Epidemiology

• Epidemiological studies involve:– determining etiology of infectious disease– reservoirs of disease– disease transmission – identifying patterns associated with outbreaks – outlining diagnostic tools and treatment options

– Morbidity rate

– Mortality rate

– Incidence

– Prevalence

– Endemic

– Sporadic

– Epidemic

– Pandemic

• In order for disease to spread:– Pathogen must have reservoir– Pathogen must be transmitted to susceptible

host

• Reservoirs of infectious disease can be– Human– Non-human animals– Environmental (non-living)

• Recognizing reservoir can help protect population from disease

• Human reservoirs– Infected humans most significant reservoirs

• May be only reservoir which makes disease easier to control• Asymptomatic carriers

– may shed organisms intermittently for long periods

• Non-human animal reservoirs– Zoonotic transmission

(zoonoses) • Disease typically more

severe in humans• Often accidental and

may be a dead end for pathogen

• Environmental reservoirs– 2 most important are

• Water• Soil

• Transmission– Successful pathogen must be passed from

reservoir to next susceptible host• Contact• Vehicle• Vector

• Contact– Direct contact

• Occurs when one person physically touches another• Hands are the main source

– Indirect contact• Transmission via inanimate objects or fomites

– Clothing, tissues, doorknobs and drinking classes– Droplet transmission

• Respiratory droplets within three feet of release

• vehicle transmission– Food, water and air

• Food contamination may originate with animal or occur during food preparation

• Waterborne disease can involve large numbers of people; prevention involves proper sanitation

• Respiratory droplets dry; creates droplet nuclei that may remain suspended or become re-suspended

• Vectors– Any living organism that can

carry a pathogen• Most common are arthropods• Mechanical or biological

– Control of vector-borne disease directed at controlling arthropod population

• Many disease occur in cycles– May be annual or occur over decades

• Flu• plague

• Herd immunity is an important factor in cycles– Low level could lead to reemergence of disease

• Small pox

• Descriptive studies– Person

• Determine profile of those who become ill

– Place• Identifies general site of contact• Give clues about potential reservoirs and vectors

– Time• Rapid rise in numbers suggest common source

epidemic• Gradual rise likely contagious

Epidemiological Studies

• John Snow – identified common

source cause of cholera epidemic

• Broad Street Pump in London

– well-designed descriptive epidemiological study

– years before the relationship between microbes and disease was established

• Analytical studies– Determines which potential factors from

descriptive study are relevant

• Experimental studies– Judge cause and effect relationship of risk

factors or preventative factors• frequently to determine effectiveness of prevention

or treatment

• Reduction and eradication of disease– Humans have been very

successful

– Efforts directed at:• Improving sanitation• Reservoir and vector control• Vaccination• Chemotherapy

– Why aren’t all infectious diseases eradicated?

• Four mechanisms public health agencies use to control disease transmission:

1. Isolation

2. Quarantine

3. Immunization

4. Vector control

Infectious Disease Surveillance

• National disease surveillance network– Network of agencies across the country

• monitor disease development

– Agencies include• Public Health Departments• Centers for Disease Control and Prevention (CDC)• World Health Organization (WHO)

• CDC– National Dept. of Health– Notifiable diseases

• Published data in weekly publication

– Morbidity and Mortality Weekly Report (MMWR)

• WHO – International disease surveillance

• Weekly Epidemiological Record• Provide global standards and guidance for human

health

Nosocomial Infections• Hospital acquired

infections– Range from mild to fatal

• Increased 36% in the last 20 years

– Leading cause of death in the US

• 100,000 deaths per year

• Reservoirs of nosocomial pathogens:– Exogenous

• Other patients• Hospital environment• Health care workers

– Endogenous• Patient’s own normal flora

• Transmission of nosocomial pathogens:

– Medical devices

– Health care workers

– Airborne