Measures of Disease Frequency

Question 1

Define the term count. List examples.

Answer 1

The number of cases or individuals affected with a condition in a given population.

• Examples:
• 14 veterinary students get diagnosed with cryptosporidium. Don’t know class size.
• 300,000 cases of cryptosporidiosis reported in USA. Don’t know timeline, etc.
• 58 cows in a herd have mastitis
• Limited use because no denominator information

Question 2

Define proportion. Give examples.

Answer 2

• A fraction in which the numerator is a subset of the denominator.
• A measure of risk or probability of a disease
• Can be measured at one point in time
• Example
• 300,000 cases cryptosporidiosis in 300 million Americans = 0.1%
• 58 cows in a herd of 100 = 58%

Question 3

Define ratio. Give example.

Answer 3

• This is a number in which the numerator is NOT a subset of the denominator (denom has both numerator and rest combined)
• Defines the relative size of two quantities expressed by dividing one (numerator) by the
  other (denominator)
• Example
• Odds of disease (a ratio) in our herd of 100 cows is 58:42 (1.38 to 1)
• 10 males out of 100 students. 100 students includes males and females.

Question 4

Define rate. Give examples.

Answer 4

• This is a fraction in which the numerator is a subset of the denominator and
  the denominator includes a measure of *time *(e.g. animal-time or people-time
  units)
• Examples - Annual mortality rate, annual birth rate
• 12 students were diagnosed with influenza out of 600 students over a 10-month period
  (12/(600*10) = 0.002 cases per student-month
• Rate is often erroneously used in a general sense to refer to all types of measures of
  disease frequency

Question 5

Define prevalence (risk).

Answer 5

You want to know right at this moment have this disease. Don’t know when they got it though - have both NEW and OLD.

Question 6

Define point prevalence.

Answer 6

Prevalence at a single point in time

Question 7

Define period prevalence.

Answer 7

• Prevalence over a specific period in time
• Includes individuals with disease at start plus new cases during follow-up period

Question 8

What is an example of prevalence (risk)?

Answer 8

Cryptorchidism in different dog breeds (Gubbels et al. 2009) examined 26,547 male dogs and found 405 were cryptorchid. What is the prevalence of cryptorchidism in their study population? Is this a Point or Period estimate?

405/26,547 = 0.0152 x 100 = 1.52% of male dogs in this pop are cryptorchid. This is a point prevalence. b/c you can’t re-develop this illness in particular

Question 9

Define incidence. What is this expressed as?

Answer 9

Think NEW disease.
* Measures how frequently initially susceptible individuals become disease
cases as they are observed in time
* Incident case = new case
- Individual changes from susceptible to diseased!
* Expressed as:
* Incidence Risk
* Incidence Rate

Question 10

Define incidence risk.

Answer 10

• Proportion of initially susceptible individuals in a population who become
  new cases during a define follow-up period.
  Risk = prevalence.

Example: Last year a herd of 121 cattle were tested for tuberculosis using the
tuberculin test and all tested negative. This year the same 121 cattle were
tested and 25 tested positive.
* The incidence risk of TB in this herd was 25/121 (0.207 = 20.7%) for the 12-month follow-
up period

Question 11

Incidence risk in a close population?

Answer 11

• For a closed population, calculating incidence risk is straightforward!
• Closed population = no additions/removals during a define follow-up period

Question 12

Incidence risk in an open population?

Answer 12

Question 13

Define incidence rate.

Answer 13

Question 14

Answer 14

days at risk = # of days you are susceptible to contracting disease.

When you have the disease, you are no longer contributing to at risk time and must remove those days from days contributed.

5 cows and you have 5 events of mastitis but not there for a whole year.

In 1 year, you expect cow to have 2.2 incidents of mastitis.

Question 15

Answer 15

have disease = no longer at risk
all the time time they are orange = have disease = not at risk.

Case A = contributed to 4 months at risk and then the remainder of the year had disease.
Case B = never developed disease and were in study for all 12 months so contributed to 12 months at risk and not a new case.
Case C = only know about at risk prior to being withdrawn

0.044 cases per animal months

if you have 1 year period, you would expect to have half a case per one animal that is exposed for one year.

Question 16

Question 17

Answer 17

point prevalence is very common

Question 18

Define attack rate. Give examples.

Answer 18

• Proportion of population developing illness during a finite period of time.
• Equivalent to incidence risk
• ‘Attack risk’ would be a better term for this statistic.
• Typically used to measure average risk during a common-source disease outbreak

Question 19

Define case fatality rate. Give examples.

Answer 19

• Proportion of individuals with a disease who die from it
• Equivalent to incidence risk
• ‘Case fatality risk’ would be a better term for this statistic

Question 20

Example of ebola outbreak.

Answer 20

This is how they reported the case fatality rate.

Question 21

Define crude mortality rate.

Answer 21

• Incidence risk of fatal cases of a particular disease in a population at risk of
  death from that disease
• Denominator includes both:
• Cases of the disease (who haven’t died yet)
• As well as individuals who are at risk of disease

Crude means unadjusted, raw. Has to do with denom = everone both cases and suceptibels. How many people ar dying out of pop regardless of whetehr you do or do not have disease.

Question 22

Define proportional mortality rate.

Answer 22

Question 23