Epidemiology-the science that evaluates the occurrence, determinants, distribution, and control of health and disease in a defined human population
Health-organism and all its parts functioning normally; a state of physical and mental well-being, not merely the absence of disease
Disease-an impairment of the normal state of an organism or any of its components, which hinders the performance of vital functions
Epidemiologist-one who practices epidemiology (a disease detective)
Epidemiological Terminology
Epidemiologists use a variety of terms to describe different types of disease occurrence
Sporadic disease-occurs occasionally at irregular intervals in a human population
Endemic disease-maintains a relatively steady low-level frequency at a moderately regular interval
Hyperendemic-a gradual increase in frequency above the endemic level, but not to the epidemic level
Epidemic-sudden increase in frequency above the endemic level
Index case-the first case in an epidemic
Outbreak-an epidemic-like increase in frequency, but in a very limited (focal) segment of the population
Pandemic-a long-term increase in frequency in a large (usually worldwide) population
Epizootology-study of the factors that influence disease in animal populations
Enzootic-moderate prevalence in animals
Epizootic-sudden outbreak of disease in animals
Zoonoses-diseases of animals that can be transmitted to humans
Measuring Frequency: The Tools of Epidemiologists
Measures of frequency are usually espressed as fractions; the numerator equals the number of individuals experiencing the event; the denominator is the number of individuals in whom the event could have occurred
Statistics-the mathematics of collection, organization, and interpretation of numerical data
Morbidity rate-the number of new cases in a specific time period per unit of population
Prevalence rate-number of individuals infected at any one time per unit of population
Mortality rate-number of deaths from a disease per number of cases of the disease
Infectious Disease Epidemiology
Infectious disease is a disease resulting from infection by microbial agents
Epidemiologists study the natural history of an infectious disease
What organism caused the disease?
What is the source and/or reservoir of the disease?
How is the disease transmitted?
What host and environmental factors facilitated development of the disease within a defined population?
How can the disease best be controlled or eliminated?
Recognition of an Infectious Disease in a Population
Recognition involves various surveillance methods to monitor the population for disease occurrence and for demographic analysis
Surveillance involves identification of the signs and/or symptoms of a disease
Signs-objective changes in the body (e.g. fever, rash, etc.) that can be directly observed
Symptoms-subjective changes (e.g. pain, appetite loss, etc.) experienced by the patient
Disease syndrome-a set of signs and symptoms that is characteristic of a disease
Characteristic patterns of an infectious disease
Incubation period-the period after pathogen entry but before signs and symptoms appear
Prodromal stage-onset of signs and symptoms but not yet clear enough for diagnosis
Period of illness-disease is most severe and has characteristic signs and symptoms
Period of decline (convalescence)-signs and symptoms begin to disappear
Remote sensing and geographic information systems: charting infectious disease-map-based tools that can be used to study the distribution, dynamics, and environmental correlates of microbial disease
Remote sensing-gathering digital images of Earth's surfaces from satellites and transforming data to maps
Geographic information system-data management system
This approach works best with diseases clearly associated with mapped environmental variables (e.g., vegetation types, elevation, precipitation)
Correlation with a single causative organism-uses methods described in chapter 34 for the organism's isolation and identification
Recognition of an Epidemic
Common-source epidemic-characterized by a sharp rise to a peak and then a rapid but not as pronounced decline in the number of cases; usually results from exposure of all infected individuals to a single common, contaminated source, such as food or water
Propagated epidemic-characterized by a gradual increase and then a gradual decline in the number of cases; usually results from the introduction of one infected individual into a population, who then infects others; these in turn infect more, until an unusually large number of individuals within the population are infected
Herd immunity-the resistance of a population to infection and to the spread of an infectious organism due to the immunity of a large percentage of the population; this limits the effective contact between infective and susceptible individuals
Antigenic shift-genetically determined changes in the antigenic character of a pathogen so that it is no longer recognized by the host's immune system (e.g., new flu strains); smaller changes are called antigenic drift; can lead to increases in disease frequency because the population of susceptible hosts increases
The Infectious Disease Cycle: Story of a Disease
What pathogen caused the disease? Epidemiologists must determine the etiology (cause) of a disease
Koch's postulates (or modifications of them) are used if possible; the clinical microbiology laboratory plays an important role in the isolation and identification of the pathogen
Communicable disease-one that can be transmitted from one host to another
What was the source and/or reservoir of the pathogen?
Source-location from which organisms are immediately transmitted to the host
Period of infectivity-the time during which the source is infectious or is disseminating the organism
Reservoir-site or natural environmental location where organism is normally found
Carrier-an infected individual who is a potential source of a pathogen
Active carrier-a carrier with an overt clinical case of the disease
Convalescent carrier-an individual who has recovered from the disease but continues to harbor large numbers of the pathogen
Healthy carrier-an individual who harbors the pathogen but is not ill
Incubatory carrier-an individual who harbors the pathogen but is not yet ill
Casual (acute, transient) carriers-any of the above carriers who harbor the pathogen for a brief period (hours, days, or weeks)
Chronic carriers-any of the above carriers who harbor the pathogen for long periods (months, years, or life)
Zoonoses-infectious diseases that occur in animals and can be transported to humans; the animals serve as a reservoir for the disease; transmission from animal to host can be direct or indirect
How was the pathogen transmitted?
Airborne transmission-suspended in air; travels a meter or more
Droplet nuclei-may come from sneezing, coughing, or vocalization
Dust particles-may be important in airborne transmission because microorganisms adhere readily to dust
Contact transmission-touching between source and host
Direct (person to person)-physical interaction between infected person and host
Indirect-involves an intermediate
Droplets-large particles that travel less than one meter through the air
Vehicle transmission-inanimate materials or objects are involved in transmission
Common vehicle transmission-a single vehicle serves to spread the pathogen to multiple hosts, but does not support its reproduction
Fomites-common vehicles such as surgical instruments, bedding, eating utensils
Food and water are also common vehicles
Vector-borne transmission-living transmitters, such as arthropods or vertebrates
External (mechanical) transmission-passive carriage of the pathogen on the body of the vector with no growth of the pathogen during transmission
Internal transmission-carried within the vector
) Harborage-pathogen does not undergo morphological or physiological changes within the vector
) Biologic-pathogen undergoes morphological or physiological changes within the vector
Why was the host susceptible to the pathogen?-depends on defense mechanisms of the host and the pathogenicity of the pathogen
How did the pathogen leave the host?
Active escape-movement of pathogen to portal of exit (e.g., many helminthes)
Passive escape-excretion in feces, urine, droplets, saliva, or desquamated cells
Virulence and the Mode of Transmission
There is evidence of a correlation between mode of transmission and degree of virulence
A pathogen that is spread by direct contact (e.g., rhinoviruses) cannot afford to make the host so ill it cannot be spread effectively
If mode of transmission does not depend on health and mobility of host, then greater degree of virulence is possible
A pathogen that is vector-borne needs to be able to replicate extensively within host; therefore, host is likely to remain healthy long enough for transmission to occur
Evidence also suggests that the pathogen's ability to survive outside the host is correlated with virulence
Pathogens that do not survive well outside the host and that do not use a vector are likely to be less virulent
Pathogens that can survive for long periods of time outside the host tend to be more virulent
Human cultural patterns and behavior are also correlated with virulence
Emerging and Reemerging Infectious Diseases and Pathogens
New diseases have emerged in the past few decades (e.g., AIDS, hantavirus pulmonary syndrome, and many "old" diseases that have increased in frequency (e.g., tuberculosis)
The Centers for Disease Control has defined such diseases as "new, reemerging, or drug-resistant infections whose incidence has increased in the last two decades or whose incidence threatens to increase in the near future"; these diseases are the focus of systematic epidemiology, which is concerned with the ecological and social factors that influence the development and emergence of disease
Reasons for increases in emerging and reemerging infectious diseases
Economic and military forces can cause population shifts and disruptions of normal public health measures
Changes in sexual behavior, use of IV drugs, and changes in food preferences affect disease transmission
Globalization of food processing and supply centers has created potential for widespread outbreaks
Increasing population density
Overcrowding increases possibility of exposure
Sanitary measures and health care systems can become overburdened
Encroachment and destruction of natural habitats exposes humans to new pathogens
Introduction of pathogens into new environments can alter transmission and exposure patterns
Drug resistance has increased dramatically in nosocomial pathogens due to excessive or inappropriate use of antimicrobial therapy
Increasing numbers of immunosuppressed individuals (e.g., AIDS patients, transplant recipients) provide a habitat in which opportunistic pathogens can evolve to them become frank pathogens
Rapid transportation systems aid in the spread of disease out of areas where they are endemic
Control of Epidemics
Reduce or eliminate the source or reservoir of infection through:
Quarantine and isolation of cases and carriers
Destruction of an animal reservoir, if one exists
Treatment of sewage to reduce contamination of water
Therapy that reduces or eliminates infectivity of individuals
Break the connection between the source and susceptible individuals through sanitization, disinfection, vector control, and other measures; examples include:
Chlorination of water supplies
Pasteurization of milk
Supervision and inspection of food and food handlers
Destruction of insect vectors with pesticides
Reduce the number of susceptible individuals-increase herd immunity
Passive immunization to give temporary immunity following exposure
Active immunization to protect individuals and host population
Role of the public health system: epidemiological guardian-a network of health professionals involved in surveillance, diagnosis, and control of epidemics (e.g., The Centers for Disease Control and the World Health Organization)
Emerging Threat of Bioterrorism
Bioterrorism is the intentional or threatened use of microorgansisms or toxins from living organisms to produce death or disease in humans, animals, and plants
Biological weapons are more destructive than chemical weapons; the United States government recently launched an initiative to create a biological weapons defense
The list of biological agents that could pose the greatest public health risk is short, and includes viruses, bacteria, parasites, and toxins
Vaccines are not a solution to bioterrorism because of the difficulty involved in vaccinating large populations and because of the broad spectrum of potential agents
In the US, the Federal Drug Administration is charged with fostering development of vaccines, drugs and diagnostic products, and other measures needed to respond to bioterrorism threats
Global Travel and Health Considerations
In developed countries, effective public health systems are in place; because of this, travel in developed countries does not pose a health risk
In underdeveloped countries, such public health systems do not exist, and people traveling in those countries are at greater risk
Travelers can take several kinds of precautions in underdeveloped countries; of these, vaccinations are very important
Space travel-zero-gravity environments impact human physiology, including immune responses; furthermore, these environments alter the pathogen (e.g., increasing resistance to antibiotics); safety of space travelers is insured by selection of healthy crew members and preflight quarantine
Nosocomial Infections
Produced by infectious agents that develop within a hospital or other clinical care facility and that are acquired by patients while they are in the facility; infections that are incubating within the patient at the time of admission are not considered nosocomial
Source
Endogenous-patient's own microbiota
Exogenous-microbiota other than the patient's
Autogenous-cannot be determined to be endogenous or exogenous
Control, prevention, and surveillance should include proper handling of the patient and the materials provided to the patient, as well as monitoring of the patient for signs of infection
The hospital epidemiologist (other terms are also used) is an individual (usually a registered nurse) responsible for developing and implementing policies to monitor and control infections and communicable disease; usually reports to an infection control committee or other similar group
