Parasites and Disease

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

Human diseases are very important and poorly understood. Millions of people killed or debilitated by disease

Debilitating diseases

Currently afflicted:

250,000,000 Malaria

250,000,000 Elephantiasis

200,000,000 Schistosomiasis (Bilharzia)

2. ParasitesParasite = 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

H supply

S su

pply

zngi – sulfate reduction bacteria

zngi –methanogenic bacteria

Competition in human gut bacteria

Safe food1. 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 systemsClass 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 Origin1342 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 concentrationsHerpes in elephants Measles in mountain gorillasMad cow disease in England

Rinderpest

5-10K yrs for domestication of cattleKeystone of Serengeti

- Canine distemper- PPV (Porcine Parvovirus)

- Peste des petits ruminants virus

- Measles- RPV (Rinderpest)

Trees of eastern North America

Franklinia -- extinctTorreya – soon goneChestnut – essentially goneElm – dominance and range greatly reducedFraser Fir – greatly reduced; future uncertainHemlock – future uncertainButternut – greatly reducedDogwood – greatly reduced range and abundanceBeech – future uncertain

6. Selection and virulenceExample of fig wasps

1.0 0.2 Proportion with single foundress

Relative reproductive success of infected wasps

1.00.8

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.