Epidemiology - Introduction

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Epidemiology - Introduction
Study of patterns, distribution of disease (or
other events)

• Cause/transmission

• Develop strategies for prevention

Epidemiologists, health detectives
Dr. Ignaz Semmelweis, Vienna (19th century),
Puerperal fever
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Epidemiology

• Principles of Epidemiology
• Epidemiological studies
• Infectious Disease Surveillance
• Trends in Disease
• Nosocomial Infections

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Principles of Epidemiology

• Rate of Disease in a population
• Reservoirs of Infection
• Transmission
• Influential Factors

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Rates of Disease in a Population
Endemic - disease constantly present in a
particular geographic area
Epidemic - disease with an unusually high
occurrence
Pandemic - worldwide severe epidemic
Morbidity - illness (morbidity rate)
Rate vs. Absolute number
Mortality - death (mortality rate)
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Outbreak cluster of cases occurring during a
brief time interval and affecting a specific
population.
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Spread/prevention of disease
Natural habitat
Reservoir
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Reservoirs of infectious agents

• Human reservoirs

Symptomatic
ex. cold virus
Asymptomatic carriers
ex. Neis. gon.,
Staph. aureus
humans only reservoir - easiest to control

• Non-human animal reservoirs

poultry - Salmonella, Campylobacter
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Reservoirs of infectious agents

• Human reservoirs

Symptomatic
ex. cold virus
Asymptomatic carriers
ex. Neis. gon,
Staph. aureus
humans only reservoir - easiest to control

• Non-human animal reservoirs

poultry - Salmonella, Campylobacter
rodents - Yersinia pestis
bats and racoons - rabies virus
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Reservoirs of infectious agents

• Human reservoirs

Symptomatic
ex. cold virus
Asymptomatic carriers
ex. Neis. gon,
Staph. aureus
humans only reservoir - easiest to control

• Non-human animal reservoirs

poultry - Salmonella, Campylobacter
rodents - Yersinia pestis
bats and racoons - rabies virus
animal reservoir - difficult to control
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Reservoirs of infectious agents

• Human reservoirs

Symptomatic
ex. cold virus
Asymptomatic carriers
ex. Neis. gon,
Staph. aureus
humans only reservoir - easiest to control

• Non-human animal reservoirs

poultry - Salmonella, Campylobacter
rodents - Yersinia pestis
bats and racoons - rabies virus
animal reservoir - difficult to control
- diseases of animals transmitted to humans
Zoonoses/zoonotic diseases

• Environmental reservoirs

soil - Clostridium species
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Transmission
Horizontal (person to person)
Vertical (mother to fetus)

• contact

direct contact
indirect contact
fomite

• inanimate object, such as clothing, doorknob
  and so on

Importance of hand washing
droplet
Large microbe-laden respiratory droplets
generally fall to the ground on farther than 3
feet. Importance of covering mouth when cough or
sneeze
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Transmission
horizontal
Transmission
Natural habitat
vertical
Reservoir

• contact

direct contact
indirect contact
fomite
- inanimate object
droplet

• food and water

• air

very difficult to control

• vectors

arthropods
flea - Yersinia pestis
mosquito - Plasmodium species (malaria)
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Black Death (Plague) - Yersinia pestis
Killed 1/4 of the population of Europe between
1346 - 1350 75 of the population in some cities.
Bubonic plague

• Flea transmits Y. pestis to a human

• Bacterium is carried to a lymph node.

• Bubo develops within days

• Y. pestis begins interfering with the
  inflammatory response - arms itself

• Multiplying bacteria spill into bloodstream
  (septicemic plague) endotoxin ? shock, DIC

• 50 - 75 mortality (if untreated)

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Influential Factors

• Dose
• There are few if any infections for which
  immunity
• is absolute.
• Incubation Period
• Population Characteristics
• Immunity, General Health, Age, Gender,
• Genetic Background

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Epidemiological studies

• Descriptive studies

Risk factors
Person Place
Time

• Analytical studies
• Experimental Studies

Which risk factors were/are most relevant?
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Epidemiological studies

• Descriptive studies

Person Place Time
Person
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Epidemiological studies

• Descriptive studies

Person Place
Time
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Epidemiological studies
1 2

• Descriptive studies

Person Place
Time

1. Rapid rise of sick people
2. Gradual rise

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Experimental studies
Experimental studies are done mostly to assess
the value of a particular intervention or
treatment, such as antimicrobial drug therapy.

• Placebo
• Double-blind

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Rates of Disease in a Population
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Rates of Disease in a Population
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Infectious Disease Surveillance
CDC - National Centers for Disease Control and
Prevention
MMWR - Morbidity and Mortality Weekly Report
WHO - World Health Organization Weekly
Epidemiological Record
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Trends in Disease

• Reduction Eradication of Disease
• Emerging Diseases

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Trends in Disease

• Reduction Eradication of Disease
• - Improved sanitation
• - Reservior vector control
• - Vaccination
• - Antibiotic treatment
• (Smallpox, eradicated globally)

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Trends in Disease

• Emerging Disease
• - Microbial Evolution, drug-resistance strain
• - Population expansion
• - Mass distribution importation of food
• - Climate change

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Nosocomial Infections(hospital-acquired
infections)5-6 patients, 4.5 billion cost

• Enterococcus species. Part of the normal
  intestinal flora
• 
  urinary, wound blood infections
• Escherichia coli. Part of the normal
  intestinal flora
• Most common cause nosocomial urinary
  infection
• Pseudomonas species. Grow in moist environment
  
• Staphylococcus species. Normal skin flora
• Common cause of nosocomial pneumonia and
  surgical site infection

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Epidemiology

• Principles of Epidemiology
• Epidemiological studies
• Infectious Disease Surveillance
• Trends in Disease
• Nosocomial Infections

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