Host Parasite Interaction - BIO214 Infectious Disease

Host-Parasite Dynamics

Host-parasite relationships are complex, dynamic interactions where both organisms continually adapt to each other. Parasites include protozoa, helminths, and opportunists, while hosts are any other organism. These interactions significantly impact host populations ecologically.

Disease Concepts

Disease results from a Disease Triad involving a Pathogen, a Host, and the Environment, including Other Microbes. Pathogenicity is a microorganism's inherent ability to cause disease, while Virulence is its degree of pathogenicity, quantified experimentally via ID{50} (Infectious Dose 50%) and LD{50} (Lethal Dose 50%). The severity of infectious disease is described by the formula:\text{Infectious Disease} = \frac{\text{No. of Organisms} \times \text{Virulence}}{\text{Host Resistance}}

Infections can follow various patterns: Acute, Persistent, Latent/Reactivating, or Slow. Pathogens reside in Reservoirs (animate or inanimate), transmit through various Modes of Transmission, and enter/exit the host via specific Portals of Entry/Exit.

Microbial Mechanisms of Pathogenicity

  • Bacteria: Employ virulence factors like capsules, cell wall components, enzymes, antigenic variations, and biofilms. They produce exotoxins (proteins secreted by mostly gram-positive bacteria, e.g., Clostridium botulinum) and endotoxins (lipid A of gram-negative LPS, released upon cell lysis, e.g., Salmonella Typhimurium).

  • Viruses: Evade immunity by growing intracellularly, attach via specific receptors, and cause cytopathic effects (CPE), leading to cell death (cytocidal) or damage (noncytocidal), and potential cell transformation.

  • Other Parasites: Fungi, protozoa, and helminths damage hosts through capsules, toxins, enzymes, and direct tissue damage.

Host Defenses and Coevolution

Hosts counter infection through Resistance (limiting pathogen burden) or Tolerance (limiting health impact despite pathogen presence). Resistance imposes strong selective pressure on pathogens, favoring more virulent strains, whereas tolerance exerts no such pressure. The ongoing evolutionary