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?

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 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

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

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

Yates et al Bioscience HHHH

Lecture outline Basic concepts / definitions Patterns of disease emergence Transmission Disease triangle Virus dynamics in grass communities

nativesinvaders resources Generalist pathogens Specialist pathogens

barley yellow dwarf virus (BYDV)

Rhopalosiphum padi (the bird cherry-oat aphid)

ELISA

Setaria lutescens (Yellow foxtail) Avena fatua (Wild oats) Digitaria sanguinalis (Hairy crabgrass) Lolium multiflorum (Italian ryegrass)

monocultures

Intraspecific transmission Setaria Digitaria Lolium Avena

Pathogen spillover in multihost community Setaria Digitaria Lolium Avena

Pathogen spillover Power and Mitchell 2004 Am Nat

quadcultures (2003) monocultures

Apparent competition SetariaDigitariaLolium Avena

pathogen host species A (reservoir) host species B + -

LoliumAvena resources pathogen

bicultures

nativesinvaders resources Generalist pathogens Specialist pathogens

Lecture outline Basic concepts / definitions Patterns of disease emergence Transmission Disease triangle Virus dynamics in grass communities