Chapter 33-Epidemiology

Principles of Epidemiology  
Epidemiology
Epidemiology is the study of disease in populations.
Includes the study of the interactions of the pathogen with the host population.
Pathogens can be selective forces in the evolution of the host and visa versa.

Etiology
Study of the cause of disease

Germ theory of disease- disease caused by infections of pathogenic microorganisms

Robert Koch developed a set of postulates one must satisfy to prove a particular pathogen causes a particular disease

Infection
Chronic (long-term) infection
Lives in balance with the host
Minimum harm
Acute infections
Characterized by rapid onset and, often, rapid recovery.
Mortality: death
Morbidity: Infection/Disease

Symptoms
Symptoms: subjective characteristic of a disease. A symptom perceived by the patient but not by the examiner
Signs: objective characteristics of disease (verifiable).
Syndrome: group of symptoms and signs that collectively characterizes a particular disease
Asymtomatic or subclinal: No observable symptoms.
Pathogenicity
Pathogenicity: the ability of a microorganism to cause disease

Virulence: the degree of pathogenicity

Virulence Factors: traits that enable pathogens to cause infections.
The Vocabulary of Epidemiology
Prevalence: the proportion of diseased individuals in a population in a given time period.
Incidence: the number of cases of an individual disease in a population in a given time period.

Definitions
Endemic disease: constantly present at low incidence in a specific population.
Epidemic: an unusually high incidence of disease which occurs in a specific population.
Pandemic: A worldwide epidemic.

Reservoirs of Infection
Most pathogens cannot survive long outside of their host
Reservoirs of infection
pathogens that are maintained as a source of infection are termed reservoirs. 3 types of reservoirs:
Animal reservoir
Human carriers
Nonliving reservoir

Opportunistic Pathogens
Normal microbiota that cause disease
Conditions that provide opportunities for pathogens
Immune suppression
Changes in the normal microbiota- changes in relative abundance of normal microbiota may allow opportunity for a member to thrive and cause disease
Introduction of normal microbiota into unusual site in the body

Animal Reservoirs
Zoonoses- diseases that are naturally spread from their usual animal host to humans
Acquire zoonoses through various routes
Direct contact with animal or its waste
Eating animals
Bloodsucking arthropods
Humans are usually dead end host to zoonotic pathogens

Vectors
Biological vectors: hosts for the multiplication of the pathogen during some stage of the pathogen’s life. Plague: Rodents: Fleas
Mechanical: passively carry pathogens to new hosts (fomites)
Some pathogens exist in soil, water, or animals.
Others exist only in humans and are maintained solely by person-to-person contact.

Severe Acute Respiratory Syndrome
Zoonosis: an infection that occurs primarily in animals and is transferred to humans.
SARS-CoV: zoonotic virus that causes severe acute respiratory syndrome (SARS).
Person-to-person spread is by respiratory means.

Infectious Disease Transmission
Transmission: A pathogen spread directly from one host to another, or indirectly by living vectors or
Fomites: inanimate objects.
Vehicles: food and water.

AIDS
AIDS is one of the most studied disease pandemics.
AIDS will continue to be a major public health problem, especially in developing countries.
There is no effective cure or immunization to prevent AIDS

Nosocomial Infections
Infections acquired while in a health care facility
Types of nosocomial infections
Exogenous- pathogen acquired from the health care environment
Endogenous- pathogen arise from normal microbiota due to factors within the health care setting
Iatrogenic- results from modern medical procedures

Control of Nosocomial Infections
Involves precautions designed to reduce the factors that result in disease

Universal (Standard) Precautions instituted by the CDC to limit exposure to pathogens

Hand washing is the most effective way to reduce nosocomial infections

Control of Disease
Public health measures that play a major role in reduction of disease incidence:

Food and water purity regulations, vector control
immunization
quarantine
disease surveillance
pathogen eradication

Virulence-enhancing Mechanisms
Virulence-enhancing mechanisms:
include the mobile genetic elements-

bacteriophages
plasmids
transposons.

Biological Warfare and Biological Weapons
Biological warfare: the use of biological agents to incapacitate or kill a military or civilian population in an act of war or terrorism.
Mode of inoculation:common sources such as food and water are the most likely
Anthrax as a Biological Weapon
Bacillus anthracis has emerged as an important pathogen because of its use as a bioweapon.
Endospores are form of transmission
> 90% mortality rate in untreated individuals
Effective treatment relies on timely observation and diagnosis of symptoms