What is infectious disease?

1. What is infectious disease?

2. Ecology of Infectious Disease&Disease in plant communities Dr. Charles Mitchell UNC Biology Department & Curriculum in Ecology

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

4. What is infectious disease?

5. What is infectious disease? • Negative effects on a host organism caused by a parasite / pathogen

6. 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 (?)

7. 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

8. 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

9. Parasites = 1/3 of Biodiversity de Meeus and Renaud 2002

10. Insect parasitoids

11. 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

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

13. 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

14. What is an emerging disease? • Newly discovered globally, or • Spreading into new host populations, or • Increasing within historical host population (“re-emerging”)

15. Human pathogens • 175 emerging / 1415 total species • Greater risk of emergence: • Viruses and protozoans • Multiple-host pathogens • Similar patterns for domestic animals

16. Examples of emerging infectious diseases of humans Morens et al. 2004

17. Causes of plant pathogen emergence

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

19. What is transmission? • The process by which a pathogen passes from a source of infection to a new host and infects that host

20. 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?)

21. 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!)

22. 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

23. Density-dependent transmission • Expected for transmission via • Direct contact (non-sexual) • Indirect contact • And sometimes for transmission via • Sex • Vector • Trophic interaction • Environmental reservoir

24. Density-dependent transmission • Can regulate host populations • Creates linkages to other variables (abiotic, competition, predation)

25. Density-dependence predicts minimum threshold density for epidemic

26. Transmission chains for contact- and vector-transmitted pathogens

27. 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 = ?

28. 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

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

31. Strengbom et al. 2002

32. H H H H Yates et al. 2002 Bioscience

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

35. Specialist pathogens Generalist pathogens natives invaders resources

36. barley yellow dwarf virus (BYDV)

37. Rhopalosiphum padi (the bird cherry-oat aphid)

38. ELISA

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

40. monocultures

41. Intraspecifictransmission Avena Digitaria Lolium Setaria

42. Pathogen spilloverin multihostcommunity Avena Digitaria Lolium Setaria

43. Pathogen spillover Power and Mitchell 2004 Am Nat

44. quadcultures (2003) monocultures

45. Apparent competition Avena Digitaria Lolium Setaria

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

47. pathogen Lolium Avena resources

48. bicultures

49. bicultures

50. Specialist pathogens Generalist pathogens natives invaders resources