Infectious Disease Epidemiology Principles of Epidemiology Lecture 7 Dona Schneider, PhD, MPH, FACE
Dec 30, 2015
Infectious Disease
Epidemiology
Principles of Epidemiology
Lecture 7
Dona Schneider, PhD, MPH, FACE
Epidemiology (Schneider)
Concepts in Infectious Epidemiology
Agent, host, environment
Classification of human infections by modes of transportation
Incubation period
Spectrum of disease
Herd immunity
Epidemiology (Schneider)
Major Factors Contributing to the Emergence of Infectious Diseases
Human demographics and behavior
Technology and industry
Economic development and land use
International travel and commerce
Microbial adaptation and change
Breakdown of public health measures
Epidemiologic Triad Concepts Infectivity – ability to invade a host
(# infected / # susceptible) X 100
Pathogenicity – ability to cause disease(# with clinical disease / # of infected) X 100
Virulence – ability to cause death(# of deaths / # with disease (cases)) X 100
All are dependent upon the condition of the host- Immunity (active, passive)
- Nutrition
- Sleep
- Hygiene
Epidemiology (Schneider)
Mode of Transmission Person-to-person (respiratory, orogenital, skin)
Examples: HIV, measles
Vector (animals, insects) Examples: rabies, yellow fever
Common vehicle (food, water) Examples: salmonellosis
Mechanical vectors (personal effects) such as doorknobs, or toothbrushes are called FOMITES
Epidemiology (Schneider)
Classification by Mode of Transmission Dynamics of Spread through Human Populations
Spread by a common vehicle Ingestion Salmonellosis Inhalation Legionellosis Inoculation Hepatitis
Propagation by serial transfer from host to host Respiratory Measles Anal-oral Shigellosis Genital Syphilis
Epidemiology (Schneider)
Principle Reservoir of Infection Man Infectious hepatitis Other vertebrates (zoonoses) Tularemia Agent free-living Histoplasmosis
Portal of Entry/Exit in Human Host Upper respiratory tract Diphtheria Lower respiratory tract Tuberculosis Gastrointestinal tract Typhoid fever Genitourinary tract Gonorrhea Conjunctiva Trachoma Percutaneous Leptospirosis Percutaneous (bite of arthropod) Yellow fever
Epidemiology (Schneider)
Cycles of Infectious Agent in Nature Man-man Influenza
Man-arthropod-man Malaria
Vertebrate-vertebrate-man Psittacosis
Vertebrate-arthropod-vertebrate-man Viral encephalitis
Complex Cycles Helminth infections River blindness
Epidemiology (Schneider)
Incubation Period
The interval between the time of contact and/or entry of the agent and onset of illness (latency period)
The time required for the multiplication of microorganisms within the host up to a threshold where the parasitic population is large enough to produce symptoms
Epidemiology (Schneider)
Each infectious disease has a characteristic incubation period, dependent upon the rate of growth of the organism in the host and
Dosage of the infectious agent Portal of entry Immune response of the host
Because of the interplay of these factors, incubation period will vary among individuals
For groups of cases, the distribution will be a curve with cases with longer incubation periods creating a right skew
Epidemiology (Schneider)
Spectrum of Disease Exposure
Subclinical manifestations
Pathological changes
Symptoms Clinical illness
Time of diagnosis
Death Whether a person passes through all these stages will
depend upon infection and prevention, detection and therapeutic measures
Iceberg Concept of Infection
Lysis of cell
CELL RESPONSE
Cell transformationor
Cell dysfunction
Incomplete viral maturation Subclinical
Disease
Exposurewithout cell entry
Clinical Disease
Fatal
Clinical andsevere disease
Moderate severityMild Illness
Infection withoutclinical illness
Exposure without infection
HOST RESPONSE
Below visualchange
Discernableeffect
Epidemiology (Schneider)
Spectrum of Disease (cont.) Example
90% of measles cases exhibit clinical symptoms 66% of mumps cases exhibit clinical symptoms <10% of poliomyelitis cases exhibit clinical
symptoms
Inapparent infections play a role in transmission. These are distinguished from latent infections where the agent is not shed
Subclinical/Clinical Ratio for Viral Infections
>>>>99%<1:10,000Any ageCNS symptomsRabies
>99%1:995 to 20 yearsRash, feverMeasles
60%1.5:1Young adultFever, coughInfluenza
50%2:15 to 20 yearsRashRubella
80% to 95%1.5:1Adult
14%7:110 to 15 years
10%11:15 to 9 years
5%20:1< 5 yearsIcterusHepatitis A
50% to 75%2:1 to 3:116 to 25 years
1% to 10%10:1 to 100:16 to 15 years
1%> 100:11 to 5 yearsMononucleosisEpstein-Barr
0.1% to 1.0%+ 1000:1ChildParalysisPolio
Clinical casesEstimated ratioAge at infectionClinical featureVirus
Epidemiology (Schneider)
Herd Immunity The decreased probability that a group will
develop an epidemic because the proportion of immune individuals reduces the chance of contact between infected and susceptible persons
The entire population does not have to be immunized to prevent the occurrence of an epidemic
Example: smallpox, measles
Investigating an Epidemic
Determine whether there is an outbreak – an excess number of cases from what would be expected
There must be clarity in case definition and diagnostic verification for each case
Epidemiology (Schneider)
Investigating an Epidemic (cont.)
Plot an epidemic curve (cases against time)
Calculate attack rates If there is no obvious commonality for the
outbreak, calculate attack rates based on demographic variables (hepatitis in a community)
If there is an obvious commonality for the outbreak, calculate attack rates based on exposure status (a church supper)
Epidemiology (Schneider)
Investigating an Epidemic (cont.) Determine the source of the epidemic
If there is no obvious commonality for the outbreak, plot the geographic distribution of cases by residence/work/school/location to reduce common exposures
If there is an obvious commonality for the outbreak, identify the most likely cause and investigate the source to prevent future outbreaks
Epidemiology (Schneider)
Index Case Person that comes to the attention of public
health authorities
Primary Case Person who acquires the disease from an
exposure
Attack rate
Secondary Case Person who acquires the disease from an
exposure to the primary case
Secondary attack rate
Epidemiology (Schneider)
Calculation of Attack Rate for Food X
64%114776%13310
Attack Rate
TotalWellIllAttack RateTotalWellIll
Did not eat the food (not exposed)Ate the food (exposed)
Attack Rate = Ill / (Ill + Well) x 100 during a time period
Attack rate = (10/13) x 100 = 76% ( 7/11) x 100 = 64%
RR = 75/64 = 1.2
Epidemiology (Schneider)
Secondary Attack Rate
Used to estimate to the spread of disease in a
family, household, dorm or other group
environment.
Measures the infectivity of the agent and the
effects of prophylactic agents (e.g. vaccine)
Secondary attack rate (%)
Total number of cases – initial case(s)Number of susceptible
persons in the group – initial case(s)
= x 100
Epidemiology (Schneider)
Mumps experience of 390 families exposed to a primary case within the family
15258415210-19
872044204505-9
501002503002-4
SecondaryPrimaryNo. susceptible before primary cases occurredTotal
Age in years
CasesPopulation
Secondary attack rate 2-4 years old =
(150-100)/(250-100) x 100 = 33%
Epidemiology (Schneider)
Case Fatality Rate
Reflects the fatal outcome
(deadliness) of a disease, which is
affected by efficacy of treatment
Case fatality rate
(%)
Number of deaths due to disease X
Number of cases of disease X= x 100
Epidemiology (Schneider)
Assume a population of 1000 people. In one year,
20 are sick with cholera and 6 die from the disease.
The cause-specific mortality rate in that year from cholera =
The case-fatality rate from cholera =
620
= 0.3 = 30%
61000
= 0.006 = 0.6%