12a - Disease Investigation Outbreak I Flashcards
Why do we investigate outbreaks?
- Service to producers and veterinarians
- Active disease surveillance tool
- Direction for research
- Valuable source of teaching material
- *infectious disease to toxicities
What is an outbreak?
- A series of events clustered in time AND space ABOVE what we expect by chance alone or history
What is outbreak investigation? (‘medical detection’)
- A systematic procedure to ID causes (risk factors) of disease outbreaks and impaired productivity
- One of the most interesting and challenging aspects of herd medicine
- *one of the first steps in a long-term herd production and health management program
What are the 3 objectives of outbreak investigation?
- Halt the progress of disease
- Determine reasons for the outbreak
- Recommend procedures to reduce the chance of future outbreaks
What are the procedures for investigating herd outbreaks?
- Define the problem: WHAT
- Define the groups: WHO, WHEN, WHERE
- Collect samples
- Establish a working diagnosis: WHY
- Take action: written report with recommendations for action
- FOLLOW-UP
Define the problem: WHAT
- Listen to the story
- SHOW ME
- Clinical exams
- Decided IF there is a problem
- Necropsies
- *a problem well-defined is ¾ of the solution
Data gathering: 3 main things
- Initial contact
- Herd visit
- Collection of sample
Data gathering: Initial contact
- Don’t diagnose herd problems over the phone
- Be a good listener
- Do NOT jump to conclusions
- Don’t ask leading or loaded questions
What does the herd owner need to prepare for data gathering and a herd visit?
- Collect and review important records
- Preliminary questionnaire?
What does the veterinarian need to prepare for data gather and a herd visit?
- Working case definition
- Review risk factors
- Contact the lab
Data gathering: herd visit
- Talk to everyone
- Timing is important
- Document your findings
o Written notes
o Camera, video
o Samples
Data gathering: herd visit ‘steps’
- Review story in chronological order and look at herd background and management practices
- Start defining important ‘groups’
- Examine the animals
a. Need a good case definition - Examine the environment
Herd visit: define important ‘groups’ (EXAM)
- Age groups, temporal cohorts, spatial cohorts
- For feed lot
o Farm of origin
o Truck loads
o Pens, sick pens - *record location of all management groups! (maps, drawings, photos)
Herd visit: examine the animals
- Clinical examination: DETAILED
- Walk through
- Distant exam
- BCS
- Count and record
- *work from healthy to sick
- *use gloves
Herd visit: need a good case definition
- Need one to compare cases to non-cases
- Comparing clinical cases to sub-clinical cases
- *if not in right category=can lead to loss of POWER for the investigation
- *iceberg concept
Herd visit: examine the environment
- Verify herd management info (ex. how often are they actually bedding?)
- Observe feeding management: mineral or feed ‘always’ out
What varies greatly across herd owners? (and influences if there is a ‘real problem’)
- Management experience
- ‘threshold of concern’
- *sometimes there is NO problem (ex. 2 abortions in 150 cows)
What are pseudo-epidemics caused by?
- Onset of producer awareness of a more common problem OR caused by a change in problem definition
- Ex. change from visual observation of abortion to early pregnancy palpation or ultrasound in a dairy herd
- *use own data or data from the literature
“The dead pile never lies”
- Necropsy ALL available cadavers
o Notes and pictures
o Use new technologies (histopathology OR immunohistochemistry to get confirmation
What are some problems collecting postmortem data?
- Necropsy material examined by more than one prosecutor
- *diagnosis based on ‘quantitative pathology’ rarely offered
- *make sure there is COORDINATION
Herd that had an 80% increase in calf mortality: 20 postmortem exams
- Need to put the information together
o Severe enterocolitis
o Villus and crypt loss and necrosis
o Lymphoid depletion - *NEED A SYSTEM FOR COLLECTING SAMPLES AND COMBINING THE RESULTS
Tissues you collect: some you may not think of
- Thyroid gland
- Thymus
- Salivary gland (Ex. Vit A deficiency=squamous metaplasia)
- Sciatic nerve
You need to ID important groups and establish herd inventory
- Body condition score
- Establish pregnancy status
- Record group affiliation
o Different management, pastures, feeding?
What is the importance of collecting samples?
- Need ADEQUATE NUMBER of animals to recognize PATTERNS ACROSS GROUPS
o Acute vs. chronic
o Diseased vs. normal
o Between locations
o Young vs. old
Lab analysis can help define the problem and ID who is affected
- Often only ONE chance to collect samples
o Timing is CRITICAL
o Further access to animals may be impossible - *look for things I can find (ex. horse)? Or start hunting (ex. zebra)?
o Ex. Salmonella is a ‘zebra’ in beef still in Canada
Collecting samples
- Right containers and coloured tubes
o Blue=trace mineral testing - Ex. Brisket tagger for a round biopsy
- *consider limitations associated with sensitivity and specificity of lab tests (know before you take the samples)
What are the 7 S’s for sampling?
- Suck: blood
- Scoop: poop
- Swab: nose, eyes, etc
- Slice: necropsies
- Spoon: feed
- Siphon: water
- Specify: ID (have tags with you, need to be able to ID the animal after the fact)
Collection of samples: talk to the lab and verify the
- Appropriated specimen
- Collection procedure
- Amount
- Container
- Storage and transport instructions
Avoid ‘fishing expeditions’ when sampling
- *sampling without an objective
o Seldom useful
o Expensive
o Loss of time and credibility
Consider sample banking
- Many can do well in freezing or liquid nitrogen
- Save the $$ to begin before you know if you need to test
Establishing or verifying the pathological and etiological diagnosis
- Recognize that in many cases ONLY establishing a definitive pathologic or etiologic diagnosis does NOT solve the producers problem
Farm with an abortion problem, colleague wants to test for N. caninum. What do you tell/ask him?
- What do we have to work with (aborted fetuses)?
- What other species?
- Other producers in the area?
- *Objective: trying to just know the reason or prevent something in the future?
How can you evaluate risk factors?
- ID important groups and look for patterns of disease
- Look for patterns and natural experiments
- Orient by subject (who), place (where, map) and time (when, epidemic curves)
- *literature review
- *path models
Epidemic curves: ‘type’ of epidemic
- Point source
- Sporadic
- Endemic problem
- Propagated
Point source epidemic
- Rapid rise in cases, then decline
- Ex. toxin
Sporadic epidemic
- Few cases scattered or time (no pattern)
Endemic problem
- Consistent over time
- Ex. staph. Aureus mastitis in dairy herds
Propagated epidemic
- Continues over time
- Initial group exposed and then another group exposed or moved in
- *need to differentiate it from point source epidemic
Path models
- Someone else has pulled together main risk factors for a particular problem
- Ex. neonatal losses due to infectious disease or due to accidents
What 2 main factors affect weaning weight? (‘risk factors’)
- Age of the calf (younger=not as heavy)
o More of a reproductive problem - Are they gaining weight like we want? (nutrition, etc.)
- *how can you differentiate between? (take age out of the equation)
Attack rate tables
- Compare percent of sick animals across suspected risk factors
o Exposed vs. unexposed - Which factor(s) are most associated with disease?
Examine the herd records?
- Do you have access to computerized records?
- Evaluate quality of records
- Have herd owner do as much as practical and possible
- *often hardest part to get paid for
*What is prevalence?
- EXISTING cases divided by population at risk
- *disease AT some point in time)
*What is incidence?
- NEW cases in period/total population at risk
- *disease DURING a given period of time
- *equals risk or attack rate
**What is the equation for relative risk (RR)
- Attack rate for positive heifers divided by attack rate for negative heifers
- *account for uncertainty with p-value (p<0.05) and confidence intervals (95% CI)
o Do not want RR=1 in CI (same as having a p>0.05) - **risk factor with the highest RR is a place to start looking for the cause of the problem
- *when you know/include all
**Odds ratio (EXAM)
- When do a case-control study
o We decided/determined control and cases=cannot use RR - *estimate of RR
o Can be poor sometimes (overestimate) - *do when we only have a subset
What are the key determinants for disease?
- The RISK FACTORS causing the problem that can be MODIFIED on this premises
- Ex. adequate bedding and crowding, nutritional deficiencies, inadequate dam immunizations
Create a list of action items
- Be very specific
- Recommendations must be appropriate for INDIVIDUAL HERD MANAGEMENT
Follow up and report to herd owner
- Written report
o Include only MOST important info
o Write for you INTENDED audience - *include plans for follow-up
- Wont always be an obvious answer
- *keep records for a minimum of 7 years (some cases go to court)
What are the 11 steps to a herd investigation?
- Minimize further losses
- Listen to the story
- Show me
- Define groups to ID risk factors
- Develop a hypothesis
- Take appropriate samples
- Examine animals
- Evaluate your hypothesis using herd records and diagnostic findings
- Draw up an action list
- Give action list to farmer, discuss hypothesis and make sure they know what to do
- Follow up
What can you look at for herd fertility?
- Pregnancy rate
- Abortions
Neosporosis abortion storm and increased risk
- Seropositive cows: 6-11x more likely to be open
- *ruled out other pathogens
When and where did Neospora infection enter the herd?
- Vertical transmission important? NO associated between cows and their dams
- *Environmental source
o No difference between pastures
o *epidemic curve supported a POINT SOURCE (fence line or a mineral mix)
Follow up to try and figure out where Neosporia came from
- did precolostral blood samples
o 64% seropositive
o 82% born from positive cows were positive at birth - *only culled the seropositive that actually aborted (not feasible to get rid of them all, and it worked out for them)
Recommendation for control of Neospora?
- Proper carcass disposal
- Minimize access of dogs to cattle feed and feed storage areas
- Raised salt and mineral feeders on pasture
- Test and culling of positive herd replacements (is it even practical?)
- *complicated and many more questions than answers on it!
