Chapter 11
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 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).
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 2010). 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.
Epidemiologic Terminology (cont.) A sporadic disease is one that occurs only occasionally within the population of a particular geographic area (e.g., tetanus). An endemic disease is one that is always present within the population of a particular geographic area (e.g., staphylococcal and streptococcal infections). 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 (e.g., the cryptosporidiosis epidemic of 1993).
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 United States); (2) the H1N1 (swine flu) pandemic of 2009 to 2010. HIV/AIDS Tuberculosis Malaria
Interactions between Pathogens, Hosts, and the Environment Whether an infectious disease occurs depends on: Factors pertaining to the pathogen (virulence of pathogen, mode of entry, and number of organisms) Factors pertaining to the host (health status, nutritional status, hygiene, age, travel, lifestyle, etc.) Factors pertaining to the environment (physical factors such as climate, season, and geographic location; availability of appropriate reservoirs; sanitary and housing conditions; and availability of potable water)
The Chain of Infection There are six 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 microbes that cause infectious diseases are many and varied; they are known as reservoirs of infection or simply reservoirs. Living reservoirshumans, pets, farm animals, insects, and arachnids (e.g., ticks and mites) Human carriers: Passive carriers Incubatory carriers Convalescent carriers Active carriers
Reservoirs of Infection (cont.)
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 (e.g., rabies, Lyme disease, and 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 include Lyme disease and malaria.
Transmission of Lyme Disease
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)
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
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; and to improve the quality of human life Investigates outbreaks of Ebola virus, etc. Eradicated smallpox Attempting to eradicate polio and dracunculiasis
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, Georgia; established in 1946 (www.cdc.gov ) Mission: “to collaborate to create the expertise, information, and tools that people and communities need to protect their health …” Certain infectious diseases, known as nationally notifiable diseases, must be reported to the CDC. Publishes Morbidity and Mortality Weekly Report (MMWR).
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
Bioterrorism and Biological Warfare 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)
Water Supplies and Sewage Disposal Water is the most essential resource necessary for the survival of humanity! The two general types of water pollution: Chemical pollution Biological pollution (e.g., fecal material and garbage) The 1993 cryptosporidiosis epidemic in Milwaukee, Wisconsin, was the largest waterborne epidemic in the United States.
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. This treatment includes primary, secondary, and tertiary sewage treatments.