What is infectious disease?

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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

Examples• AIDS• Malaria• Measles• Influenza (the flu)• Anthrax• Tapeworm infection• SARS

Non-examples• Asthma• Cancer (?)• Heart attacks (?)

What is a parasite / pathogen?• An organism that exploits a single host

individual per life-history stage, causing disease

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 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

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

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?)

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

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

Strengbom et al. 2002

Yates et al. 2002 Bioscience

H H H H

Lecture outline

natives invaders

resources

Generalistpathogens

Specialistpathogens

barleyyellowdwarfvirus(BYDV)

Rhopalosiphum padi(the bird cherry-oat aphid)

Setarialutescens

(Yellow foxtail)

Avena fatua(Wild oats)

Digitariasanguinalis

(Hairy crabgrass)

Loliummultiflorum

(Italian ryegrass)

Avena Digitaria Lolium Setaria

100 monocultures

Intraspecifictransmission

SetariaDigitaria Lolium

Pathogen spilloverin multihostcommunity

+ Avena- Avena

P<0.05

Pathogen spillover

Power and Mitchell 2004 Am Nat

quadcultures (2003)

100monocultures

Apparent competition

pathogen

hostspecies A(reservoir)

hostspecies B

Lolium Avena

resources

pathogen

- Avena + Avena

bicultures

*- virus + virus

bicultures

natives invaders

resources

Generalistpathogens

Specialistpathogens

Lecture outline

What is infectious disease?

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