Parasites and zoonoses

  1. Symbiosis=living together

    Mutualism/Cooperation + +
    Spite - -
    Predation + -
    Parasitism + -


  2. Types - parasites can be classified along a variety of dimensions
    1. Microparasites (viruses, bacteria, fungi) or Macroparasites (platyhelminths, nemotodes, arthropods)
    2. Obligate vs. facultative
    3. Endoparasites vs. Ectoparasites


  3. Characteristics of parasites
    1. Usually smaller than host
    2. Usually physiologically dependent on host
    3. May have specificity for particular host species
    4. Usually do not kill host (but may debilitate it)
    5. May live all or part of life cycle on host, and may have complex life cycle

      1. Life cycle of deer tick
      2. Life cycle of tapeworm
      3. Life cycle of menigeal worm
      4. Life cycle of Toxoplasma


  4. Zoonotic disease are those carried by nonhuman animals but transmissible to humans.

  5. Case studies

    1. Lyme disease

      1. Disease is caused by a bacterium, Borrelia burgdorferi
      2. Vector is an arachnid, the deer tick
      3. Symptoms of the disease

        1. bulls-eye rash, low grade fever, muscle aches, lethargy
        2. cardiac problems, arthritis, neurological symptoms


      4. Life cycle of deer tick and how it becomes infected

        1. Eggs (laid on ground) are not infected
        2. Eggs hatch in summer into larvae (6 legged), feed on small mammals
        3. White-footed mouse is primary host and is the reservoir for the bacteria causing Lyme disease
        4. Larvae get one blood meal then drop off to metamorphose into nymphs. They are likely to become infected at the larval stage if they feed on an infected host.
        5. Nymphs (now often infected) get a blood meal. They can pass the bacterium to their host if attached to it for at least 24 hours.
        6. Nymphs drop off and metamorphose into adults. They take one more blood meal prior to laying eggs. This is usually from their primary host, the deer.


      5. Humans are most likely to get the disease from nymphs
        1. Seasonal activity of the nymphs coincides most closely with when humans are likely to be in tick infested areas (summer)
        2. Although adult ticks are often infected, they don't usually pass the disease on to humans because they are usually detected before they have been attached for 24 hrs.
      6. Incidence of Lyme disease is affected by ecological factors (see Fig. 1) in a complex and spatially dependent way
        1. Factors that come into play include
          • acorn production (food for deer mice)
          • small mammal biodiversity (relative density of reservoir hosts vs dilution hosts, presence of predators)
          • degree of reforestation of previously agricultural land
          • Socioeconomic factors of human populations
          • abiotic factors
            1. climate change
            2. relative humidity


  6. Toxoplasmosis
    Watch this interview with primatologist Robert Sapolsky on toxoplasmosis (watch up to about 20:00).
    Or this one
    Here is an additional popular science article that gives some background on the links between toxoplasmosis and human behavioral disorders.