Lecture One-Introduction

Question 1

What is a naive population?

Answer 1

One that is vulnerable or susceptible to a disease.

Question 2

What was John Snow’s contribution to epidemiology?

Answer 2

He first mapped cholera-affected households during an 1854 epidemic, determining the source as a communal water pump. This assisted in the rejection of the miasma (bad air) theory.

Question 3

What is the interplay between the states of Immune, Diseased and Susceptible?

Answer 3

An individual who was diseased can recover with Immunity, or recover without Immunity, becoming Susceptible.
A Susceptible individual becomes Diseased with its incidence, but can become Immune through management (vaccination, biosecurity).
An Immune individual can become Susceptible with the loss of Immunity over time.

Question 4

What are the Measures of Disease Frequency?

Answer 4

1. Counts- the number of cases in a given population. It needs to be relative to population size and include a time period or the individuals infected.
2. Percentages-
3. Proportion (synonymous with %)-the numerator is a subset of the denominator = a / (a + b)
4. Ratio (synonymous with Odds)-the numerator is not a subset of the denominator = a / b (Or a : b). Values range from 0 to infinity, they cannot be expressed as a %.
5. Rate-REQUIRES A TIME FACTOR. The denominator is the number of animal-time units at risk. Rates form the basis of comparisons between population groups.

Question 5

What is Prevalence?

Answer 5

The proportion of a population affected by a disease at a given point in time. It qualifies existing cases, counting the number of existing cases at a certain time point.
Prevalence = # of prevalent cases/population present at a given time
P = randomly selected individual having the disease at a certain point in time.

Question 6

What are the issues with using Prevalence?

Answer 6

As it is a measure of existing cases (new and old), there is no time factor for when new cases occurred.
Diseases of long duration have a high prevalence
Diseases of short duration have a low prevalence

Question 7

What is Incidence?

Answer 7

A measure of NEW cases that occur within a given time period. It is expressed as a relative measure with respect to the population at risk

Question 8

What is the Population at Risk?

Answer 8

The animals that can get the disease or can be diagnosed to have the disease. It can be open or closed.

Question 9

What is a ‘Closed’ Population?

Answer 9

There are no additions during the study, with few to no losses in the time period. Only disease-free animals at the beginning of the study are considered ‘at risk’, as an animal is not usually at risk of contracting the disease.
Withdrawals are those animals lost to follow-up. To deal with this, subtract half the number of withdrawals from the population at risk. This assumes that, on average, animals are removed from the Middle of the study period.

Question 10

What is a withdrawal in a study, and how is it accounted for in results?

Answer 10

Withdrawals are those animals lost to follow-up. To deal with this, subtract half the number of withdrawals from the population at risk. This assumes that, on average, animals are removed from the Middle of the study period.

Question 11

What is an ‘Open Population’?

Answer 11

Animals enter and leave through the study period. It is considered stable if the rate of additions and withdrawals are relatively constant over time.

Question 12

What are the two measures of Incidence?

Answer 12

Incidence Risk-the probability for a disease

Incidence Rate-the rapidity of disease occurrence

Question 13

What is Incidence Risk?

Answer 13

It is the Probability for a disease.
It relates the number of new cases to the size of the population at risk in the beginning of the time period studied. It is an accumulative incidence.
The proportion of disease-free individuals developing the disease over a specified time.
It has no unit as it is a probability estimate.
Animals have to be disease-free at the beginning of the time period as it is interpreted as the individuals risk of developing the disease within the period in question. Here animals may develop the disease several times during the time period, only the first occurrence is counted.
It assumes a CLOSED population.

Question 14

What is Incidence Rate?

Answer 14

It is the rapidity of disease occurrence.
It is related the number of new cases to the animal-time (units) at risk. It takes into account changes to the population at risk during the follow-up period.
It quantifies the number of cases per animal unit of time.
It is an instantaneous concept, the rate at which cases occur at a given instant. It does not have an interpretation at an individual level.

Question 15

What is epidemiology?

Answer 15

The investigation and explanation of the patterns, causes and effects of disease in defined populations.

Question 16

Define;
Epidemic
Endemic
Pandemic
Outbreak
Sporadic

Answer 16

Epidemic-Disease occurrence at a level higher than ‘normal’ or expected.
Endemic-Disease which is normally present within a population.
Pandemic-Widespread epidemic, usually on a global scale.
Outbreak-Sudden increase in the number of cases.
Sporadic-single cases or clusters of disease which are not normally present in an area.

Question 17

What are the differences between Incidence and Prevalence?

Answer 17

Incidence always requires a duration, prevalence may not.
In incidence, the unit of analysis is the event, in prevalence it is the animal.
Prevalence includes new and old cases, incidence only new.
Incidence is more important when thinking of aetiology (associations) of the disorder, prevalence when thinking of the burden of the disorder.
Prevalence can never exceed 100%.
Incidence may exceed 100% as the event may recur, I.e. Clinical mastitis. It is measured in those at-risk of the disease.

Question 18

What is Attack Rate?

Answer 18

Number of new cases / initial population at risk.
It is the same as incidence risk
Short time interval
Used in outbreak investigations
Calculated for different exposures and compared

Question 19

What is Secondary Attack Rate?

Answer 19

(Number of cases - initial case(s) / initial population ‘at risk’
Used to describe the ‘infectiousness’, contact with known cases.
Difficult to distinguish between primary cases and secondary cases when the latent period is long.

Question 20

What are the factors affecting Interpretation of data?

Answer 20

1. Temporal sequences
2. Disease duration
3. Real vs Apparent prevalence
4. Case definition
5. Dangling numerators
6. Population at risk
7. Adjusting rates

Question 21

Discuss the factors affecting the Interpretation of data?

Answer 21

1. Temporal sequences
   Can be used to examine possible causal relationships between risk factors and population health status, but not measured over time. Therefore we can not infer if factor or outcome came first.
2. Disease duration
   Prevalence study may miss cases where disease is of short duration (or fatal), resulting in higher incidence than prevalence.
   Chronic diseases detected for long periods are more likely to be detected in a prevalence study, resulting in higher prevalence than incidence. Anything that increase the duration of disease will increase the prevalence.
3. Real vs Apparent prevalence
   Real prevalence assumes that disease testing results are based on tests that are not 100% correct. True prevalence can be calculated by knowing test characteristics and test results. Test results are considered the apparent prevalence.
4. Case definition
   It is difficult to define a set of disease signs that will include all true cases of the disease.
5. Dangling Numerators
   Absolute numbers (I.e. Counts) do not take into account the consideration of the population at risk (older vs younger animals), so this does not tell us anything about the risk of it becoming a case (incidence) or being a case (prevalence).
6. Population at Risk
   Must be interpreted within the context of the population at risk, similar to the patients being diagnosed.
7. Adjusting rates
   Crude disease rates include population compositions that may interfere with interpretation. It is necessary to disentangle two factors before meaningful comparisons can be made between comparison groups.