202 General Microbiology II

Epidemiology and Public Health Microbiology

Definitions

• Epidemiology - study of the occurrence of disease in a human population, especially the cause (etiology) and transmission of disease
  
• Epidemic - literally, "upon the people" (epi = upon; demos = people); commonly refers to an unusually high incidence of a disease in a community (population) at one time
  
• Prevalence - the proportion (or percentage) of diseased individuals in a population at one time
  
• Incidence - the number of diseased individuals in a population at one time

  Did ya know...?

  In 1995, the Top 10 Notifiable Infectious Diseases in the US were:

  • Chlamydia
  • Gonorrhea
  • AIDS
  • Salmonellosis
  • Hepatitis A
  • Shigellosis
  • Tuberculosis
  • Syphilis
  • Lyme Disease
  • Hepatitis B

  By 2004, the Top 10 Notifiable Infectious Diseases in the US had shifted to:

  • Chlamydia
  • Gonorrhea
  • Salmonellosis
  • AIDS
  • Pertussis
  • Varicella
  • Lyme Disease
  • Giardiasis
  • Shigellosis
  • Tuberculosis
  
  
• Pandemic - literally, "all the people" (pan = all; demos = people); indicates an epidemic involving more than one continent or a worldwide epidemic
  
• Endemic - literally, "in the people" (en = in; demos = people); a disease is constantly present, usually at low incidence, in a population
  
• Outbreak - appearance of several cases of a disease, usually in a short period of time, in an area previously experiencing no cases or only sporadic (scattered or isolated) cases of the disease; "mini" epidemic
  
• Morbidity - disease; sickness; clinical illness
  
• Mortality - death
  
• Reservoir - a site in which infectious agents remain viable (alive) and from which infection of individuals may occur
  
• Carrier - an infected individual which is not showing obvious signs or symptoms of clinical disease, but which is shedding the etiologic agent for a long period of time (greater than six months)
  
• Zoonoses - diseases which occur primarily in animals but are occasionally transmitted to people (plague, Lyme disease, rocky mountain spotted fever)
  
• Vector - living agent which transmits infectious agent (ticks, fleas, flies mosquitoes)
  
• Fomite - non-living object that transmits infectious agents (pencil, doorknob, handkerchief); fomites such as food or water are called vehicles

Transmission of Infectious Diseases

• Stages in transmission
  • escape from old host
  • travel to new host
  • entry into new host
• Modes of transmission
  • direct - transmission via
    • close, but not intimate, contact (shaking hands, etc.)
    • intimate contact (sexual contact)
  • indirect - transmission via
    • vectors
    • vehicles or fomites

Types of Epidemics

• Common-source
  • infection or intoxication of many people from a single contaminated source
  • characterized by rapid onset, "sharp" peak and rapid decline in incidence
• Propagated
  • introduction of an infected person into a susceptible population leads to transfer of the etiologic agent to others, who transfer it to many others
  • characterized by slow onset, "blunted" peak and slow decline in incidence

Nosocomial (hospital-acquired) Infections

Hospitals are good places to acquire infections due to the presence of many patients with infectious diseases

• Iatrogenic infection - infections caused as a result of medical procedures (catheterization, injection, surgery, etc.) which have a good potential to introduce microbes into patient tissues
• Transmission of microbes between patients by hospital personnel
• Crowding leads to cross-infection of patients by direct and indirect means
• Immunocompromised people are more susceptible than normal ones due to:
  • disease - lower resistance due to damage caused by diseases such as AIDS
  • treatment - weakened resistance caused by drug therapy for cancer, etc.
  • immaturity - newborns are highly susceptible because their immune systems have not yet had a chance to mature and become fully functional
  • stress - decreases resistance and immune response development
• Antibiotic-resistant strains - routine use of antibiotics selects for these in hospitals (these strains are frequently resistant to multiple antibiotics)

Public Health Measures for Control of Epidemics

Involves strategies which break the "chain of transmission"

• Immunization of people
  • boosters frequently required
  • 100% immunization is not necessary (herd immunity)
• Blocking vehicle-mediated transmission
  • water purification
  • effective cooking
  • prevention of food contamination with infectious agents
• Quarantine
  • limiting freedom of movement of individuals carrying infectious agents
  • minimum time required for a quarantine period is equal to longest period of communicability (transmissibility) of the disease
• Elimination of animal reservoirs
  • immunization of animals that act as reservoirs of diseases such as bovine tuberculosis, brucellosis (highly effective when monitored properly)
  • eradication of animals that act as reservoirs of diseases such as tularemia, plague, Lyme disease (sometimes effective, difficult to monitor properly)