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Parasites and Disease Very important and poorly understood Millions of people killed or debilitated by disease Human diseases are very important and poorly understood. Millions of people killed or debilitated by disease Disease Symptoms Deaths 1997 Haemophilus influenzae Influenza 3,900,000 Mycobacterium tuberculosis Tuberculosis 2,900,000 Vibio cholerae Cholera 2,500,000 Human immunodeficiency virus AIDS 2,300,000 Plasmodium falciparum Malaria 2,600,000 Morbillivirus Measles 1,000,000 Hepatitis B virus Hepatitis B 600,000 Bordetella pertussis Whooping cough 400,000 Clostridium tetani Tetanus 300,000 Falvirus Dengue Fever 150,000 Debilitating diseases Currently afflicted: 250,000,000 Malaria 250,000,000 Elephantiasis 200,000,000 Schistosomiasis (Bilharzia) 2. Parasites Parasite = organism that obtains nutrients from one or a very few host individuals, normally causing harm but not death. Diversity of parasites - Microparasites - Macroparasites - Hemiparasites - Brood and social parasites 3. The role of ecology in understanding disease -- Lyme Disease example • Heavy acorn crop 2-6 yr cycle • Attracts deer to oak-dominated forests • Deer are the preferred host for deer ticks, which need blood meal • While on deer, ticks mate and then drop off • Outbreak of larval ticks summer following mast • 99% larval ticks hatch from eggs free of spirochete • White-footed mouse also eat acorns with population peak year after mast • Just when larval ticks emerge, find favorite host – WFMouse Anthropogenic factors: loss of predators, changes in habitat, fragmentation of habitats. 4. Darwinian medicine 1. The delicate balance Dependence of disease organism on host to regulate environment, yet it harms the host 2. Importance of regulating resource ratios - Gut bacteria that compete for H - Glutamine competition - Iron deficiency Competition in human gut bacteria S supply zngi – sulfate reduction bacteria zngi –methanogenic bacteria H supply Safe food 1. Spices in cooking - Plant secondary compounds, typically powerful antimicrobial agents. - As mean annual temperatures increase, increase in the proportion of recipes with spices, the number of spices per recipe, and use of the most potent antibacterial spicies. - Within a country, meat dishes spicier than vegetable dishes. - Proximate reason for spices is to enhance palatability. Ultimate is likely that spices cleanse foods of pathogens. Selection for people who enjoy their flavors. 2. Morning Sickness 5. Importance of Genetic Diversity - Monocultures have more frequent diseases; - Strong selection for diversity - Mixtures of species and genotypes more stable (agriculture, forestry) - Evolution of microbial organisms is rapid - Mortality rate generally higher if transmitted from relative - This may be the strongest selection for sex Time Lags in Pathogen-Host Systems • Immune responses can create cycles of infection in certain diseases: – measles produced epidemics with a 2-year cycle in pre-vaccine human populations: • two years were required for a sufficiently large population of newly susceptible infants to accumulate – other pathogens cycle because they kill sufficient hosts to reduce host density below the level where the pathogens can spread in the population: American Indians Following European contact, roughly 56,000,000 died. Some populations dropped to 10% of original size No evidence of genetic inferiority Explanation #1 – Novel diseases (Why so few the other direction?) Diversity in immune systems Class I MHC genes - responsible for host histocompatibility antigens; - each individual has some that protect against specific molecular forms; - immune system selects against viruses with peptides that match the MHC antigens. - different individuals have different set of MHC genes. N Alleles Origin 1342 40 SubSaharan Africans 1069 37 European Caucasians 4061 34 Eastern Asians 1163 17 North Amerinds 1944 10 South Amerinds 12243 14 Polynesians 5499 10 Papua New Guineans Pathogen pollution - Rinderpest with cattle into Africa from Asia - Brucellosis in Yellowstone elk and bison - West Nile virus - Wild dogs in Serengeti extinct in 1991 - The great frog die-offs - Risks of zoos as concentrations Herpes in elephants Measles in mountain gorillas Mad cow disease in England Rinderpest 5-10K yrs for domestication of cattle Keystone of Serengeti - Canine distemper - PPV (Porcine Parvovirus) - Peste des petits ruminants virus - Measles - RPV (Rinderpest) Trees of eastern North America Franklinia -- extinct Torreya – soon gone Chestnut – essentially gone Elm – dominance and range greatly reduced Fraser Fir – greatly reduced; future uncertain Hemlock – future uncertain Butternut – greatly reduced Dogwood – greatly reduced range and abundance Beech – future uncertain 6. Selection and virulence Example of fig wasps Relative 1.0 reproductive success of infected 0.8 wasps 1.0 0.2 Proportion with single foundress Examples of selection acting on virulence - E. coli - Daphnia magna & Pleistophora - Water-born vs insect transmitted diseases of humans HIV Simian immunodeficiency virus in 26 species of African nonhuman primates. Two of these, 1 from chimps (HIV1) and 1 from sooty mangabeys, are sources of human HIV. At least 7 events Must maintain access to new hosts. One option is to remain latent by incorporating in chromosome and thus avoiding triggering an immune response. When more frequent sexual encounters, selection for more rapid rate of increase. HIV1 is more virulent than HIV2, and it dominates in east & central Africa where social disruption But in west, different virulence in HIV2 associated with high prostitution cities.
