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Lecture 12: Disease

Lecture 12: Disease

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Lecture 12: Disease

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  1. Lecture 12: Disease EEES 3050

  2. Disease…

  3. Disease: an interaction in which a an organism lives on or within a host plant or animal, to the benefit of the disease agent and the detriment of the host. Microparasites bacteria, viruses (disease agents) Macroparasites just larger agents, such as tapeworms, flukes, arthropods, etc. differ from predators...how? Parasitism and Disease

  4. Disease transmission • directly transmitted: • Organism to organism contact • Vector transmission: • Third party carrier/vector • E.g. mosquitoes, rodents, etc.

  5. Parasite Diversity • Microparasites • multiply within host • virus, bacteria, protozoa • directly transmitted: • STDs, influenza • vector-transmitted: • sleeping sickness, malaria

  6. Parasite Diversity • Macroparasites • grow in/on host & infect other hosts • external and internal • directly-transmitted macroparasites: • intestinal nematodes: roundworms, hookworms • lice and fleas: external, mobile • mildews and other plant-infecting fungi

  7. Parasite Diversity • Macroparasites (grow in/on host & infect other hosts) • indirectly transmitted macroparasites: • tapeworms: ingested by carnivores • schistosomes: aquatic snail host->swim to human • filarial nematodes: by blood-sucking insects

  8. Transmission and distribution • the appropriate units of study: • number of macroparasites • number of hosts infected with microparasites • Hosts as islands: transmission requires agent • transmission by contact: host densities & agent characteristics are important

  9. When are diseases most influential? • New strains/introductions • AIDS, West Nile Virus, • Read Guns, Germs, and Steel • Critical threshold • Most diseases have a threshold host population density that is needed for the continued presence of the pathogen. • In general parasites and diseases are rarely lethal – why?

  10. Why are disease and parasites important topics for ecology? • Again – Typically non-lethal • They reduce fitness • Lower reproduction • Less tolerant to extremes in environment • Easier to capture.

  11. Examples of effects of disease on individuals. • How does disease impact individuals? • Reproduction: • Lizards • Egrets • Mortality • Harbor seals

  12. Examples of effects of disease on populations. • Examples include: • Brucellosis in ungulates • Rabies in wildlife. • Bovine TB • Rabies • All mammals are susceptible. • For humans, without vaccination before or soon after infection, there is no cure… • Except…

  13. Girl Is First to Survive Rabies Without a Shot – Nov. 2004 • By ELISABETH ROSENTHAL – New York Times • Wisconsin teenager is the first human ever to survive rabies without vaccination, the Centers for Disease Control and Prevention said yesterday, after she received a desperate and novel type of therapy. • Last month, doctors at the Children's Hospital of Wisconsin in Wauwatosa, a suburb of Milwaukee, put the critically ill girl into a drug-induced coma and gave her antiviral drugs, although it is not clear which, if any, of the four medicines contributed to her surprising recovery. • Dr. Charles Rupprecht of the disease control agency called the recovery "historic." But even the doctors who took care of the girl said the result would have to be duplicated elsewhere before the therapy could be considered a cure or a treatment. • "You have to see this therapy repeated successfully in another patient," said Dr. Rodney Willoughby, the associate professor of pediatrics who prescribed the cocktail of medicines for the sick girl, Jeanna Giese, 15. "Until then, it is a miracle."

  14. West Nile Virus: Human positives in 2006

  15. Marjorie Wonham1,2 Tomás de Camino-Beck2 Mark Lewis1,2 An epidemiological model for West Nile virus: Invasion analysis and control applications 1 Department of Mathematical & Statistical Sciences 2 Department of Biological Sciences University of Alberta

  16. West Nile Virus: What we know • Pathogen: • Encephalitic • Arboviral • Arthropod Borne • Flavivirus • – related to Japanese, St. Louis, • equine, Murray Valley encephalitis • Morbidity & mortality effects: • Ecological • – birds (0-100%) • Economic • – horses (~40%) • Public health • – humans (<0.1%)

  17. Humans Bird reservoir host Mosquito vector secondary transmission http://www.47custer.com/ virus amplification cycle Horses West Nile Virus: What we’d like to know • Effective control: • Reduce mosquitoes? • - spray adults? • - remove larval habitat? • Reduce birds? • Modelling goals: • Simple, accurate system • Robust parameter estimates • Insights for control http://www.cdc.gov/

  18. Basic Disease Model: SIR

  19. Basic Disease Model: SIR

  20. Basic Disease Model: SIR

  21. Removed birds Susceptible mosquitoes Biting rate x Transmission probability Infectious birds Infectious mosquitoes Susceptible birds Removed mosquitoes West Nile Virus model

  22. Dead birds Recovered birds Susceptible mosquitoes Biting rate x Transmission probability bird death rate (WNV) Infectious birds Infectious mosquitoes Susceptible birds Removed mosquitoes West Nile Virus model

  23. Mosquito modifications 1. Life cycle time delays Larva ~15 d Adult ~35 d

  24. Larval mosquitoes West Nile Virus model Dead birds Recovered birds Susceptible mosquitoes Biting rate x Transmission probability bird death rate (WNV) Infectious birds Infectious mosquitoes Susceptible birds Removed mosquitoes

  25. WNV titer (log(10)pfu) Days since inoculation Mosquito modifications 1. Life cycle time delays Larva ~15 d Adult ~35 d 2. Viral incubation period Larva ~15 d Infectious Susceptible Exposed Incubation ~9.4 d

  26. mosquito death rate (natural) Larval mosquitoes Susceptible mosquitoes mosquito birth rate Exposed mosquitoes Infectious mosquitoes Infectious mosquitoes Removed mosquitoes West Nile Virus model Dead birds Recovered birds Biting rate x Transmission probability bird death rate (WNV) Infectious birds Susceptible birds

  27. Larvae birth death maturation Susceptible infection death maturation dI S = - m - B B abI I gI M V B B dt N B Exposed infection death viral incubation Infectious death viral incubation West Nile Virus model infection Susceptible Infectious infection death recovery Recovered recovery Dead WNV death

  28. Larvae birth death maturation Susceptible infection death maturation dI S = - m - B B abI I gI M V B B dt N B Exposed infection death viral incubation Infectious death viral incubation West Nile Virus model infection Susceptible Infectious infection death recovery Recovered recovery Dead WNV death

  29. LM EM Susceptible SM IM Mosquitoes per crow Larval Exposed Infectious Numerical simulations match observed infection levels SB IB Susceptible DB Proportion of crows Infectious Dead Time ( =9.4 d)

  30. Desired bird survival next year Observed end-of-season relative bird abundance Estimated initial relative mosquito abundance Required proportional mosquito mortality Disease Invasion Thresholds 1.0 Final Susceptible birds SB/NB 0.8 0.6 0.4 0.2 0 10 20 30 40 Initial susceptible mosquitoes per bird

  31. Disease Invasion Thresholds 2. Threshold of initial mosquitoes per bird: = 3.4 mosquitoes/bird (for NYC 2000) Control goal: Reduce mosquitoes below SM* Strategy: Do not have to kill all mosquitoes Question: How much mosquito control needed?

  32. b b Disease growth rate () 0 a a threshold for constant pop. = 3.4 Disease Invasion Thresholds 3. Disease growth rate: R0 1 Susceptible mosquito population

  33. General concept infection rate vs. no. of susceptibles.

  34. General concept infection rate vs. no. of susceptibles.

  35. General concept: contact rate vs. pop’n density.

  36. General concept: contact rate vs. pop’n density.

  37. Schistosomiasis • Commonly called swimmer's itch or snail fever. • A common ailment in much of the developing world.

  38. Health effects of schistosomiasis • Abdominal pain • Cough • Diarrhea • Eosinophilia - extremely high white blood cell count. • Fever • Fatigue • Hepatosplenomegaly - the enlargement of both the liver and the spleen.

  39. Life cycle

  40. Schistosomiasis • What can be done to control? • Medical treatment • Molluscicides • Both of these are expensive. • Other means? • Crayfish.

  41. IMPACT OF THE CRAYFISH PROCAMBARUS CLARKII ON SCHISTOSOMA HAEMATOBIUM TRANSMISSION IN KENYA GERALD M. MKOJI, BRUCE V. HOFKIN, ARMAND M. KURIS, ALLAN STEWART-OATEN, BENJAMIN N. MUNGAI, JIMMY H. KIHARA, FRANCIS MUNGAI, JOSEPHAT YUNDU, JANE MBUI, JUMA R. RASHID, CURTIS H. KARIUKI, JOHN H. OUMA, DAVY K. KOECH, AND ERIC S. LOKER

  42. IMPACT OF THE CRAYFISH PROCAMBARUS CLARKII ON SCHISTOSOMA HAEMATOBIUM TRANSMISSION IN KENYA • Observations: • Crayfish rarely if ever coexisted with known molluscan intermediate hosts of human schistosomes • Hypothesis: • Introduction of crayfish would lower the transmission rate of schistosomias. • Experiment: • Introduce crayfish into selected locations.

  43. Schistosomiasis • Methods: • Study sites needed to have: • 1) a local school • 2) high enough disease prevalence for a crayfish effect to be detected • 3) at least one human-made water impoundment harboring snails • 4) no other ongoing schistosomiasis study • 5) agreement of local people and authorities.

  44. Schistosomiasis • Methods: • 6 locations selected and combined to 3 pairs. • 1 school from each pair was randomly selected to receive crayfish in the ponds. • What would you measure? • Prevalence of infection.

  45. Schistosomiasis: Results • Group 1:

  46. Schistosomiasis: Results • Group 1

  47. Schistosomiasis: Results • Group 2:

  48. Schistosomiasis: Results • Group 3

  49. Schistosomiasis: Results • Group 3:

  50. Summary • Disease: • an interaction in which a an organism lives on or within a host plant or animal, to the benefit of the disease agent and the detriment of the host. • Different types of transmission • West Nile Virus: • SIR models • Kenya: • Interaction between schistosomiasis, snails and crayfish.