Epidemiology Flashcards

Epidemiology

Learning Objectives

• Define epidemiology and its uses.
• Understand Koch’s postulates.
• Recognize patterns of infection and types of epidemics.
• Know important epidemiological statistics.
• Explain herd immunity and the role of vaccines.
• Define nosocomial infections.
• Describe control measures for infections and their modulation.

What is Epidemiology?

• Epidemiology is the science that evaluates the:
  • Occurrence
  • Determinants
  • Distribution
  • Control of health and disease in a defined human population.
• John Snow:
  • Considered the first epidemiologist.
  • Studied cholera in London.

Koch's Postulates

• A set of criteria to establish a causative relationship between a microorganism and a disease:
  1. The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
  2. The microorganism must be isolated from a diseased organism and grown in pure culture.
  3. The cultured microorganism should cause disease when introduced into a healthy organism.
  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

John Snow and Cholera

• John Snow was a British doctor in the mid-1800s.
• He aimed to determine the cause of cholera outbreaks in London.
• Based on observations, he suspected cholera spread through contaminated water.
• He shut down a water pump, leading to a dramatic decrease in cases in 1849.
• The water was found to be contaminated with raw sewage.
• People supplied by one company using contaminated water had an >80 times higher chance of dying from cholera in 1854.

Why We Need Epidemiology

• Old Diseases:
  • Cholera
  • Typhoid
  • Smallpox
  • Influenza
• Modern Diseases:
  • HIV/AIDS
  • SARS
  • Bird flu
  • Ebola
  • Zika

Centers for Disease Control and Prevention (CDC)

• Located in Atlanta, GA.
• Functions as a national focus for:
  • Developing and applying disease prevention and control.
  • Environmental health.
  • Health promotion and health education activities designed to improve public health.
• The World Health Organization (WHO) in Geneva, Switzerland, is the worldwide counterpart.

Epidemiology - Determinations

• Causative agent.
• Source and/or reservoir of the disease agent.
• Mechanism of transmission.
• Host and environmental factors that facilitate disease development within a defined population.
• Best control measures.

Epidemiology Terminology

• Sporadic disease: Occurs occasionally and at irregular intervals.
• Endemic disease: Maintains a relatively steady low-level frequency at a moderately regular interval.
• Hyperendemic diseases: Gradually increase in occurrence frequency above the endemic level but not to the epidemic level.

Epidemiology Terminology – 2

• Outbreak:
  • A sudden, unexpected occurrence of disease.
  • Usually focal or in a limited segment of the population.
• Epidemic:
  • An outbreak affecting many people at once.
  • A sudden increase in frequency above the expected number.
  • Index case: The first case in an epidemic.
• Pandemic: Increase in disease occurrence within a large population over a wide region (usually worldwide).

Epidemiological Methods

• Public health surveillance:
  • Protecting populations/improving the health of communities via education, promotion of healthy lifestyles, and prevention of disease and injury.
  • Methodical approach to identify issues.
    • Review of death certificates
    • Field investigation of epidemics
    • Investigation of actual cases

Public Health Surveillance - Proactive Evaluation

• Genetic backgrounds
• Environmental conditions
• Human behavior
• Emerging diseases
• Antibiotic resistance

Measuring Infectious Frequency

• To determine if an outbreak, epidemic, or pandemic is occurring, epidemiologists measure disease frequency at single time points and over time.
• Statistics – mathematics dealing with the collection, organization, and interpretation of numerical data.
• Need accurate counts of population size, people exposed, and affected people.
• Three important statistical measures of disease frequency:
  • Morbidity rate
  • Prevalence rate
  • Mortality rate

Morbidity Rate

• An incidence rate.
• Number of new cases in a specific time period per unit of population.
• Formula: \frac{\text{# new cases during a specific time}}{\text{# individuals in population}}
• Example: If in 1 month, there are 700 new cases per 100,000 individuals, it's 700 per 100,000 or 0.7%.

Prevalence Rate

• Total number of individuals infected at any one time.
• Depends both on incidence rate and duration of illness
• Formula: \frac{\text{Number of current cases}}{\text{Total population}}

Mortality Rate

• Number of deaths from a disease per number of cases of the disease.
• Formula: \frac{\text{# deaths due to given disease}}{\text{size of total population with disease}}

Patterns of Infectious Disease in a Population

• Infectious disease: Disease resulting from an infection by microbial agents.
• Communicable disease: Can be transmitted from one person to another.
• Two types of epidemics:
  • Common source epidemic: Single common contaminated source (e.g., food).
  • Propagated epidemic: One infected individual into a susceptible group, infection propagated to others.

Herd Immunity

• Herd immunity: The level of resistance of the population to infection and microbe spread because of the immunity of a large percentage of the population.
• The level can be altered by the introduction of new susceptible individuals into the population.
• The level can be altered by changes in the pathogen:
  • Antigenic shift: A major change in the antigenic character of the pathogen.
  • Antigenic drift: Smaller changes.

Emerging Infections

• Infectious disease mortality has increased since 1982 in the U.S.
• Increase in incidence of infectious disease due to emerging microbial populations.
• Some are novel or reemerging infectious diseases.
• Hot spots of emerging infectious diseases include:
  • Northeast U.S.
  • West Europe
  • Japan
  • Southeast Australia.

Emerging and Reemerging Infectious Diseases and Pathogens

• Examples of emerging and reemerging infectious diseases include but are not limited to:
  • Bacteria: Bartonella henselae, Ehrlichia spp., Heliobacter pylori, Borrelia burgdorferi, Clostridium difficile, Streptococcus pyogenes, Staphylococcus aureus
  • Fungus: Encephalitozoon spp., Cryptococcus gattii strains
  • Protist: Acanthamoeba spp., Babesia spp.
  • Virus: Australian bat lyssavirus, Ebola virus, Hendra virus, Hepatitis C virus, Hepatitis E virus, Human herpesvirus 8, Human herpesvirus 6, Parvovirus B19, MERS-COV, Avian influenza A (H7N9), Bourbon virus, Chikungunya virus, Measles virus, Mumps virus, Enterovirus A71, Enterovirus D68, Polio virus, Dengue virus

Systematic Epidemiology

• Focuses on ecological and social factors that influence the development and spread of emerging and reemerging diseases.
• Numerous factors have been identified.

Reasons for Increases in Emerging and Reemerging Infectious Diseases

• World population growth
• Increased international travel
• Habitat disruption
• Microbial evolution and development of resistance
• Inadequate public infrastructures

Reasons for Increases in Emerging and Reemerging Infectious Diseases– 2

• Changes in ecology and climate
• Social unrest, wars, and bioterrorism
• Changes in food processing and agricultural practices
• Changes in human behavior, technology, and industry
• Medical practices that lead to immunosuppression

Nosocomial Infections

• Hospital-acquired infections from pathogens within a hospital or other clinical care facility, acquired by patients in the facility
• Approximately ~5% of all hospital patients acquire a nosocomial infection
• Often caused by bacteria that are members of normal microbiota
• Many hospital strains are antibiotic-resistant

Control, Prevention, and Surveillance

• Nosocomial infections:
  • Prolong hospital stays by 4–14 days
  • Result in additional $28 to $33 billion per year to direct healthcare costs
  • Result in approximately 99,000 deaths annually
• Proper training of personnel in basic infection control measures:
  • Handling of surgical wounds
  • Hand washing
• Monitoring of patients for signs and symptoms of nosocomial infection

Healthcare-Associated Infections (HAIs)

• Examples of HAIs include infections of the:
  • Bloodstream
  • Ventilator-Associated Pneumonia
  • Intestinal Tract and Liver
  • Urinary Tract
  • Surgical Sites
  • Caused by various bacteria, fungi, and viruses

Sources of Nosocomial Infections

• Endogenous pathogen: Brought into hospital by patient or acquired when patient is colonized after admission.
• Exogenous pathogen: Microbiota other than the patient’s.
• Autogenous infection: Caused by an agent derived from the microbiota of the patient, regardless of whether it became part of patient’s microbiota following admission.

Prevention and Control of Epidemics

• Three types of control measures:
  • Reduce or eliminate source or reservoir of infection
  • Break connection between source and susceptible individual
  • Reduce the number of susceptible individuals

Reduce or Eliminate Source or Reservoir

• Quarantine and isolation of cases and carriers
• Destruction of animal reservoir
• Treatment of sewage
• Therapy that reduces or eliminates infectivity of cases

Break Connection Between Source and Susceptible Individuals

• Chlorination of water supplies
• Pasteurization of milk
• Supervision and inspection of food and food handlers
• Destruction of insect vectors with pesticides

Reduce Number of Susceptible Individuals

• Raises herd immunity
• Passive immunity following exposure
• Active immunity for protection

Vaccines and Immunization

• Vaccine: A preparation of microbial antigens used to induce protective immunity.
  • Consists of killed, living, weakened (attenuated) microbes or inactivated bacterial toxins (toxoids), purified cell material, recombinant vectors, or DNA.
• Immunization: Result obtained when a vaccine stimulates immunity.
• Vaccines attempt to induce antibodies and activated T cells to protect the host from future infection.
• Vaccinomics: The application of genomics and bioinformatics to vaccine development.

Adjuvants

• Mixed with antigens in vaccines to enhance the rate and degree of immunization.
• Can be any nontoxic material that prolongs antigen interaction with immune cells and stimulates the immune response to the antigen.
• Several types are available.

Immunized Hosts – Who Needs Vaccines, and When?

• Vaccination of children should begin at ~2 months.
• Further vaccination depends on relative risk:
  • Living in close communities
  • Reduced immunity
  • International travelers
  • Health-care workers

Vaccination History

• Edward Jenner (1749-1823)
  • Observed that people who contracted cowpox never contracted smallpox.
  • Smallpox had a mortality rate of 10-40%.
  • Cowpox is a relatively mild infection.
  • Jenner took pus from a cowpox lesion and injected it into a boy.
  • He then injected the boy with smallpox, and the boy did not develop smallpox.

Bioterrorism Preparedness

• Bioterrorism is defined as "intentional or threatened use of viruses, bacteria, fungi, or toxins from living organisms to produce death or disease in humans, animals, and plants."

Bacteria

• Examples of CDC-Defined Select Agents and Toxins (BSATs) include:
  • Bacillus anthracis
  • Brucella abortus
  • Brucella melitensis
  • Brucella suis
  • Burkholderia mallei
  • Burkholderia pseudomallei
  • Coxiella burnetii
  • Clostridium botulinum
  • Francisella tularensis
  • Rickettsia prowazekii
  • Yersinia pestis

Examples of Intentional Uses of Biological Agents

• 1984 in Dalles, OR:
  • Salmonella typhimurium in 10 restaurant salad bars.
• 1996 in Texas:
  • Intentional release of Shigella dysenteriae in a hospital lab break room.
• 2001 in seven eastern U.S. states:
  • Use of weaponized Bacillus anthracis spores delivered through the U.S. postal system.
• All of the above caused hospitalizations, while the anthrax episode resulted in five deaths.

Choosing Biological Agents as Weapons

• Biocrime is the term used when chosen as a means for a localized attack, versus bioterrorism when targeting mass casualties.
• Characteristics that favor their use:
  • Invisible, odorless, and tasteless.
  • Difficult to detect.
  • Take hours or days before one becomes aware that they have been used.
  • Fear and panic associated with the anticipation that they were used.

U.S. Biological Weapons Defense Initiative

• Procurement of specialized vaccines and medicines for a national civilian protection stockpile.
• Invigoration of biodefense research including genome sequencing, vaccine, and therapeutic research.
• Development of improved detection and diagnostic systems.
• Preparation of health care professionals to be members of the “first responder” team.

Additional Government Responses

• 2002 – The Public Health Security and Bioterrorism Preparedness and Response Act:
  • Identified “select” agents whose use is tightly regulated.
  • Revised in 2012 to identify “tier 1” threats.
    • Can produce mass casualty or economic devastation.
    • Highly communicable with low infectious dose.
    • Easily weaponized.
• 2003 – The Department of Homeland Security was established to coordinate the defense of the U.S. against terrorist attacks.
  • Responsible for developing/maintaining a National Incident Management System to monitor large-scale hazards.

Partnerships to Protect the U.S. Population

• CDC partnering with academic institutions to educate health care providers.
• Establishment of Centers for Public Health Preparedness.
• The Laboratory Response Network (LRN):
  • Ensuring effective laboratory response to terrorism by improving U.S. public health lab infrastructure.
  • A CDC-managed program.
  • Partners with the FBI and the Association of Public Health Laboratories (APHL).
• A CDC-managed program.

Key Indicators of a Bioterrorism Event

• Sudden increased numbers of sick people, especially with unusual diseases for that place and/or time of year.
• Sudden increased numbers of zoonoses, diseased animals, or vehicle-borne illnesses.