Epidemiology Flashcards
___________ are concerned with the health of an individual; ___________ are concerned with
the collective health of the population in one or other area.
Clinicians; epidemiologists
the study of the distribution and determinants of health-related states or events in specified
populations and the application of this study to the control of health problems.
Epidemiology
Father of Epidemiology
• The first person known to have examined the relationships between the occurrence of disease and environmental influences.
• He coined the term ‘epidemic’ in his essay entitled “On Airs, Waters, and Places,”
Hippocrates, 400 BC
a London haberdasher who published his landmark analysis of mortality data in 1662. He was the first to quantify patterns of birth, death, and disease occurrence, noting male-female disparities, high infant mortality, urbanrural differences, and seasonal variations.
John Graunt
____________ wrote a book ‘Cholera’. He began to systematically collect and analyze Britain’s mortality statistics. He considered the father of modern vital statistics and surveillance, developed many of the basic practices used today in vital statistics and disease classification. He extended the epidemiologic analysis of morbidity and mortality data, looking at the effects of marital status, occupation, and altitude. He also developed many epidemiologic concepts and techniques still in use today.
William Farr
the ongoing systematic collection, analysis, and interpretation of outcome-specific data for use in planning, implementation, and evaluation of public health practice.
Surveillance
A fundamental science of public health, goal is to promote health, prevent and control disease, is achieved through studies that examine the relationship of disease and factors in the population.
Epidemiology
outbreak of disease in human population
Epidemic
outbreak of disease in animal population
Epizootic
outbreak of disease in bird population
Epornitics
to visit a community, I.e., a diseases which comes periodically, not present continuously
Epidemein
resides within (the diseases continuously present in a population)
Endemein
is the study of health-event, health-characteristic, or healthdeterminant patterns in a population. ___________ is the study of the distribution and determinants of healthrelated states or events in specified
populations, and the application of this study to the control of
health problems.” (Last JM, 1988)
Epidemiology
______________ inferentially acknowledges the epidemiologic character of population phenomena, for as its etymology indicates, (epi, upon;demos, people; logos, study), epidemiology is the study of what “comes upon” groups of people. More specifically epidemiology is concerned with the distribution of disease and death, and with their determinants and consequences in population groups.
Mayer
a) a quantitative basic science built on a working knowledge of probability, statistics, and sound research methods;
b) a method of causal reasoning based on developing and testing hypotheses pertaining to occurrence and prevention of morbidity and mortality; and
c) a tool for public health action to promote and protect the public’s health based on science, causal reasoning, and a dose of practical common
sense.
Epidemiology
Investigation and assessment of health and productivity related events in animal population.
Veterinary Epidemiology, Epizootology
Uses of Epidemiology
- Identify and measure the importance of health
problems, describe the high risk groups, and explain the cause of these problems. - Understand the natural history of disease
- Essential for disease surveillance and control
- Contributes to the planning, monitoring and evaluation of health services.
- Key instrument in the formulation of health/animal health policies.
Epidemiology assists
- “to identify the etiology or cause of a disease and the risk factors.”
- “to study the natural history and prognosis of disease”.
- “to evaluate new models of health care delivery.”
- “to provide a scientific foundation for regulatory decisions relating to health or environmental problems.
- “to provide a clue to changes taking place over time…”
- “to identify subgroups in the population who are at high risk for disease.”
- to determine the best or most appropriate types of primary and secondary prevention.’
– primary prevention prevents disease in healthy individual (vaccination).
– Secondary prevention limits disease by early detention usually through screening programs.
Goals of an epidemiologist
- Identify factors that cause disease or disease
transmission. - Prevent the spread of communicable and
non-communicable diseases and conditions. - He/she identifies and prevents disease in given population, while a clinician identifies and treats disease in an individual.
disease has not yet developed, but there are factors that favor its occurrence.
e.g., poor eating habits and fatigue resulting
from lack of sleep are risk factors that favor common cold and could occur if exposure to an
agent occurs at this time.
Susceptibility
Initial responses of the cell or immune system. If these adaptation responses are successful, then no disease occurs and the process is arrested.
Adaptation
No symptoms indicating the presence of illness.
Adaptation, however, has been unsuccessful and
pathogenic changes have begun. Changes may be
detectable by sophisticated laboratory tests, are called ______________ because they are below the level of the clinical horizon, an imaginary line dividing the point where there are detectable signs and symptoms from that where there are not.
Early pathogenesis; subclinical
- Disease is detectable through symptoms or signs during a physical examination.
- Sufficient anatomical or functional changes have occurred.
- Range from early clinical disease to advanced disease, and death.
- Possible outcomes: complete recovery, residual defect that produces some degree of disability or death.
Clinical disease
- Follows exposure of the host to an agent
- Occurs during the stage of susceptibility.
- Organism multiples to sufficient numbers to produce a host reaction and clinical symptoms.
- Relatively short, usually hours to months.
Incubation period
The end of the incubation period is _______________, whether by screening or by appearance of clinical signs and symptoms.
the point of disease detection
The natural history of a disease provides the basis for planning intervention to halt or reverse the process of pathological change as early as possible, thus preventing further damage. Goal of intervention at each level is to prevent the pathogenic process from evolving further.
Levels of Prevention
Intervention before pathological changes have begun, during the stage of susceptibility. Measures to (1) prevent spread of the infectious agent from those environments that harbor it to individuals who are susceptible and who may be exposed, and (2) increase host resistance.
Primary prevention
Activities that modify or eliminate the environment in which the infectious agent lives and that interfere with the means of transmission to the
human host. Health education on good nutrition, hygiene, environmental sanitation
Primary prevention: General health promotion
Increase host resistance, protection against specific agents.
e.g., Immunization, removal of harmful agents from the environment, sewage treatment, pasteurization of milk, or chlorination of water.
Specific protection
Seeks to detect disease early, treat promptly, and
cure disease at its earliest stage. Stage of early pathogenesis or on the very early stage of clinical disease.
Secondary prevention
aim to control infectious diseases
- quickly identify new cases
- follow up existing cases to prevent exposure of
susceptibles
- Institute specific treatments to limit the period of
communicability and progression of pathology.
Surveillance programs
Includes limitation of disability and rehabilitation of the individuals where residual damage already exists. Plays a smaller role in infectious disease programs than in non-infectious programs because infectious disease less often results in permanent disability.
Tertiary prevention
is multi-factorial in nature and that many factors and events contribute to ill health.
Disease
a model of disease causation
Epidemiologic triangle or ecologic triad
Epidemiologic Triad
Host, Agent, Environment
Agents of Disease
a. Biologic (microorganisms)
b. Chemical (toxins, poisons)
c. Nutritional (excess food, lack of food, vitamin deficiency
d. Physical forces (automobiles)
e. Energy ( ionizing radiation)
Host Factors
a. Biologic traits – genetic characteristics age, race, sex
b. Social traits – marital status, lifestyle, diet, habits
Environmental Factors
a. Physical factors – climate, setting (urban, rural), pollution
b. Biologic factors – necessary to maintain the agent or allow for its transmission.
c. Social factors – political, social, economic
Infectious Disease Process
- Etiologic Agents
- Reservoirs
- Portal of Entry
- Portal of Exit
- Mode of Transmission
- Susceptible host
Etiologic Agents
protozoa, metazoa, fungi, bacteria, viruses
Reservoirs
2.1. Humans
a. Clinical cases
b. Carriers
2.2. Animals
2.3. Environment
Mode of Transmission
a. Direct
a. Person-to-person; animal-to-animal spread
b. Droplet spread
b. Indirect
a. Vector spread (direct, mechanical)
b. Vehicle spread
c. Airborne spread
Susceptible host
a. General factors of resistance (intact skin, cough reflex, gastric juices, normal bacterial flora)
b. Specific factors of resistance (leukocytes, serum factors, immune system (cell-mediated immunity; circulating antibodies – active and passive immunity)
Characteristics of agents affect their ability to produce disease in the host
Infectivity
Pathogenicity
Pathogenetic mechanisms
Virulence
Immunogenicity
Ability of an agent to invade and multiply (produce infection) in a host. The minimum number of agents required to establish infection in 50 percent of a group of hosts. Number varies with the agent, route of administration of the agent, source of the agent, and with many host factors such as age and race.
Infectivity
• Ability to produce clinically apparent illness
• Proportion of infections resulting in clinical disease.
• Affected by host and environmental factors,
dose, route of entrance of the infection, and source of the infection.
Pathogenicity
Pathogenetic mechanisms
- direct tissue invasion
- production of a toxin
- immunologic enhancement
- Persistent or latent infection
- Enhancement of host susceptibility to drugs of otherwise minimal toxicity.
- Immune suppression
When some stress, hormonal, or environmental factor alters the host-cell regulatory mechanisms, then multiplication occurs and may result in clinical disease.
Persistent or latent infection
proportion of clinical cases resulting in severe clinical manifestations, sequelae, death.
Virulence
• Infection’s ability to produce specific immunity.
• humoral immunity, cellular immunity, or a mixture of both.
• Virus in GIT may produce only local and not systemic, immune responses.
Immunogenicity
Individual animal in whom an agent produces disease.
The host
Immunity
(humoral, cellular; active, passive )
Inherent resistance
anatomic, physiological; genetics, acquired, rest, nutrition
• Living organisms or inanimate matter (such as soil) in which an infectious agent normally resides and multiplies.
• Consist of human beings, animals, and environmental sources.
• An essential component of the cycle by which an infectious agent maintains and perpetuates itself.
• Intimately related to the life-cycle of that agent in nature.
Reservoirs
Most viral and bacterial respiratory diseases,
most staphylococcal and streptococcal infections, diphtheria, venereal diseases, childhood exanthems, mumps, amebiasis, etc
Humans as Reservoirs
Leptospirosis (from rodents), rabies (from dogs, bats, foxes, other wild animals). Human is not an essential part (usual reservoir) of the life-cycle of the agent.
Animals as reservoirs
infections transmissible under natural conditions from vertebrate animals to man.
Zoonosis
Serve as intermediate hosts of different developmental stages of the agent.
E.g., tapeworm infestations, schistosomiasis,
malaria, and vector-borne infections.
Animals as reservoirs
Three levels of infection
- Colonization
- Inapparent infection
- Infectious disease
Agent may be present on the surface of the body and propagate at a rate sufficient to maintain its numbers without producing identifiable evidence of any reaction in the host. e.g., Staphylococcus aureus on the nasal mucosa.
Colonization
Organisms not only multiply in the host, but also cause a measurable reaction that, however, is not clinically detectable.
Inapparent infection
Clinical (overt) disease with symptoms, physical findings, or both
Infectious disease
An infected person who does not have apparent clinical disease, (colonization or inapparent infection) but is a potential source of infection to others.
Carriers
Types of carrier
- Inapparent throughout: Polio virus, meningococcus, hepatitis viruses
- Incubatory carrier: Viruses of chickenpox, measles, hepatitis
- Convalescent carrier: C. diphtheriae, hepatitis B, Salmonella species
- Chronic carrier: S. typhosa, hepatitis B virus
Means of transmission
Direct
a. Person-to-person/ Animal-to-animal spread
b. Droplet spread
Indirect
a. Vector spread (direct, mechanical)
b. Vehicle spread
c. Airborne spread
Immediate transfer of an infectious agent from an infected host or reservoir to an appropriate portal of entry.
Direct transmission
occurs over short distances
Droplet spread
arthropod carries agent mechanically in its feet or probroscis, multiplication of agent in the
vector does not occur.
Mechanical vector
agent multiplies in the arthropod before it is transmitted. Incubation period in the arthropod before the arthropod can become infective.
Biological vector
Indirect transmission
• Vector – spread or vector – borne
• Vehicle – spread, or vehicle - borne
• Air-borne
avenue by which the agent enters the host
Portal of entry
avenue by which the agent exits from the host
Portal of exit
• Occurrence in a community or region of cases of an illness clearly in excess of normal expectancy.
• Simply, it is an increase in the number of cases over past experience for a given population, time and place. This time period may be either short or long so that both acute and chronic illness, irrespective of the incubation or induction period, may occur in epidemics or in epidemic cycles.
• E.g. Food poisoning, dengue fever, influenza, AIDS
Epidemic
Type of epidemic
- Common source outbreaks
- Propagative or progressive epidemics
- Mixed epidemics
Exposure of a susceptible population or group to a common source of the pathogen. When a single common source is involved and all persons are exposed at the same time, then an explosive outbreak results.
Common source outbreaks
involve the transfer of the pathogenic agent from one host to another. This usually entails multiplication and excretion of an infectious agent in the host and sometimes includes intermediate animal/human multiplication cycles in the spread of the organism.
Propagative or progressive epidemics
involve both a single, common exposure to an
infectious agent and secondary, propagative spread to other individuals, usually by person-to-person transmission. E.g.,many food-borne pathogens (Salmonella, typhoid, hepatitis A) and airborne organisms (Mycobacterium tuberculosis).
Mixed epidemics
Investigation of an epidemic
- Verify the diagnosis of disease investigated.
- Establish the existence of an epidemic
- Characterize the distribution of cases
- Develop a hypothesis
- Test the hypothesis
- Formulate a conclusion
- Institute control measures
Resistance of a group to invasion and spread of an infectious agent, based on the immunity of a high proportion of susceptible and immune individuals members of the group. Epidemics or outbreaks of disease occur when the proportion of susceptible individuals is high and disappear as the proportion of immune individuals increases.
Herd immunity
The proportion of immune individuals within a group that is necessary to prevent an outbreak of disease varies according to the disease and its mode of transmission. In general, diseases spread by the airborne route require a higher proportion of immune individuals to prevent an outbreak than diseases spread by direct contact.
Herd immunity
Process of determining the frequency with which certain diseases occur in the community by collection, consolidation, analysis, and
dissemination of relevant data.
Objectives: Know what diseases are occurring so effective control can be made. Evaluate the effectiveness of the control programs. Increase our knowledge of disease processes.
Disease surveillance
• Can be active (occurs when the animal health dept. calls health care providers to see if they have seen cases of particular disease), or passive (animal health dept. simply waits for health care providers to report cases).
• Sources: individual case reports, laboratory
reports, emergency room visit records, hospital discharge summaries, case investigations, death certificates, surveys.
Disease surveillance
Requires the total annihilation of the agent so that the epidemiologic triangle will never occur again, Smallpox was eradicated because of unique epidemiologic features of this disease: virus lived only in humans and always caused clinical disease, a susceptible host could be made immune to this disease by immunization, surveillance identified all known cases, and mass immunization campaigns around these cases eliminated the susceptible hosts so that the virus had no reservoir in which to reside.
Disease Eradication
These are factors that influence the occurrence of health-related events in animal
populations. Determinants can be biological (e.g., pathogens), environmental (e.g., climate), or related to management practices (e.g., housing conditions)
Determinants
These include any health-related events or states, such as the incidence of disease, changes in productivity, or mortality rates
Outcomes
Continuous monitoring of animal populations to detect the early emergence of diseases, allowing for timely intervention
Disease Surveillance
The number of new cases of a disease in a population over a specific period.
Incidence
The total number of cases of a disease in a population at a given time
Prevalence
A disease that is regularly found among particular populations or in a certain area
Endemic Disease
A sudden increase in the number of cases of a disease above what is normally expected.
Epidemic
An epidemic that has spread over several countries or continents, usually affecting a
large number of people
Pandemic
