·         Disease description: symptoms, signs

·         Disease classification criteria

·         Time characteristics of disease: trends, duration, natural history

·         Place characteristics of disease: urban/rural, boundaries (political & natural), institutions

·         Person characteristics of disease: age, gender, race/ethnicity, SES, marital status

·         Epidemicity: epidemic, endemic, pandemic, epidemic curve, slow and acute epidemics, visibility of the epidemic

·         Measures of disease occurrence and excess disease risk

Key Words and Terms:

·         Disease classification

• Disorder classification
• Epidemic disease
• Ethnicity
• Etiology of disease
• Heredity
• Infection
• Morbidity
• Mortality
• Nomeclature of disease
• Nosology
• Risk ratio
• Risk difference
• Incidence rate ratio
• Incidence rate difference
• Odds ratio
• Cumulative incidence
• Morbidity
• Mortality

 OUTLINE

1.0 CLASSIFICATION OF DISEASE

1.1 History:

1.2 Purposes:

1.3 Criteria:

2.0 DISEASE DESCRIPTION

2.1 Time description of disease

2.2 Place description of disease

2.3 Individual variation of disease occurrence

2.4 Time-place interactions

3.0 DISEASE MEASUREMENT

3.1 Disease incidence (a measure of new disease occurrence)

3.2 Prevalence (a measure of existing disease both new and old)

3.3 Measures of excess disease occurrence

3.4 Attributable risk

3.5 Impact of disease

1.0 CLASSIFICATION OF DISEASE

1.1 History:

In 1853 William Farr introduced standard nomenclature for causes of death.

In 1946 WHO introduced an International Classification of Diseases, Trauma, and Cause of Death (ICD). The ICD classification has gone through several revisions as new disease entities are discovered or as old disease entities are described in new ways.

1.2 Purposes:

Disease classification is useful for explanation & description, prediction of disease course, prognosis, planning treatment, and disease prevention.

1.3 Criteria:

The following criteria are used in disease classification: manifestational (symptoms and signs), causal (eg HIV), operational/pragmatic (eg ICD).

Classification can also be based on the etiologic agent eg viral disease), the disease process (eg neoplastic disease), the organ system (eg heart disease), method of transmission (e.g sexually transmitted), and and portal of entry (eg respiratory).

2.0 DISEASE DESCRIPTION: what, why, when, how, where, and who

2.1 Time description of disease

Time can be described as calendar time (on the interval scale) or cohort time (on the ratio scale).

Time trends are biorhythmic (nocturnal / diurnal) or periodic (monthly, annual, seasonal), linear, and curvilinear.

Disease durations can be acute (<3 months), sub-acute, and chronic >3 months).

Natural history is progression from susceptibility; sub-clinical disease; clinical disease; and recovery, disability, or death.

Time intervals are induction (causal action to disease initiation), incubation, and latent periods (disease initiation to disease detection).

2.2 Place description of disease

Location:  as rural, urban, sub-urban, and slums/shacks (septic fringe

Boundaries: political (between countries) and natural (such as mountains and rivers)

Institutions: hospital, home, school, factory, farm, and outer space

2.3 Individual variation of disease occurrence

Heredity: familial

Age:  fetal, infant, child, adolescent, young adults, middle age, elderly

Gender: male and female

Socio-economic status: rich/poor, educated/illiterate, professional/laborer

Marital status: married, single, divorced, widowed

Ethnicity/race: Malay, Chinese, Indian, Orang Asli

2.4 Time-place interactions

Clustering is excessive concentration of events at a point in time or a place.

A disease outbreak is excessive disease occurrence of a lesser degree than an epidemic.

An endemic disease has high prevalence in an area.

An epidemic is excessive incidence over a brief period of time.

An epizootic is an epidemic disease in animal populations. Epizootics can become epidemics in human populations.

An enzootic is an endemic disease among animals. An epizoodemic is an epidemic involving both human and animal populations.

A pandemic is an epidemic affecting several countries.

A point source epidemic originates from one person or place. A common source epidemic has more than one origin.

Some epidemics occur without public awareness (eg death from lung cancer). An epidemic is visible in the following cases: cases are few but the disease is rare, the case number is large, the disease is distinctive, swift shift from non-epidemic to epidemic status, and disease duration is short.

The occurrence of an epidemic can be ascertained by changes in trend over time, comparing incidence in epidemic and non-epidemic places, and comparing disease incidence among population sub-groups of the same country.

Investigating an epidemic consists of establishing the diagnosis, case definition, determination is made whether an epidemic exists, characterizing the epidemic by place, time, and person, drawing a spot map showing cases, computing incidence rates by location, identifying clusters, developing hypotheses about the source and route of infection, testing the hypotheses laboratory and case control studies.

Control measures are then instituted and may include sanitation, prophylaxis, diagnosis and treatment, and vector control. Surveillance is continued in the post epidemic period.

3.0 DISEASE MEASUREMENT

3.1 Disease incidence (a measure of new disease occurrence)

Incidence rate (IR) = incident number (new cases) / total person-time.

Cumulative incidence = incident number / susceptible population at the start.

3.2 Prevalence (a measure of existing disease both new and old)

Prevalence proportion = # cases of illness at a particular time (old and new) / # of individuals in the population at the same time. Prevalence proportion = incidence rate x average duration of disease.

Prevalence can be point prevalence (one point in time), period prevalence (over a short interval of time)

Prevalence is useful for administrative purposes because it shows the total burden of disease

Prevalence is not used for etiological studies because the time sequence is not obvious.

Prevalence changes due to changes in incidence and duration.

3.3 Measures of excess disease occurrence

Excess disease risk measured as an absolute effect using Rate Difference or Risk Difference

Excess disease risk measured as a relative effect uses Relative Risk, Rate Ratio, Risk Ratio, Prevalence Ratio, Cumulative Incidence Ratio, Incidence density Ratio, Odds Ratio, and Standard Mortality Ratio

The odds ratio is the most commonly used. It is approximately equal to the risk ratio. It is interpreted as follows. up to 0.3 strong benefit, 0.4 – 0.5 moderate benefit, 0.6 – 0.8 weak benefit, 0.9 – 1.1 no effect, 1.2 –1.6 weak hazard, 1.7 –2.5 moderate hazard, and >=2.6 strong hazard.

OR values range from 0 to infinity.

OR is a good estimator of risk ratio if the disease is rare and the cases and controls are randomly selected from the population.

3.4 Attributable risk

The proportion of disease due to a particular exposure is measured by various parameters of attributable risk (AR) that takes into consideration the population at risk.

Proportional mortality studies are used to compare the proportion of deaths among the exposed to the proportion of deaths among the non-exposed.

3.5 Impact of disease

A common measure of disease impact is the years of potential life lost (YPLL).

The life expectancy is a measure of survival

EXERCISES (TRUE/FALSE)

1. The following statements are true about classification of disease

A. William Farr introduced standard nomenclature for causes of death.

B. In 1946 WHO introduced an International Classification of Diseases (ICD).

C. The ICD classification is based only on rational criteria

D. Very common diseases need not be classified

E. All disease classifications are in the English language

2. Disease classification is useful in the following functions

A. Explanation & description

B. Prediction of disease course

C. Prognosis

D. Planning treatment,

E. Disease prevention.

3. The following are criteria used in disease classification

A. Manifestational criteria

B. Causal criteria

C. Abstract criteria

D. Operational/pragmatic criteria

E. Economic criteria

4. Disease classification may be based on the following

A. Etiologic agent

B. Disease process

C. Social attitudes

D. Transmission route

E. Portal of entry.

5. Disease description covers the following

A. What

B. When

C. How

D. Where

E. Who

G. Why

6. The following statements are true about time description of disease

A. Time can be described on the interval scale as calendar time

B. Time can be described as cohort time on the ratio scale.

C. Time trends cannot be biorhythmic

D. Time periods can be periodic (monthly, annual, seasonal)

E. Time can never be described as a linear relation.

7. The following statements are true about time description of disease

A. Acute disease lasts <3 months

B. Sub-acute disease is always mild

C. Chronic disease lasts >3 months

D. Acute disease never becomes sub-acute

E. Sub-acute disease may turn into chronic disease

8. The following are the main stages in the natural history of disease

A. Susceptibility

B. Sub-clinical disease

C. Clinical disease

D. Recovery or disability

E. Death.

9. The following are time intervals are used in disease description

A. The induction period is from the causal action to disease initiation

B. The incubation period is the same as the induction period

C. The latent period is from disease initiation to disease detection

D. All diseases have the same time intervals in their natural history

10. The following are used in disease description by geography

A. Urban

B. Rural

C. Sub-urban

D. Boundaries: political and natural

E. Institution: hospital, home, school, factory, farm, and outer space

11. The following are used in person description of disease

A. Heredity

B. Gender

C. Age

D. Marital status

E. Ethnicity/race.

F. Religious beliefs

G. Political philosophy

12. The following terms are used to describe time-place interactions of disease

A. Clustering

B. Disease outbreaks

C. Epidemics

D. Endemics

E. Demographics

F. Pandemics.

13. The following statements are true about time-place interactions

A. Clustering is excessive concentration of events at a point in time or a place.

B. Disease outbreak is excessive disease occurrence of a lesser degree than an epidemic.

C. An epidemic disease has high prevalence in an area all the time.

D. An endemic is excessive incidence over a brief period of time.

E. A pandemic is an epidemic affecting several countries at the same time.

14. The following statements are true about epidemics

A. A point source epidemic originates from one person or place.

B. A common source epidemic has more than one origin.

C. In epidemic progression new infections occur after the initial ones.

15. An epidemic is visible in the following situations

A. Cases are few but the disease is rare

B. The case number is large

C. The disease is distinctive

D. If the shift from non-epidemic to epidemic status is swift

E. Disease duration is short.

16. The occurrence of an epidemic can be ascertained by the following

A. Changes in trend over time

B. Comparing incidence in epidemic and non-epidemic places

C. Comparing disease incidence among population sub-groups of the same country.

17. The following measures are involved in investigating an epidemic

A. Establishing the diagnosis

B. Case definition

C. Determination of whether an epidemic exists

D. Characterizing the epidemic by place, time, and person

E. Drawing a spot map showing cases

17. The following measures are involved in investigating an epidemic

A. Computing incidence rates by location

B. Identifying clusters

C. Developing hypotheses about the source and route of infection

D. Testing causal hypotheses by laboratory tests

E. Testing causal hypotheses by case control studies.

18. The following measures are used to control an epidemic

A. Sanitation

B. Prophylaxis

C. Diagnosis and treatment

D. Vector control.

E. Surveillance in the post epidemic period.

19. The following statements are true about epidemics involving animals

A. An epizootic is an epidemic disease in animal populations.

B. Epizootics can become epidemics in human populations.

C. An enzootic is an endemic disease among animals.

D. An epizoodemic is an epidemic involving both human and animal populations.

E. Asian bird flu is an enzootic disease

20. The following statements are true about disease incidence and prevalence

A. The incidence rate (IR) = incident number/ total person-time.

B. Cumulative incidence = incident number / susceptible population at the start.

C. Point prevalence is the number cases (old and new) at a point in time

21. The following statements are true about disease incidence and prevalence

A. Prevalence proportion = (new + old cases) / total population

B. Prevalence proportion = incidence rate x average duration of disease.

C. Prevalence is useful for administrative purposes.

D. Prevalence is not used for etiological studies because the time sequence is not obvious.

E. Prevalence changes due to changes in incidence and duration.

22. The following statements are absolute measures of excess disease

A. Rate Difference

B. Rate Ratio

C. Odds Ratio

23. The following are relative measures of disease excess

A. Relative Risk

B. Risk Difference

C. Risk Ratio

24. The following are relative measures of disease excess

A. Prevalence Ratio

B. Cumulative Incidence Ratio

C. Incidence density Ratio

D. Standard Mortality Ratio

25. The following statements are true about the interpretation of the odds ratio

A. OR values range from 0 to infinity

B. OR > 0.3 indicates a strong benefit

C. 0.4 < OR < 0.5 indicates moderate benefit

D. 0.6 < OR < 0.8 indicates weak benefit

E. 0.9 < OR < 1.1 indicates no effect

EXERCISES (COMPUTATIONS)

A medical team came to Kampung Emas to study the disease situation. They stayed in the kampong for 1 year (January to December) and at the end of the year produced a report. When they first arrived they carried out a census to determine the total population. They found that there were 200 adult males, 300 adult females and 600 children divide equally among the 2 genders. They then carried out a cross sectional study and found that there were 40 cases of hypertension in adult males, 30 cases of hypertension in adult females and no cases in children. They visited each house in the village once a month and checked everybody for hypertension. They found new cases of hypertension in adult males as follows: January 2, February 0, March 0, April 3, May 4, June 0, July 1, August 3, September 1, October 3, November 4, December 0. No new cases of hypertension were found in adult females or in children all through the year. They became intrigued when villagers told them that it was well known in the village that eating eggs caused hypertension. They wanted to investigate the claim. They divided the cases detected with new hypertension into 2 groups according to whether they are eggs or did not eat eggs. They found that 75% of men with hypertension ate eggs whereas 25% of those without hypertension ate eggs.

Compute the following statistics from the narrative above:

(1)   Prevalence of HT in the village at the start and in June

(2)   Incidence of HT in the year

(3)   Cumulative incidence of HT by the month of July

(4)   Odds ratio of developing HT in egg eaters

(5)   The attributable risk of eating eggs