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DiseaseEcology

I. Howdocoevolutionary armsraceswork?

Malariaasamodel(parasitismby4speciesofPlasmodium)

Kills~1millionpeople/year

Sickens~300millionpeople/year

Plasmodium cellsarepassedfrominfectedmosquitoes

o umans,an rom n ec e umans omosqu oes

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A.ArmsraceI:Malariaandpublichealthofficials

UseofDDTtokillmosquitoes:

Currentemphasis:useofinsecticideimpregnatedbed

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B.ArmsraceII:Evolutionaryresponsesinhumans

Question:In

areas

where

malaria

is

common,

does

naturalselectionfavorindividualswithresistantalleles?

Hemoglobin(Hb)

is

aprotein

that

carries

oxygen

in

red

bloodcells;thenormalalleleisHbA

whenitisheterozygous;causessicklecelldiseasewhen

homozygous

AA AS SS

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Recentdata:studyHb allelesinhealthyandPlasmodium

ObserverelativelyhighfrequencyofHbC allele

1.Inhealthy

people,

HbC genotypes

are

in

HW

proportions

2.AmongPlasmodiuminfectedpeople,observeno

HbCC genotypes.

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Estimate:Riskofinfectionreducedby29%inHbAC

n v ua s; y n n v ua s

Note:AfollowupstudyinMalishowednoprotection

against

severe

malaria

in

HbAC

heterozygotes.

HowisselectionactingonHbC alleles?

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II.Canparasitesmanipulatetheirhosts?

Doesnatural

selection

favor

alleles

that

allow

parasites

to

manipulatehostbehaviorinawaythatincreasestheir

transmissiontonew osts

HostmanipulationbyPlasmodium?

19humanvolunteers(Tanzania);eachindividualhas

Sleepinthesamehouse

Collectmosquitoes

the

following

day

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Underthehostmanipulationhypothesis:

1. Whatpercentage

of

uninfected

mosquitoes

bit

more

thanoneperson?

2.Whatpercentageofinfectedmosquitoesbitmorethan

oneperson?

Claim:Plasmodium ismanipulatingmosquitoesinaway

thatmakes

them

more

likely

to

bite

multiple

individuals

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Q:Whenyousneeze,isyourbodyriddingitselfofcold

viruses oriscoldvirusmani ulatin outoincrease

transmissionto

new

hosts?

ww u y u y u y

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.

Underwhatconditionsareparasitesexpectedtoevolveto

be

extremely

virulent?

=

Observation

2:

Virulence

is

a

function

of

parasite

w u .

Observation3:Theprobabilityofparasitetransmission

isafunction

of

the

number

of

parasites

present.

virulence

(it

can

respond

to

selection).

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Question:Give

two

reasons

why

condom

use

and

sexual/maritalfidelityshouldbeeffectiveinhaltingthe

.

Circumcisionas

aprophylaxis?

Pros:

Cons:

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A. SurveyincidenceofcholerabacteriuminwatersoffPeru

a

s

ere a ons p

e ween

seawa er

empera ure

andVibrio cholerae concentration?

ans

CooloceanwaterSe

in

hum

wa

ter

nrate

temp

nfectio

ratue

I

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B.Surveyincidenceofmicethatcarryhantavirus,asa

rodents,speciesthatpreyonmice)

FiveparksinPortland,Oregon