PHA3109 Parasitology Overview

Parasitology Overview

• PHA3109 Microbiology & Immunology Q3 2025
• Chapters covered
  • Chapter 51: Intestinal and Urogenital Protozoa
  • Chapter 52: Blood and Tissue Protozoa
  • Chapter 54: Cestodes
  • Chapter 55: Trematodes
  • Chapter 56: Nematodes

Parasite Classification

• Parasites thrive at the host's expense, coming in two eukaryotic forms
  • Single-celled protozoa
  • Multicellular metazoa (worms = helminths)
• Protozoa categorized by infection site
• Metazoa categorized by worm type (2 phyla):
  • Platyhelminthes (flatworms)
  • Nemathelminths (roundworms)

Protozoa Groups

• Sarcodina (amoebas): unicellular, move by pseudopodia
• Sporozoa (sporozoans): unicellular, nonmotile
• Mastigophora (flagellates): unicellular, flagellated
• Ciliata (ciliates): unicellular, ciliated
  • Trematoda (flukes) -Cestoda (tapeworms)

Protozoa Terminology

• Trophozoite: active, feeding, reproducing protozoan form with a flexible membrane
• Cyst: dormant, non-reproducing stage; surrounded by a thick wall, enabling survival in the environment
• Promastigotes/trypomastigotes: flagellated forms of certain protozoa
• Amastigotes: non-flagellated forms of certain protozoa

Transmission and Management

• Intestinal protozoa transmission
  • Ingestion of cysts
• Blood protozoa transmission
  • Insect vectors
• Helminth transmission
  • Ingestion of eggs
  • Penetration of skin by larvae
  • Insect bite
• Disease management focuses on interrupting transmission
  • Sanitation
  • Water Purification
  • Insect Management
  • Personal Protective Measures

Intestinal and Urogenital Protozoa

• 1. Intestinal tract:
  • Entamoeba histolytica (amoeba)
  • Giardia lamblia (flagellate)
  • Cryptosporidium hominis (sporozoan)
• 2. Urogenital tract:
  • Trichomonas vaginalis (flagellate)

Intestinal Protozoa - Entamoeba histolytica

• Causes amebic dysentery and liver abscess
• Symptoms range from dysentery to liver abscess
• Important properties
  • Trophozoites and cysts
  • Cysts have four nuclei
• Diagnosis
  • Examine stool for trophozoites or cysts
• Treatment
  • Metronidazole or tinidazole
• Prevention
  • Handwashing, avoid contamination of food and water.
  • Cyst-killing techniques: boiling, filtration

Intestinal Protozoa - Giardia lamblia

• Causes giardiasis: watery, foul-smelling, non-bloody diarrhea
• Pathogenesis involves ingestion of cysts and excystation in duodenum, resulting in trophozoite attachment with inflammation & malabsorption
• Diagnosis
  • Examine stool for trophozoites or cysts (string test)
• Treatment
  • Tinidazole, metronidazole
• Prevention
  • Boil or filter water and avoid consumption of contaminated foods

Intestinal Protozoa - Cryptosporidium hominis

• Causes cryptosporidiosis: severe watery diarrhea in immunocompromised populations and milder self-limiting symptoms in healthy individuals
• Pathogenesis: ingestion of oocysts, excystation in small intestine, and trophozoite attachment to gut wall.
• Treatment
  • Nitazoxanide
• Prevention
  • Filtered or boiled water, hygiene

Blood/Tissue Protozoa

• Incidence data (by specific protozoan) for the U.S. and worldwide is provided.

Blood/Tissue Protozoa - Malaria

• Malaria is caused by four species of Plasmodium. P. falciparum is the most severe. Significant morbidity and mortality.
• Lifecycle involves the mosquito vector, human liver, and human blood stages.
• Symptoms include fever, chills, headache, and other systemic symptoms. Can cause extensive brain and kidney damage.
• Complicated by developing drug resistance.

Blood/Tissue Protozoa - Toxoplasma gondii

• Causes toxoplasmosis.
  • Congenital toxoplasmosis (in uteros) can have severe outcomes.
  • Immunocompetent adults are typically asymptomatic.
  • Immunocompromised individuals may develop severe, life-threatening disease.
• Pathogenesis involves cyst rupture in the gut, macrophage engulfment, and tachyzoite proliferation into tissues.
• Treatment:
  • Sulfadiazine, pyrimethamine.

Blood/Tissue Protozoa - Pneumocystis jiroveci

• Formerly classified as a fungus, now known to be a related form of yeast.
• Causes pneumocystis pneumonia in immunocompromised individuals.
• Transmission: inhalation, infection in the lungs
• Treatment
  • Trimethoprim-sulfamethoxazole, and Pentamidine
• Prevention
  • Immunosuppression control

Parasitic Helminths - Platyhelminthes

• Platyhelminthes are flatworms, classified into cestodes (tapeworms) and trematodes (flukes)
• Important structure
  • Scolex: head with hooks and suckers
  • Proglottids: body segments; contain eggs

Cestodes - Taenia solium

• Adults cause taeniasis; larvae cause cysticercosis
• Pathogenesis involves ingestion of eggs, development in the gut, and migration to tissues.
• Symptoms
  • Taeniasis: mostly asymptomatic, but can include pain, weight loss, blockage.
  • Cysticercosis: variable ranging from headache and vomiting to blindness and neurological damage.
• Treatment
  • Praziquantel, albendazole (sometimes surgical excision)
• Prevention
  • Thoroughly cooking pork and managing human waste.

Cestodes - Treatment

• Praziquantel: Increases schistosome membrane permeability for calcium ions
• Albendazole: Binds beta-tubulin, inhibiting microtubule assembly