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