lInvestigating Disease Outbreaks
MED 6301- Chp. 3
Dr. John Kowalczyk
Spring 2009
lInfectious Disease Process
“Chain of Infection”
-Infectious Disease Process
-Results from the interactions of the 3 components of the
epidemiological triangle:
-Agent
-Host
-Environment
-Chain of infection includes 6 chain links which all must
happen in succession- break any link in the chain, the
disease spread will cease
Chain of infection-1
Causative Agent:
-The entity capable of causing disease
-Agents are one of three types:
1.Biological- protozoa, metazoa, bacteria, viruses,
fungi, rickettsia
2.Chemical- pesticides, additives, industrial
chemicals
3.Physical- heat, light, ionizing radiation, noise,
vibrations
*note- most diseases are caused by biological agents;
chemical and physical agents cause chronic long term
illnesses
Chain of infection-2
Reservoir of Agent
-The normal habitat in which an infectious agent lives,
multiplies, and grows (not all diseases have one)
-These habitats include humans, animals, and environment
-2 major categories of human sources of infection:
-Acute clinical cases. Less likely to cause transmission due
to early diagnosis and treatment (going to doctor)
-Carriers- person who harbors infection but has no overt
signs or symptoms
Why is a carrier dangerous? Don’t express Sx but spread
to others.
-Chain of infection-3.
-Portal of exit of agent from Host
There are 5 means of exit from the human body:
1.Respiratory tract- very common but difficult to control
Common cold, Influenza, TB, Airborne droplets
2.Genito-urinary tract- urine, semen
Syphilis, Gonorrhea, Schistosomiasis (blood
flukeworm inside body, eggs leave thru urine-
urinate in the lake; eggs enter skin while wading
in water, enters bloodstream
3.Alimentary tract- mostly related to food- by mouth; also
vomitus, feces, saliva
Rabies- dog bite
Cholera-intestinal tract
4.Skin- Superficial wounds- cuts, gashes, burns, puncture
wound—or smallpox
5.Transplacental mode (mother to fetus)- pregnant woman
passes disease thru placenta-umbilical cord to baby
Rubella, Syphilis, Hepatitis B, HIV+
Chain of infection-4.
Mode of transmission of agent to new host
-Direct transmission (person to person)- touching, biting,
kissing, sexual intercourse
-Indirect transmission-Vehicle borne (inanimate) blood, water, food,
surgical equipment, eating utensils, clothing, etc
-Vector borne- feces, eggs, biting (saliva)
-Chain of infection-5.
Portal of entry into new Host
Basically the same as #3- portal of exit
Mouth, nose, Skin, Genital/anal, transplacental
Chain of infection-6.
Host susceptibility
-Depends upon:
-Genetic factors
-General resistance
-Specific acquired immunity (natural or thru
vaccinations)
+skin toughness
-malnutrition
+gastric acidity
-poor health
+cough reflex
-repressed immune
Surveillance of Disease
-Entire process of collecting, analyzing, interpreting, and
reporting data concerning incidence of disease, injuries, or
death
-USA- Centers for Disease Control (CDC) federal agency
responsible for surveillance activities of acute and
infectious diseases
-Local and state health departments also responsible on
local and state level for surveillance
Baseline Data
-Endemic levels of disease- usual level of disease in a
community
-Evaluation of time trends:
-Long term secular trends- over time period a
disease is looked at to determine trends
-Seasonal variation trends- based on route of
spread; i.e. respiratory route for influenza is
during winter and early spring; insect vectors in
summer time for Lyme or West Nile;
Epidemic
-Disease outbreak at an unusual or unexpected frequency;
above and beyond usual endemic level
-Pandemic- epidemic outbreak affecting several countries
or continents; pandemic flu will likely be pandemic over
many countries in the world when it happens
-Epizootic- ‘upon animals’- unusual pattern of disease in
animal populations
Procedures- 5 steps
1. Define the problem and establish diagnosis:
determine from outset whether this epidemic
(outbreak) is real. What needs to be looked at?
-Surveillance
- Endemic levels
-Foodborne Outbreak Definition
-Two or more persons (except for botulism and
chemical poisoning it may be only one person
affected)
-Who all ate the SAME food at the same location…..
-AND ALL became ill at about the SAME time, and
with the SAME symptoms.
-Oh, yeah.. The Poo-Poo Story! Outbreak in residence hall,
dinner party, all ate subs, 45/60 sick. Sx 6-12 incubation,
V/D, nausea, cramps, headache, Staph aureas infection!
Baby poop in kitchen.
2. Appraise the existing data:
evaluate known distribution of cases with respect to
person, place, and time
-Case identification- find all potential cases involved in the
outbreak; start with index case- first person to get sick and
transmit to others; how did it spread?
-Clinical observations- record number, types, and patterns
of symptoms associated with disease
-Tabulation & spot maps- cases of disease are plotted on a
map (Dr. John Snow- Cholera); these can be done by
date/time of disease onset of symptoms, geographical
clustering; graphing can show time of onset over period of
time
-Plot an epidemic time curve graph
-Identification- determine the responsible agent
(biological, chemical, physical)- How?
3. Formulate a hypothesis:
-What are possible sources of infection?
-What is likely agent?
-What is likely method of transmission? (spread of
disease)
-What is best approach to control the outbreak?
4. Test the hypothesis:
-collect data needed to confirm or refute your initial
suspicions;
-continue to look for more cases; evaluate alternative
sources of data;
-begin laboratory investigations
5. Initiate Control Measures and Draw conclusions to
formulate practical applications
-Sanitation
-Prophylaxis
-Diagnosis and treatment
-Control of disease vectors
-based on results of investigation, likelihood of
new programs, policies, or procedures will need to
be implemented
-long term surveillance and prevention efforts to
prevent recurrence of similar outbreaks
Measures of Disease Outbreaks
-Attack rate- same as ‘incidence rate’; looking at number
of new cases of disease per unit of population per unit of
time.
-Occurrence of disease in population increases greatly over
a short period of time, often related to specific exposure
-Attack rate= # ill persons
x 100%
# ill + # well persons
-Attack Rate Table- to find specific food responsible for
outbreak
A (ate the food)
ill
not ill
‘A’ total
attack rate%
ex: 10 + 3
13
77%
B (did not eat food)
ill not ill
‘B’ total
attack rate%
ex: 7 + 4
11
64%
Identifying Food that Caused Outbreak
1.List all foods consumed at event
2.Persons involved in outbreak:
-A (ate food)
-B (did not eat the food)
3.Calculate attack rates for each food item with well and ill
persons
4.After calculating AR, find difference in attack rates A-B
between those who ate and those who did not eat
-Identifying Food that Caused Outbreak- cont’d
5.Repeat process for each food item suspected in outbreak
6.Foods that have greatest difference in attack rates may
be the foods that were responsible for the illness
-Secondary Attack Rate
-S.A.R.= Number of cases of an infection that occur among
contacts within the incubation period following exposure
to a primary case in relation to the total number of
exposed contacts
-Denominator is restricted to susceptible contacts when
these can be determined
-Is a measure of contagiousness and useful in evaluating
control measures
-Secondary Attack Rate Formula
SAR%= # new cases in grp - initial case(s)
# susceptible persons in grp – initial case x100
Initial case- index case + co-primaries
Index case- case that first comes to attention of public
health officials
Co-primaries- cases related to index case so closely in
time- same generation of cases
-Epidemic Curve
-Defined as a graph in which cases of a disease that have
occurred during an epidemic period are graphed according
to the time of onset of illness in the cases
-Provides a simple visual display of the outbreak’s
magnitude and trends
-How to Draw an Epidemic Curve
-You must first know time of onset of illness
-Number of cases plotted on the y-axis (vertical axis)
-Unit of time on x-axis (hortizontal axis)
-Time is usually based on hours
-Show pre and post epidemic period on your graph
-Interpreting Epidemic Curve
-Consider overall shape- this will determine pattern of
epidemic- common source or person-to-person
transmission
-Curve with steep up  slope and gradual down slope
indicates a point source epidemic where people were
exposed to same agent over brief period of time
-Person-to-person transmission spread will have series of
progressively taller peaks one incubation period apart
-Cases that stand apart are called ‘outliers’ and also should
be looked at closely
Case: hillside elementary school. Potluck picnic 135
students, 315 parents. – 275 parents & 125 students ate all
of the students.
45 students & 65 parents staph food poisoning
90 ate potato salad, 105 ate lasagna, 50 had only drinks.
Calc Attack Rate. Food specific AR for lasanga & potato
salad.
AR:
Attack rate= # ill persons x 100%
# ill + # well persons
total = 450
ill # = 110
AR = 24.4%
Food specific attack rate:
Lasanga AR = 105/400 x 100 = 26%
Potato salad AR = 90/400 x 100 = 22%
Denominator – people who ate only
Culprit = lasanga had highest AR