Measures of disease frequency - Caribvet

Measures
of Disease
Frequency

Why measure disease
• To quantify or measure the occurrence of
disease is fundamental in understanding:
• Impact of disease in the population
• Possible associations between factors and
disease
• Possible mechanisms of spread
• Effect of control measures

Ratios, Proportions, Rates
• Ratios: the numerator is not included in the
denominator:
a/b (a and b are mutually exclusive)

Ratio Example
Ratio of bulls to cows: 2/4= 0.5

Ratio Example
• Feed conversion ratio 3:1 (3 units of feed
per each unit in weight gain)
:

Ratios, Proportions, Rates
• Proportions: the numerator is included in the
denominator; they express a fraction:
a/b (a is included in b)

Proportion of males in a flock is 3/23 or 0.13

Proportion Example
Proportion of bulls in a herd: 2/6= 0.33

Ratios, Proportions, Rates
• Rates: special types of proportions; they
express the relationship of an event and a
defined population at risk in a specified time
period
a/b
(a is included in b and
b represents population time)

Measures of disease
• Prevalence
• Incidence

Prevalence
• Describes what proportion of the population is
affected by a disease at one point in time
• All cases old and new
• Also called point prevalence
• Defined as:
prevalence =
number of cases at time
population at risk
t

Prevalence
Disease duration P= 3/N

Prevalence
Concerns when using this measure:
• Is a static measure
• Only active cases can be included
• Seasonality
• Underestimate
• Short duration
• High fatality
• Sick animals culled
• Overestimate
• Diseases of long duration

Incidence
• Frequency of new cases during a period of
time for a population at risk
• Describes the flow of individuals from
disease-free to diseased
• Can use to predict future impact of a disease
• Two measures:
• Cumulative incidence
• Incidence rate

Incidence
(cumulative incidence rate (CIR))
• Proportion of a population that becomes
diseased during a period of time
• Probability of developing the disease over the
time period
• Length of observation period affects CIR
• Usually measured over short periods of time
I (CIR) =
number of new cases during period
number of disease free individuals at start of
period

Incidence Rate
• Also called incidence density rate
• Measure of the speed of disease
IR =
sum of
number of new cases
time periods for all disease - free individuals
at risk
• Denominator expressed in “animal-time at
risk”
• Example 180 cows, 2 week period = 360 cowweeks

Incidence Rate
• Example: 180 calves are followed for 4 weeks
• 12 develop disease, 6 on the 1 st week, 3 on
the second and 3 on the third
• Numerator:
• 12 new cases over the 4 week-period
• Denominator:
• (6 x1)+(3x2)+(3x3)+(168x4)=693 animal-weeks
• IR=12/693=0.017 cases per animal-week

Incidence Rate
Incidence rate = 6 cases/animal time-units
Disease duration

Incidence
Concerns when using this measure:
• Recurrent diseases
• Need to find best time interval
• Several ways to define population at risk

Incidence vs Prevalence
Incidence = 6/N Prevalence = 8/N
Disease duration
Observation
period

Crude and specific rates
• Crude incidence rate
• Total number of cases and population time at risk
• Ignore the influence of host factors or
management
• Specific incidence rate
• Frequency of disease for specific sub/populations
(age specific, sex specific, etc.)

Incidence vs prevalence
• Prevalence depends on the incidence of
disease and the duration of disease
Prevalence ~ incidence x duration
• For a given incidence, prevalence will be
_____ higher if the duration is long

Other measures of disease
• Case fatality rate/lethality
deaths by cause x
population affected by cause
x
• Cause-specific mortality rate
• Proportional mortality
deaths by cause x
population at risk
deaths by cause x
deaths in the population

Other measures of disease
frequency
• Attack Rate (really a proportion):
• Number of new cases during a specific time period
in a specific group
• Denominator is number of animals at risk in the
start of an outbreak
• Secondary Attack Rate:
• Proportion of cases that develop due to contact
with the primary case

Epidemic Curves
• Plot the frequency of distribution of the
number of new cases occurring over time
• Give pictorial information about time and type
of exposure, as well as disease spread
• Easier to interpret than tables of numbers

Propagative epidemic
Endemic disease
35
10
30
9
25
8
7
casos
20
15
casos
6
5
10
4
3
5
2
0
1 2 3 4 5 6 7 8 9 10
semanas
1
1 2 3 4 5 6 7 8 9 10
semanas
Sporadic disease
Point epidemic
10
12
10
8
8
6
casos
casos
6
4
4
2
2
0
1 2 3 4 5 6 7 8 9 10
semanas
0
1 2 3 4 5 6 7 8 9 10
Días

200
150
100
50
0
Vesicular Stomatitis in
the United States
1997-2004
Sep
Jan
M ay
Sep
Jan
May
Sep
Jan
May
Sep
J an
May
Sep
Jan
May
Sep
Jan
M ay
Sep
Jan
May

120
100
80
60
40
20
0
apr
jul
oct
jan
Vesicular stomatitis
Colombia 1996-1999
apr
jul
oct
jan
apr
jul
oct
jan
oct
jul
apr
jan
Casos

Temporal Pattern of 2003/4 and 2004/5
Temporal Pattern of 2003/4 and
AI Epidemics
2004/5 AI Epidemics
300
250
200
150
100
Tet holiday
Feb 9-11, 2004 Tet holiday
Feb 11-13, 2005
no. outbreaks
50
0
10/1/04
10/2/04
10/3/04
10/4/04
10/5/04
10/6/04
10/7/04
10/8/04
10/9/04
10/10/04
10/11/04
10/12/04
10/1/05
10/2/05
10/3/05
Source: Dirk Pfeiffer

BSE in the United Kingdom
40,000
35,000
30,000
25,000
casos
20,000
15,000
10,000
5,000
0
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Measures of Disease
The measures used will depend upon:
• The data available
• The study used
• The type of disease process being studied