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.