answer from the powerpoint

Epidemiology and Cycle of Microbial Disease

Public Health 4030

Chapter 8 Chapter 8

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Epidemiology and Cycle of

Microbial Disease

• Concepts of Epidemiology

– Epidemiology is an investigative branch of medicine that deals with the source, cause, and possible control of infectious disease and other public health problems

– Epidemiologists are challenged to determine why an outbreak of disease occurs at a particular time and/or particular place

• Population-based disease control

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Epidemiology and Quantification

• Leading cause of mortality

• 1900 compared to 2015

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Epidemiology is more than infectious and chronic diseases

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Epidemiology is more than infectious and chronic diseases

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Zika Virus 2015-20116

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Epidemiology’s early history

• Hippocrates (460–377 B.C.) linked malaria, yellow fever, and swamps – Father of Medicine and first

epidemiologist

– Movement away from supernatural

• Edward Jenner’s late 1700s observations regarding cowpox led to smallpox vaccine

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Ignaz Semmelweis • Mid 1800’s

• Described as the “savior of mothers”

• Early pioneer of antiseptic procedures

• Proved that childbed fever resulted from physicians not washing their hands after dissections

• Could not “prove” and many disregarded recommendation until Louis Pasteur and germ theory

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John Snow

• 1849 – Broad Street Pump

• Investigated cholera outbreak that occurred in Broad Street, Golden Square, London.

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• Linked cholera epidemic to contaminated water supplies • Used a spot map of cases to determine contaminated pump 9/7/2017 11Ch. 08 - Epidemiology

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Terms for classification of diseases

• Sporadic—occur only occasionally and in an unpredictable fashion (tetanus, etc.)

• Endemic—regularly found at a steady level in a particular location (common cold, etc.)

• Epidemic—sudden increase in morbidity (illness rate) and mortality (death rate) above the norm (plague, etc.)

• Pandemic—epidemics that spread across continents (1918 influenza, HIV/AIDS)

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There are two recognized types of epidemic

• Common-source epidemics involve contact

with a single contamination source

(contaminated water)

• Propagated epidemics result from person-to-

person contact (mumps and chicken pox)

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Herd Immunity (Group Immunity)

• Refers to the proportion of immunized

individuals in a population

– The smaller the number of susceptible individuals,

the less opportunity for contact between them and

infected individuals.

• Public health officials strive to maintain high

levels of herd immunity

– VACCINATE…VACCINATE…VACCINATE

• Societies debate about vaccines?

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Herd Immunity cont.

• Figure 08.04: (a) Cows immunized and herd is protected. (b) Cows not immunized become infected and sick (lack of herd immunity).

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Reproduction rate (R0 or R-nought)

• Measure of the potential for transmission

• Mean number of secondary cases, occurring in a nonimmunized (susceptible) population in the wake of a particular infection.

– Population density, duration of contagiousness, other factors considered

– R0 must be greater than 1 to spread; if less, it will die out.

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• Table 08.01: Basic Reproduction Rate (R0) Within Human Populations.

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Graphs, charts, maps, rates…are used by

epidemiologists to illustrate the frequency and

distribution of diseases

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Cycle of Microbial Disease

• A chain of linked factors (reservoirs, modes of

transmission, portals of entry and exit)

contributes to the cycle of microbial disease

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Reservoirs of Infection

Reservoir is a site in nature in which microbes survive (and possibly multiply) and from which they may be transmitted • All pathogens, to exist, must have one or more

reservoirs

• Reservoirs are prime targets for preventing, minimizing, and eliminating existing and potential epidemics

• Humans are the only known reservoir for pathogens that cause smallpox, gonorrhea, measles, polio, etc.

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Reservoirs of Infection (cont.)

• Active carriers are individuals who have a microbial disease

• Healthy carriers have no symptoms and may unwittingly pass the disease on to others

– Typhoid Mary (Mary Mallon) carrier of pathogen that causes typhoid fever

– Continue to harbor the microbe after recovery, this state can continue indefinitely without illness

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To keep track of infectious diseases in the United States, physicians

are required to report cases of certain notifiable diseases

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Carriers

• Active carriers are individuals who have a microbial disease.

• Healthy carriers have no symptoms and unwittingly pass the disease onto others.

– Typhoid Mary

– West Nile virus and in-apparent infection

– Carriers play a significant role in spread of TB.

• Chronic carriers harbor the pathogen for long periods of time after recovery, without ever becoming ill again.

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Zoonoses • Zoonoses are diseases in which animals serve

as reservoirs and carriers

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Zoonoses are diseases of animals that can affect

humans. Animals act as reservoirs of pathogen.

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Nonliving Reservoirs

Some organisms are able

to survive and multiply

in nonliving

environments, such as

soil and water

• Spore formers, like the

group of Clostridium

bacteria (cause of

tetanus and botulism)

can survive for many

years in soil

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Transmission

• Transmission is the mechanism by which an

infectious agent is spread to a susceptible

person

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• Direct transmission

– Contact (kissing, sneezing, sex, etc.)

– Animal Bites

– Transplacental • Horizontal: person-to-person, touch (including sex),

droplets, etc.

• Vertical: mother to child (transplacental, breast milk,

birth canal)

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• Indirect transmission, microbes pass from reservoir (or source) to an intermediate agent and then to a host

– Vehicleborne: via food, water, biological products (organs, blood, blood products), and fomites (inanimate objects)

– Airborne: aerosols of water or dust particles (less than 4 µm) in the air; unlike droplets (10 µm or larger) aerosols remain airborne for extended periods

– Vectorborne: Arthropods (i.e., ticks, flies, mosquitoes, lice, and fleas) or insects (Chagas’ kissing bug, etc.) • Mechanical passive transmission on feet, etc.; microbes

do not invade, multiply, or develop in the vector

• Biological vectors are a necessary part of the life cycle of a pathogen, transmission is an active process

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Transmission

Figure 08.08: Droplet transmission (sneezing).

Figure 08.09: Exercise machines can be reservoirs for microbes.

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Indirect Transmission is by vectors

(e.g. Arthropoda)

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Table 08.07: Diseases Transmitted by Arthropod Bites.

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Arthropods • Act as insect vectors

• Capable of transmitting bacteria, virus and protozoa infectious

• Examples: mosquitos, ticks, flies, mites

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What is happening with vector borne diseases?

• Lack of resources for vector control

• Lack of political will

• Lack of research

• Global trends of human growth

• Climate change

Geographic distribution of Aedes aegypti in the Americas. (Gubler D., 2004)9/7/2017 41Ch. 08 - Epidemiology

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The portal of entry can be important in the

outcome of infection; a portal of exit is

required for transmission to a new host

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Nosocomial (Hospital-Acquired)

Infections

Hospital Environment as a Source of Nosocomial

Infections

• Many patients are ill and may have a weakened

immune system

• Hospitals foster the development of antibiotic-

resistant strains of bacteria

• Invasive procedures and insertion of needles,

catheters, tubes, etc., facilitate the direct

transmission of microbes

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CDC reports that approximately 1 out of every 20 hospitalized patients will contract an HAI

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Catheter - UrinaryCentral line Ventilator Surgical Site

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Figure 08.13: Body Site Distribution of Nosocomial Infections.

Data from: CDC, National Nosocomial iInfection Surveillance 9/7/2017 48Ch. 08 - Epidemiology

Table 08.10: Nosocomial Infection Caused by Antibiotic-Resistant Bacteria.

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