Chapter 14 - Parasitism and Infectious Diseases

Key Concepts in Parasitism and Infectious Diseases

• Types of Parasites

  • Ectoparasites:
    • Definition: Parasites that live on the outside of the host organism.
    • Examples: Nematodes, Fleas, Lice, Mites, Ticks.
  • Endoparasites:
    • Definition: Parasites that live inside the host organism.
    • Examples: Viruses, Prions, Helminths (roundworms and flatworms).

• Key Vocabulary:

  • Infection Resistance: The ability of a host to prevent an infection from occurring.
    • Importance: Hosts can avoid infection much like predator-prey dynamics.
  • Infection Tolerance: The ability of a host to minimize the harm once an infection has occurred.
    • Importance: Beneficial for those hosts that cannot prevent an infection and need to cope with it.
  • Parasite Load: The number of parasites of a given species that an individual host can harbor.
    • Selection pressure rewards hosts that can tolerate higher parasite loads.
  • Emerging Infectious Disease: A disease that is newly discovered or has increased significantly in occurrence after being rare.
  • Horizontal Transmission: Movement of parasites between individuals other than parents and offspring.
  • Vertical Transmission: Transfer of parasites from a parent to its offspring.
  • Reservoir Species: Species that carry a parasite without suffering from the disease it causes in other species.
    • Example: Certain bird species that can carry avian malaria but do not get sick from it.

• Modeling Parasite Dynamics:

  • Susceptible-Infected-Resistant (S-I-R) Model:
    • Components:
      • S: 100% of the population is susceptible.
      • I: Some individuals become infected.
      • R: Some infected individuals develop immunity.
      • b: Rate of infection between infected and susceptible individuals.
      • g: Rate of recovery from the infection.

• Interactions Between Parasites and Hosts:

  • Parasites evolve offensive strategies to infect hosts.
  • Hosts develop defensive strategies to combat parasites:
    • Avoidance of infected feces.
    • Removal of ectoparasites.
    • Production of antifungal and antibacterial chemicals (e.g., some amphibians release antimicrobials through their skin).
    • Example of self-medication: Chimpanzees chew bitter leaves and stems to reduce their parasite infections.

• Preferred Habitats of Parasites:

  • Brain: Afflicted by prions causing Creutzfeldt-Jakob disease.
  • Lungs: Bacteria causing Tuberculosis.