Chap. 13 - Epidemiology


 Epidemiology - study of when & where diseases occur & how they are transmitted in human populations (focuses on groups of people rather than individuals); the modern definition does not limit this study to that of epidemic diseases; knowing the source of the disease can help prevent transmission even while the causative microorganism (etiologic agent) is still unknown.


Epidemics - a pattern of disease transmission that affects many members of a population within a short time (ex. cholera in South America, flu, etc.).


Pandemic - an epidemic that spreads world wide (ex. AIDS, flu).


Endemic - numbers stay too low to constitute a public health concern (ex. chicken pox).


Sporadic - diseases occurring only occasionally in a population (ex. tetanus, trichinosis).




A.  Sources of Information

1.    vital statistics - birth, death, marriage, & divorce records

2.    census data - number of people living in an area & their distribution by age, race, sex, marital status.

3.    disease reports - doctors are required to report certain diseases to public health dept.; local public health stats are forwarded to state agencies (ex. Texas Dept. of Health) and the Centers for Disease Control & Prevention (CDC); the CDC prepares the Morbidity & Mortality Weekly Report (MMWR)  for the U.S.; stats included in this report:

a.     morbidity rate – the number of individuals affected by a disease during a set period in relation to the total number in the population (expressed as number of cases per 100,000 people per year).

b.    mortality rate – the number of deaths due to a disease in a population during a specific period in relation to the total population (expressed as number of deaths per 100,000 people per year).

4.    other sources - surveys, questionnaires, interviews, hospital records.


B.  Use of Stats

1.    incidence rate – number of new cases within a set population during a specified period of time divided by the total number of people in the population; incidence rates measure the growth or spread of a disease; ex. this stat tells us how many people develop AIDS in the U.S. per year.

2.    prevalence rate - number of people who have a certain disease at any particular time (old and new cases) divided by the total number of people in the population; ex. this stat tells us how many people currently have AIDS in the U.S.




Among patients admitted to hospitals each year about 10% (2 million) acquire a nosocomial infection; about 20,000 of those infected die from their infection.


A.   Organisms Causing Nosocomial Infections

Escherichia. coli, Enterococcus, Staphylococcus aureus, and Pseudomonas are responsible for one half of all nosocomial infections.


B.  Factors Fostering Nosocomial Infections

1.    immunocompromised patients – people with AIDS, organ transplant recipients (they take immunosuppressants so that the organ will not be rejected by their body), the elderly, cancer patients, patients taking steroids (ex. those with asthma).


2.    invasive medical procedures - ex. blood drawing, i.v.'s, urinary catheters, endoscopes, implants, coronary bypass surgery, hemodialysis, gynecological equipment, tooth extractions, injections


3.    antibiotic resistance - many bacteria found in hospitals have developed antibiotic resistance.


C.  Types of Nosocomial Infection:  (From most common to least common)

1.    UTI's (urinary tract infections) - usually E. coli, Proteus, Klebsiella, Enterobacter; can be from catheterization; more commonly results from improper hygiene (wiping the wrong way).

2.    surgical wound infections - most commonly Staphylococcus aureus  & enterics; at least 10% of surgery patients develop an infection despite scrubbing, etc.!

3.    respiratory tract  (ex. pneumonia) - include Streptococcus, Staphylococcus, Pseudomonas aeruginosa, enterics.

4.    skin infections - particularly in newborns (usually Staphylococcus aureus ) & burn victims (usually

Pseudomonas aeruginosa ).


D.  Nosocomial Infection Control

1.    hospitals hire hospital epidemiologists.

2.    once an epidemic is recognized, take cultures from hospital workers.

4.    patient isolation; reverse isolation separates infection-prone patients from sources of infection (ex. the boy in the plastic bubble).

            4.   enforce CDC program.

       5.   treat every patient as if they are infected with AIDS.




Public health deals with disease prophylaxis (prevention); 2 methods of prophylaxis: 

            1.)  decrease or eliminate the reservoir or interrupt disease transmission.

            2.)  immunization - artificially augments the body's natural immune defenses.


A.  Decrease or Eliminate the Reservoir or Interrupt Disease Transmission

1.    Clean Water - diseases such as cholera, typhoid fever, & diarrhea can be spread when human sewage contaminates the water supply.

2.    Clean Food - pasteurization, boiling, adequate cooking, refrigeration prevent food poisoning, trichinosis (roundworm), salmonellosis, tapeworm infection, etc.

3.    Personal Cleanliness - hand washing of #1 importance.

4.    Insect Control - to decrease mosquito populations early programs drained swamps, screened living areas, used mosquito netting, used insecticides such as DDT (until it was found to be carcinogenic to humans!); now efforts concentrate on educating the public to remove stagnant water; biological control is also used - ex.  Gambusia, the mosquito fish, was introduced to the U.S. – this fish feeds on mosquito larvae

5.    Prevention of STD's - public education, limit sexual exposure, use of condoms.

6.    Prevention of Respiratory Diseases - isolate infected individuals, wear face masks; most effective way is immunization.


B.    Immunization


1.    Active Immunization  (= Immunization or Vaccination)

a.     Active Immunization Defined - a person's own immune system is stimulated, memory cells are produced to protect against future natural infection.


b.    Vaccine Defined - an agent containing antigen capable of inducing active immunity without causing disease; vaccines must be safe & immunogenic (stimulate an immune response strong enough to confer protection against natural infection); vaccines can be given orally, subcutaneously (below skin), or intramuscularly; some stimulate both Ab & cell mediated immune responses, other stimulate primarily Ab mediated immunity.


              c.  Types of Active Vaccines

1.) attenuated – Live, weakened viruses or bacteria; virus is cultivated in the lab until it loses its virulence; the organism is then injected into a human and allowed to multiply; may cause a limited infection, usually without serious illness; provides strong & long-lasting immunity.  Ex. tuberculosis (b), oral Sabin polio (v), mumps (v), measles (v), rubella (German measles) (v).  The latter 3 are referred to as MMR.  The fairly new chicken pox vaccine is also attenuated.  This type of vaccine is not recommended for those who are immunocompromised.


2.) inactivated (killed) - By heat or chemical agents such as formalin, phenol, or acetone; process can destroy the Ag's that stimulate immunity (ex. heat denaturation of protein Ag's); inactivated microorganisms can't multiply in host so vaccine dose must contain enough Ag to produce a protective immunologic reaction; usually requires a booster; Ex. pertussis (b), typhoid fever (b), rabies (v), Haemophilus influenzae  type B (b) (causes meningitis), injectable Salk polio (v) (sometime referred to as IPV – inactivated polio vaccine), cholera (b), viral influenza


Haemophilus influenzae  type B - combined polysaccharide Ag with a protein to make it more powerful (polysaccharides are weak stimulants of Ab production); called a protein conjugate vaccine.


3.) genetically engineered – Genetic engineering & recombinant DNA technology have allowed us to use bacteria to produce the protein antigens found in the capsids of certain viruses and the cell envelopes of bacteria.  Scientists determine the genetic code for these antigens & insert the gene into the chromosome of bacterial cells. The bacteria produce the antigens coded for on the inserted genes when they go through their regular process of protein synthesis.  These antigens can then be injected as a vaccine (your body doesn't care if the protein antigens are in the real viral capsid or if they were made by a bacterium; they are the same proteins & your body's immune system will respond to these antigens in the same way).  These vaccines do not pose the same risks as inactivated and attenuated viruses!  Examples:


Pertussis (b) – the inactivated vaccine contains many Ag's that contribute to the frequent undesirable side effects of this vaccine; an acellular, genetically engineered  vaccine has recently been licensed; it has fewer side effects, but may not stimulate vigorous immunity.


Hepatitis B (v) - originally produced from recovering viruses from the serum of infected patients, which could have other diseases like AIDS (made people nervous); now a genetically engineered vaccine.


4.)  toxoids - for diseases caused by exotoxins rather than the microorganisms themselves, vaccines are made of toxoids (toxins that have been modified by heat or chemical agents to render them harmless); toxoids stimulate the production of Ab's called antitoxins; ex. tetanus (b), diphtheria (b).

        2.  Passive Immunization


a.     Defined - Ab's from an immune person or animal are transferred to a patient; like an Ab transfusion!


              b.  Preparation

1.    gamma globulin, a collection of Ab's from the pooled serum of many different donors;

2.    special preparations contain high titers of specific Ab's; ex. varicella zoster (v), (chickenpox & shingles), tetanus (b), mumps (v), measles (v), hepatitis A & B (v), rabies (v), pertussis (b)


c.     Advantages - even severely immunosuppressed patients can be protected & protection is immediate.


d.    Disadvantages - protection lasts only as long as the Ab molecules survive in the recipient – months if from a human, only weeks if from an animal; also a risk of serum sickness.


Serum sickness - Occurs when proteins from animal serum are used in medical therapy; ex. horse antiserum is used in the treatment of venomous snake bites.  Small concentrations of venom are injected into the horse to get it to produce antibody against the toxin.  Patients then receive an infusion of these horse antibodies to bind to the snake venom antigen in their blood.   The patients may produce antibodies against the horse antibodies, forming large complexes that are deposited in the tissues.



3. Boosters – Immunity is not always life-long.  Booster shots boost immunity by greatly    increasing the  numbers of antibody.




These are infectious diseases that are potentially harmful to the public’s health and must be reported by physicians to the CDC.  Make sure you can list some of these!


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