Chapter 14

EPIDEMIOLOGY

PATHOLOGY

• Study of disease
• Etiology

• Cause of disease
• Which organism(s) caused it?

• Pathogenesis

• Development of disease
• How did it start and how did it progress?

• Final Effects

• What damage was caused and why?

Infection vs. Disease

• Infection

• Invasion by microbes
• Can occur without causing disease

• HIV infection; AIDS disease

• Pathogens or opportunistic pathogens

• Opportunists will be discussed later

• Disease

• Abnormal state

    - Changes in health; damage to host

Normal Flora

• Born sterile
• Normal flora develops the first few weeks and gains permanent residence
• 1x10¹⁴ versus 1x10¹³ body cells
• Some are pathogenic
• Normal versus transient flora

Relationships Between Humans and Microbes

• Symbiotic

• Relationship between normal microbiota and the host

 

• Commensalism

• One benefits
• Corynebacteria in eyes

 

• Mutualism

• Both benefit
• E. coli in the gut

 

• Parasitism

• One benefits and other harmed
• Any pathogen

Relationships Between Humans and Microbes – Fig 14.2

• Opportunistic Pathogens

• Some normal microbiota are opportunistic pathogens
• 3 circumstances for gaining control/disease

 

• Normal flora in wrong environment

• E. coli in urinary tract or wounds

 

• Compromised immune system

• Pneumocystis carinii and AIDS

 

• Disruption of homeostasis

• Secondary infection after the flu

Normal Flora

• Microbial antagonism

• Normal flora prevents growth of many pathogens (transient pathogens)
• Competition for nutrients
• Bacteriocins
• pH
• Oxygen

• Examples

• Vaginal flora and yeast

• Douche and antibiotic effects

 

• GI flora and Clostridium difficile

• Antibiotics

 

Probiotics

ETIOLOGY – Fig 14.3

• Cause of disease
• Koch’s postulates are used to prove the cause of an infectious disease

 

 

 

ETIOLOGY

• Summary of Koch’s postulates

• Suspected pathogen present in every case of disease
• Suspected pathogen isolated in pure culture from diseased organisms
• Pure cultures of suspected pathogen must cause disease in healthy organisms when inoculated
• Suspected pathogen must be re-isolated from inoculated organisms that contracted disease

• Exceptions

• Obligate intracellular parasites (Treponema pallidum)
• Same disease/symptoms and different organisms (pneumonia)
• Same organism and different diseases (Streptococcus pyogenes)
• Ethical issues of postulate #3

Classifying Infectious Diseases

• Symptoms

• Ex. pain

• Signs

• Ex. fever

• Syndrome

• Groups of symptoms and signs characterizing disease

Classifying Infectious Diseases

• Communicable disease

• A disease that is spread from one host to another
• Chicken pox

• Contagious disease

• A disease that is easily spread from one host to another

• Noncommunicable disease

• A disease that is not transmitted from one host to another

• Tetanus

Occurrence of Diseases

• Prevalence

• Fraction of a population having a specific disease at a given time

• Range
• The prevalence of AIDS in 1999 was 700,000

• Incidence

• Fraction of a population that contracts a disease during a specific time

• Spread
• The incidence of AIDS in 1999 was 45,000

Classifying Diseases

• Sporadic — occurs occasionally

• Plague

• Endemic — constantly present in a population

• Cold

• Epidemic — many people in a given area acquire in a short period

• Flu season

• Pandemic — epidemic that occurs worldwide

• AIDS

Severity or Duration of a Disease

• Acute — develops rapidly

• Flu

• Chronic—develops slowly and reactions less severe over long periods

• Hepatitis

• Latent—causative agent inactive and become active later

• Shingles, herpes, HIV

 

IMMUNITY

• Lowers rate of spread
• Lowers incidence
• Vaccination to increase immunity

• Small pox and zero incidence

• Herd immunity

• Immunity in most of a population
• 70% immunity=population immunity

Host Involvement

• Local Infection — Invaders in small area

• Boils and Staphylococcus aureus

• Systemic infection—spread by lymph or blood

• Measles

• Focal—When local enters lymph and travels to other specific area

• Bacterial endocarditis

 

• Bacteremia

 

• Septicemia

 

• Toxemia

 

• Viremia

 

• Primary versus secondary infections

• Acute versus opportunistic

 

• Subclinical infection is not apparent

• Hepatitis A

Patterns of Infection

• Reservoir à transmission à Invasion à pathogenesis
• Transmission

• Resistance of host

• Immune system, vaccine, previous exposure
• Protection from another factor (sickle cell and malaria)

 

• Predisposing Factors

• Increases susceptibility

Patterns of Infection – Fig 14.5

• Invasion and Pathogenesis

• Pathogen overcomes host’s defense

 

• Incubation = time between infection and signs

 

• Prodromal = early mild symptoms

 

• Period of illness = most overt signs and symptoms

• Most contagious
• Immune system most active

 

• Period of Decline = decrease in signs and symptoms

• Secondary infection possible

 

• Period of Convalescence = recovery and repair

 

• Spread can occur at any period

Spread of Infection

• Reservoirs = sources of infection

• Human

• Diseased or carrier

 

• Zoonoses: animal to human

• Direct contact or indirect (contact with feces)

 

• Rabies virus or Lyme disease

 

• Nonliving reservoirs (fomites)

• Soil and water

• Clostridium tetani, Clostridium botulinum, ringworm

Transmission of Disease – Fig 14.6

• Direct versus indirect

 

• Droplet = water droplets in air

• Sneezing, coughing

 

• Vehicle: water, food, air (spores)

• Infects many

 

• Vector

• Arthropods that carry from one host to another
• West Nile from mosquito
• Mechanical versus biological

Nosocomial Infections – Fig 14.10

• Hospital acquired—very common! (5-15%)
• Reasons

• Patients are reservoirs!

• Higher concentration of diseased individuals

 

• Weakened resistance

• Compromised patients
• Babies underdeveloped immune systems
• Immunosupression for transplant patients

 

• Ventilation system

 

• Breaching the skin barrier

• Urinary catheters!
• Burn patients, surgery, radiation therapy

 

• Resistant bacteria

Compromised Host

• AIDS patients

• Due to loss of T cells

• Therapy

• Radiation, steroid, immunosuppressed

• Burn patients

• Loss of skin barrier

• Infants

• Underdeveloped immune system

• Other diseases

• Diabetes, leukemia, etc.

• Specialized care units

• Burn wards
• Oncology units
• Hemodialysis
• Intensive care

Chain of Transmission

• Direct contact

• Hospital staff to patient
• Patient to patient

• Indirect contact

• Fomites

• Urinary catheter
• Needles
• Scalpel

• HVAC system

Emerging Infectious Disease

• Sudden prevalence

 

 

 

• Not always a new organism (reemergence)

 

• Reasons

• Genetic recombination
• Germ warfare
• Evolution of new strains
• Ecological change
• Inappropriate use of antibiotics and pesticides
• Changes in weather patterns
• Modern transportation
• Failures in public health

EPIDEMIOLOGY – Fig 14.11a

• The study of when and where diseases occur and how they are transmitted
• How the disease affects different types of people
• Methods of controlling disease

 

• 3 types of investigation

• Descriptive = describes the occurrence of the disease

 

• Analytical = to determine probable cause

 

• Experimental: hypothesis testing

 

 

Case reporting	Health care workers report specified disease to local, state, and national offices
Nationally notifiable diseases	Physicians are required to report occurrence

 

Epidemiological Terms

• Morbidity

• Number of affected by disease in a population

 

• Mortality

• Number that dies from disease in population

 

• Morbidity rate

        - Number of people infected in relation to the total population in a given time period

• Mortality rate

         - Number of deaths from a disease in relation to the population in a given time

• Centers for Disease Control (CDC)

• MMWR