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What is infectious disease?
What is infectious disease?
Ecology of Infectious Disease&
Disease in plant communities
Dr. Charles MitchellUNC
Biology Department & Curriculum in Ecology
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
What is infectious disease?
What is infectious disease?• Negative effects on a host organism
caused by a parasite / pathogen
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 (?)
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 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
Parasites = 1/3 of Biodiversity
de Meeus and Renaud 2002
Insect parasitoids
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
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
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
What is an emerging disease?
• Newly discovered globally, or• Spreading into new host populations, or• Increasing within historical host population
(“re-emerging”)
Human pathogens
• 175 emerging / 1415 total species• Greater risk of emergence:
– Viruses and protozoans– Multiple-host pathogens
• Similar patterns for domestic animals
Examples of emerging infectious diseases of humansMorens et al. 2004
Causes of plant pathogen emergence
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
What is transmission?• The process by which a pathogen passes from
a source of infection to a new host and infects that host
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
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
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
Density-dependent transmission
• Expected for transmission via– Direct contact (non-sexual)– Indirect contact
• And sometimes for transmission via– Sex – Vector – Trophic interaction– Environmental reservoir
Density-dependent transmission
• Can regulate host populations• Creates linkages to other variables
(abiotic, competition, predation)
Density-dependence predicts minimum threshold density for epidemic
Transmission chains for contact- and vector-transmitted pathogens
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 = ?
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
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
Strengbom et al. 2002
Yates et al. 2002 Bioscience
H H H H
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
natives invaders
resources
Generalistpathogens
Specialistpathogens
barleyyellowdwarfvirus(BYDV)
Rhopalosiphum padi(the bird cherry-oat aphid)
ELISA
Setarialutescens
(Yellow foxtail)
Avena fatua(Wild oats)
Digitariasanguinalis
(Hairy crabgrass)
Loliummultiflorum
(Italian ryegrass)
Avena Digitaria Lolium Setaria
Viru
s pr
eval
ence
0
25
50
75
100 monocultures
Intraspecifictransmission
SetariaDigitaria Lolium
Avena
Pathogen spilloverin multihostcommunity
SetariaDigitaria Lolium
Avena
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
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
Apparent competition
SetariaDigitaria Lolium
Avena
pathogen
hostspecies A(reservoir)
hostspecies B
+ -
Lolium Avena
resources
pathogen
Loliu
m v
irus
prev
alen
ce (%
)
0
20
40
60
- Avena + Avena
*
bicultures
Effe
ct o
f Ave
na o
nLo
lium
bio
mas
s (%
)
-100
-75
-50
-25
0
*- virus + virus
bicultures
natives invaders
resources
Generalistpathogens
Specialistpathogens
Lecture outline
• Basic concepts / definitions• Patterns of disease emergence• Transmission• Disease triangle• Virus dynamics in grass communities
