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%