DISEASE

Understanding population dynamics and evolution is important to controlling the spread of diseases.

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Reading Assignment

Chapter 28.1

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Spread of Disease

A disease is called infectious if it is caused by an organism.
Organisms that live on other species (including parasites) must have a way to get from one individual to other individuals (colonization).

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Infection Size

Bacteria are small. They infect people thru air, water, food, animals and sex.
Bacterial infections do not normally start with a single cell. Usually infections are ‘successful’ only when a large number of cells, say 105, enter the body.

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Human Diseases caused by Bacteria

Disease name Species How enters body

Plague Yersinia pestis flea
Cholera Vibrio cholerae water
Gonorrhea Neisseria gonorrhoeae sex
Anthrax Bacillus anthracis air
Tetanus Clostridium tetani cuts
Dysentery Shigella dysenteriae water & food

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Vectors

An animal that delivers a disease organism to a host individual is called a vector.
Mosquitoes are vectors for many human diseases. Ticks and flea also may serve as vectors.
Control of the vector organism can be effective in controlling spread of disease.

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Koch’s Postulates to establish a microorganism causes a disease

1) Organism always found in diseased individuals.
2) Organism can be isolated and grown in pure culture.
3) Pure culture of organism can cause disease in healthy host.
4) Organism can be recovered from newly infected host.

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Koch’s Postulates

If all four conditions are fulfilled, Koch accepted the hypothesis that the microorganism was the cause of that specific disease.
There are many species of microorganism that can be isolated from a sick individual, so all parts were necessary to accept organism as the cause.

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Disease Terminology

A contagious disease is a disease that can spread directly from person to person (via air or contact.)
- Bubonic plague was infectious but not contagious (required a flea vector).
Quarantine of infected individuals is a method of control of contagious disease.

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Measures of Disease Abundance

Prevalence is the proportion of a population that are cases at a point in time.
- Unsuitable for acute disorders.
Incidence is the rate at which new cases occur in a population during a specified period.

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WHY BACTERIAL DISEASES DISAPPEARED

Diseases transmitted by water were dramatically reduced by creation of a treated water supply and treatment of wastewater.
People learned to quarantine (isolate) diseased individuals.
Because of the large phylogenetic gap between bacteria and humans, it was possible to find compounds that kill bacteria.

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WATER TREATMENT

Water supply infrastructure
- Water filtered, treated with biocides before entering a sealed delivery system.
Wastewater Treatment
- Where are plants of Water Reclamation District?
- What is special about Chicago system?

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Wastewater & Chicago River

Episodes of Cholera before 1889.

Decision was made to reverse the flow of the Chicago River from into Lake Michigan to into the Des Plaines River by digging the Ship and Sanitary Canal.

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CHICAGO SHIP & SANITARY CANAL

Water from Lake Michigan now flows into the Chicago River and on to Des Plaines River.
Wastewater is collected in sewers and transported to one of 3 plants where it is treated (removing most bacteria among other things) and then released into one of the canals. Chlorination is done to kill bacteria.
The canals feed into the Des Plaines and then into the Illinois River.

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Solutions and Problems

The diversion of water to the Mississippi drainage resulted in protests by Canada and other states bordering the Great Lakes. The supreme court limited the amount of water that could be diverted.
Treated wastewater from Chicago creates problems for downstream communities.

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Antibiotics

Antibiotics are compounds that are able to inhibit the growth of bacteria.
Fungi are the major source of antibiotics for they compete with bacteria for resources.
The discovery of penicillin in 1928 and the ability to produce stable preparations in 1939 revolutionized public health.

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ANTIBIOTICS

Penicillin inhibits the growth of the cell wall of some bacteria.
Many antibiotics inhibit bacterial ribosomal function. The bacterial ribosome is smaller than eukaryotic ribosome and sensitive to different compounds.
- Erythromycin
- Streptomycin

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EVOLUTION OF RESISTANCE

As an antibiotic comes into widespread use, the advantage enjoyed by a resistant mutant increases.
Some diseases, especially sexually transmitted infections (STI), are now difficult to treat, because the disease organism has evolved resistance to almost all antibiotics.

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Public Health versus Individual Perception

There is a conflict between maximizing the benefit for people collectively and the demands of individuals.
Antibiotics proved to be so effective that individuals asked to be treated with antibiotics even in situations where antibiotics do not good (viral diseases). This inappropriate use increases the abundance of resistant strains.

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Minimizing the abundance of resistant strains

There are things that can be done that will minimize the spread of bacteria resistant to antibiotics.
- Limiting antibiotic use is important.
  - Antibiotics do not effect viral diseases, but they are often prescribed in such cases to “do something” when there is nothing to help.
  - It is a common practice of people to think if some is good more is better, but this is not true.

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Take Home Problem

A contagious disease can be spread from individual to individual.
If an individual is no longer infectious when dead, predict whether evolution will tend to favor acceleration of death versus prolongation of the period between infection and death.

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Vocabulary

Infectious disease
Contagious disease
Antibiotic
Water treatment
Vector
Resistance

Wastewater
Incidence
Prevalence
Koch’s postulates
Public health
Quarantine

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