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.
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.
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).
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!