What is infectious disease?
Ecology of Infectious Disease
&
Disease in plant communities
Dr. Charles Mitchell
UNC
Biology Department &
Curriculum in Ecology
Lecture outline
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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
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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 humans
Morens 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
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!)
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 contactand 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 rate
g = 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
H
Yates et al. 2002 Bioscience
HHH
Lecture outline
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•
•
•
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Basic concepts / definitions
Patterns of disease emergence
Transmission
Disease triangle
Virus dynamics in grass communities
Specialist
pathogens
Generalist
pathogens
natives
invaders
resources
barley
yellow
dwarf
virus
(BYDV)
Rhopalosiphum padi
(the bird cherry-oat aphid)
ELISA
Avena fatua
(Wild oats)
Digitaria
sanguinalis
Lolium
multiflorum
Setaria
lutescens
(Hairy crabgrass) (Italian ryegrass) (Yellow foxtail)
Virus prevalence
100
monocultures
75
50
25
0
Avena Digitaria Lolium Setaria
Intraspecific
transmission
Avena
Digitaria
Lolium
Setaria
Pathogen spillover
in multihost
community
Avena
Digitaria
Lolium
Setaria
Non-Avena
virus prevalence (%)
25
Pathogen spillover
P<0.05
20
15
10
5
0
- Avena
Power and Mitchell 2004 Am Nat
+ Avena
Virus prevalence
100
quadcultures (2003)
75
50
25
0
Avena Digitaria Lolium Setaria
Virus prevalence
100
monocultures
75
50
25
0
Avena Digitaria Lolium Setaria
Apparent
competition
Avena
Digitaria
Lolium
Setaria
pathogen
+
host
species A
(reservoir)
-
host
species B
pathogen
Lolium
Avena
resources
Lolium virus prevalence (%)
60
bicultures
*
40
20
0
- Avena + Avena
Effect of Avena on
Lolium biomass (%)
0
-25
-50
-75
-100
bicultures
- virus
*
+ virus
Specialist
pathogens
Generalist
pathogens
natives
invaders
resources
Lecture outline
•
•
•
•
•
Basic concepts / definitions
Patterns of disease emergence
Transmission
Disease triangle
Virus dynamics in grass communities