Parasitism

1
Parasitism

• Smith Smith 6th Edition
• Chapter 17, p. 306-321
• October 30, 2000

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Parasitism Definition

• A condition in which 2 organisms live together
  but one derives its nourishment at the expense of
  another
• They rarely directly kill their host but often
  weaken them allowing to fall prey to predators,
  infection, emaciation, sterility, and stunted
  growth.

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Parasitoids

• Parasitic larva in many insects that devours the
  soft tissue of their host killing the host.
• They are essentially predators.

4
Leopolds View on Disease

• He felt that the role of disease in wildlife was
  greatly underestimated.
• Parasites can control both predator and prey
  populations.
• Unfortunately, parasites are very difficult to
  study due to the fact that they are not directly
  responsible for the death of their host and many
  effects are indirect.

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Characteristics of Parasites

• A heavy load of parasites is considered an
  infection and the outcome of infection is
  disease.
• Disease Any condition of a plant or animal
  that deviates from normal well-being
• Not all parasites will cause disease

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Types of Parasites

• Ectoparasites Live on the outside of the host
• Endoparasites Live within the body of the host
• Full-time vs. Part-Time Parasites
• Microparasites vs. Macroparasites

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Hosts as Habitat

• Hosts are homes for parasites
• Much like larger ecosystems there is Niche
  Partitioning where different species use
  different tissues or parts of tissues.)
• Major problem is entering and exiting the host

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Life Cycles Introduction

• Have to move from host to host (Can not move at
  will in most cases)
• Definitive host The host in which the parasite
  becomes an adult
• Intermediate Hosts Harbor some developmental
  stage of a parasite

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Life Cycles Direct Transmission

• Definition The transfer of a parasite from one
  host to another by direct contact or through a
  carrier or vector.
• Most microparasites are transmitted directly
• Examples Rabies through saliva, Elm phloem
  necrosis transmitted by root grafts or by a
  leafhopper species.

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Direct Transmission Animal Macroparasites

• Parasitic Nematodes Lay many eggs which are
  expelled in feces, feces ingested by another
  host, travels through the system to invade
  another host
• Other examples Lice, Ticks, mange mites, fleas

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Direct Transmission Plant Macroparasites

• Holoparasites Lack chlorophyll and draw water,
  nutrients, and carbon from the roots of a host
  plant (i.e. Squawroot on oak)
• Hemiparasites They are photosynthetic but draw
  water and nutrients from a host plant (i.e.
  Mistletoe)

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Squawroot (Conopholis Americana) on an Oak, a
Holoparasite
(Smith Smith 2001)
Note Book has the picture labeled incorrectly
as a Hemiparasite but text has squawroot
(correctly) as a holoparasite
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Life Cycles Indirect Transmission

• Different stages of the life cycle require
  different hosts.

(Smith Smith 2001)
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Indirect Transmission

• Some parasites need 2 intermediate hosts
• More common in animals than plants
• Examples Black grub in fish, many forms of tree
  rusts

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Black Grub Life Cycle

• The fluke (Uvulifer ambloplitis) is actually
  white but fish lay down a black pigment causing
  the black appearance we see
• The parasite passes from the Kingfisher, to
  snails, then enter a fish by active searching
• The fish is eaten by a kingfisher and the cycle
  is repeated

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Black Grub in Fish
(Smith Smith 2001)
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Indirect Transmission Plants

• Uncommon except with tree rusts
• See Figure 17.5, Page 312 in the 6th Edition

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Dynamics of Transmission

• The impact of parasites on host populations
  depends the nature of transmission and the
  density and dispersal ability of the host.
• May help to limit host populations
• Microparasites need dense host populations

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Disease Outbreaks

• Epizootic Rapid spread of a viral or bacterial
  disease in an animal population (e.g. Rabies in
  Europe)
• Epidemic Rapid spread of a viral of bacterial
  disease in humans (e.g. Bubonic plague)
• Much more common with direct transmission
  parasites

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Spread of Rabies in Europe
(Smith and Smith 2001)
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Overdispersion

• A minority of the host population hosts a
  majority of the parasite
• Parasites may act as top predators (Grenfell 1992)

(Smith Smith 2001)
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Host Response Biochemical

• Defensive responses as an attempt to prevent or
  reduce the effects of the parasite
• Immune Response Inflamations, increased WBCs
  to attack infection, antibodies produced in
  response to antigens

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Host ResponseAbnormal Growths

• Most common in plants (Cysts, limb cankers, root
  knots, root nodules)
• Galls abnormal growth hurt plants but increase
  predation of the parasite
• Animals Malaria may increase spleen size, pearl
  formation in mollusks

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Galls in Plants
(Smith Smith 2001)
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Host Response Sterility

• Parasite infection may cause sterility of host
• Several Drosophilia are parasitized by a nematode
  causing up to 100 sterility in females depending
  on fruit fly species
• In plants some fungal parasites inhibit flowering
  but stimulate vegetative reproduction, increasing
  potential number of hosts

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Host ResponseBehavioral Changes

• Tularemia in rabbits causes sluggishness
• Rabid foxes and raccoons may be overly aggressive
• Whirling disease in fish
• Grooming and preening a response to ectoparasites

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Host ResponsesMate Selection

• Hypothesized (Hamilton and Zuk 198, Zuk 1991,
  1992) that male birds with bright plumage have
  fewer parasites and thus healthier. That is why
  females are choosing them
• No direct evidence of this.

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Parasite Population Dynamics

• We can view hosts as islands with a finite amount
  of resources for potential parasites.
• Same topics weve covered earlier in class
  Inter and Intra-specific competion, population
  dynamics, etc. can be applied to parasites.
• 3 populations (Host, Adult parasite, ineffective
  transmission stage

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Intraspecific Competition
(Smith and Smith 2001)
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Parasite Death Rate
(Smith and Smith 2001)
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Evolutionary Responses

• Host and parasite have co-evolved. We often see
  problems when hosts or parasites are introduced
  into a new location.
• Both parasites and hosts are constantly adapting.
  
• The Catch-22 A virulent parasite that kills its
  host wont persist however, neither will a
  parasite that cant defend itself against its
  host.

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Myxomatosis in European Rabbits

• Introduced into Australia, the rabbits
  population exploded unchecked
• The government introduced Myxomatosis virus into
  the population killing 97-99.
• However, overtime the virus was less
  successfulthe species were co-evolving.

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Social Parasitism

• An organism parasitically depends on the social
  organization of another.
• Brood Parasitism One organism used another to
  incubate eggs or care for young. (e.g.
  brown-headed cowbirds)
• Kleptoparasitism The forcible theft of prey by
  the parasite from the host (e. g. bald eagles
  from osprey)

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Brood Parasitism

• Temporary vs. Permanent In permanent brood
  parasites, the parasite spends its entire life
  cycle within the nest.
• Facultative vs. Obligatory Obligatory brood
  parasites can not raise their own brood, I.e.
  some birds do not/can not build nests.
• Many waterfowl are temporary facultative

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Interspecific Brood Parasitism
Intraspecific Brood Parasitism
Brown-headed cowbird lays eggs in other birds
nests, cowbird fledges first and out-competes the
mothers progeny.
Cliff swallows nesting in colonies increases
brood parasitism. Woodducks
(Smith and Smith 2001)
(Smith and Smith 2001)