Veterinary Biometrics and Epidemiology

1
Veterinary Biometrics and Epidemiology

• Mark Stevenson
• Institute of Veterinary, Animal, and Biomedical
  Science
• M.Stevenson_at_massey.ac.nz
• Alisdair Noble
• Institute of Information Sciences and Technology
• A.D.Noble_at_massey.ac.nz

2
Housekeeping

• Biometrics
• the measurement of life
• Epidemiology
• the study of disease in populations

3
Housekeeping

• 38 lectures
• 31 biometrics (Alasdair N)
• 7 epidemiology (Mark S)
• Mondays (0900 1000) Vet 1
• Tuesdays (1400 1500) Vet 1
• Fridays (1000 1100) AH 2

4
Housekeeping

• Assessment
• Assignment 1 due 24 July
• Assignment 2 due 11 August
• Assignment 3 due 6 October
• Quiz 15 September
• Final exam

5
Housekeeping

• Resources
• Petrie and Watson (2005) Statistics for
  Veterinary and Animal Science. London Blackwell
  Science
• course notes
• sample exercises and problems (see EpiCentre web
  page)
• material on web sites listed in course notes
• questions Alasdair or myself
• email to arrange a suitable time to visit

6
Housekeeping

• How to do well in this paper
• attend lectures
• look at past exam papers
• think of examples that illustrate concepts that
  were talking about (not just the examples we
  give you )
• evidence of additional reading
• Lectures
• we probably wont cover all material intended
• youll need to read your notes

7
Introduction to Epidemiology

• Mark Stevenson
• EpiCentre, IVABS, Massey University, Palmerston
  North
• M.Stevenson_at_massey.ac.nz

8
Roadmap

• What is epidemiology?
• Disease in individuals and populations
• individuals host, agent, environment
• populations individual, place and time
• Temporal patterns of disease

9
What is epidemiology?

• Epidemiology ? the study of disease in
  populations
• What epidemiologists do
• describe the amount of disease in populations
• identify subgroups of the population with a high
  risk of disease and subgroups with low risk
• identify characteristics and activities
  associated with a high (or low) risk of disease
• modifying exposure to these characteristics
  and/or activities provides a rational basis for
  controlling disease

10
What is epidemiology?

• Well-known examples
• smoking ? lung cancer
• high fat diet, sedentary livestyle ? heart
  disease
• thalidomide ? birth defects

11
What is epidemiology?

• What is the relevance of this to the
  veterinarian?
• services provided by veterinarians should be
  regarded as an investment, not a cost
• preventing disease ? investment
• application of epidemiological principles to
  clinical practice ? a preventive approach
• established paradigm in food animal medicine, but
  also important in companion animal practice
• cancer eye in a herd of hereford cattle
• recurrent FUS in a family pet

12
What is epidemiology?

• This approach works well for diseases where the
  aetiology is known, but also for diseases where
  the aetiology is not known

13
What is epidemiology?

• For example
• November 1986 BSE first diagnosed in Great
  Britain
• to date there have been 170,000 confirmed cases
• after the first 200 cases, epidemiological
  investigations identified that meat and bone meal
  was the vehicle of transmission
• the relatively prompt ban on feeding meat and
  bone meal in July 1988 prevented an epidemic much
  larger than the one observed

14
What is epidemiology?

• So,
• if we can manage risk, we can manage disease
• even if we dont know the exact cause

15
Roadmap

• What is epidemiology?
• Disease in individuals and populations
• individuals host, agent, environment
• populations individual, place and time
• Temporal patterns of disease

16
Disease in individuals and populations

• Whether or not disease occurs in an individual
  depends on an interplay of three factors
• the host
• the agent
• the environment
• Useful concept when explaining to clients why
  some animals become sick and others do not

17
Disease in individuals and populations

• The level of disease in a population depends on
  an interplay of three factors
• characteristics individuals within the population
• age structure, breed, sex
• spatial factors
• where is the disease especially common or rare,
  and what is different about those places?
• temporal factors
• calendar time
• subject-referent

18
Disease in individuals and populations

• Individual factors that influence patterns of
  disease
• age structure of population
• distribution of genotypes
• immunity of population
• population dynamics and migration

19
Disease in individuals and populations

• Place (spatial) factors that influence patterns
  of disease
• proximity to pollutants
• proximity to infectious agents
• proximity to disease risks

20
Disease in individuals and populations

• Time factors that influence patterns of
  disease
• calendar time
• examples influenza in humans, bovine ephemeral
  fever in cattle
• subject-referent time
• examples milk fever in dairy cattle, gestational
  diabetes in humans
• epidemic curves are a useful way to visualise
  temporal patterns of disease

21
Counts of BSE submissions by 6-month period,
November 1986 to June 1997.
22
Roadmap

• What is epidemiology?
• Disease in individuals and populations
• individuals host, agent, environment
• populations individual, place and time
• Temporal patterns of disease

23
Temporal patterns of disease

• Terms used to describe the temporal pattern of
  disease in a population
• endemic
• disease occurs at expected frequency
• epidemic
• disease occurs at greater than expected frequency
• pandemic
• huge epidemic (international)
• sporadic
• single case or cluster of cases

24
Temporal patterns of disease

• Endemic
• disease occurs at expected frequency
• disease present in population or region at all
  times
• level of disease usually low and predictable
• examples
• lameness in dairy cattle
• long bone fractures in lt 10 year olds

25
Temporal patterns of disease

• Temporal patterns of onset can provide insight
  into nature of epidemic
• common source
• propagated

26
Temporal patterns of disease

• Common source epidemics
• disease arises from a single source of exposure
  to a causal agent
• epidemic curve shows a steep initial rise in case
  numbers and then a rapid falling off in the tail
• examples
• batch of contaminated feed causing an outbreak of
  salmonellosis in feedlot cattle
• milk vacuum problem causing an outbreak of
  clinical mastitis in a herd of dairy cows
• foodborne disease outbreaks

27
Temporal patterns of disease

• Propagated epidemics
• occur when the agent is transmitted through the
  population from host to host (typically
  infectious conditions)
• nature of epidemic depends on
• characteristics of agent (virulence) and host
  (susceptibility)
• contact rate
• population density
• examples
• influenza in humans
• contagious mastitis in dairy cattle

28
Epidemic curve typical of a common source
epidemic.
29
Epidemic curve typical of a propagated epidemic.
30
Temporal patterns of disease

• Other uses for temporal displays of disease data
• identify seasonal trends
• identify long-term trends

31
Monthly reports of human leptospirosis cases in
the USA, 1980 - 1995
32
Roadmap

• What is epidemiology?
• Disease in individuals and populations
• individuals host, agent, environment
• populations individual, place and time
• Temporal patterns of disease