Chapter 7

EPIDEMIOLOGY

PATHOLOGY

n    Study of disease

n    Etiology

•  Cause of disease

•   Which organism(s) caused it?

n    Pathogenesis

•  Development of disease

•  How did it start and how did it progress?

n    Final Effects

•  What damage was caused and why?

Infection vs. Disease

n    Infection

•  Invasion by microbes

•  Can occur without causing disease

n  HIV infection; AIDS disease

•  Pathogens or opportunistic pathogens

n  Opportunists will be discussed later

n    Disease

•  Abnormal state

n  Changes in health; damage to host

Normal Flora (Microbiota)

n    Born sterile

n    Normal flora develops the first few weeks and gains permanent residence

n    1x10¹⁴ versus 1x10¹³ body cells

n    Some are pathogenic

n    Normal versus transient flora

Relationships Between Humans and Microbes

n     Symbiotic

•   Relationship between normal microbiota and the host

•   Commensalism

n   One benefits

n   Corynebacteria in eyes

•   Mutualism

n   Both benefit

n   E. coli in the gut

•   Parasitism

n   One benefits and other harmed

n   Any pathogen

Relationships Between Humans and Microbes

n          Opportunistic Pathogens

•         Some normal microbiota are opportunistic pathogens

•         3 circumstances for gaining control/disease

n        Normal flora in wrong environment

•      E. coli in urinary tract or wounds

n        Compromised immune system

•      Pneumocystis carinii and AIDS

n        Disruption of homeostasis

•      Secondary infection after the flu

Normal Flora

n     Microbial antagonism

•    Normal flora prevents growth of many pathogens (transient pathogens)

•    Competition for nutrients

•    Bacteriocins

•    pH

•    Oxygen

n     Examples

•    Vaginal flora and yeast

n   Douche and antibiotic effects

•    GI flora and Clostridium difficile

n   Antibiotics

n     Probiotics

ETIOLOGY

n     Cause of disease

n     Koch’s postulates are used to prove cause of an infectious disease

•    Some pathogens can cause several disease conditions

•    Some pathogens cause disease only in humans

ETIOLOGY

n         Summary of Koch’s postulates

n      Suspected pathogen present in every case of disease

n      Suspected pathogen isolated in pure culture from diseased organisms

n      Pure cultures of suspected pathogen must cause disease in healthy organisms when inoculated

n      Suspected pathogen must be re-isolated from inoculated organisms that contracted disease

n         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

n    Symptoms

•  Ex. pain

n    Signs

•  Ex. fever

n    Syndrome

•  A specific groups of signs and symptoms that accompany a disease

Classifying Infectious Diseases

n    Communicable disease

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

n  Chicken pox

n    Contagious disease

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

n    Noncommunicable disease

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

n  Tetanus

Occurrence of Disease

n     Prevalence

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

n   Range

n   The prevalence of AIDS in 2005 was 984,155

n     Incidence

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

n   Spread

n   The incidence of AIDS in 2006 was 56,300

•   Article

Occurrence of Diseases

n     Sporadic — occurs occasionally

•   Plague

n     Endemic — constantly present in a population

•   Cold

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

•   Flu season

n     Pandemic — epidemic that occurs worldwide

•   AIDS

Severity or Duration of a Disease

n    Acute — develops rapidly

•  Flu

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

•  Hepatitis

n    Latent—causative agent inactive and become active later

•  Shingles, herpes, HIV

IMMUNITY

n    Lowers rate of spread

n    Lowers incidence

n    Vaccination to increase immunity

•  Small pox and zero incidence

n    Herd immunity

•  Immunity in most of a population

•  70% immunity=population immunity

Host Involvement

n     Local Infection — Invaders in small area

•    Boils and Staphylococcus aureus

n     Systemic infection—spread by lymph or blood

•    Measles

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

•    Bacterial endocarditis

n    Sepsis

•    Toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection

n     Bacteremia

n     Septicemia

n     Toxemia

n     Viremia

n     Primary versus secondary infections

•    Acute versus opportunistic

n     Subclinical infection is not apparent

•    Hepatitis A

Patterns of Infection

n    Reservoir à transmission à Invasion

     à pathogenesis

n    Transmission

•  Resistance of host

n  Immune system, vaccine, previous exposure

n  Protection from another factor (sickle cell and malaria)

•  Predisposing Factors

n  Increases susceptibility

The Stages of a Disease

n     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

n   Most contagious

n   Immune system most active

•   Period of Decline = decrease in signs and symptoms

n   Secondary infection possible

•   Period of Convalescence = recovery and repair

•   Spread can occur at any period

The Stages of a Disease

Spread of Infection

n    Reservoirs = sources of infection

•  Human

n  Diseased or carrier

•  Zoonoses:  animal to human

n  Direct contact or indirect (contact with feces)

n  Rabies virus or Lyme disease

•  Nonliving reservoirs (fomites)

n  Soil and water

•  Clostridium tetani, Clostridium botulinum, ringworm

Transmission of Disease – Fig 14.6

n     Direct versus indirect

n     Droplet = water droplets in air

•   Sneezing, coughing

n     Vehicle: Water, food, air (spores)

•   Infects many

n     Vector

•   Arthropods that carry from one host to another

•   West Nile from mosquito

•   Mechanical versus biological

Nosocomial Infections

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

n     Reasons

•    Patients are reservoirs!

n   Higher concentration of diseased individuals

•    Weakened resistance

n   Compromised patients

n   Babies underdeveloped immune systems

n   Immunosupression for transplant patients

•    Ventilation system

•    Breaching the skin barrier

n   Urinary catheters!

n   Burn patients, surgery, radiation therapy

•    Resistant bacteria

Relative Frequency of Nosocomial Infections

Common Causes of Nosocomial Infections

Compromised Host

n     AIDS patients

•   Due to loss of T cells

n     Therapy

•   Radiation, steroid, immunosuppressed

n     Burn patients

•   Loss of skin barrier

n     Infants

•   Underdeveloped immune system

n     Other diseases

•   Diabetes, leukemia, etc.

n     Specialized care units

•   Burn wards

•   Oncology units

•   Hemodialysis

•   Intensive care

Chain of Transmission

n    Direct contact

•  Hospital staff to patient

•  Patient to patient

n    Indirect contact

•  Fomites

n  Urinary catheter

n  Needles

n  Scalpel

•  HVAC system

Emerging Infectious Disease

n     Sudden prevalence

•   Not always a new organism (reemergence)

n     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

n     The study of when and where diseases occur and how they are transmitted

n     How the disease affects different types of people

n     Methods of controlling disease

n     3 types of investigation

•   Descriptive = describes the occurrence of the disease

•   Analytical = to determine probable cause

•   Experimental:  hypothesis testing

Epidemiology

?  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

n     Morbidity

•   Number of affected by disease in a population

n     Mortality

•   Number that dies from disease in population

n     Morbidity rate

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

n     Mortality rate

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

n     Centers for Disease Control (CDC)

•   MMWR