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Epidemiology
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• Epidemiological studies involve:– determining etiology of infectious disease– reservoirs of disease– disease transmission – identifying patterns associated with outbreaks – outlining diagnostic tools and treatment options
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– Morbidity rate
– Mortality rate
– Incidence
– Prevalence
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– Endemic
– Sporadic
– Epidemic
– Pandemic
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• In order for disease to spread:– Pathogen must have reservoir– Pathogen must be transmitted to susceptible
host
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• Reservoirs of infectious disease can be– Human– Non-human animals– Environmental (non-living)
• Recognizing reservoir can help protect population from disease
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• 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
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• Non-human animal reservoirs– Zoonotic transmission
(zoonoses) • Disease typically more
severe in humans• Often accidental and
may be a dead end for pathogen
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• Environmental reservoirs– 2 most important are
• Water• Soil
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• Transmission– Successful pathogen must be passed from
reservoir to next susceptible host• Contact• Vehicle• Vector
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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
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• 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?
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• Four mechanisms public health agencies use to control disease transmission:
1. Isolation
2. Quarantine
3. Immunization
4. Vector control
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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)
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• CDC– National Dept. of Health– Notifiable diseases
• Published data in weekly publication
– Morbidity and Mortality Weekly Report (MMWR)
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• WHO – International disease surveillance
• Weekly Epidemiological Record• Provide global standards and guidance for human
health
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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
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• Reservoirs of nosocomial pathogens:– Exogenous
• Other patients• Hospital environment• Health care workers
– Endogenous• Patient’s own normal flora
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• Transmission of nosocomial pathogens:
– Medical devices
– Health care workers
– Airborne