DISEASE E&E (Measuring Disease 3) Flashcards
Two components of animal health/disease surveillance:
- Early detection
- Early response
Animal health surveillance:
-identifies important disease events in animal populations
-allows farmers, vets and government control agencies to develop timely and appropriate responses to eradicate, control or mitigate adverse effects
Surveillance defined by WHO:
ongoing systematic collection, collation, analysis, and interpretation of data and the sharing of that info to those who need to know so action can be taken
Early detection:
disease monitoring system
-a defined level or threshold of disease at which an intervention should take place
Early response:
-defined set of interventions that will be taken if and when the threshold is reached
-interventions will vary depending on disease and its public health or economic significance
4 general purposes of disease surveillance:
- Demonstrating freedom from disease
- Early detection of disease
- Measuring the level of disease
- Finding cases of disease
For disease that are not currently present: (purposes)
-early detection
-demonstrate freedom
Diseases that are present: (purposes)
-case finding
-measure level of disease
Diseases that are NOT currently present or normally present:
-exotic diseases
-emerging diseases
-new diseases
-epidemic diseases
- poopy pants
Why demonstrate freedom?
- Trade access
- Trade barriers
- Stopping control or eradication measures that have been in place
- Public health measures could be lifted
- Political reasons
Why early detection?
-early response
-prevention of spread to other countries
Diseases that are present:
-recognized endemic diseases
-measure other facts that might limit market access (antimicrobial usage, resistance, animal welfare practices)
*define what is normal at the population level
Doing single measures at one point in time:
*prevalence
-prioritization of disease/risk analysis
>which disease have greatest economic impact
Doing multiple measures of disease for comparison:
-spatial distribution of disease
-temporal distribution
Spatial distribution of disease:
-understanding spatial risk factors
-establishing disease free zones
Ex. maybe could have a control point in Manitoba if FMD broke out in eastern Canada
Temporal distribution:
-monitoring control programs
-early detection of changes in endemic disease (trends)
-identify risk factors for disease and production practices that influence disease status
Finding cases of disease:
-identifying individual cases or individual outbreaks
-usually part of a disease control program
Concepts and principles to meet surveillance objections:
- Timeliness
- Population coverage
- Representativeness of surveillance sample
Representativeness of surveillance sample:
-representation (prevalence in sample=prevalence in population
-risk based (prevalence of sample > prevalence in population)
-biased (prevalence in sample (not equal) prevalence in population
Early detection surveillance system should:
-be continuous
-have comprehensive coverage of the population
-be sensitive with a very low design of prevalence
Demonstration of freedom surveillance system:
-does NOT need to be continuous, but may be intermittent
-can use risk-based sampling to increase efficiency
-uses a higher design prevalence than early detection
Measuring disease surveillance system:
-can be periodic
-should be based on representative sampling to avoid bias
-needs a large enough sample to give adequate precision
Case finding surveillance system:
-use comprehensive coverage of population
>risk based approaches can focus on high risk parts of the population
-be ongoing
-have good individual herd or animal sensitivity
Surveillance systems are characterized by:
-timing of data collection
-source of data
-sampling method
-level of detail/case definition
Types of surveillance systems:
- Passive surveillance
- Active surveillance
Passive surveillance
-using existing data that is available
-“observer initiated” data collection
-less expensive
-requires significant effort to collect and analyze data
-used to identify new and emerging diseases
*continuous but NOT representative data
Steps in a diagnosis being made:
- Disease animal shows clinical signs of disease
- Producer recognizes animal is sick
- Producer consults vet
- Vet examines animal and takes samples
- Samples submitted to diagnostic lab
- Appropriate tests conducted to detect disease
- Diagnosis made
Disadvantages of passive surveillance systems:
-samples are NOT representative of entire population
-info about population is lacking
-can’t always extrapolate to the general population
-certain diseases are suited for it (stigmatised, mild, ones with unfavorable cost-benefit ratios)
Active surveillance:
-purposeful collection of info often targeting a specific disease
-“investigator-initiated” data collection
-more expensive
-more accurate estimate of disease frequency
Ex. surveys, mass screening programs
Larry Brilliant and Smallpox surveillance:
*great example of active surveillance
-early detection…early response!
Risk based or targeted surveillance:
-target at animals or populations that are more likely to have the disease or in an area that it is more likely to be introduced
-usually used for freedom from diseases or early detection
-helps to optimize use of surveillance resources
Ex. surveillance for BSE in Canada
Surveillance for BSE in Canada:
-disease is very rare
-random sampling would be very INEFFICIENT way of finding cases
-4D: diseased, down, dying or dead
-*OSE has a point classification system for risk-based surveillance
Sentinel surveillance:
a “hazard-specific” surveillance system
-smaller numbers of animals of know immune status
-“canary in a coal mine” type of surveillance
-sentinel units are chosen to intensively monitor over a period of time
Observe sentinel units over a period of time:
-repeated visits to sentinel sites or sentinel animals
-antibody status often used to determine exposure
-measuring freedom from disease
-early warning system
-geographic distribution
Abattoir surveillance:
-inexpensive as animals are already being inspected and processed
-can cover a large number of animals
-relatively constant supply of data
-can be both active and targeted
-skilled observers are present
-potential problem with biased sample
Syndromic surveillance:
-group clinical diseases into syndromes rather than specific diagnoses
-indicator-based surveillance
-utilize clinical reports
-large amounts of data required
-simple, cheap and easy to obtain
-useful for early detection
*need to follow up suspicious events
-false alarms will occur
Ex. diarrhea
Random sampling:
-every member of the population should have an equal probability of being sampled in the surveillance system
*method used will be dependant on goals of the surveillance system
Sampling considerations?
-what is the primary objective of the surveillance system?
-do we need a representative sample or a risk-based sample?
-what is the sampling unit?
-how accessible is the population?
Simple random sampling:
-select a fixed percentage using a formal random process
-requires a SAMPLING FRAME (ex. list)
-will provide an unbiased estimate of prevalence
-larger sample sizes provide more precise estimates
-every individual has an equal opportunity of being sampled
Systemic random sampling:
-sampling units are selected from a sampling frame at regular intervals
-Ex. every fifth farm or every third animal
-calculate the required sample size
*divide expected population by the sample size required to get the appropriate interval
-can introduce bias if animals don’t enter facility randomly
Stratified random sample:
- source population divided into strata based on factors know to affect the outcome (ex. breed, gender, herd size)
>systematic sampling is used for selecting the sample units
-more flexible than simple random
-can have equal numbers or sample proportional to the size of the strata
Cluster sampling:
-each sample units is an aggregate of study units (ex. herds, litters)
-individual animal is still unit of interest
-randomly sample herds via simple, systematic or stratified random methods
*then sample all individual units within each unit
-infectious disease clusters within herds
Advantage of cluster sampling:
-less cost to sample more individuals
Disadvantages of cluster sampling:
-may lead to an underestimate of the variability of the disease in the broader populations of individuals
Multistage sampling:
-random sampling methods at 2 levels
-randomly sample herds (primary sampling units)
-randomly sample individuals within the herds (secondary sampling unit)
If more variation between herds: multistage sampling
- may want to sample more herds and less individuals
Sample size calculations:
-number of formulas that can be used
-if rare disease=have to sample more animals to try and find it
-if want an accurate estimate=must sample more animals
Challenge in building good surveillance systems:
-buy in and participation
-data integration and analysis
-confidentiality and data sharing
-maintaining vigilance
-diagnostic tests for identifying disease
Who is on the front line of disease surveillance?
-practicing vets and knowledgeable producers
Classical surveillance activities:
- Mandatory reporting (federally and provincially reportable diseases)
- Active surveillance for specific diseases
- Passive surveillance at labs, abattoirs, markets and dead stock
- Export testing and certification
Canadian animal health surveillance network (CAHSN):
-tried to standardize and merge data from different places
-an independent, member-drive “network of networks” for effective, responsive, and integrated animal health surveillance in Canada
Provincial or regional activities:
-Alberta Vet Surveillance Network
-western Canadian animal health network (WeCAHN
Sector or animal type activities:
-Canadian cow-calf surveillance network
Zoonotic disease and public health:
-rabies surveillance and control programs
-tick surveillance for lyme disease
