Herd health investigation 1

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Herd health investigation 1

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

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Herd health

• What this series of lectures will cover
• Lecture 1
• multifactorial nature of disease
• investigating problems and implementing
  interventions
• Lectures 2 and 3
• causation
• measures of health
• Lecure 4
• case studies
• tools for herd health investigations

3
Roadmap

• Background
• Multifactorial nature of disease
• Investigating problems and implementing
  interventions

4
Background

• What is a herd?
• a population of animals
• sheep, cattle, pigs, fish, litters, live animal
  exports
• share common features
• ownership
• management
• environment

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Background

• What is health?
• freedom from clinical (obvious) disease
• scouring, coughing, rubbing, limping
• freedom from subclinical (not obvious) disease
• decreased appetite, decreased growth rates
• Being healthy
• bright, alert, responsive, eating, drinking
• producing to potential

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Background

• An investigation is the process of
• identifying problems
• identifying causes of problems
• creating solutions
• in many situations cook book solutions dont
  work
• skill required to identify ways to minimise
  impact of problems under a variety of individual
  situations

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Some problems are obvious
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Some problems are less obvious
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Some problems are less obvious
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Some problems are less obvious
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Background

• Herd health investigations
• detective work
• because disease does not occur at random, we must
  look for patterns
• What kind of patterns?
• space, time, animal
• This allows us to identify risk factors for
  disease
• Once risks are identified, they can be managed

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Roadmap

• Background
• Multifactorial nature of disease
• Investigating problems and implementing
  interventions

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Multifactorial nature of disease

• What causes sunburn?
• the sun!
• While the sun is a necessary cause of sunburn, it
  is not the only thing involved

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Multifactorial nature of disease

• Whether or not we get sunburn depends on
• Host factors
• colour of skin, level of previous exposure
• Agent factors
• season, time of day, length of time in sun
• Environment factors
• amount of clothing, use of suntan lotion

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What causes E. coli scours?
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Multifactorial nature of disease

• What causes E. coli scours?
• E. coli
• While the E. coli is a necessary cause of
  colibacillosis, it is not the only thing involved

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Multifactorial nature of disease

• Whether or not pigs get colibacillosis depends
  on
• Host factors
• immune status of sows, colostrum intake, birth
  order
• Agent factors
• strain of organism
• Environment factors
• stress (chilling), environmental contamination,
  management

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What causes E. coli scours?
Host
Management
Agent
Environment
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What causes E. coli scours?
Nutrition Vaccination Medication
Host
Management
Agent
Environment
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What causes E. coli scours?
Nutrition Vaccination Medication
Host
Management
Agent
Environment
Biosecurity Strain of organism
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What causes E. coli scours?
Nutrition Vaccination Medication
Host
Management
Agent
Environment
Biosecurity Strain of organism
Air quality Environmental temperature Level of
faecal contamination
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Multifactorial nature of disease

• How do pigs manage it?
• many animals in a confined space
• all breathing the same often poor quality air
• constant exposure to faeces and urine
• competition for feed and water
• constant exposure to an enormous number of
  mircoorganisms (bacteria, fungi, viruses)

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Theyre amazing
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Roadmap

• Background
• Multifactorial nature of disease
• Investigating problems and implementing
  interventions

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Investigating problems

• Step 1 What is the problem?
• aim to characterise cases in the form of a
  working definition
• ill thrift in recently weaned calves
• sudden death in grower pigs
• throughout the investigation, this working
  definition will evolve into a case definition

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Investigating problems

• Step 2 Is there a true excess of disease?
• obtain objective data to document and verify the
  magnitude of the problem
• do not rely only on the memories and perception
  of management and employees (perception usually
  incorrect)
• subjective perceptions of those involved are
  valuable sources of hypotheses about risk factors
  your task is to support or refute these and
  other hypotheses with objective evidence

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Investigating problems

• Step 2 Is there a true excess of disease?
  (cont.)
• compare the actual number of cases to the
  expected number to determine whether or not the
  frequency is excessive
• be careful of
• dangling numerators, that is counting the
  number of cases without considering the number of
  animals actually at risk of becoming a case
  during that time period
• what the producer accepts as normal (chronic
  problems lead to an increased tolerance of what
  is normal)

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Indonesian feedlot.
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Shewhart control chart of pull hazard as a
function of calendar date.
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Investigating problems

• Step 3 Establish a case definition
• if you can make a definitive pathological
  diagnosis, then the case definition is easy
• Pasteurella pneumonia in recently-weaned calves
• if you cant make a definitive diagnosis, it
  should still be possible to provide a case
  definition
• Undefined respiratory disease recently-weaned
  calves

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Investigating problems

• Step 3 Establish a case definition (cont.)
• recognise that in many cases establishing a
  definitive pathological and aetiologic diagnosis
  does not solve the producer's problem
• calf scour agents are ubiquitous and often
  sampling scouring calves will only confirm this
  fact
• the real question is Why does this farm have a
  problem with this agent when many others do not,
  even though the infection is most likely present
  there as well?

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Investigating problems

• Step 4 Enhance surveillance
• intensive surveillance required to accumulate
  enough cases on which to draw conclusions
• intensive surveillance required to document
  success (or failure) to recommended interventions
• at the national level the farming press, radio
  and television are effective tools for enhancing
  surveillance BSE

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Investigating problems

• Step 5 Describe the problem in terms of animal,
  place and time

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Investigating problems

• Animal
• compare affected animals with unaffected animals
• what are the characteristics of affected and
  unaffected animals in terms of exposure to
  potential risk factors, age, production level,
  stage of production cycle?

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Investigating problems

• Place
• where are the affected and unaffected animals
  located?
• because different groups or pens of animals often
  have different levels of exposures (e.g.
  different amounts of feed ingredients, different
  water sources, different housing, different
  pasture, different origins, different stages of
  the production cycle) a dose-response
  relationship exists for many aetiologic agents,
  this is an important set of clues

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Investigating problems

• Time
• when in calendar time did the problem actually
  begin?
• what is the pattern of performance over time?
• do not rely on human recollections alone verify
• be careful of pseudoepidemics caused by the
  onset of producer awareness of a more chronic
  problem or caused by a change in problem
  definition (or change of staff)

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Total daily rainfall (mm) and instantaneous
hazard of respiratory disease as a function of
calendar date.
Instantaneous hazard
Daily rainfall (mm)
Calendar date
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Investigating problems

• Step 6 Hypotheses
• based on history and information collected,
  develop hypotheses about what key determinants
  might be involved
• if necessary, search the literature for
  hypotheses about plausible risk factors
• subjective observations by the producer and other
  professionals are often valuable sources of
  hypotheses
• prioritise your hypotheses and focus your efforts
  on those with the highest priority

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Implement interventions

• The goal
• what can I do today to reduce the impact of this
  problem in this herd
• hands on, practical advise
• A secondary intervention may be to develop a
  means for monitoring the problem
• while monitoring what is going on is important,
  it shouldnt be the only advise that you give
• Farmer All of my calves have died what can I
  do?
• Vet I suggest that you do a better job
  recording case details

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Summary

• Background
• Multifactorial nature of disease
• Investigating problems and implementing
  interventions