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

Epidemiology

• Principles of Epidemiology

• Epidemiological studies

• Infectious Disease Surveillance

• Trends in Disease

• Nosocomial Infections

Principles of Epidemiology

• Rate of Disease in a population

• Reservoirs of Infection

• Transmission

• Influential Factors

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)

Mortality - death (mortality rate)

Rate vs. Absolute number

Outbreak– cluster of cases occurring during a brief time interval and affecting a specific population.

Spread/prevention of disease

Reservoir

Natural habitat

Transmission

Reservoirs of infectious agents

•Human reservoirsSymptomatic

Asymptomatic carriers

•Non-human animal reservoirs

Reservoir

Natural habitat

Transmission

ex. cold virus

ex. Neis. gon.,

poultry - Salmonella, Campylobacter

**humans only reservoir - easiest to control**

Staph. aureus

Reservoirs of infectious agents

•Human reservoirsSymptomatic

Asymptomatic carriers

•Non-human animal reservoirs

Reservoir

Natural habitat

Transmission

ex. cold virus

ex. Neis. gon,

poultry - Salmonella, Campylobacter

rodents - Yersinia pestis

bats and racoons - rabies virus

**humans only reservoir - easiest to control**

Staph. aureus

Reservoirs of infectious agents

•Human reservoirsSymptomatic

Asymptomatic carriers

•Non-human animal reservoirs

Reservoir

Natural habitat

Transmission

ex. cold virus

ex. Neis. gon,

poultry - Salmonella, Campylobacter

rodents - Yersinia pestis

bats and racoons - rabies virus

**humans only reservoir - easiest to control**

**animal reservoir - difficult to control**

Staph. aureus

Reservoirs of infectious agents

•Human reservoirs

Symptomatic

Asymptomatic carriers

•Non-human animal reservoirs

Zoonoses/zoonotic diseases

•Environmental reservoirs

Reservoir

Natural habitat

Transmission

ex. cold virus

ex. Neis. gon,

poultry - Salmonella, Campylobacter

rodents - Yersinia pestis

bats and racoons - rabies virus

- diseases of animals transmitted to humans

**humans only reservoir - easiest to control**

**animal reservoir - difficult to control**

soil - Clostridium species

Staph. aureus

Transmission

Horizontal (person to person)

•contactdirect contactindirect contact

fomite

droplet

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

Vertical (mother to fetus) Reservoir

Natural habitat

Transmission

Importance of hand washing

Large microbe-laden respiratory droplets generally fall to the ground on fartherthan 3 feet.

Importance of covering mouth when cough or sneeze

Transmission

horizontal

•contactdirect contactindirect contact

fomite

droplet

•food and water

•air

•vectors

Reservoir

Natural habitat

Transmission

- inanimate object

very difficult to control

flea - Yersinia pestis

mosquito - Plasmodium species (malaria)

vertical

arthropods

“Black Death” (Plague) - Yersinia pestis

fleaBubonic plague

Killed 1/4 of the population of Europe between 1346 - 1350; 75% of the population in some cities.

•Flea transmits Y. pestis to a human

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

•Bubo develops within days

•Bacterium is carried to a lymph node.

•50 - 75% mortality (if untreated)

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

rodent rodent

flea

flea

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

Epidemiological studies

•Descriptive studies

•Analytical studies

•Experimental Studies

Which risk factors were/are most relevant?

Person

PlaceTime

Risk factors

Epidemiological studies

•Descriptive studies

Person

Place

Time

Person

•Descriptive studies

Person

PlaceTime

Epidemiological studies

Epidemiological studies

•Descriptive studies

Person

PlaceTime

12

1. Rapid rise of sick people2. Gradual rise

Experimental studies

• Placebo

• Double-blind

Experimental studies are done mostly to assess the value of a particular interventionor treatment, such as antimicrobial drugtherapy.

Rates of Disease in a Population

Endemic

Epidemic

Pandemic

Morbidity

Mortality

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)

Mortality - death (mortality rate)

Infectious Disease Surveillance

CDC - National Centers for Disease Control and Prevention

MMWR - Morbidity and Mortality Weekly Report

WHO - World Health Organization

Weekly Epidemiological Record

Trends in Disease

• Reduction & Eradication of Disease

• Emerging Diseases

Trends in Disease

• Reduction & Eradication of Disease

- Improved sanitation

- Reservior & vector control

- Vaccination

- Antibiotic treatment

(Smallpox, eradicated globally)

Trends in Disease

• Emerging Disease - Microbial Evolution, drug-resistance strain

- Population expansion

- Mass distribution & importation of food

- Climate change

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

Epidemiology

• Principles of Epidemiology

• Epidemiological studies

• Infectious Disease Surveillance

• Trends in Disease

• Nosocomial Infections