Measuring disease

Question 1

Disease quantification

Answer 1

Important to describe the ‘amount’ of disease
- # of animals affected by disease aka morbidity
- mortality: # of deaths caused by disease

When and Where
- when: temporal distribution
- where: geographic (spatial) distribution
- helps us estimate the ‘population at risk’

Question 2

Closed vs Open populations

Question 3

Closed population

Answer 3

• no additions to population being assessed, during study period
• few to no losses (some loss to follow up)
  
  • bias small is loss is relative to population size
• disease free animals at the start of the study are “at risk”

preferred in epidemiological research

Question 4

open population

Answer 4

• animals enter and leave population throughout study period
• stable if additions/withdrawals are stable over time
• can’t calculate disease risk from open

Question 5

prevalence

Answer 5

number of instances of disease in a known population at a specific point in time
- more meaningful to express as # of diseased in relation to # at risk

P = # w/ disease / # in pop

• point prevalence: amount of disease at particular point in time (more common)
• period prevalence: number of cases that occurred during specific time period (uncommon)

Question 6

cumulative incidence

Answer 6

proportion of non-diseased animals at the beginning of the study period that become diseased during the study period

CI = # animals w/ disease / # healthy animals in pop at beginning of study period

• only calculated for 1st incidence of disease per animal
• additional animals added to population can’t be added to initial pop
  
  • open: calculate for short periods of time w/ no coming/going of the healthy population

Question 7

incidence

Answer 7

number of new cases in a known population over a specified period of time

I = # of animals diseased during particular period / # healthy animals in pop at beginning of period

Question 8

Incidence rate (I) - closed population

Answer 8

measures how quickly new cases of disease develop over time

I = # new cases / sum of all animal’s time at risk of developing disease

• diseased animals no longer contribute to denominator

Question 9

approximate Incidence rate - open population

Answer 9

new cases / (start - 1/2sick - 1/2wth - 1/2add) x time

start = # at risk at start of period
sick = # developing disease
wth = # withdrawn fr. population
add = # added to population
time = length of study period

Question 10

Difference b/twn prevalence & incidence of disease

Answer 10

prevalence: doesn’t have time componet
- “a snapshot”
- measures EXISTING (new & old) cases of disease

incidence: have time component
- measures NEW cases of disease

Question 11

When to calculate prevalence and incidence

Answer 11

prevalence
- disease w/ long durations
- identifying common disease problems in a population
- evaluating disease control strategies

incidence
- diseases w/ short durations
- understanding disease development & transmission (temporality)
- predicting risk of disease (change in health status)

Question 12

Study Designs

Question 13

descriptive (describes)

Answer 13

• summarize and describe data
• no comparisons made
• hypothesis generating

Question 14

case report

Answer 14

• rare or new conditions/disease
• 1 or 2 animals

Question 15

case series

Answer 15

• collection of animals w/ same rare condition/disease
• 4 or more animals

Question 16

descriptive surveys

Answer 16

• want to estimate frequency & percentages of variables of interest

ex: morbidity, mortality, management practices etc.

Question 17

analytical (comapres)

Answer 17

• make comparisons b/twn groups
• explore relationships and associations
• test hypothesis

Question 18

experimental

Answer 18

• control which animals get what treatment
• researcher manipulate animals and/or environment

Question 19

observational

Answer 19

• researchers don’t control who gets what treatment
• doesn’t manipulate animal or environment
• ‘natural environments’
• focused on exposures and outcomes