Epidemiology

Question 1

Epidemiology definition

Answer 1

The study of patterns of disease and health in
populations.

Question 2

Key Concepts of disease

Answer 2

1. Disease is usually the result of multiple causes.
2. Disease never occurs in a random manner.
3. Disease can be considered an interaction between an agent, a host and the environment.

• Health is a state of equilibrium between agent, host and environmental factors
  (each of which may involve a number of factors).
• Factors that alter the interaction between agent, host and the environment may
  upset the balance and result in disease.
• Agent, host and environmental factors can all be determinants of disease (and alter
  the likelihood of disease).

Question 3

The epidemiology triangle

Answer 3

HPE and Time

Question 4

WHat are the modes of transmission

Answer 4

Direct (aerosol, contact)
Indirect (fomites, vectors, air-borne)

Question 5

What are the directions of transmission?

Answer 5

Horizontal
Vertical (mother to baby)

Question 6

What are the different types of hosts

Answer 6

Definitive host: a host in/on which the agent reaches sexual maturity

• Intermediate host: a host essential for the agent’s lifecycle, but in/on which it is unable
  to complete full development
• Reservoir host: a host where the agent develops or multiplies, but infection is not
  apparent
• Abnormal/dead-end host: a host where development - but not transmission - may
  occur

Vectors - play a critical role in the transmission of many agents either by mechanical
transfer or by acting as an intermediate host

Question 7

How is crude mortality calculated?

Answer 7

Total # deaths from all causes in time period/
Ave number of animals at risk

Question 8

How is case fatality rate calculated

Answer 8

Total # deaths from disease/
Total number of animals developing the disease

Question 9

What does disease influence in animal populations?

Answer 9

 Increased mortality
 Reduced yield and quality of product
 Reduced length and quality of life
 Changes to the structure of the population
 Effect on human and animal welfare

Question 10

What are the 3 types of disease durations?
What are the 2 types of disease presentations?

Answer 10

 Peracute - of very short duration
 Acute - a short course of 12-24 hours
 Chronic - of long to very long duration

 Clinical - with clinical signs
 Subclinical - without any clinical signs

Question 11

Patterns of disease occurrence

Answer 11

Sporadic - disease occurs infrequently, and without discernible pattern

Endemic - the disease is virtually always present, often at low occurrence

Epidemic - the number of cases in a population clearly exceeds what is normal

Pandemic - a widespread epidemic spanning countries and/or continents

Question 12

Types of epidemics

Answer 12

 a point source
 an intermittent source
 a continuous source

Propagating epidemic - an epidemic caused by an infectious agent in which initial
(primary) cases excrete the agent, and thus infect susceptible individuals that become
secondary cases

Question 13

Why do epidemics occur?

Answer 13

 Changes to host
o Susceptible animals are introduced into a
population
o There is a change in host susceptibility or
response
 Changes to agent
o There is a change in agent
infectivity/pathogenicity /virulence
o There are changing modes of transmission
 Changes to host-agent relationship
o Agent is introduced into susceptible population
o There is increased level of effective contact

Question 14

Key principles and methods of control for infectious diseases

Answer 14

Reduce the proportion of susceptible population
Reduce adequate contact between animals

Question 15

Key features of a disease control program

Answer 15

 Features of the disease – epidemiological and clinical
 Diagnostic capability
 Veterinary infrastructure
 Zoonotic implications
 Ecological implications
 Trade implications
 Availability of replacement stock
 Views of producers and society
 Legislation to enforce program and to provide compensation
 Economic costs of disease and of control options

Question 16

What are the 2 forms of surveillance and what are they for?

Answer 16

 A single disease – Targeted surveillance
 All diseases – General surveillance

Question 17

Describe sensitivity?

Answer 17

the ability of a test to detect disease in a diseased animal.

Question 18

Describe specificity

Answer 18

he ability of a test to detect the absence of disease in a healthy animal.

Question 19

How do you calculate sensitivity?

Answer 19

A/
A + C

A = true pos
c = false negs

Question 20

How do you calculate specificity?

Answer 20

D/
B + D

B = False pos
D = True neg

Question 21

How do you calculate true prevelance?

Answer 21

A + C/
A+B+C+D

A = true pos
B = false pos
C = false neg
D = true neg

Question 22

How do you calculate apparent prevelance

Answer 22

A + B/
A+B+C+D

A = true pos
B = false pos
C = false neg
D = true neg

Question 23

What is a positive predictive value?

Answer 23

probability that the disease of interest is present in
an animal with a positive test result.

Question 24

WHat is a negative predictor value?

Answer 24

probability that the disease of interest is absent in
an animal with a negative test result.

Question 25

How do you calculate PPV?

Answer 25

. A
PPV = /
A + B

A = true pos
B = false pos

Question 26

How do you calculate NPV?

Answer 26

. D
NPV = /
C + D

C = false pos
D = true neg