Parasitology Fundamentals: Relationships, Factors, and Disease Transmission

Understanding Parasites

A parasite is any living animal or plant that spends part or all of its existence inside or outside another living being (the host), feeding at its expense and potentially causing apparent or unapparent damage.

What is Parasitology?

Parasitology is the scientific discipline that studies organisms (parasites) that live inside or outside another host, extracting food and shelter. This association is not always harmful to the host.

Defining Parasitism

Parasitism occurs when a living organism (the parasite) lives on or in another organism of a different species (the host), from which it obtains nourishment. This relationship encompasses a wide range of organisms, from viruses to arthropods.

Zooparasites

Zooparasites are parasites from the Animal Kingdom that affect humans, causing infections or parasitic diseases.

Key Epidemiological Factors

Factors Influencing Parasitic Diseases

• Biological Characteristics: Mechanisms of invasion, location in the body, pathology.
• Intervention Measures: Treatment, prevention, and control strategies.

Environmental & Social Determinants

• Fecal contamination
• Environmental conditions
• Rural living
• Hygiene and education levels
• Food habits
• Migration patterns
• Immunosuppression

Ecological Relationships

Relationships Among Beings of the Same Species

• Societies: Individuals retain their individuality.
• Colonies: Organisms do not retain their individual identity.

Relationships Between Different Species

• Parasitism: One individual lives at the expense of another, causing harm.
• Commensalism: One individual lives at the expense of another without causing harm.
• Inquilinism (Renters): One organism (the inquiline) lives within or on another organism (the host) without being parasitic. Example: Birds like woodpeckers making homes in tree holes.

Types of Symbiotic Relationships

• Mutualism: A relationship where both organisms benefit.
• Phoresis: One organism uses another for transportation. Example: Remoras attached to sharks.
• Metabiosis (or Tanatocresia): An indirect dependency where one organism uses something produced by another, often after the first organism's death. Example: A hermit crab using a discarded conch shell for protection.

The Ecological Triad in Disease

The ecological triad describes the interaction between three key components in the occurrence of disease:

• Contagious Agent & Reservoir: The pathogen itself and where it naturally lives and multiplies (e.g., human, animal, inanimate objects).
• Host (or Guest) & Individual Variation: The organism susceptible to the agent, influenced by factors such as:
  • Genetic structure
  • Immunity
  • Age, nutrition, and vaccination status
• Environment: External factors influencing transmission, including:
  • Mechanism of transmission (e.g., via infective source)
  • Transmission probability (e.g., overcrowding, living standards, habits, medical therapy)

Key Terms in Epidemiology

Zoonosis

Zoonosis refers to infections naturally transmitted from vertebrate animals to humans, and vice versa.

Reservoir

A reservoir is the primary source of an infection. It is the natural habitat (human, animal, or even inanimate matter like soil) where an infectious agent normally lives and multiplies, depending on it for survival, and from which it can be transmitted to a susceptible host.

Infecting Source

An infecting source is a person, animal, object, or substance from which an infectious agent is transmitted to a host.

Fomite

A fomite is an inanimate object that can transmit an infectious agent from an infecting source to a susceptible host.