what is infectious disease?
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What is infectious disease?. Ecology of Infectious Disease & Disease in plant communities. Dr. Charles Mitchell UNC Biology Department & Curriculum in Ecology. Lecture outline. Basic concepts / definitions Patterns of disease emergence Transmission Disease triangle - PowerPoint PPT PresentationTRANSCRIPT
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What is infectious disease?
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Ecology of Infectious Disease&
Disease in plant communities
Dr. Charles MitchellUNC
Biology Department & Curriculum in Ecology
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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What is infectious disease?
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What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
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What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
Examples• AIDS• Malaria• Measles• Influenza (the flu)• Anthrax• Tapeworm infection• SARS
Non-examples• Asthma• Cancer (?)• Heart attacks (?)
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What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
What is a parasite / pathogen?• An organism that exploits a single host
individual per life-history stage, causing disease
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What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
What is a parasite / pathogen?• An organism that exploits a single host individual per life-history stage, causing disease
Examples• HIV -> AIDS• Plasmodium spp. -> malaria• Taenia spp. -> tapeworm infection
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Parasites = 1/3 of Biodiversity
de Meeus and Renaud 2002
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Insect parasitoids
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What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
What is a parasite / pathogen?• An organism that exploits a single host
individual per life-history stage, causing disease
What is infection?• The process by which a parasite exploits its
host, signified by its presence in the host
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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Disease and society: history
• Biblical human and crop “plagues”• Plague of Athens -> end of Golden Age• Smallpox and measles -> Euro colonization• Irish potato famine -> migration to U.S.• Early 1900’s: vaccines and antibiotics• 1967: “The war against infectious diseases
has been won” – U.S. Surgeon General• 1980 - present: rise of emerging diseases
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What is an emerging disease?
• Newly discovered globally, or• Spreading into new host populations, or• Increasing within historical host population
(“re-emerging”)
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Human pathogens
• 175 emerging / 1415 total species• Greater risk of emergence:
– Viruses and protozoans– Multiple-host pathogens
• Similar patterns for domestic animals
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Examples of emerging infectious diseases of humansMorens et al. 2004
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Causes of plant pathogen emergence
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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What is transmission?• The process by which a pathogen passes from
a source of infection to a new host and infects that host
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Why is it crucial?(Why is it the central ecological challenge for pathogens?)
What is transmission?• The process by which a pathogen passes from
a source of infection to a new host and infects that host
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Why is it crucial?• Host individuals are spatially discrete• Hosts defend themselves (resistance)• Hosts die (especially if infected!)
What is transmission?• The process by which a pathogen passes from
a source of infection to a new host and infects that host
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Modes of transmission• Direct contact (e.g. handshake)
– Common cold• Indirect contact (e.g. sneezing)
– Measles• Sex
– AIDS• Vector (species that transmits pathogen without
experiencing disease; usually arthropods)– Malaria
• Trophic (from prey to predator)– Schistosomiasis
• Environmental reservoir (free-living stage)– Cholera
• Vertical (from parent to offspring)– Syphilis
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Density-dependent transmission
• Expected for transmission via– Direct contact (non-sexual)– Indirect contact
• And sometimes for transmission via– Sex – Vector – Trophic interaction– Environmental reservoir
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Density-dependent transmission
• Can regulate host populations• Creates linkages to other variables
(abiotic, competition, predation)
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Density-dependence predicts minimum threshold density for epidemic
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Transmission chains for contact- and vector-transmitted pathogens
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R0 – the basic reproductive ratio
• The number of individuals infected by a single infectious host introduced into a population of uninfected hosts
• Critical value of R0=1• Simplest (of many) theoretical formulas:
R0 = β/g, where
β = ?g = ?
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R0 – the basic reproductive ratio
• The number of individuals infected by a single infectious host introduced into a population of uninfected hosts
• Critical value of R0=1• Simplest (of many) theoretical formulas:
R0 = β/g, where
β = transmission rateg = rate infected individuals recover or die
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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Strengbom et al. 2002
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Yates et al. 2002 Bioscience
H H H H
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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natives invaders
resources
Generalistpathogens
Specialistpathogens
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barleyyellowdwarfvirus(BYDV)
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Rhopalosiphum padi(the bird cherry-oat aphid)
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ELISA
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Setarialutescens
(Yellow foxtail)
Avena fatua(Wild oats)
Digitariasanguinalis
(Hairy crabgrass)
Loliummultiflorum
(Italian ryegrass)
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Avena Digitaria Lolium Setaria
Viru
s pr
eval
ence
0
25
50
75
100 monocultures
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Intraspecifictransmission
SetariaDigitaria Lolium
Avena
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Pathogen spilloverin multihostcommunity
SetariaDigitaria Lolium
Avena
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Non
- Ave
navi
rus
prev
alen
ce (%
)
0
5
10
15
20
25
+ Avena- Avena
P<0.05
Pathogen spillover
Power and Mitchell 2004 Am Nat
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Viru
s pr
eval
ence
0
25
50
75
100
Avena Digitaria Lolium Setaria
quadcultures (2003)
Avena Digitaria Lolium Setaria
Viru
s pr
eval
ence
0
25
50
75
100monocultures
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Apparent competition
SetariaDigitaria Lolium
Avena
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pathogen
hostspecies A(reservoir)
hostspecies B
+ -
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Lolium Avena
resources
pathogen
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Loliu
m v
irus
prev
alen
ce (%
)
0
20
40
60
- Avena + Avena
*
bicultures
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Effe
ct o
f Ave
na o
nLo
lium
bio
mas
s (%
)
-100
-75
-50
-25
0
*- virus + virus
bicultures
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natives invaders
resources
Generalistpathogens
Specialistpathogens
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Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities