Parasite ExamplesAssassin bugs (Triatoma)MalariaTapeworms (Cestodes)Cholera (Shigella) transmission via dysenteryToilet seats, elevator buttons, shopping carts...Molecular mimicry“eclipsed antigens” resemble host antigenshence do not elicit formation of host antibodiesMajor Histocompatibility ComplexTrypanosoma shed coats, change antigensFilariasis Elephantiasis (blocked lymph nodes,nematode worms carried by mosquitos)
Botfly
Botfly Botfly larva
Filariasis Elephantiasis (blocked lymph nodes,nematode worms carried by mosquitos)
Nematode (Roundworm)Dracunculus medinensis
Dracunculus medinensis
Host Altered BehaviorRabies virus — rabid animals bite
Lancet fluke Trematode Dicrocoelium dentriticum Cercaria —> Metacercariae encyst on ant’s brain
Sheep ingest an ant and get infected
Starlings, Pill bugs, and AcanthocephalansDucks, Amphipods, and Acanthocephalans
STDs —> increased sexual activity?
Ectoparasites, endoparasites
Social parasites
Parasite–Predator spectrum
Microparasites
Macroparasites
Parasitoids
Predators
Mode of transmission & virulence
Comparison of Ecological Characteristics That Vary Along a Parasite–Predator Spectrum_________________________________________________________________________Characteristic Microparasite Macroparasite Parasitoid Predator_________________________________________________________________________ Body size Much smaller Smaller than Mature stages Larger
than hosts hosts similar in size than preyIntrinsic rate Much faster Faster than Comparable Usuallyof population than hosts hosts but slightly slowergrowth slower than preyInteraction with One host One host One host can Many preyhost individuals usually supports supports a few support seve- items arein natural several popula- to many indivi- ral individuals eaten bypopulations tions of different duals of different each predator
species speciesEffect of the Mildly to fairly Variable, not Eventually Usuallyinteraction on deleterious too virulent to fatal immediatelyhost individual definitive; can fatal
be intermediateStability of the Intermediate High Intermediate Usually lowInteraction
Ability to reg- Moderate Low Fairly high Highulate lowerTrophic level__________________________________________________________________
Challenges facing Parasites, hosts as islands, how to infect new ones?
High specificity, high fecundities, exploitation of vectors (mosquitoes)
Intermediate and final hosts, host altered behavior (rabies, etc.)Assassin bugs (Triatoma)MalariaTapeworms (Cestodes), Nematodes (roundworms)Cholera (Shigella) transmission via dysenteryToilet seats, elevator buttons, door knobs, shopping carts...etc.Molecular mimicry: “eclipsed antigens” resemble host antigenshence do not elicit formation of host antibodiesMajor Histocompatibility Complex (MHC) Trypanosoma shed coats, change antigensFilariasis Elephantiasis (lymph nodes blocked by nematodes carried by mosquitoes)BotfliesDracunculus medinensis, caduceus symbol of medicine
Evolution of Virulence (benign parasites allow hosts to live)Host altered behaviorRabies virus — rabid animals bite, passes on virus to new hostLancet fluke Trematode Dicrocoelium dentriticumCercaria —> Metacercariae encyst on ant’s brainSheep ingest an ant and get infectedStarlings, Pill bugs, and AcanthocephalansDucks, Amphipods, and AcanthocephalansSTDs —> increased sexual activity?Ectoparasites (fleas, ticks, lice), endoparasitesSocial parasites (thievery, brood parasitism)
Parasitoids: Ichneumonid wasps
Microparasites—> macroparasties—>parasitoids—>predator spectrum
and many correlates thereof, such as relative sizes, rates of increase,
number of parasites per host, virulence, stability, and ability to
regulate lower trophic level
Ebola zaire
Ebola reston
Parasitism ——> Commensalism ——> Mutualism(+, –) 0 <—— (+, 0) <—— (+, +)
Host-Altered Behavior
Evolution of Virulence
Biological Control
Brazilian cottontail rabbit Sylvilagus brasilensiscarried a benign myxoma virus which, wheninjected into an Oryctolagus cunniculus harecaused cancers that quickly killed the rabbits.
Released in 1951, epidemic killed 99.9% of bunniesbut, rabbits being rabbits, bred like bunnies, andsoon there were as many as ever. Second epidemiconly killed 70% and the third only 50%. ResistentRabbits evolved, but so did the virus — as it wasspreading through the rabbit population, the virusevolved reduced virulence.
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