Introduction to Parasitology

Introduction to Parasitology

• Parasitology is the area of biology studying the dependence of one living organism on another.
• Parasitology deals with parasites that infect us, the diseases they cause, the responses we generate against them, different diagnostic methods, and prevention and treatment strategies.

Parasites

• A parasite is an organism entirely dependent on another organism (host) for its cycle and metabolic needs.
• Microparasites are small, unicellular organisms that multiply within vertebrate hosts, often inside cells, e.g., Protozoa.
• Macroparasites are large, multicellular organisms that do not directly reproduce within vertebrate hosts, e.g., Helminths.

Hosts

• A host is an organism that harbors a parasite, providing nourishment and shelter.
• Different types of hosts include:
  • Definitive host: Harbors the adult parasite and, if applicable, where the parasite replicates sexually; the mammalian host is the definitive host if it is not obvious.
  • Intermediate host: Alternates with the definitive host, harboring larval or sexual stages of the parasite; some parasites need two intermediate hosts to complete their cycle.
  • Paratenic host: A host where the larval stage of a parasite survives but does not develop further; often not a necessary part of the life cycle.
  • Reservoir host: Harbors the parasite and serves as a source of infection for other susceptible hosts.
  • Compromised host: An individual with impaired defense mechanisms, such as those with AIDS, congenital deficiencies, or bypassed barriers like skin penetration.
  • Zoonosis: An animal infection naturally transmissible to humans directly or via a vector; examples include leishmaniasis and South American trypanosomiasis.
  • Vector: An agent, usually an insect, that transmits an infection from one human host to another.

Host-Parasite Relationships

• Symbiosis is when both host and parasite are so dependent on each other that they cannot live without the other, and neither is harmed.
• Commensalism is when only the parasite benefits without causing injury to the host; the commensal can live independently, feeding on waste products.
• Parasitism is when a parasite benefits and the host is harmed, receiving nothing in return; the degree of dependence on the host varies.

Safety

• Fresh specimens must be handled with care as they can contain infectious agents like bacteria, viruses, fungi, and parasites.
• Proper labeling of fixatives and use of proper containers is important.

Fresh-Specimen Collection

• Procedures for recovering intestinal parasites should be done before barium is used for radiological exams.
• Wait 5 to 10 days after barium is given before collecting specimens.
• Avoid mineral oil, bismuth, antibiotics, antimalarials, and non-absorbable antidiarrheal treatments.
• Protozoa numbers may decrease with antibacterial therapy.

Collection of the Specimen

• Use clean, wide-mouth containers for fecal specimens to avoid spillage and maintain moisture.
• Do not contaminate specimens with water or urine because water may contain free-living organisms mistaken for human parasites and urine destroys motile organisms.
• Proper labeling is important.
• Three specimens should be separately collected for proper diagnoses.

Specimen Type, Specimen Stability, and Need for Preservation

• Fresh specimens are mandatory for recovering motile trophozoites (amebae, flagellates, ciliates).
• Examine liquid specimens within 30 minutes of passage and arrival; if not possible, place in fixatives.
• Soft specimens may contain protozoan trophozoites and cysts; examine within 1 hour; if not possible, use preservatives.
• Immediate examination of formed specimens is not critical, as protozoan cysts remain intact up to 24 hours after passage.
• Trophozoites are usually in liquid specimens, trophozoites and cysts in soft specimens, and cysts in formed specimens.
• Fecal specimens should NEVER be incubated or frozen before routine microscopy.

Fixatives

• Various types of fixatives include Formalin, Merthiolate (thimerosal)-iodine formalin (MIF), Sodium acetate-acetic acid-formalin (SAF), Schaudinn’s fluid (with or without polyvinyl alcohol [PVA]), single-vial systems, and Universal Fixative.