Introduction to Medical Parasitology

Medical Parasitology Introduction

• Medical parasitology is the study of organisms (parasites) that infect humans and cause disease.
• A parasite is a living organism that lives in or on another organism (host). It derives nutrients from the host without benefiting the host.
• A host is a living organism that harbors a parasite.

Types of Hosts

• Definitive host: The host that harbors the mature stage of the parasite.
• Intermediate host: The host that harbors the immature stage of the parasite.
• Reservoir host: An animal that harbors the mature stage of the parasite.
• Vector: An arthropod (e.g., insect) that carries the parasite from one host to another.

Classification of Medical Parasitology

• Helminthology: The study of parasitic worms including:
  • Class: Trematoda
  • Class: Cestoda
  • Class: Nematoda
• Protozoology: The study of single-celled parasitic organisms (protozoa)
• Arthropodology: The study of parasitic arthropods

Classification of Parasites

• Ectoparasites: Live on the surface of the host, e.g. fleas, ticks
• Endoparasites: Live inside the host's body, e.g. Leishmania
• Temporary parasites: Visit the host to obtain nutrients, e.g. mosquitoes
• Permanent parasites: Live on or in the host their entire life, e.g Ascaris lumbricoides
• Facultative parasites: Can live as parasites or free-living. Strongyloides stercoralis

Obligatory and Specific Parasites

• Obligatory parasites: Cannot live without a host, e.g. lice
• Specific parasites: Affect one specific host, e.g. Entrobius vermicularis
• Nonspecific parasites: Live in an animal host but can inhabit humans without completing their complete life cycle. E.g Avian schistosomes and Toxocara canis
• Opportunistic parasites: Cause disease only in immunocompromised individuals. e.g Cryptosporidium

Sources of Infection

• Human sources: Blood, excreta (stool, urine, sputum) containing the infectious stage of the parasite.
  • Examples: toxoplasmosis, amoebiasis, enterobiasis
• Animal sources:
  • Raw or undercooked meat containing the infectious stage of the parasite, e.g. Taenia saginata & Taenia solium
  • Animal stool containing different stages of the parasite
• Contaminated soil
• Contaminated water

Modes of Transmission

• Ingestion: Contaminated food, water or vegetables carrying infectious parasite stages.
• Penetration: Parasite larvae penetrate skin or mucous membranes. E.g. Strongyloides stercoralis, Hookworm
• Sexual contact: Certain parasites are transmitted sexually. E.g Trichomonas vaginalis.

Vertical Transmission

• Transmission of parasites from mother to fetus

Blood Transfusion

• Transmission of parasites through blood transfusions.

Autoinfection

• Parasites can transmit to the same person through self-contamination methods

Inhalation

• Inhalation of dust containing infective parasite stages
• Contact with infected patients
• Bite of vectors

Diagnosis of Parasitic Diseases

• Clinical diagnosis: History, symptoms, and signs
• Laboratory diagnosis:
  • Direct: Identifying the parasite in the sample (stool, urine, sputum, blood)
  • Indirect: Identifying indicators of the presence of a parasite

Methods of Laboratory Diagnosis

• Intradermal skin tests
• Immunodiagnostic tests (detecting antigens and antibodies)
• Molecular methods (DNA probes and PCR)
• Imaging techniques (X-ray, ultrasound)
• Biopsy

Trematodes (Fasciola and Heterophyes )

• Fasciola hepatica/gigantica:
  • Disease: Fascioliasis
  • Worldwide distribution, especially tropical areas.
  • Habitat: Bile ducts and gall bladder
  • Definitive Host: Herbivorous animals (sheep, cattle, camels); humans can be infected
  • Intermediate Host: Snails
  • Adult worm description
  • Egg description diagnostic parameter
  • Encysted metacercaria (infective stage) description
  • Life Cycle
  • Clinical Picture: Acute and Chronic
  • Diagnosis
  • Treatment
  • Prevention/control
• Heterophyes heterophyes:
  • Geographical distribution: Fish-eating countries, habitat: Small intestine
  • Definitive Host: Man and fish-eating animals
  • Intermediate Host: Snails and fish
  • Adult worm description
  • Egg description- diagnostic parameters
  • Infective stage: Encysted metacercaria
  • Life Cycle- important details
  • Clinical Picture: Mild to no symptoms, intermittent symptoms,
  • Diagnosis methods
  • Treatment-drug of choice
  • Prevention/control