Principles of Epidemiology and Host Defenses Study Guide

Chapter 14: Principles of Epidemiology

  • Epidemiology Defined: The study of disease within a population.

  • Etiology: The specialized study concerned with determining the actual cause of a disease.

  • Occurrence of Disease:

    • Incidence: The number of people who develop a specific disease during a particular time period.

    • Prevalence: The number of people who have a disease at a specified time, regardless of when it first appeared. This metric is comprehensive, taking into account both old and new cases.

  • Vocabulary for Disease Occurrence:

    • Sporadic Disease: A disease that occurs only occasionally.

    • Endemic Disease: A disease that is constantly present within a specific population.

    • Epidemic: A disease acquired by many people in a given area within a short period of time.

    • Pandemic: An epidemic that has spread worldwide.

  • Severity and Duration Vocabulary:

    • Infection Fatality Rate (IFR): Calculated by dividing the number of deaths attributed to a disease by the total number of infected individuals within a specific time period. A lower IFRIFR indicates a lower fatality level.

    • Case Fatality Ratio (CFR): The proportion of individuals diagnosed with a disease who die from that disease within a certain amount of time.

    • Comparative Fatality Examples:

    • The Ebola infection fatality rate is approximately 50%50\%.

    • The infection fatality rate of measles in developed countries with medical care is approximately 0.3%0.3\%.

    • Discussion Point: Considerations on whether the Ebola infection fatality rate would be lower if it occurred in the United States involve factors such as medical infrastructure and access to care.

  • Predisposing Factors (Susceptibility):

    • Factors that make individuals more susceptible to disease include nutrition, sex, genetic inheritance, climate, environment, vaccination status, age, lifestyle, and being a compromised host.

  • The Five Stages of Disease:

    1. Incubation Period: The interval between the initial infection and the first appearance of signs and symptoms.

    2. Prodromal Period: The period following incubation characterized by early, mild, and non-specific symptoms.

    3. Period of Illness: The phase where the disease is at its most severe level.

    4. Period of Decline: The phase where signs and symptoms begin to subside.

    5. Period of Convalescence: The phase where the body returns to its pre-diseased state and recovery occurs.

  • Spread of Infection:

    • Continual Reservoirs of Infection:

    • Human Reservoirs: Includes carriers (persons who carry the disease but may not exhibit any symptoms).

    • Animal Reservoirs: Source of zoonotic diseases (diseases transmitted from animals to humans).

    • Environmental Reservoirs: Soil, water, and food.

  • Modes of Transmission:

    • Contact Transmission:

    1. Direct: Person-to-person contact.

    2. Congenital: Transmission from mother to fetus or newborn.

    3. Indirect: Involves a fomite (non-living object that can harbor and transmit pathogens).

    4. Droplet Spread: Transmission via airborne droplets.

    • Vehicle Transmission (Non-living):

    1. Airborne.

    2. Waterborne.

    3. Foodborne.

    • Vectors (Arthropods):

    1. Mechanical Translation.

    2. Biological Translation.

  • Healthcare-Associated (Nosocomial) Infections:

    • Definition: Infections acquired while in a hospital setting.

    • Causes: The convergence of microorganisms in the hospital environment, a compromised host, and a chain of transmission.

    • Example Organisms: Escherichia coli (E. coliE.\,coli), Candida auris, and Staphylococcus aureus.

  • Precautionary Measures:

    • Universal Precautions: Designed to reduce microbe transmission in healthcare and long-term care settings.

    • Standard Precautions: Basic minimum practices applied to everyone regardless of diagnosis.

    • Transmission-Based (Supplemental) Precautions: Designed for known or suspected infections that are highly transmissible or involve epidemiologically important pathogens.

  • Controlling Healthcare-Associated Infections:

    • Implementation of contact, droplet, and airborne precautions.

    • Reducing the number of pathogens.

    • Prescribing antibiotics only when strictly necessary.

    • Avoiding invasive procedures when possible.

    • Minimizing the use of immunosuppressive drugs.

  • Historical Figures in Epidemiology:

    • John Snow: Mapped the occurrence of cholera in London (Descriptive epidemiology).

    • Ignaz Semmelweis: Demonstrated that handwashing significantly decreased the incidence of puerperal sepsis (Experimental epidemiology).

    • Florence Nightingale: Showed that improved sanitation decreased the incidence of epidemic typhus (Analytical epidemiology).

  • Types of Epidemiology:

    • Descriptive: The collection and analysis of data regarding a disease.

    • Analytical: Analyzes a specific disease to determine its probable causes or risk factors.

    • Experimental: Involves a hypothesis and the execution of controlled experiments.

  • Notifiable Diseases and Reporting:

    • Notifiable Disease: An infectious disease that healthcare providers and laboratories are required by law to report to public authorities.

    • Reporting Path: Hospitals report to the State Health Department, which then coordinates with public health agencies.

  • Key Health Organizations:

    • CDC: Centers for Disease Control and Prevention; the main public health agency of the United States.

    • WHO: World Health Organization.

    • MMWR: Morbidity and Mortality Weekly Report.

Chapter 16: Innate Host Defenses

  • Innate vs. Adaptive Defenses:

    • Innate Immunity: Defenses you are born with; provides immediate protection.

    • Adaptive Immunity: Immunity developed after exposure to a pathogen or via vaccination.

  • Types of Innate Defenses:

    • First Line (Prevent Entry): Physical and chemical factors that prevent pathogens from gaining access to the body.

    • Second Line (Cells of the Immune System): Destroy pathogens that have entered the body before they can spread.

  • First Line of Defense: Physical and Chemical Barriers:

    • Skin:

    • Consists of the dermis and epidermis.

    • Mechanism: Sheds cells to remove microbes, inhibits microbial growth, and acts as a physical barrier.

    • Mucous Membranes:

    • Line the openings of the digestive, respiratory, and genitourinary tracts.

    • Mechanism: Bathed in secretions that move microbes toward areas where they can be eliminated.

    • Chemical Factors:

    • Sebum: Lowers skin pH\text{pH} and protects the skin.

    • Lysozyme: Enzyme that digests bacterial cell walls; found in saliva, tears, tissue fluid, and urine.

    • Gastric Juice: Extremely low pH\text{pH} that destroys most bacteria and toxins.

    • Microbiome:

    • Competes with pathogens and produces toxic compounds.

    • Essential for the development of the immune system.

  • Second Line of Defense: Hematopoiesis and Blood Cells:

    • Hematopoiesis: The formation and development of the formed elements of blood, occurring in the red bone marrow.

  • White Blood Cell (WBC) Classifications and Functions:

    • Granulocytes:

    • Neutrophils: Perform phagocytosis; most active in early infection stages; can leave the blood and enter tissues.

    • Basophils: Responsible for the production of histamine; active in allergic responses.

    • Eosinophils: Produce toxic proteins against certain parasites.

    • Agranulocytes:

    • Monocytes: Mature into macrophages in tissues where they become highly phagocytic (the "clean-up crew").

    • Dendritic Cells: Perform phagocytosis and initiate adaptive immune responses by presenting antigens; found in skin, mucous membranes, and thymus.

    • Lymphocytes:

      • T cells: Responsible for cell-mediated immunity.

      • B cells: Develop into plasmacytes (plasma cells) that produce antibodies.

      • Natural Killer (NK) cells: Play a role in adaptive immunity and attacking infected cells.

  • The Lymphatic System:

    • Function: Returns interstitial fluid to the blood and filters lymph to remove microbes.

    • Components: Lymph plasma, spleen, thymus, intestines, and lymph nodes.

  • Phagocytosis Process:

    • Definition: The process by which certain WBCs (Neutrophils, Macrophages, Dendritic cells) destroy pathogens and debris. Lymphocytes (B and T cells) do NOT perform phagocytosis.

    • Steps:

    1. Chemotaxis: Movement of WBC toward the infection site in response to chemical signals.

    2. Adherence: Phagocyte attaches to the pathogen surface.

    3. Ingestion: Phagocyte surrounds the pathogen with pseudopods.

    4. Digestion: Formation of a phagolysosome; the phagosome fuses with a lysosome filled with digestive enzymes.

  • Inflammation:

    • Definition: A local defensive response triggered by tissue damage.

    • Signs and Symptoms (PRISH): Pain, Redness, Immobility, Swelling, and Heat.

    • Goal: Destroy the injurious agent, limit its effects, and repair/replace damaged tissue.

    • Duration: Acute (short-acting) vs. Chronic (long-acting).

    • Mediators: Histamine causes vasodilation and increased permeability of blood vessels.

    • Steps: Vasodilation and increased permeability →\rightarrow Phagocyte migration and phagocytosis →\rightarrow Tissue repair.

  • Cell Communication: Cytokines:

    • Cytokines: Small proteins used for communication between immune cells.

    • Chemokines: Attract immune cells to infection sites.

    • Interferons: Released by virus-infected cells to warn nearby healthy cells.

    • Interleukins: Aid communication specifically between leukocytes.

    • Cytokine Storm: An out-of-control release of cytokines causing significant tissue damage.

  • Fever:

    • Mechanism: Cytokines stimulate the release of pyrogens, which act on the hypothalamus to raise body temperature.

    • Pyrogen: The substance that induces fever.

    • Benefits: Helps the body fight infection.

  • The Complement System:

    • Definition: A group of over 3030 plasma proteins produced mostly by the liver that circulate in an inactive form.

    • Results of Activation:

    1. Opsonization: Marking pathogens for phagocytosis.

    2. Inflammation.

    3. Cell Lysis: Destruction of the pathogen cell.

Chapter 17: Adaptive Immunity

  • Overview: Adaptive immunity is acquired specifically through infection or vaccination.

  • Humoral vs. Cell-Mediated Immunity:

Feature

Humoral Immunity

Cell-Mediated Immunity

Cell Types

B lymphocytes (Plasma cells, Memory B cells)

T lymphocytes (Helper T, Cytotoxic T, Memory T)

Origin/Maturation

Produced in red bone marrow; Mature in bone marrow

Produced in red bone marrow; Mature in thymus

Function

B cells recognize antigens →\rightarrow Plasma cells produce antibodies. Antibodies neutralize toxins/viruses, cause agglutination, and opsonization.

Helper T cells activate B cells and macrophages via cytokines. Cytotoxic T cells kill infected/cancerous cells via apoptosis.

Memory

Memory B cells (long-term)

Memory T cells (fast future response)

Primary Site

Body fluids

Inside infected tissues and cells

  • Key Definitions:

    • Antigen: A substance that triggers an immune response.

    • Antibody: Proteins (Immunoglobulins, Ig) produced by B cells that bind specifically to antigens.

    • Interleukins: Facilitate communication between leukocytes.

    • Chemokines: Induce migration of immune cells.

    • APC (Antigen-Presenting Cell): Includes dendritic cells, macrophages, and B cells.

    • MHC (Major Histocompatibility Complex):

    • MHC Class I: Found on all nucleated cells.

    • MHC Class II: Found on APCs.

  • Immunoglobulins (Ig):

    • IgG: Most abundant; can cross the placenta.

    • IgM: First to be produced during an initial infection.

    • IgA: Found in secretions (saliva, breast milk).

  • Clonal Processes:

    • Clonal Selection: Identifying the specific lymphocyte that matches an antigen.

    • Clonal Expansion: The rapid proliferation of that specific lymphocyte.

  • Results of Antigen-Antibody Interaction:

    1. Agglutination.

    2. Opsonization.

    3. Neutralization.

    4. Antibody-dependent cell-mediated cytotoxicity.

    5. Activation of complement.

  • Immune Response Characteristics:

    • Immunological Memory: Ability to remember previous pathogens.

    • Immunological Tolerance: The ability of the immune system to not attack the body's own cells.

    • Primary Response: Initial contact with an antigen.

    • Secondary (Anamnestic) Response: Faster, more effective response upon subsequent exposure.

Chapter 18: Practical Applications of Immunology

  • Vaccines History:

    • Variolation: An early method of immunization involving the deliberate infection with smallpox material; works against Smallpox.

    • Vaccination Etymology: Derived from "vacca," the Latin word for cow (linked to Jenner's use of cowpox).

  • Vaccination Principles:

    • Vaccine: A suspension of organisms or fractions of organisms used to induce immunity.

    • Mechanism: Works by triggering a primary immune response, leading to the formation of memory cells without causing the full disease.

    • Herd Immunity: When a high percentage of the population is immune, the spread of the disease is limited, protecting those who are not immune.

  • Vaccine Types:

    • Attenuated Vaccines: Live, weakened pathogens.

    • Inactivated Vaccines: Killed pathogens.

    • Subunit Vaccines: Use only specific fragments/antigens of a pathogen.

    • Toxoids: Inactivated toxins produced by pathogens.

    • mRNA Vaccines: Use genetic material to instruct cells to produce an antigen.

  • Diagnostic Applications and Monoclonal Antibodies:

    • Monoclonal Antibodies: Laboratory-produced molecules engineered to serve as substitute antibodies; used for research, diagnostics, and therapy.

    • ELISA (Enzyme-Linked Immunosorbent Assay):

    • Direct ELISA: Measures the presence of antigens.

    • Indirect ELISA: Measures the presence of antibodies.