Chapter 3 Distribution of Disease

Section 1 Epidemic Strength of Disease

Definition: epidemic strength of disease refers to the variability and characterization of incidence of a certain disease in certain place, certain population, and in certain period time.

There are three terms to describe the epidemic strength of disease.

1.SporadicThat means the number of incidence

is not lot, and there is no obvious transmitting relationship among patients, or incidence rate is no more than general level in a certain place.

general level:average incidence rate of former three years

Reasons of sporadic:(1)Some diseases are epidemic often in a

certain place, or people in this place have been vaccinated , so they have high level of immunity. Measles

(2)Infectious diseases in which covert infection are mainly, most patients have no obvious symptom,we can not find them. HB

(3)Infectious diseases which have a long incubation period ,the spread speed is very low. lepra

2. EpidemicEpidemic refers to the incidence rate

of disease in some place is obviously higher than sporadic level.

For example:For cholera: time incidence

strength30~40 years 100/million

sporadicago Today same

epidemic

3.Outbreak If many cases with the same

symptom suddenly occurred in small community or collective unit in short time, we can tell this phenomenon as outbreak.

Short time : refers to the longest incubation period of some disease. Such as food poisoning, HA.

Reasons: All of cases maybe contact with the

same source of infection, or infected through the same route.

SARS

Section 2 Distribution of Disease

Definition

Distribution of Disease descript the frequency and distribution of diseases within different time, place and population.

Population

AgeSexRaceBehavior

1 Age

Age is the most important determinant among the personal variables.Mortality and morbidity rates of almost all conditions show relation to this variable.

Aim to study distribution of disease for age:

① To help explore the determinants and provide clues for study the cause of disease.

②To help pose important protecting objects and find high risk population.

Mortality rate ---Age

Figure 6-1

Incidence rate---Age

Many infectious diseases such as measles, are considered childhood disease,that is to say, the highest frequency of occurrence is in the younger age groups.

incidence rate---Age

Figure 6-2

2 Sex

The mortality and morbidity rates of most diseases have also relationship with sex. But there is an interesting phenomenon. Death rates are higher for males than females, but morbidity rates are generally higher in females.

Figure 6-1

This results may be due to sex linked inheritance or to differences in hormonal balance, environment or habit patterns.

The reasons of relatively high morbidity and low mortality in women are:

① the women seek medical care more freely and early and they can be diagnosed early and then have a suitable treatment .

② the same disease will tend to have a less lethal course in women than in men.

A good example: depression

Rates of depression were twice as high in women as in men, and the rate of attempted suicide is also higher in women.

However, completed suicide are more common in men.

3 Race

Mortality and morbidity rates of some diseases are different among variable races. This probably due to different inheritance, environment, customs, religious ,and so on.

For example: United States

Blacks: higher rates of deaths caused by hypertensive heart disease, cerebrovascular accidents, tuberculosis, syphilis, and accidental death.Whites: higher rates of deaths from arteriosderotic heart disease ,suicide, and leukemia.

4 Behavior

Such as smoking, drinking, taking drugs ,and so on.

Smoking---lung cancer ,stomach cancer, liver cancer, esophagus cancer, and so on.

AIDS: These years,the incidence of AIDS

increases more quickly. Why?The important reason is that they

took drugs with syringe and being infected with HIV.

Place

The frequency of disease often varies by geographic location. It implies that these variations in frequency provide etiologic clues.

two aspects :One is native environment

factors,such as special graphic location,weather condition.

The other is social environment including custom habit and social culture background.

1.International Comparisons

In different countries, disease and death rates are also different.

They also may provide clues to causation of disease.

Figure 6-12

Age-adjusted death rate for breast cancer,1966-1967.

Figure 6-13

Such marked contrasts provide opportunities to formulate etiologic hypotheses and test them.

2.Urban-Rural Difference

The distribution of disease in urban or rural is very different because they have different characteristics.

In rural, there are a number of health problems because of working farms, such as:

(1) farm accident--- cause disability and death(2)skin cancers--- because farmers exposure

usually to ultraviolet radiation.(3) exposure to pesticides---poisoned(4)exposure to a variety of microorganisms –

cause different kinds of infectious disease.(5) health condition is bad---infectious disease

in intestines

In urban ,people also face with a variety of hazards to health.

Air pollution is very serious---cause disease of respiratory system, such as lung cancer, bronchitis, and so on.

But in urban, health condition is better than rural ,so infectious disease in intestines less than in rural.

Time

Study of disease occurrence by time is a basic aspect of epidemiologic analysis.There are 3 major kinds of change with time.

The first consists of long-term variations called secular trend.

The second is periodic fluctuations on an annual or other basis,cyclic change.

Finally,there is short-term fluctuation talked about in other chapter.

1.Secular Trend

This term refers to changes over a long period of times,years or decades.Such trend may occur in both infectious and noninfectious diseases.

Figure 6-14

Figure 6-15

Apparent secular trends in morbidity or mortality may be due to these reasons as below:

change of diagnostic methods;lung cancer

change of treatment methods;stomach cancer

increasing of life level; CHD

2.Cyclic variation

Cyclic variation refers to that diseases occur every several years.

For example,measles : two or three years. influenza A :two- to three- year influenza B :four to six years.

Cyclic variation of some disease is not invariable. It can be changed by use of immunization.

For example, Measles: every 2 or 3 years before immunization,

every 5 to 7 years after immunization. The epidemic peak from February to April.That is because the level of immunization

in population increases, then the susceptibility in population decreases.

Seasonal Variation :

Seasonal variation refers to incidence rates of some diseases increase in certain season in every year.

Infectious diseases:(1) Strict seasonality: occur only in

some certain months, in other months rarely, such as malaria transmitted by mosquito, only occurs in July to September because mosquito can live only in summer.

(2) Not strict seasonality:Some diseases can occur all of one

year, but increase in certain seasons.For example, bacterial dysentery can

occur every season but the epidemic peak is in summer and autumn.

For measles, the incidence rate increases in December to next year April. But this characterization can change because of immunization.

(3)No seasonality:The diseases which are spread

through touching in daily-life,such as tuberculosis, lepra, can occur in every month.

Noninfectious diseases: no obvious seasonality.

The reason of seasonal variation maybe related to temperature, humidity, insect vector, people’s activity, custom and so on.

Important consents:1.What is distribution of disease.2.Conception of sporadic, epidemic

and outbreak.