Transcript Disease ecology - Biology Department
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 • 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 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 contact and vector-transmitted pathogens
R 0 – the basic reproductive ratio • The number of individuals infected by a single infectious host introduced into a population of uninfected hosts • Critical value of R 0 =1 • Simplest (of many) theoretical formulas: R 0 = β/g, where β = ?
g = ?
R 0 – the basic reproductive ratio • The number of individuals infected by a single infectious host introduced into a population of uninfected hosts • Critical value of R 0 =1 • Simplest (of many) theoretical formulas: R 0 = 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 H H H
Yates et al. 2002
Bioscience
Lecture outline • 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
(Hairy crabgrass)
Lolium multiflorum
(Italian ryegrass)
Setaria lutescens
(Yellow foxtail)
100 75 50 25 monocultures 0
Avena Digitaria Lolium Setaria
Intraspecific transmission
Avena Digitaria Lolium Setaria
Pathogen spillover in multihost community
Avena Digitaria Lolium Setaria
25 Pathogen spillover P<0.05
20 15 10 5 0 -
Avena
Power and Mitchell 2004 Am Nat +
Avena
100 75 50 25 quadcultures (2003) 0 100
Avena Digitaria Lolium Setaria
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
60 bicultures * 40 20 0 -
Avena
+
Avena
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