Disease Outbreak Investigation Flashcards
Why investigate outbreaks?
-service to producers and vets
-active disease surveillance tool
-direction for research
-valuable source of teaching material
Outbreak
A series of events clustered in time and space
Outbreak investigation
A systematic procedure to ID causes (risk factors) of disease outbreaks and impaired productivity
Objectives of outbreak investigation
- Halt the progress of disease
- Determine reasons for the outbreak
- Recommend procedures to reduce the chance of future outbreaks
How to investigate herd outbreaks?
- Define the problem
-the What? Establish the existence of outbreak or issue - Define the groups
-Who affected? When? Where?
-collect samples - Why?
-Take action and report with recommendations for action. Follow up!
Defining the problem
-listen to the story
-show me (the problem)
-clinical exams
-decide if there is a problem
-necropsies
Data gathering
- initial contact
- the herd visit
- collection of the samples
Initial contact of data gathering
-don’t diagnose on the phone, ask open ended questions! No leading questions!
-collect and review records; standardized questionnaire
-Vet: working case definition, review risk factors, contact the lab
Herd visit of data gathering
-talk to everyone,
- timing is key; review story in chronological order
- document any findings (camera, samples. written notes)
-define important groups
-examine the animals
-compare cases vs. non cases
-examine environment; observe feed management
-show me (often situation does not make sense until see whats happening on the farm… ex. sick beef calves being house with dairy calves)
Defining important groups
-age groups
-temporal cohorts (when calving occurred)
-spatial cohorts (pen, pasture)
-Feedlots (farm of origin, truck loads, pens, sick pens, have pens been mixed?)
Examination of animals
-clinical exam- start with healthy animals, exam more sick last to avoid spread
-walk through
-distant exam
-BCS
-count and record
Comparison of cases vs non-cases
-Need a good case definition to compare cases to non-cases
*comparing clinical cases to sub-clinical cases can lead to a loss of power in the investigation
Recording location of management groups
-use maps, drawings, photographs
Is it a herd problem or a head problem?
Is it actually an issue or an anxiety issue?
-sometimes just a one off thing, not a herd problem
management experience and threshold of concern varies among owners
Pseudo-epidemics
-caused by the onset of producer awareness of a more common problem or caused by a change in the problem definition
eg. start using ultrasound for pregnancy checks
-have an increase in abortion rates BUT likely that detecting early embryonic losses that were happening anyway but not detectable with just palpation
Standard deviations
2 or 3 SD away is likely abnormal
-Falls outside 95% to 99% of the time
Necropsy
Necropsy all available cadavers- take notes and pictures, tissues fixed in formalin
-stillbirths- difficult to find answers
Problems with post mortem data collection
-necropsy material examined by more than one prosector
*can result in various interpretations
-diagnosis based on quantitative pathology rarely offered
Identifying important groups
Establish herd inventory
-BCS
-establish pregnancy status
-record group affiliation
Collect samples
*need adequate number of animals to recognize patterns across groups
-acute vs chronic
-diseased vs normal
-between locations
-young vs old
*Ensure all animals have individual identification eg. tags
Lab analysis
-helps define the problem and ID who is affected
*often only one chance to collect samples
*timing is important
*further access to animals may be impossible
7 S’s for sampling
Suck blood
Scoop poop
Swab nose, eyes
Slice necropsies
Spoon feed
Siphon water
Specify identify
Discussion with lab
-talk to lab
verify:
-appropriate specimen, collection procedure, amount, container, storage and transport instructions
Sample collection limitations
- limitations of lab tests (sensitivity/specificity)
*what are you going to do with false negatives/positives
-time consuming
-expensive
-don’t sample without an objective
Does establishing a definitive pathologic or etiologic diagnosis solve the problem?
NO
-need recommendations
Evaluate risk factors
-ID important groups and look for patterns of disease
*Who, Where, When
*epidemic curves
-use literature reviews and path models
*calculations and tools to pull apart components and simplify
-attack rate tables
Patterns in space
Where
-on site issues
-off site issues
Patterns in time
When
-Point source epidemic
-Sporadic epidemic
-Endemic problem
-propagated epidemic
Point source epidemic
rapid rise in cases, that then taper. Everyone who was going to get exposed, got exposed.
Endemic problem
-often goes on in the background
-no large peaks and valleys
Propagated epidemic
-everyone in initial exposure group is exposed
-next group- aged in, moved in, or were not exposed originally
Sporadic epidemic
-not really an issue
-few cases here and there
Attack Rate tables
Compare % of sick animals across suspected risk factors
*Exposed vs. unexposed
*method of difference
Herd records
-Best to have computerized records
-but need to evaluate quality of existing records
**often hardest part to get paid for
Prevalence
=# existing cases/population at risk
**Disease at some point in time
Incidence
=# new cases in period/total population at risk
=Risk or Attack Rate
**During a given period of time
Method of difference in attack rates
Look for which risk factor has the most difference between the exposed and unexposed
Case control study- evaluate risk factors
Calculate odds ratio!
-estimate of relative risk (can be poor sometimes)
What are the key determinants of disease?
Key determinants are the risk factors causing the problem that can be modified on the premises
eg. nutritional deficiences or toxin ingestion OR crowded calving areas
Follow up
-report to herd owner
-written report; only include important information
-write for intended audience
-include plans for followup
**not always an obvious answer- consider prospective monitoring or clinical trials
Court cases
-keep records for minimum of 7 years
Herd investigation summary
- Minimize further losses
- Listen to story (detailed history)
- Show me (herd records, observe the environment, distant exams)
- Define groups to ID risk factors
- Develop a hypothesis
6, Take appropriate samples - Examine animals (clinical and post mortem exams)
- Evaluate hypothesis using herd records and diagnostic findngs
9.Draw up action list - Give action list to the farmer, discuss hypothesis and make sure they know what to do
- Follow up
