Parasitology Notes

Bristol Zoo Project

  • Date: 10th April
  • Coach departs from the main building around 9:30 am (to be confirmed).
  • Departs Bristol Zoo at 3 pm to return to Cardiff.
  • Travel and entry costs are covered by Biosciences.
  • Bring lunch and notepads.

Job Opportunity: Parasites in the Arctic

  • Paid summer work with Dr. Sophie Watson.
  • Summer 2025, flexible working offered.
  • Deadline: 09.04.25
  • Pay: £12.45/hour, max 200 hours.

Host-Parasite Adaptations

  • Morphological: Size and shape.
  • Physiological: Coping in extreme environments (e.g., gut).
  • Immunological: Evading detection.
  • Behavioral: Facilitating transmission.
  • Evolutionary arms race between host and parasite.

Why Study Parasites?

  • Parasites infect and harm hosts.
  • Malaria: 198 million cases in 2013.
  • Soil-transmitted helminth infections: Affect over 1.5 billion people (24% of the world's population).
  • Cause morbidity rather than mortality.

What is a Parasite?

  • Macroparasites: helminths (endoparasites) and ectoparasites.
    • 50% of all animal species are parasitic
  • Microparasites: protozoa, viruses, bacteria.
    • 1/4 of all animal families are parasites

Where Are Parasites Found?

  • Parasites are found in almost every part of the host's body and in the environment.
  • Examples of parasites and their locations within a host's body:
    • Brain: Tapeworm larvae
    • Lungs: Lungworm, Tapeworm larvae
    • Blood: Flukes, Filarial worms
    • Muscle: Tapeworm larvae
    • Skin: Filarial worm
    • Large Intestine: Nematodes
    • Abomasum: Nematodes
    • Liver: Fluke, Tapeworm larvae
    • Small Intestine: Nematodes, Adult tapeworms

Examples of Parasites

  • Cymothoa exigua: tongue-eating louse.
  • Leucochloridium paradoxum: green-banded broodsac (helminth).
  • Bot Fly (Cuterebra emasculator).

Morphological Adaptations

  • Structures for penetration and attachment (hooks, suckers, teeth).
  • Needle-like or barbed mouthparts.
  • Dorso-ventrally flattened shape (common in ectoparasites).
  • Immunomodulatory activities inhibiting host defense reactions.
  • Safety in numbers.

Physiological Adaptations

  • Reduction in unnecessary structures (sense organs, nervous system, locomotion, digestive system).
  • High fecundity (e.g., tapeworm proglottids).
  • Resistant life cycle stages (e.g., cryptobiosis in human hookworm).
  • Strong, impermeable cuticle (nematodes) or tegument (cestodes).
  • pH of gastric acid is 1.5 to 3.5.

Adaptations – Evading Immunity

  • Helminths produce excretory/secretory proteins (ESPs) to suppress host immunity.

Survival and Transmission

  • Parasites have to survive in multiple environments.
  • Life cycles and transmission routes are varied and specialized
  • Direct and indirect life cycles exist.

Immunological Adaptations

  • Molecular mimicry: Blood fluke (Schistosoma mansoni) uses sugars present in snails to avoid detection by the snail's immune system (lectins).

Behavioral Adaptations

  • Parasites adapt their behavior and those of their hosts to persist; transmission is the ultimate goal.
    • Host finding behaviors.
    • Periodic behaviors.
    • Host modifying behaviors.

Host Finding Behaviors

  • Ticks questing: waiting on vegetation for a host, stimulated by carbon dioxide, heat, and movement.
  • Mosquitoes, helminths, bed bugs also have stimuli for host-finding.

Periodic Behaviors

  • Filarial worms: microfilariae move to peripheral blood during biting hours of vectors (mosquitoes or flies).

The Extended Parasite Phenotype

  • Changes to the host’s phenotype (coloration, morphology, and/or behavior) must produce a fitness benefit for the parasite.
  • Example: Carpenter ants infected with fungus O. unilateralis are manipulated to head to an area suitable for fungal growth.

Parasites Modifying Host Behaviors

  • Complex life cycles (indirect) involve intermediate and definitive hosts.
  • Parasites manipulate the behavior of intermediate and/or definitive hosts to aid transmission known as trophic transmission.
  • Example: Trematode Curtuteria australis infects cockles, making them more vulnerable to predation by birds (definitive host).

Modifying Host Behaviors

  • Toxoplasma gondii can manipulate host behavior to facilitate completion of its life cycle.
  • Example: Leucochloridium paradoxum, green-banded broodsac (helminth).

Host-Parasite Coevolution: Arms Race

  • Red Queen Hypothesis: fitness of species remains the same despite constant evolution.
  • Example: Rabbits in Australia. Introduction of Myxoma virus in 1950 reduced the rabbit population, but resistance evolved. A second biocontrol agent, RHDV, was introduced, and again, resistance evolved.

What Happens When There Are No Parasites?

  • Enemy Release Hypothesis.