Parasites and diseases

Introduction to parasites

  • Topic: parasitism and disease; focus on parasites, their transmission, coevolution with hosts, and how transmission strategies shape virulence and life histories.
  • Opening example (video content): a snail ingesting parasites that hijack the snail's tentacles and brain, turning the snail into a 'zombie' to extend the parasite's life and offspring; the parasite aims to reach a bird definitive host by causing the snail to move into sunlit areas where birds prey on it; parasite reproduces in the bird, then the life cycle continues when snails eat bird droppings containing parasite stages.
  • Key idea from the video: behavior modification by the parasite to increase transmission.
  • Concept preview: intermediate host vs definitive host; how parasites maximize transmission; general life-history logic of parasitism.

Core concepts introduced

  • Behavior modification as a transmission strategy: parasites can manipulate host behavior to increase contact with the next host.
  • Intermediate host vs definitive host:
    • Intermediate host: hosts the parasite for part of its life cycle; not the site of reproduction.
    • Definitive host: where the parasite reproduces.
  • Visual analogy: an image with an ant infected by a nematode; the ant’s abdomen swells and resembles a fruit to attract birds; birds eat the ant, parasite reproduces in the bird; cycle continues via feces.
  • Transmission cycle summary for this ant example: parasite passes via bird feces to the ant, then the ant ingests the feces and the cycle restarts.
  • Ecological significance: parasitism may be widespread; up to about half of Earth’s species are parasites.

Parasite prevalence and diversity

  • Observation: high parasite loads in free-living species are common; example given: a study sampling 76 mammal species to assess parasite loads.
    • Notes on table data (described): mean number of parasites per host and the number of parasite species per host population; some hosts show high parasite counts, others low.
    • Example parasite group: spiny-headed worms (acanthocephalans) among lab or wild mammals.
  • General conclusion: parasites are common and many species are specialized to particular hosts.
  • Specialization:
    • Many parasite species are specialists, infecting only one or a few host species (evidence shown via a green wasp and its ant host).
    • Specialization drives coevolution between parasite and host.

Coevolution between parasites and hosts

  • Definition: coevolution is the genetic change in one species in response to changes in another, particularly when species interact closely (e.g., plants and pollinators; mutualisms; parasites and hosts).
  • Detecting coevolution: use quantitative methods to compare branching patterns on phylogenetic trees of parasite and host; visual inspection alone is insufficient.
  • Concept of parallel branching: when host speciation is followed by parasite speciation (post-speciation coevolution), their phylogenies show congruent patterns.
  • Example discussed: primates and parasitic nematodes show broadly matching branching patterns, indicating linked evolutionary histories.
  • Practical point: even when visual similarity exists, robust inference requires quantitative tree-based analyses.

Life cycle, host interaction, and adaptation

  • General pathway for parasite life cycles:
    1) Host harboring: parasite finds a host that provides a relatively safe, harboring environment (inside the host).
    2) Establishment: parasite invades or attaches to the host (e.g., penetrating skin or moving into the gastrointestinal tract).
    3) Immune evasion and behavioral defenses: parasite evades the host immune system and copes with host defenses (grooming, swatting).
    4) Growth and reproduction: parasite grows and reproduces within the host or moves to another host for reproduction.
    5) Dispersal: parasite leaves the current host to find a new one, continuing the cycle.
  • Adaptation note: adaptations to one host are rarely effective in a different host due to host-specific biology and defenses.
  • Example of life cycles: freshwater mussel larva parasite on fish scales; Schistosoma-like life cycle (reviewed):
    • Humans (definitive host) shed parasite eggs in waste.
    • Eggs reach water and hatch into larval stages that infect snails (intermediate host).
    • Snails release larval stages that infect humans by penetrating skin, propagating within human tissues, and reproducing, continuing the cycle