Outbreak investigation

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Outbreak investigation

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

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Roadmap

• Background
• Verify the outbreak
• 1. What is the illness (problem)?
• 2. Is there a true excess of disease?
• Investigate the oubreak
• 1. Establish a case definition
• 2. Enhance surveillance
• 3. Describe outbreak according to individual,
  place and time
• 4. Develop hypotheses
• 5. Conduct analytical studies
• Implement interventions

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Background

• An outbreak is a series of disease events
  clustered in time
• During an outbreak the investigator asks the
  questions
• what is the problem?
• can something be done to control it?
• can future occurrences be prevented?
• May involve part of a herd, a whole herd, or an
  entire country
• clinical mastitis in a group of dairy heifers
• BSE, foot-and-mouth disease, classical swine fever

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Background

• Although the term outbreak implies a sudden
  (and possibly spectacular) event, be aware that
  outbreaks can be of a more insidious nature some
  causing subclinical losses in a population of
  animals over an extended period before being
  identified, characterised and investigated

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Background

• Because individuals are grouped on a herd basis
• we can compare affected animals, both clinical
  and subclinical, with unaffected animals, in both
  cross-section (at one point in time) and over
  time to determine the differences between both
  the animals themselves and the factors affecting
  them

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Background

• This search is often multithreaded
• meaning that a number of problems may be
  occurring at once, and
• The process is recursive
• meaning that new information causes a previous
  step to be revisited

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Background

• Stevensons First Law of Veterinary Science
• in individual animals, multiple clinical signs
  tend to have a single aetiology
• in populations, a single syndrome (e.g.
  production inefficiency, disease) tends to have
  multiple aetiologies

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Background

• Three types of problems
• acute
• e.g. management or husbandry error
• additive or cyclic
• e.g. combination of management or husbandry
  errors over time
• chronic
• e.g. long action of management or husbandry
  errors that required the passage of time for
  before the consequences became of sufficient
  magnitude to be recognised

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Roadmap

• Background
• Verify the outbreak
• 1. What is the illness (problem)?
• 2. Is there a true excess of disease?
• Investigate the oubreak
• 1. Establish a case definition
• 2. Enhance surveillance
• 3. Describe outbreak according to individual,
  place and time
• 4. Develop hypotheses
• 5. Conduct analytical studies
• Implement interventions

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Verify the outbreak

• 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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Verify the outbreak

• 2. Is there a true excess of disease?
• obtain objective data to document and verify the
  magnitude of the problem
• dont rely only on the memories and perception of
  management and employees (perception usually
  incorrect)
• perceptions of employees and managers and
  practitioners are valuable sources of hypotheses
  about risk factors

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Verify the outbreak

• 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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Roadmap

• Background
• Verify the outbreak
• 1. What is the illness (problem)?
• 2. Is there a true excess of disease?
• Investigate the oubreak
• 1. Establish a case definition
• 2. Enhance surveillance
• 3. Describe outbreak according to individual,
  place and time
• 4. Develop hypotheses
• 5. Conduct analytical studies
• Implement interventions

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Investigate the outbreak

• 1. 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
• Acute respiratory disease in recently weaned
  calves

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Investigate the outbreak

• 1. Establish a case definition (cont.)
• recognise that in many cases establishing a
  definitive pathological and aetiologic diagnosis
  doesnt 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 to be
  present there as well?

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Investigate the outbreak

• 1. Establish a case definition (cont.)
• case definition of Legionnaires' disease
• the clinical criteria required that a person have
  onset between 1 July and 18 August 1976, an
  illness characterised by cough and fever
  (temperature of 38.9 degrees or higher) or any
  fever and chest x-ray evidence of pneumonia
• to meet the epidemiologic criteria, a patient
  either had to have attended the American Legion
  Convention held 21 24 July 1976, in
  Philadelphia, or had to have entered Hotel A
  between 1 July 1976 and the onset of illness

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Investigate the outbreak

• 2. 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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Investigate the outbreak

• 3. Describe the occurrence of disease in terms
  of
• INDIVIDUAL
• PLACE
• TIME

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Investigate the outbreak

• 3. Describe outbreak according to the individual
• 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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Investigate the outbreak

• Attack rate
• numerator number of individuals with disease
• denominator number of individuals exposed
• Risk ratio (? relative risk)
• numerator incidence risk in exposed
• denominator incidence risk in unexposed
• Attributable fraction
• the closer AF is to 100, the greater the
  likelihood that exposure accounted for the
  outbreak

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Measures of effect

• Attributable fraction (AF)
• is the proportion of disease in the exposed that
  is due to the exposure

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Investigate the outbreak
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Investigate the outbreak
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Investigate the outbreak
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Investigate the outbreak
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Investigate the outbreak
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Investigate the outbreak

• 3. Describe outbreak according to place
• where are the affected and unaffected animals
  located?
• because different groups or pens of animals often
  have different levels of exposures a
  dose-response relationship exists for many
  aetiologic agents, this is an important set of
  clues

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Investigate the outbreak

• 3. Describe outbreak according to place (cont.)
• clustering ? an aggregation of disease events in
  space
• but often there will be large numbers of cases of
  disease where the size of the population at risk
  is large
• cases of disease per square kilometre
• vs
• cases of disease per head of population per
  square kilometre

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Investigate the outbreak

• 3. Describe outbreak according to 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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Temporal patterns of disease

• 3. Describe outbreak according to time (cont.)
• point source epidemics
• propagated epidemics

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Investigate the outbreak

• 4. Develop hypotheses about the nature of
  exposure
• generate hypotheses
• if necessary, search the literature
• subjective observations by the producer and other
  professionals are often useful
• prioritise your hypotheses by their likelihood
  and focus your efforts on those with the highest
  priority until they are either more fully
  supported or are refuted

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Investigate the outbreak

• 4. Develop hypotheses about the nature of
  exposure (cont.)
• a common error is to jump to generating
  hypotheses without first developing the
  quantitative information (the who, when, where
  counts) beyond vague clinical impressions
• note that for additive, cyclic or chronic
  problems, the initial occurrence of the
  underlying management or husbandry deficiencies
  are usually not close in time to the recognition
  of the problem

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Investigate the outbreak

• 5. Test your hypotheses
• predict what you should find in other animals,
  such as test results or production effects, and
  proceed accordingly to test your predictions
• finding what you predict supports your
  hypotheses not finding what you predict weakens
  your hypotheses

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Roadmap

• Background
• Verify the outbreak
• 1. What is the illness (problem)?
• 2. Is there a true excess of disease?
• Investigate the oubreak
• 1. Establish a case definition
• 2. Enhance surveillance
• 3. Describe outbreak according to individual,
  place and time
• 4. Develop hypotheses
• 5. Conduct analytical studies
• Implement interventions

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Implement interventions

• The goal
• what can I do today to reduce the impact of this
  problem in this herd
• has to be 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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Implement interventions

• As professionals, we should always aim to provide
  a written report of our findings
• gives client evidence of your efforts
• provides a valuable record for future reference
• How do I write a good report?
• Gardner I (1990) Reporting Disease Outbreaks.
  In Epidemiology at Work Refresher Course for
  Veterinarians, Quarantine Station, North Head,
  NSW, 1990, pp. 29 - 42.

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Roadmap

• Background
• Verify the outbreak
• 1. What is the illness (problem)?
• 2. Is there a true excess of disease?
• Investigate the oubreak
• 1. Establish a case definition
• 2. Enhance surveillance
• 3. Describe outbreak according to individual,
  place and time
• 4. Develop hypotheses
• 5. Conduct analytical studies
• Implement interventions