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Burton's Microbiologyfor the Health Sciences

Chapter 11.Epidemiology and Public Health

Chapter 11 Outline

• Epidemiology

• Interactions Among Pathogens, Hosts and the Environment

• Chain of Infection

• Reservoirs of Infection

• Modes of Transmission

• Public Health Agencies

• Bioterrorism and Biological Warfare Agents

• Water Supplies and Sewage Disposal

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Epidemiology

• Introduction

– Epidemiology can be loosely

defined as the study of

disease.

– Epidemiologists study the

factors that determine the

frequency, distribution, and

determinants of diseases in

human populations.

– Epidemiologists also develop

ways to prevent, control, or

eradicate diseases in

populations.

Epidemiologic Terminology

• A communicable disease is an infectious disease that can be transmitted from one person to another

• A contagious disease is a communicable disease that is easilytransmitted from person-to-person.

• Zoonotic diseases are diseases that humans acquire from animal sources.

• The incidence of a particular disease is the number of new cases of that disease in a defined population during a specific time period.

• The morbidity rate is the number of new cases of a particular disease that occurred during a specified time period per a specifically defined population (usually per 1,000, 10,000 or 100,000 population).

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Epidemiologic Terminology

Epidemiologic Terminology, cont.

• Prevalence

– Period prevalence is the number of cases of a disease existing in a given population during a specific time period (e.g., during the year 2009).

– Point Prevalence is the number of cases of a disease existing in a given population at a particular moment in time (e.g., right now).

• Mortality rate is the ratio of the number of people who died of a particular disease during a specified time period per a specified population

HIV Prevalence

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Epidemiologic Terminology, cont.

• A sporadic disease is one that occurs only occasionally within the population of a particular geographic area; example, tetanus.

• An endemic disease is one that is always present within the population of a particular geographic area; example, gonorrhea.

• An epidemic disease is defined as a greater than usual number of cases of a disease in a particular region, usually within a short period of time; example, the Legionnaire’s disease epidemic of 1976.

http://news.bbc.co.uk/2/hi/health/6959583.stm

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Epidemiologic Terminology, cont.

• A pandemic is a disease that is occurring in epidemic proportions in many countries simultaneously. Examples include:– Influenza

• Examples: (1) the Spanish flu pandemic of 1918 during which more than 20 million people were killed worldwide (500,000 in the U.S.); (2) the H1N1 (“swine flu”) pandemic of 2009-2010.

– HIV/AIDS– Tuberculosis– Malaria

Interactions Among Pathogens, Hosts, and the Environment

• Whether an infectious disease occurs depends on:

– Factors pertaining to the pathogen (e.g., virulence of pathogen, mode of entry, number of organisms)

– Factors pertaining to the host (e.g., health status, nutritional status, hygiene, age, travel, lifestyle, etc.)

– Factors pertaining to the environment (e.g., physical factors such as climate, season, geographic location; availability of appropriate reservoirs; sanitary and housing conditions; and availability of potable water

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The Chain of Infection

There are 6 components in the infectious disease process:● a pathogen● a source of

the pathogen (a reservoir)● a portal of

exit● a mode of

transmission● a portal of

entry● a susceptible

host

Reservoirs of Infection

• The sources of microorganisms that cause infectious diseases are many and varied; they are known as reservoirs of infection or simply reservoirs.

– Living reservoirs –humans, pets, farm animals, insects, arachnids

– Human carriers:

• Passive carriers

• Incubatory carriers

• Convalescent carriers

• Active carriers

http://www.trutv.com/library/crim

e/criminal_mind/forensics/typhoid

_mary/8.html

“Typhoid Mary”

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Reservoirs of Infection, cont.

• Animals– Infectious diseases

that humans acquire from animal sources are called zoonotic diseases or zoonoses.

– Zoonoses may be acquired by direct contact with an animal, inhalation or ingestion of the pathogen, or injection of the pathogen by an arthropod.• Examples:

• Rabies• Lyme disease• Many others

Reservoirs of Infection, cont.

• Arthropods

– Many different types of arthropods serve as reservoirs of infection, including insects (e.g., fleas, mosquitoes, lice) and arachnids (e.g., mites and ticks)

– When arthropods are involved in the transmission of infectious diseases they are referred to as vectors.

• Examples of arthropod-borne diseases:

• Lyme disease

• Malaria

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Reservoirs of Infection, cont.

• Nonliving Reservoirs

– Air, soil, dust, contaminated water and foods, and fomites

– Fomites -inanimate objects capable of transmitting pathogens (e.g., bedding, towels, eating and drinking utensils, hospital equipment, telephones, computer keyboards, etc.)

Inanimate Vectors of Infection (Fomites)

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Most Common Modes of Transmissionof Infectious Diseases• Direct skin-to-skin contact

• Direct mucous membrane-to-mucous membrane contact by kissing or sexual intercourse

• Indirect contact via airborne droplets of respiratory secretions, usually produced by sneezing or coughing

• Indirect contact via food and water contaminated by fecal matter

• Indirect contact via arthropod vectors

• Indirect contact via fomites

• Indirect contact via transfusion of contaminated blood or blood products or by parenteral injection using nonsterile syringes or needles

Public Health Agencies

• World Health Organization (WHO)

– A specialized agency of the United Nations founded in 1948; www.who.org

– Missions: to promote technical cooperation for health among nations; to carry out programs to control and eradicate diseases; to improve the quality of human life

• Investigates outbreaks of Ebola virus, etc.

• Eradicated smallpox

• Attempting to eradicate polio and dracunculiasis

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Public Health Agencies, cont.

• Centers for Disease Control and Prevention (CDC)

– A federal agency administered by the U.S. Department of Health and Human Services; located in Atlanta, GA; established in 1946; www.cdc.gov

– Certain infectious diseases, known as nationally notifiablediseases must be reported to the CDC.

– Publishes Morbidity and Mortality Weekly Report(MMWR).

http://www.cdc.gov/obesity/data/index.html

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Public Health Agencies, cont.

• Measures for prevention and control of epidemics:– Increase host resistance

through the development and administration of vaccines that induce active immunity and maintain it in susceptible persons

– Ensure that persons exposed to a pathogen are protected against the disease

– Segregate, isolate and treat those who have contracted a contagious infection to prevent the spread of the pathogen to others

– Identify and control potential reservoirs and vectors of infectious diseases

http://www.meningvax.org/cont

rol-epidemics.php

Bioterrorism and BiologicalWarfare Agents

• Microbes purposely used to harm others in wartime are called biological warfare (bw) agents.

• Pathogens used to create fear, chaos, illness, and death in situations other than war are called bioterrorism agents. Examples:

– Bacillus anthracis (the cause of anthrax)

– Clostridium botulinum (the cause of botulism)

– Smallpox virus (Variola major)

– Yersinia pestis (the cause of plague)

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Water Supplies and Sewage Disposal

• Water is the most essential resource necessary for the survival of humanity!

• The 2 general types of water pollution:

– Chemical pollution

– Biological pollution (e.g., fecal material and garbage)

• The 1993 cryptosporidiosis epidemic in Milwaukee, WI, was the largest waterborne epidemic in the U.S.

Water Supplies and Sewage Disposal, cont.

• Water Treatment

– The major steps in water treatment are sedimentation, coagulation, filtration, and chlorination

– Water is tested for fecal contamination by checking for the presence of coliform bacteria (coliforms), such as E. coli and other members of the family Enterobacteriaceae

• Sewage Treatment

– Raw sewage consists mainly of water, fecal material, garbage, and bacteria

– Includes primary, secondary, and tertiary sewage treatments