Parasitism Notes

Final Topics

• Evolution and Selection
  • Trait and genetic variation
• Optimality theory
  • Game theory and behavioral ecology
• Population dynamics
  • Population growth models and metapopulations
• Species interaction
  • Parasitism (predator and prey dynamics)
  • Epidemiology and disease dynamics
  • Evolution and cancer

Species Interaction

• Impact on population density: increase (+), reduction (-), or no change (0).

• Symbiosis: Living in intimate contact with other organisms (marked with * above).

Darwin’s Finches

• Ground finches example with cactus (G. scandens), small (G. fuliginosa), medium (G. fortis), and large (G. magnirostris) sizes.
• Beak sizes and seed types are correlated, as shown by De León et al. 2014.
• The concept of "Imperfect generalism" in Darwin's finches, referencing De León et al. 2014.

Parasitism

• A major factor regulating natural populations.
• Plays an important role in ecosystem function.
• Parasites constitute roughly 50% of the species on Earth.
• Tapeworm (Taenia solium) example
  • Intestinal parasite with hooks and suckers to attach to its host.

Predator vs. Parasitoid vs. Parasite

• Predator:
  • Usually the same size or larger than prey.
  • Feeds on many prey species - generalist.
  • Relates to optimal foraging theory.
• Parasitoids:
  • Mud dauber wasp females paralyze hosts & lay eggs in/on them
  • Specialized in their choice of host species.
  • Eggs or larvae are usually laid in, on, or near host.
  • Immatures remain on or in host and kill it.
  • Usually, only one host is needed to complete the life cycle
• Parasites:
  • Usually smaller than hosts (“prey”).
  • Multiple parasites can live on/in the same host.
  • May require multiple hosts to complete its life cycle.
  • Highly diverse strategies.

Parasite Types

• Cuscuta pentagona
  • Obligate parasitic plant.
• Macroparasites
  • Large, such as arthropods and worms.
• Microparasites
  • Microscopic, such as protozoa, bacteria, and viruses.
• Endoparasites
  • Live within the host, in the alimentary canal, or within cells or tissues (e.g., Cordyceps and Zombie ants).
• Ectoparasites
  • Live on the host’s surface (e.g. Cat flea).

Ectoparasitism vs. Endoparasitism

• Ectoparasitism
  • Advantages:
    • Ease of dispersal
    • Safe from host's immune system
  • Disadvantages:
    • Vulnerability to natural enemies
    • Exposure to external environment
• Endoparasitism
  • Advantages:
    • Ease of feeding
    • Protected from external environment
    • Safer from enemies
  • Disadvantages:
    • Vulnerability to host's immune system
    • Feeding more difficult
    • Dispersal difficult

Parasite Life Cycle

• Two key factors:
  • Transmission: The movement of the parasite or its descendants from one host to another.
  • Infection: The capacity for the parasite to successfully find and enter the host, and affect the health of the host.
    • The parasite lives off the now infected host, which itself become a source of transmission.

Modes of Parasite Transmission

• Direct transmission
  • From one host to another.
  • Examples:
    • Sexually transmitted diseases
    • Rabies (microparasites)
    • Fleas (macroparasites)
• “Passive” or indirect transmission
  • Via air as aerosol (flu, colds viruses) or contact with feces.
• Via vectors
  • With possible intermediate species or reservoir hosts.

Examples of Transmission

• Direct Transmission:
  • Ebola Virus:
    • Average case fatality rate is around 50%.
    • Case fatality rates have varied from 25% to 90% in past outbreaks.
• Passive or indirect transmission
  • COVID-19 Coronavirus
  • Measles
    • Measles virus breaks through epithelial cell barriers to achieve transmission (J Clin Invest DOI: 10.1172/JCI36251).
• Transmission via Vector: Phoresis or Phoresy
  • Botfly (Dermatobia hominis):
    • Eggs are laid on a blood-sucking vector.
    • Larvae enter the host after the vector takes a blood meal.
    • Pupae develop in the soil.

Transmission in Disturbed Environments

• Environmental change, deforestation, urbanization, migration
• Forest-urban virus cycling leading to spillover events.

Malaria Transmission

• Malaria is caused by the parasite Plasmodium sp.
  • Life cycle Involves two hosts.
    • Female Anopheles mosquito inoculates sporozoites into the human host.
    • Sporozoites mature inside the human liver and blood cells.
    • Sexual stages (gametocytes) are ingested by the Anopheles mosquito during a blood meal.
    • They complete their cycle and multiply as sporozoites and then migrate to the mosquito salivary glands.
  • Vector borne transmission.

Parasites and Behavior

• Parasites can affect behavior.