the various measures of morbidity and mortality such as prevalence, incidence, and mortality rate. mapping and application of measures of diseases. rate, ratio, and proportion which are the basic tools to measure the disease in a a population. Relationship between prevalence and incidence rate.

1. Measurements of disease
Amit Gangwar
MVSc. scholar

2. Tool of measurements
● RATIO
● RATE
● PROPORTION

3. RATIO
● The value obtained by dividing one
quantity by other - X/Y.
● relations between two quantities.
● Numerator is not a part of denominator.
Ex. - Male : Female ratio
WBC : RBC ratio

4. RATE
● expresses a change in one quantity (the numerator) with respect to
another quantity (the denominator).
● ‘Time’ is included in the denominator.
● Ex. - Velocity (e.g., 10 m per second) is a rate.

5. Proportion
● Relation between two quantities.
● Numerator always parts of denominator.
Ex. - prevalence, case fatality are proportions.

6. Measurements of morbidity
Incidence
● Occurrence of new cases
Prevalence
● Existence of all new & old cases.

8. Prevalence
number of individuals having a disease at a particular point in time
P = ——————————————————————————————
number of individuals in the population at risk at that point in time

9. Prevalence
Point prevalence
● the amount of disease in a population at a
particular point in time.
Period prevalence
● number of cases that are known to have
occurred during a specified period of time.
● Ex. - a year (annual prevalence).

10. INCIDENCE
Two components:
1 - the number of new cases;
2 - the period of time over which the new cases occur.

11. Cumulative incidence Incidence Rate
● Proportion
number of individuals that become diseased during a particular period
———————————————————————————————
number of healthy individuals in population at beginning of that period
● Rate
number of new cases of disease that occur in a population during a
particular period of time
—————————————————————————————————
the sum, over all individuals, of the length of time at risk of developing
disease

12. The relationship between prevalence and
incidence rate
● P = I x D
● This means that a change in prevalence can be due to:
1 - a change in incidence rate.
2 - a change in the average duration of the disease.
3 - a change in both incidence rate and duration.

13. Attack rate Secondary attack rate
● describe the proportion of animals that develop
the disease, when period of risk is brief.
● Rate at which disease is spreading.
● Expressed in %.
● proportion of cases of a transmissible disease
that develop when contact with the primary
case.

14. Mortality
● When the relevant outcome is death.
● Rather than new case of a specific
disease.

15. Cumulative mortality
● estimated same as cumulative incidence.
● number of individuals that die during a particular period
number of individuals in the population at the beginning of the period

16. Mortality rate
● Calculated same as incidence rate.
number of deaths due to a disease that occur in a population particular
period of time
M=
the sum over all individuals, of the length of time at risk of dying

17. Death rate
● The death rate is the total mortality rate for all diseases
● rather than one specific disease, in a population.

18. Case fatality rate
● The tendency for a condition to cause the death of affected animals in a
specified time is the case fatality.
● proportion of diseased animals that die.
number of deaths
number of diseased animals

19. Measures
CRUDE
● Crude prevalence, incidence and mortality
values are an expression of the amount of
disease and deaths in a population as a whole.
● no account of the structure of the population
affected.
SPECIFIC
● describe disease occurrence in specific
categories of the population.
● related to host such as age, sex, breed and
method of husbandry, and, in man, occupation
and socio-economic group.
● convey more information than crude measures
on the pattern of disease.
● can indicate categories of animal that are
particularly at risk of disease, and can provide
evidence on its cause.

20. Measures
Adjusted (standardised)
● Remove confounding of different age structures.
● Adjusting the crude values.
● reflect the values that would be expected if the potentially confounding
characteristics were similarly distributed in the two study populations.

21. Mapping
● displaying the geographical (spatial) distribution of disease by
drawing maps (cartography).
● recording of areas where diseases exist but also in investigating
the mode and direction of transmission of infectious diseases.

22. POINT/DOT MAP
● illustrate outbreaks of disease in
discrete locations, by circles, squares,
dots or other symbols.
● Qualitative.
● Direction of spread of disease.

23. DISTRIBUTION
MAP
● show the area over which disease
occurs.

24. PROPORTIONAL
CIRCLE MAP
● Morbidity and mortality can be
depicted using circles whose area is
proportional to the amount of disease
or deaths.
● Shading is used to give the
impression of spheres on a two-
dimensional map.

25. CHOROPLETHIC
MAP
● Greek: choros = ‘an area’, ‘a region’;
plethos = ‘a throng’, ‘a crowd’, ‘the
population.
● display quantitative information as
discrete shaded units of areas.
● graded in intensity to represent the
variability of the mapped data.

26. ISO PLETHIC
MAP
● True boundaries between
different values is depicted by
joining all points of equal value
by a line.

27. Why should we
measure it?
● Describe the extent and nature of
disease n community - assist in
establishing priorities.
● Provide essential data for research.
● Starting point for etiological disease,
help in PREVENTION.
● Monitoring and evaluation of disease
control activities.

28. Thanks!
Reference:
Veterinary epidemiology, 3rd edition
(Michael thrusfield)