how do you know when its an outbreak? (3)
- Clear-cut epidemic - recognized train wreck (with no obvious (to
the client) cause - Cumulative chronic problem
(Exacerbation of endemic disease, Build-up of subclinical disease (e.g. Johne’s)) - Newly recognized problem
what is an outbreak investigation
“a thorough search for and interpretation of clues with the goal of arriving at a solution quickly and efficiently”
epidemic vs outbreak
- Epidemic (epizootic)– Occurrence of disease at a higher than expected rate
- Outbreak – a cluster of events related in time and space with a markedly higher than expected incidence
how to determine if there is actually a real problem? (4)
- Counts vs. risks/rates (increased denominator or number at risk may in itself be a risk factor)
- Attempt to get or estimate denominators (number at risk)
- Compare rates or risks (with farm-specific history, with local or industry norms)
- Even if there is no real problem that you can quantify, the “outbreak” may represent a teachable moment
important considerations for data gathering in relation to preparation (4)
- Give yourself time (but make time to start in the moment if the initial call was not for an outbreak)
- +/- review key aspects of your diagnostic hypotheses to remind you or focus your questions and sampling
- It may be useful to draft a written plan or checklist for the main visit, but don’t let it drive the visit
- Speak with all the involved people, preferably in the same visit
main tasks of an investigation (6)
- Define the problem
- Define groups
- Collect samples
- Establish a working diagnosis
- Take action
- Do the follow-up
who do we need to observe in outbreak investigations?
affected AND unaffected animals
things to consider to establish a case definition (5)
- Simple and objective
- Mutually exclusive categories (usually 2)
- e.g. if possible, separate actual signs from treatment (UBRD vs UF vs pulled)
- Signs that occurred in specified time frame
- e.g. “abortion = unexpectedly not pregnant now” – Were they ever? When did they abort?
why is it important to consider pathobiology related to certain diseases in cows for example
- IBR aborted fetuses were infected 2-4 weeks ago
- BVD-PI’s born now were infected 5-6 months ago
- Clinical Johne’s cases were infected 2-5 years ago
how can you define affected and unaffected groups? (7)
- Age/parity
- Location/Group
- Exposures/risk factors
- Production level or stage
- Source (purchased or raised on the farm)
- Feed & water source
- Vaccination & treatment history
method of agreement vs difference
- “Method of agreement” – what do most or all affected animals
have in common? - “Method of difference” – what is different between affected and
unaffected animals?
you can approach hypotheses about key determinants by comparing what? (2)
- Exposure risks in affected and unaffected
- Disease risks in exposed and unexposed
what are some caveats with outbreak investigation (2)
- One effect may have several causes
- One cause may have several effects; this hypothesis is supported if several predicted effects are present
what is the very rough rule of thumb for collecting samples?
- Very rough thumb rule: collect 5 samples (5-10) per group
- This may mean 2 groups (affected vs. unaffected), or numerous groups
(exposed or unexposed to 1 or more potential risk factors)
some types of samples you may need to collect (7)
- Suck - Blood
- Scoop - Poop
- Swab - Orifices
- Slice - Necropsies
- Spoon - Feed
- Siphon - Water
- Specify - Identify
what is the take home message related to observations and confidence intervals?
- Take-home: when the number of observations is low, confidence interval will be wide (i.e. precision is low)
does outbreak investigation rely on finding statistically significant differences
- Outbreak investigation does not rely on finding statistically significant differences
- Statistically significant association does not equal causation
- Important differences may not be statistically significant
does infection = disease
no
what is the key question for a working diagnosis
the key question is, “Why is there a problem (associated with the bug) when others don’t have the problem?”
why do outbreaks occur?
More susceptibles
* Introduction of naïve animals
* Lack of specific immunity
* Lower innate resistance
Higher dose/greater challenge
* Clinicals generally shed»_space; subclinicals
* Environmental conditions that favour multiplication of pathogens
what does an outbreak indicate vs not indicate
An outbreak of clinical disease does not imply that a
* new agent
* or mutated/more virulent strain
has been introduced
- More commonly, unrecognized changes have occurred in the challenge from, or response to, endemic agents
key principles/critical control points for taking action (5)
- Reduce shedding by infected host(s); Treat or cull
- Reduce environmental contamination; Sanitation; cleanliness of feed, water, tools
- Reduce exposure to vectors; Fly and vermin control
- Reduce exposure of susceptibles; Isolation; segregation; ventilation; stocking density
- Increase resistance of susceptibles; Nutrition; vaccination; passive transfer
4 key points to remember when taking action
- Avoid the pitfall of intervening only in affected animals; They may no longer be at risk, Others are likely exposed and perhaps subclinical
- Be clear, concise, and concrete
- Prioritize; NOT “Improve passive transfer”, NOT “ Do a better job feeding colostrum”… Rather “Feed 4L of colostrum with a sanitized tube feeder within 6 hours of birth”
- If animal movements or segregation are required, walk around and decide where to move animals and how to house, feed, and water them
5 key questions to ask when doing outbreak investigation
- Who
- What
- When
- Where
- Why
- Use tools of descriptive epidemiology to answer these
