NURSING 205

Introduction to Epidemiology

Lecture 2

Natural History of Disease

Natural History of Disease

The course of a disease from onset to resolution, in the absence of intervention

“disease” refers to any health outcome.

Four Stages of Disease

Susceptibility Stage

Period before the onset of the diseaseDisease exists in the population, but a particular

individual in the population does not yet have itOnly the risk factors are present.

Example: a person having unprotected sex in a sexual network where Chlamydia infections are present is in the susceptibility stage.

Sub-Clinical PhaseEtiological factors present in the body and are causing pathological

changes- no signs or symptoms. For infectious diseases called the incubation period.

Example:

1. IMMEDIATE: influenza, 1-2 days,

2. INTERMEDIATE: chicken pox, 2-3 weeks or

3. EXTENDED: (e.g. (AIDS), up to 15 years or >

can still transmit the disease

Clinical Phase

Onset of signs or symptoms of the disease. Ideally, a diagnosis is made as soon as the clinical stage appears, but this does not always happen

Can range from mild, moderate to severe

Recovery, Disability or Death Phase

Final stage for any given disease or condition Influenced by a number of factors

Example: MI can result in recovery, recovery associated with significant activity restrictions, or death

Skills Enhancement for Health Surveillance (PHAC, 2006)Skills Enhancement for Health Surveillance (PHAC, 2006)

J. Moseley 2013

Prepathogenesis Period

Period of Pathogenesis

Stanhope et al. (2011, p. 236)

Measuring Disease Occurrence

Ratio, proportion, rate = x/y x Ratio, proportion, rate = x/y x 10 ⁿ 10 ⁿ

In this formula, x and y are the two quantities that are being compared. The formula shows that x is divided by y. 10 ⁿ is a constant that we use to transform the result of the division

into a uniform quantity. The size of 10 ⁿ may equal 1, 10, 100, 1000 and so on depending on

the value of n. Example: 102 = 10 x 10 = 100 103 = 10 x 10 x 10 = 1000 105 = 10 x 10 x 10 x 10 x 10 = 100,000

RATIO & PROPORTIONRATIO & PROPORTION

A ratio is used to compare the occurrence of a variable in two different groups.

These may be two completely independent groups, or one may be included in the other.

For example, we could compare the sex of children attending an immunization clinic in either of the following ways:

Female/male or Female/allIn the first example, x (female) is

completely independent of y (male). In the second example, x (female) is included in y (all). This second type of ratio, called a proportion

RatioRatio

Usually simplified by reducing the numbers so that the smallest number becomes 1….

Lets work through one……

Independent x and y:During the first 9 months of national surveillance for eosinophilia-myalgia syndrome (EMS), CDC received 1,068 case reports that specified sex; 893 cases were in females, 175 in males. What is the method for calculating the female-to-male ratio for EMS?

ProportionsProportions

Tell us what fraction of the population is affected by the disease

Based on the same data , how will you calculate the proportion of EMS cases that were male.

Rates

Rates: Measure of the frequency of a health event in a specified population in a defined time period.

Used to make comparisons among populations or to compare a subgroup of the population to the total population.

:

Numerator: Actual number of events

Denominator: Total population at risk

Multiply by a standard base number

Calculate how many teenage pregnancies Calculate how many teenage pregnancies per 100,00 adolescents aged 14 -19per 100,00 adolescents aged 14 -19

City A: 125 teenage pregnancies in an at risk population of 120,602 female teenagers (ages 14-19)

City B: 492 teenage pregnancies in an at risk population of 194,301 female teenagers (aged 14-19)

Population at risk….Population at risk….

Comprises those for whom there is some finite probability of experiencing that event

Which one of the following best describes a rate?

A. A measure of the frequency of a health event in a defined population during a specified period of time

B. An investigation of the causes and associations between factors, events, and health

C. Simultaneous investigation of outcomes and exposures for a characteristic of interest

D. A measure of the level of occurrence of disease exceeding expectations

A measu

re of t

he freque...

An investi

gation of t

he ...

Simulta

neous inve

stigat..

.

A measu

re of t

he leve

l of...

25% 25%25%25%

Find the proportion of deaths caused by heart disease Find the proportion of deaths caused by heart disease in 2000. In 2000, there were 2,404,624 deaths recorded in 2000. In 2000, there were 2,404,624 deaths recorded

in Country X, of which 709,894 were reported as in Country X, of which 709,894 were reported as caused by CVD caused by CVD

Rank Responses

1

2

3

4

5

6 Other

1 2 3 4 5 6

17% 17% 17%17%17%17%

Calculate the birth rate for Country X. Country X Calculate the birth rate for Country X. Country X has 37,850 births in 2005 with the total has 37,850 births in 2005 with the total

population being 3,970,000 .population being 3,970,000 .

Rank Responses

1

2

3

4

5

6 Other

1 2 3 4 5 6

17% 17% 17%17%17%17%

SummarySummary

All three of these frequency measures are calculated in basically the same way. In practice, we use: – a ratio to compare two independent groups, – a proportion to compare one group with a larger one to

which it belongs, and – a rate to measure an event in a population over time.

Ratios, proportions, and rates are used in infectious disease epidemiology to describe morbidity (disease) and mortality (death).

Measures of Morbidity

Incidence RateNumber of NEW cases of a disease that occur during

a specified period of time in a population at risk for the disease

Incidence Rate per 1000 =

# of new cases of a disease occurring

in the population during a specified period of time

X 1000

No. of persons who are at risk of

developing the disease during that period of time

Twenty people attended a church picnic on the weekend. By Monday, four individuals exhibited symptoms of food poisoning. On Tuesday, the CHN records the addition of two new cases. What is

the incidence rate for this occurrence?

A. Two new cases divided by 16 at risk

B. Two new cases divided by 20 at risk

C. Six cases divided by 20 at risk

D. Four cases divided by 16 at risk

Two new case

s divi

ded b...

Two new case

s divi

ded b...

Six ca

ses d

ivided by 20 at ..

.

Four c

ases d

ivided by 16 a...

25% 25%25%25%

PrevalenceNumber of affected people present in the

population at a specific time divided by the number of people in the population.

# of cases of a disease present in a population at a specified time

# of people in a population at a specified

time

PrevalenceGood measure of burden of disease in a

communityFor planning health servicesOnly way to decrease prevalence is CURE or

DEATH

A breast cancer screening program screened 8,000 women and discovered cancer in 35 women who had

previously been diagnosed with breast cancer and in 20 women with no previous history of breast cancer. What is the prevalence rate of breast cancer in this scenario?

A. Current and past breast cancer events in this population of women

B. Newly diagnosed cases of breast cancer in this population of women

C. Past breast cancer events in this population of women

D. Population of women who had no evidence of breast cancer

Current a

nd past bre

ast ...

Newly diagnose

d case

s o...

Past b

reast ca

ncer e

vents ..

Population of women w

..

25% 25%25%25%

Incidence & Prevalence

Incidence & Prevalence

Incidence & Prevalence

Public Health ImplicationsEffect of lowering prevalence through either

death or cure underlies an important issue in public health 

Paradox – new measure is introduced that enhances survival but net effect ↑ in prevalence

Implications for policy-makers - difficult to convince some people that a program is successful if the prevalence of the disease that is the target of the program actually increases.

Other conceptsOther concepts

Epidemic- an outbreak of a disease, injury, or other condition that exceeds the usual (endemic) level of that condition. Therefore the incidence of the disease has increased.

Pandemic- a worldwide outbreak of an epidemic disease based on the rate of the disease, injury, or other condition that exceeds the usual level in the population and is widespread geographically.

SurveillanceFundamental role of public healthTo monitor changes in disease frequency and risk

factorsProvides a lot of information for morbidity and

mortalityUseful for monitoring infectious diseases, congenital

anomalies, cancer, asthma, environmental issues etcHealth services allocation

Measures of Mortality

Measures of Mortality

Rate can be expressed for the total population (crude or adjusted rate) or for a subgroup of the population (specific rate)

Mortality Rates

Crude Annual mortality rate or mortality from all causes

Total # of deaths from all causes in 1 yr

No of people in the population at midyearX 100,000

Mortality RatesAge specific

# of deaths from all causes in one year

in children younger than 10 year of age

# of children in the population younger

than

10 years of age at midyear

X 100,000

Mortality RatesCause Specific

# of deaths from lung cancer in one year

# of people in the population at midpointX 100,000

Case Fatality Rate

The case fatality rate is often used as a measure of the severity of an infectious disease. It is the proportion of known cases who died of the disease in question. For example, in an outbreak of 20 cases of invasive meningococcal disease on a large university campus, 3 patients died. The case fatality rate was 3/20 or 15%.

Infant mortality rateInfant mortality rate

Used all over the world as an indicator of overall health and availability of health care services

Diagnostic and Screening Tests

“A normal individual is a person who has not been sufficiently

examined” anonymous

Importance of Screening

To understand how a disease is transmitted and develops it is necessary to distinguish between people in the population who have the disease and those who do not- secondary prevention

Characteristics of a Successful Characteristics of a Successful Screening ProgramScreening Program

Valid (accurate)- high probability of correct classification of persons tested.

Reliable- Results are consistent from place to place, time to time and person to person.

Facility for large group administration Innocuous- Few if any side effects, and the test is

minimally invasive High yield- can detect enough new cases to justify the

effort and expense