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Transcript
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.