Disease Outbreak Investigation

Question 1

Why investigate outbreaks?

Answer 1

-service to producers and vets

-active disease surveillance tool

-direction for research

-valuable source of teaching material

Question 2

Outbreak

Answer 2

A series of events clustered in time and space

Question 3

Outbreak investigation

Answer 3

A systematic procedure to ID causes (risk factors) of disease outbreaks and impaired productivity

Question 4

Objectives of outbreak investigation

Answer 4

1. Halt the progress of disease
2. Determine reasons for the outbreak
3. Recommend procedures to reduce the chance of future outbreaks

Question 5

How to investigate herd outbreaks?

Answer 5

1. Define the problem
   -the What? Establish the existence of outbreak or issue
2. Define the groups
   -Who affected? When? Where?
   -collect samples
3. Why?
   -Take action and report with recommendations for action. Follow up!

Question 6

Defining the problem

Answer 6

-listen to the story
-show me (the problem)
-clinical exams
-decide if there is a problem
-necropsies

Question 7

Data gathering

Answer 7

1. initial contact
2. the herd visit
3. collection of the samples

Question 8

Initial contact of data gathering

Answer 8

-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

Question 9

Herd visit of data gathering

Answer 9

-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)

Question 10

Defining important groups

Answer 10

-age groups
-temporal cohorts (when calving occurred)
-spatial cohorts (pen, pasture)
-Feedlots (farm of origin, truck loads, pens, sick pens, have pens been mixed?)

Question 11

Examination of animals

Answer 11

-clinical exam- start with healthy animals, exam more sick last to avoid spread
-walk through
-distant exam
-BCS
-count and record

Question 12

Comparison of cases vs non-cases

Answer 12

-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

Question 13

Recording location of management groups

Answer 13

-use maps, drawings, photographs

Question 14

Is it a herd problem or a head problem?

Answer 14

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

Question 15

Pseudo-epidemics

Answer 15

-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

Question 16

Standard deviations

Answer 16

2 or 3 SD away is likely abnormal
-Falls outside 95% to 99% of the time

Question 17

Necropsy

Answer 17

Necropsy all available cadavers- take notes and pictures, tissues fixed in formalin
-stillbirths- difficult to find answers

Question 18

Problems with post mortem data collection

Answer 18

-necropsy material examined by more than one prosector
*can result in various interpretations

-diagnosis based on quantitative pathology rarely offered

Question 19

Identifying important groups

Answer 19

Establish herd inventory
-BCS
-establish pregnancy status
-record group affiliation

Question 20

Collect samples

Answer 20

*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

Question 21

Lab analysis

Answer 21

-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

Question 22

7 S’s for sampling

Answer 22

Suck blood

Scoop poop

Swab nose, eyes

Slice necropsies

Spoon feed

Siphon water

Specify identify

Question 23

Discussion with lab

Answer 23

-talk to lab

verify:
-appropriate specimen, collection procedure, amount, container, storage and transport instructions

Question 24

Sample collection limitations

Answer 24

• 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

Question 25

Does establishing a definitive pathologic or etiologic diagnosis solve the problem?

Answer 25

NO
-need recommendations

Question 26

Evaluate risk factors

Answer 26

-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

Question 27

Patterns in space

Answer 27

Where
-on site issues
-off site issues

Question 28

Patterns in time

Answer 28

When
-Point source epidemic
-Sporadic epidemic
-Endemic problem
-propagated epidemic

Question 29

Point source epidemic

Answer 29

rapid rise in cases, that then taper. Everyone who was going to get exposed, got exposed.

Question 30

Endemic problem

Answer 30

-often goes on in the background
-no large peaks and valleys

Question 31

Propagated epidemic

Answer 31

-everyone in initial exposure group is exposed

-next group- aged in, moved in, or were not exposed originally

Question 32

Sporadic epidemic

Answer 32

-not really an issue
-few cases here and there

Question 33

Attack Rate tables

Answer 33

Compare % of sick animals across suspected risk factors
*Exposed vs. unexposed
*method of difference

Question 34

Herd records

Answer 34

-Best to have computerized records
-but need to evaluate quality of existing records

**often hardest part to get paid for

Question 35

Prevalence

Answer 35

=# existing cases/population at risk

**Disease at some point in time

Question 36

Incidence

Answer 36

=# new cases in period/total population at risk
=Risk or Attack Rate

**During a given period of time

Question 37

Method of difference in attack rates

Answer 37

Look for which risk factor has the most difference between the exposed and unexposed

Question 38

Case control study- evaluate risk factors

Answer 38

Calculate odds ratio!
-estimate of relative risk (can be poor sometimes)

Question 39

What are the key determinants of disease?

Answer 39

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

Question 40

Follow up

Answer 40

-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

Question 41

Court cases

Answer 41

-keep records for minimum of 7 years

Question 42

Herd investigation summary

Answer 42

1. Minimize further losses
2. Listen to story (detailed history)
3. Show me (herd records, observe the environment, distant exams)
4. Define groups to ID risk factors
5. Develop a hypothesis
   6, Take appropriate samples
6. Examine animals (clinical and post mortem exams)
7. Evaluate hypothesis using herd records and diagnostic findngs
   9.Draw up action list
8. Give action list to the farmer, discuss hypothesis and make sure they know what to do
9. Follow up