and invade the intestinal mucosa. The larvae then travel through the portal and systemic circulation to the lungs. From the lungs, they either:

  • Ascend the bronchi, reach the throat, and are swallowed, or
  • Penetrate the alveoli

Adult worms live in the small intestine.

Ascaris life cycle
Ascaris life cycle

Infection may be asymptomatic. Heavy infections may present with abdominal pain and discomfort, intestinal obstruction (and sometimes perforation), growth retardation, and malnutrition in children - especially vitamin A deficiency and protein-energy malnutrition.

Migrating adult worms can:

  • Occlude the biliary tree
  • Cause appendicitis
  • Be extruded from the nasopharynx

Loeffler’s syndrome is caused by larvae migrating through the lungs. It presents with fever, dry cough, dyspnea, eosinophilia, and urticaria.

Diagnosis is made by demonstrating eggs in stool samples.

Ascaris egg
Ascaris egg

Unfertilized egg of A. lumbricoides in an unstained wet mount, 200x magnification.

Acscaris egg
Acscaris egg

Fertilized egg of A. lumbricoides in an unstained wet mount of stool.

Transmission and Pathogens

  • Infection via ingestion of fertile eggs (contaminated hands, food, water)
  • Main species: Ascaris lumbricoides (humans), Ascaris suum (pigs)

Life Cycle

  • Eggs hatch in small intestine, larvae invade mucosa
  • Larvae migrate via portal/systemic circulation to lungs
  • Larvae ascend bronchi to throat (swallowed) or penetrate alveoli
  • Adult worms reside in small intestine

Clinical Manifestations

  • Often asymptomatic
  • Heavy infection: abdominal pain, intestinal obstruction/perforation, growth retardation, malnutrition (vitamin A deficiency, protein-energy malnutrition)
  • Migrating worms: biliary tree occlusion, appendicitis, nasopharyngeal extrusion

Loeffler’s Syndrome

  • Caused by larval migration through lungs
  • Symptoms: fever, dry cough, dyspnea, eosinophilia, urticaria

Diagnosis

  • Identification of eggs in stool samples
  • Distinction between fertilized and unfertilized eggs under microscopy
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Ascaris

Ascaris

Infection spreads when you ingest fertile eggs from contaminated hands, food, or water. The common pathogens are Ascaris lumbricoides and Ascaris suum. A. suum is transmitted from pigs.

After you ingest fertile eggs, they hatch in the small intestine and invade the intestinal mucosa. The larvae then travel through the portal and systemic circulation to the lungs. From the lungs, they either:

  • Ascend the bronchi, reach the throat, and are swallowed, or
  • Penetrate the alveoli

Adult worms live in the small intestine.

Infection may be asymptomatic. Heavy infections may present with abdominal pain and discomfort, intestinal obstruction (and sometimes perforation), growth retardation, and malnutrition in children - especially vitamin A deficiency and protein-energy malnutrition.

Migrating adult worms can:

  • Occlude the biliary tree
  • Cause appendicitis
  • Be extruded from the nasopharynx

Loeffler’s syndrome is caused by larvae migrating through the lungs. It presents with fever, dry cough, dyspnea, eosinophilia, and urticaria.

Diagnosis is made by demonstrating eggs in stool samples.

Unfertilized egg of A. lumbricoides in an unstained wet mount, 200x magnification.

Fertilized egg of A. lumbricoides in an unstained wet mount of stool.

Key points

Transmission and Pathogens

  • Infection via ingestion of fertile eggs (contaminated hands, food, water)
  • Main species: Ascaris lumbricoides (humans), Ascaris suum (pigs)

Life Cycle

  • Eggs hatch in small intestine, larvae invade mucosa
  • Larvae migrate via portal/systemic circulation to lungs
  • Larvae ascend bronchi to throat (swallowed) or penetrate alveoli
  • Adult worms reside in small intestine

Clinical Manifestations

  • Often asymptomatic
  • Heavy infection: abdominal pain, intestinal obstruction/perforation, growth retardation, malnutrition (vitamin A deficiency, protein-energy malnutrition)
  • Migrating worms: biliary tree occlusion, appendicitis, nasopharyngeal extrusion

Loeffler’s Syndrome

  • Caused by larval migration through lungs
  • Symptoms: fever, dry cough, dyspnea, eosinophilia, urticaria

Diagnosis

  • Identification of eggs in stool samples
  • Distinction between fertilized and unfertilized eggs under microscopy