Medical Parasitology: Introduction to Parasites

Introduction to medical parasitology and parasitic diseases

• Medical parasitology focuses on human parasites.
• A parasite is an organism dependent on another organism (host) for its life cycle.
• Human parasitology studies organisms that parasitize humans.
• Parasites include viruses, bacteria, fungi, protozoa, and metazoa that infect hosts.
• Microbiology incorporates viruses, bacteria, and fungi, for historical reasons.
• Parasites are of two types: microparasites and macroparasites.
• Microparasites are small, unicellular organisms that multiply within a vertebrate host, often inside cells; protozoa are examples.
• Macroparasites are large, multicellular organisms that do not directly reproduce within a vertebrate host; helminths are examples.

Biological Associations

• Host-parasite relationships exist.
• Symbiosis is an association where both host and parasite are so dependent on each other that one cannot live without the other, and neither partner suffers harm.
• Mutualism is an association between two different organisms where life apart is impossible, with both partners benefiting from each other.
• Commensalism is an association where one partner benefits, while the other is neither benefited nor injured.
• Parasitism is a relationship in which the parasite harms or lives at the expense of one of the participants, the host.

Parasite Location

• A parasite is a living organism that lives in or on another organism and has some metabolic dependence on that organism, known as a host.
• Parasites can be divided based on location into Ectoparasites and Endoparasites.
• Ectoparasites live on the surface of the body, such as lice, ticks, and mites.
• Infection by ectoparasites is known as infestation.
• Ectoparasites are vectors that transmit pathogenic microorganisms.
• Endoparasites live within the body of the host.
• Protozoan and helminthic parasites of man are endoparasites e.g. Entamoeba histolytica.
• Invasion by endoparasites is known as infection.

Types Of Parasites

• Obligate parasites are completely dependent on the host during part or all of their life cycle, such as Plasmodium spp.
• Accidental parasites are not normally parasitic but can survive for a limited period within another organism, like Hymenolepis diminuta (tapeworm).
• Erratic parasites wander into an organ in which they are not usually found, such as Entamoeba histolytica in the liver or lung of humans.
• Temporary (intermittent) parasites remain parasitic for limited periods for feeding or reproduction.
• Adaptive parasites (facultative parasites) have the capacity to live both as free-living or parasitic organisms.

Life cycle elements for Parasites

• A parasite's life cycle involves growth, development, and reproduction, which may proceed in one or more hosts, depending on the species.
• A direct life cycle involves only a single host.
• An indirect life cycle involves more than one host.
• The infective stage is when a parasite can invade a human body and live in it.
• The infective route is the specific entrance through which the parasite invades the human body.
• The infective mode refers to how the parasite invades the human body.
• An example of this is cercariae penetrating the skin of a swimmer.
• Another example is Ascaris eggs being swallowed.

Hosts

• A host in parasitism supplies nourishment and shelter to the parasite and is the injured partner.
• A carrier harbors the parasite without clinical symptoms, serving as a source of infection in epidemiology.
• There are different types of hosts:
  • Definitive host: parasites reach sexual maturity, and the adult form resides or sexual stages of reproduction occur.
  • Intermediate host: immature or larval form resides, and parasite undergoes asexual reproduction.
  • Transport host: the parasite does not undergo development but remains alive and infective to another host.
  • Reservoir host: any animal that harbors an infection that can be transmitted to humans.
  • Compromised host: normal defense mechanisms are impaired e.g. (AIDS) and are extremely susceptible to pathogens.

Parasite transmission

• Host specificity refers to which hosts can be infected by a given parasite.
• Some parasites have very specific hosts, like Ascaris lumbricoides.
• Others have a wide range of hosts, like Toxoplasma gondii.
• The routes of parasite transmission include:
  • Congenital transmission from mother to infant, e.g., Toxoplasmosis.
  • Contact transmission, e.g., Trichomonas vaginalis.
  • Food transmission involving parasites in contaminated food.
  • Water transmission through drinking or contacting contaminated water.
  • Soil transmission.
  • Arthropod transmission.

Vectors and Protozoa

• A vector is an agent, usually an insect, that transmits infection from one human host to another.

• There are two types of vectors:

  • Mechanical vector: a vector that assists in transferring parasitic forms between hosts but is not essential in the life cycle of the parasite.
  • Biological vector: essential in the life cycle.

• Zoonosis describes a disease communicable from animals to humans, such as enzootic infection acquired under natural conditions.

Protozoa

• Protozoa are unicellular animal organisms.
• Each protozoon performs all functions of life.
• Protozoa range in size from 1 to 150 micro.
• Protozoa have cytoplasm and a nucleus.
• The cytoplasm is differentiated into ectoplasm and endoplasm.
• The ectoplasm functions in protection, locomotion, ingestion of food, excretion, and respiration.
• Locomotion occurs either by pseudopodia, cilia, or flagella.
• There are modes of nutrition:
  • Holozoic.
  • Absorption of liquid food through the body surface.
  • Ingestion of solid particles by pseudopodia or through the cytostome.
• Reproduction may be asexual or sexual.

Protozoa Classification

• Human parasites in the kingdom Protista, subkingdom Protozoa, are classified under four phyla.
• Sarcomastigophora contains amoeba and flagellates.
• Apicomplexa contains Sporozoa.
• Ciliophora contains Ciliates.
• Microspora.