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M265 – Medical Microbiology

Epidemiology and Public Health

Ref: Burton’s Microbiology for the Health Sciences, 9th Ed

Ch.11

Lecture # 13

Dr. Samer Swedan

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

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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 easily transmitted 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 same as incidence but is usually expressed per 1,000, 10,000 or 100,000 population.

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

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.

• 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.

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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; (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 relating to the pathogen (e.g., virulence of pathogen, mode of entry, number of organisms)

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

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

The Chain of Infection, cont.

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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 [completely recovered from disease but still carry the pathogen for a long time]

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

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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

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.)

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Inanimate Vectors of Infection (Fomites)

Most Common Modes of Transmission of 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

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Modes of Disease Transmission

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

•Eradicated smallpox

•Investigates outbreaks of Ebola virus, etc.

•Attempting to eradicate polio and guinea worm disease

Required

Self

Reading

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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 (e.g. passive immunization)

– 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

Bioterrorism and Biological Warfare Agents

• Microbes purposely used to harm others in wartime are called biological warfare 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)

Required

Self

Reading