Parasites

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Parasites Parasitism
Jo Hamilton Parasitology BS31820
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Course overview resources

• Explore animal associations
• Major parasitic groups
• Life strategies / cycles
• Infection immunity
• Web http//www.aber.ac.uk/parasitology/Edu/EduInd
  ex.html
• Exam comp q.

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Learning outcomes.

• Explore animal associations
• Consider
• Symbiosis
• Mutualism
• Commensalism
• Parasitism
• Key definitions

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Introduction the 3rd great environment?

• Aquatic terrestrial
• Parasitic?
• How successful is parasitism?
• depends on definition
• major animal groups parasitic members.
• gt 50 all known species parasitic at some stage.
• 100 individual plants animals parasitized.

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Animal associations symbiosis.

• Parasitism - form of animal association.
• Definitions
• Symbiosis - living together.
• DeBary (1876) - describe two species organisms
  that lived together.
• range intimate interactions
• E.g. mutualism, commensalism parasitism.
• Overarching (umbrella) term.

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Animal associations mutualism.

• Mutualism
• highly interdependent association
• cannot survive without one another.
• E.g. ruminants rumen protozoa
• protozoa - enzymes - convert cellulose to glucose
• host provides low redox potential environment
• Two way benefit, no harm Symbiosis some authors.

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Animal associations commensalism.

• Commensalism - 1 way benefit
• no harm exerted in either direction.
• E.g. Clown Fish sea anemone.
• Sea anemone - protection against predators
• Clown fish highly evolved to survive cnidarian
  nematocysts.
• Mucus - thicker lacks sialic acid groups which
  trigger nematocyst discharge.
• Commensalism usually involves feeding
  relationship.

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Animal associations parasitism.

• Parasitism - Classical definition - intimate
  relationship in which 1 (parasite) lives on, off
  or at expense of other (host).
• Implies 1 partner benefits, other harmed.
• But harm difficult to quantify
• assumptions evidence?

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Animal associations parasitism paradigms.

• E.g. 1 - Rats infected with Spirometra grow
  larger than uninfected rats.
• The larva produces analogue of vertebrate growth
  hormone
• Growth boost harming host or good for it?
• E.g. 2 many molluscs infected with Digenetic
  flukes develop thicker, heavier shells
• An advantage?

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Animal associations parasitism paradigms.

• Some classic examples of mutualism - more like
  armed standoff than mutual benefit.
• Depending on external conditions associations can
  switch between mutualism parasitism.
• E.g. tree roots fungal mycorrhiza.

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Animal associations parasitism paradigms.

• Simple definition of parasitism not really
  adequate
• does not differentiate between parasite
  predator or parasite micro-predator.
• Concept of host-parasite relationship expands
  classical definition
• 'parasite metabolically or physiologically
  dependent on host
• 'genetic complementation between parasite
  host'.

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Parasitism a refined definition.

• More recent description parasitism - Crofton.
• Ecological relationship between two organisms
  (parasite host).
• Parasite physiologically / metabolically
  dependent upon host.
• Heavily infected hosts killed by their parasites.
  
• Reproductive potential of parasite greater than
  hosts.
• Parasite population not evenly distributed
  amongst host population.

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Parasitism a refined definition.

• Parasitism defined in terms of 2 different
  species who form a regular association
• Rules out foetus/mother relationship.
• Size might also be a consideration
• Parasites generally smaller than host
• Predators generally larger than host
• Always exceptions!

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Parasitism a refined definition.

• Physiological / metabolic dependence parasite on
  host v. important
• Does not distinguish parasitism from mutualism.
• Heavily infected hosts killed
• Concept of cost to host population
• Parasites have higher reproductive capacity than
  hosts
• Distinguishes parasites from predators

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Parasitism a refined definition.

• Overdispersed / clumped frequency distribution
  important
• Helps exclude micro-predators from definition.
• No definition of parasitism completely
  satisfactory
• Always exceptions grey areas

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• Hostparasite relationship - subtle interplay
  between parasite survival host defence.

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Animal associations not a linear sequence.

• Animal associations - not linear sequence
• Not Commensalism parasitism mutualism.
• Each association independent - different
  endpoints.

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Evolution of parasitism animal associations.

• Arguments
• 2 species living in close association
• Prey-predator relationship?
• Recently evolved parasites virulent?
• Host evolves tolerance?
• Sequence Acute chronic- mutualism?
• Or
• Recently evolved parasites less virulent?
• Parasite virulence evolves later?
• Not linear sequence.

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Evolution of the parasite niche.

• Concept of parasite niche
• Host body divided up into separate niches?
• Niche exists whether parasitised or not.
• Or
• parasite creates its own niche.
• parasite determines niche, not host.
• Niche description of parasites requirements, not
  host attributes.

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The parasite niche.

• Parasites have not exhausted all possible host
  niches
• new parasite species!
• Parasites can influence host evolution
• affect reproductive success of hosts
• remove resources / kill hosts

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Parasite influence on host evolution.

• Macroevolutionary level evolutionary change
  above species level
• E.g. are parasites a driving force in evolution
  of sex?
• Microevolutionary level deterministic, within
  species phenomenon.
• Role of parasites in mate choice?
• Parasites may influence host evolution.

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Useful definitions.

• Ectoparasite - lives on surface.
• Endoparasite - lives inside host.
• Mesoparasite - penetrates external openings (E.g.
  buccal cavity, cloaca, ear).
• Definitive host - where parasite reaches sexual
  maturity.

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Useful definitions.

• Intermediate host - required by parasite to
  complete its life cycle.
• Usually undergoes morphological / physiological
  change.
• Paratenic host - optional transport host
• no morphological change in parasite.
• Vector - host that plays an active role in
  transmission
• can be definitive / intermediate host.

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Summary.

• Explored animal associations
• Consider
• Symbiosis
• Mutualism
• Commensalism
• Parasitism
• Key definitions