Host-Parasite Relationships Overview

Host-Parasite Relationships

• A host is a larger organism harboring a smaller organism (parasitic, mutualistic, or commensal).
• A parasite obtains nutrition and/or shelter from the host.
• Parasites can live on or in the host, resulting in potential damage and disease.

Parasite Types

• Non-living: prions, viroids
• Non-living/living: viruses
• Prokaryotes: bacteria
• Eukaryotes: fungi, protozoa, helminths

Introduction

• Host and parasite interaction is dynamic, determined by the characteristics of both.
• Parasite virulence factors enable invasion, damage, and resistance to host defenses.
• Host defenses vary in their ability to resist parasitic infection.

Host Defenses

• A microorganism must overcome host defenses to invade.
• Specific defenses include skin, mucosal secretions, non-specific local responses (e.g., pH), non-specific inflammatory responses, and specific immune responses (e.g., lymphocytes).

Pathogen Defenses

• Pathogens have virulence factors which facilitate disease formation.
• Factors include the ability to adhere to, colonize, and evade host defenses.
• Higher virulence equates to a greater chance of infection.

Host-Pathogen Battles

• A visual representation of the struggle between a bacteria and a leukocyte (white blood cell).

Infectious Disease

• Infectious disease is caused by pathogenic microorganisms.
• Pathogenicity is the ability to cause disease in another organism.
• Virulence is the degree or intensity of pathogenicity.
• Disease is an abnormal state or deviation from wellness/health.
• Contamination is the presence of microorganisms.
• Infection is the increase in parasitic microorganisms within or on the host.

Host-Pathogen Interactions

• Symbiosis describes host-organism interaction.
• Three forms exist at an anatomical level:
  • Mutualism: both members benefit. Example: lactic acid bacteria in the vagina.
  • Commensalism: one member benefits, the other is neither helped nor harmed. Example: microorganisms in the eyes.
  • Parasitism: one member benefits, the other is harmed. Example: parasites in the gastrointestinal tract.

Bacterial Pathogenesis

• Pathogens are microorganisms (or viruses) which cause disease.
• Pathogenicity is the microorganism's ability to cause disease.
• The body's normal flora includes resident (long-term) and transient (temporary) bacteria.
• Normal flora protects against pathogens by:
  • Covering adherence sites.
  • Producing toxic compounds.
  • Preventing pathogen nutrient consumption.
  • Producing nutrients (e.g., vitamins B and K).

Mechanisms of Pathogenesis

• Bacterial infection stages in order:
  • Transmission
  • Adherence
  • Invasion
  • Colonization
  • Evasion of host defenses
  • Damage to host tissues
  • Exit from the body
  • Survival in the environment

Opportunistic Infections

• Infections caused by non-pathogenic microorganisms that become pathogenic when the host's immune system is impaired.
• Examples include those caused by Candida albicans, associated with oral thrush and gastrointestinal infection.

Virulence Factors

• Bacteria need virulence capabilities to infect a host:
  • Adherence and colonization
  • Invasion
  • Toxins
  • Enzymes
  • Capsules
  • Pili

Adherence and Colonization

• Adherence is essential for bacterial colonization in a new host.
• Many gram-negative bacteria use fimbriae as adhesins.

Biofilm

• Normal flora bacteria create a polysaccharide matrix (glycocalyx) to form biofilms attached to surfaces.
• Biofilms resist antibiotics, trap nutrients, and adhere to surfaces.

Stages of Biofilm Formation

• Stage 1: Attachment
• Stage 2: Growth

Healthy vs. Diseased Tooth/Gum

• Visual comparison demonstrating the pathogenic buildup on teeth and gums.

Invasion

• Bacteria invade eukaryotic cells at mucosal surfaces.
• Some bacteria are obligate intracellular pathogens (e.g., Rickettsia, Chlamydia).
• Capsule formation helps bacteria resist phagocytosis and intracellular killing.

Toxins

• Bacterial toxins are classified as exotoxins or endotoxins.
• Exotoxins are released immediately, while endotoxins are released after bacterial cell death.
• Exotoxins and endotoxins have varying toxicity and immunogenicity.

Infection

• Infection involves disease-causing agents invading tissues, multiplying, and host tissues reacting to the agents and toxins.
• Infections occur by various means.
• Source of infection: people, environments, dust.
• Susceptible person
• Transmission

Chain of Infection

• Infection spread within a community described as a "chain" of interconnected steps defining the pathogen's movement.
• Links in the chain include the infectious agent, reservoir, portal of exit, mode of transmission, portal of entry, and susceptible host.

Reservoir

• Reservoirs are the environments where infectious agents live.
• Reservoirs include humans, animals, and the environment.

Human Reservoirs

• Human reservoirs may or may not exhibit illness, e.g., asymptomatic carriers.
• Types of carriers include asymptomatic, incubatory, convalescent, and chronic.

Animal Reservoirs

• Humans can be susceptible to animal-borne diseases (zoonotic).
• Many zoonotic diseases are transmitted between animals, with humans being incidental hosts.

Environmental Reservoirs

• Environmental reservoirs, such as plants, soil, and water, can harbor and transmit infectious agents.
• Many fungal agents live and multiply in the soil (e.g. histoplasmosis).

Modes of Transmission

• Direct Contact: infection through direct contact with a person or their skin/body fluids (kissing, sexual intercourse).
• Indirect Contact: infection through an intermediary (contaminated objects, inanimate items), called fomites. Examples include using a contaminated doorknob or shared surface (toys).
• Droplet Transmission: infection spread via particles larger than 5 micrometers over short distances (up to 2 meters).
• Airborne Transmission: infection spread via particles 5 micrometers or smaller over longer distances and remain suspended in the air for a period of time. Examples include the airborne route; tuberculosis, rubeola, varicella.
• Fecal-Oral Transmission: infection when pathogens in feces contaminate food or water, consumed by another.
• Vector-Borne Transmission: infection transmitted through an arthropod vector (e.g. mosquito, flies, ticks).
• Food and water transmission (contaminated food or water).
• Through body fluids or blood (e.g., hepatitis B&C, HIV).

Explore the intricate dynamics of host-parasite interactions in this quiz. Learn about different types of parasites, the various host defenses, and the characteristics that influence these relationships. Test your understanding of how parasites obtain nutrients and the virulence factors that enable them to invade their hosts.