2012 International PRRS Symposium National Swine Improvement ...
Iowa State University College of Veterinary Medicine Diagnostic sampling strategies and...
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Iowa State University College of Veterinary Medicine
Diagnostic sampling strategies and classification of breeding
herds for PRRS virus
Pilot training school in PRRS diagnostics, 2012 Zagreb, Croatia
Dr. Derald Holtkamp
September 24, 2012
Iowa State University College of Veterinary Medicine
Objective
• Provide a herd classification system for describing the PRRSV status of herds, based upon a set of definitions that reflect the biology and ecology of PRRS virus
• Why?– Required for implementation of regional and national efforts
towards PRRSV control and elimination – Necessary to facilitate communication between veterinarians,
swine producers, genetic companies, and other industry participants
Iowa State University College of Veterinary Medicine
Background
• A definitions committee was formed to develop a herd classification system for describing the PRRS virus status of herds in July of 2009
• The committee was formed jointly by– American Association of Swine Veterinarians (AASV) – United States Department of Agriculture (USDA) PRRS-CAP
Iowa State University College of Veterinary Medicine
Background
• Committee was chaired by Dr. Bob Morrison and composed of researchers and veterinarians from private practice, industry, and representatives from the American Association of Swine Veterinarians (AASV) and National Pork Board (NPB)
– Drs. Dyneah Augsburger, Lisa Becton, Steve Henry, Derald Holtkamp, Dale Polson, Max Rodibaugh, Bob Rowland, Harry Snelson, Barb Straw, Montse Torremorell, Paul Yeske, Jeff Zimmerman (in alphabetical order)
• Input was also received from the AASV PRRS Task Force
Iowa State University College of Veterinary Medicine
Background
• The herd classification system proposed by the committee was approved by the AASV Board of Directors on March 9, 2010
– A recommendation from the AASV PRRS Task force was made to the Board to approve the herd classification system
• A manuscript describing the herd classification system and related terminology has been published in the Journal of Swine Health and Production (JSHAP) as a peer reviewed commentary
Holtkamp D.J., Polson D.D., Torremorell M. and committee members Morrison B. (chair), Classen D.M., Becton L., Henry S., Rodibaugh M.T., Rowland R.R., Snelson H., Straw B., Yeske P., Zimmerman J. 2011. Terminology for classifying swine herds by PRRS virus status. J. Swine Health Prod. 19:1 44-56.
Iowa State University College of Veterinary Medicine
Basis for classification of a herd
• The herd classification system is based upon the shedding and exposure status of the herd
– Shedding status is negative, uncertain or positive– Exposure (current or past infection) status is negative or
positive
Iowa State University College of Veterinary Medicine
Diagnostic evidence is based upon periodic herd tests
• Rely upon PCR and ELISA
• For the purpose of establishing a negative or uncertain shedding status and negative exposure status, no distinction is made between diagnostic results that are positive due to vaccine usage versus those due to wild-type virus exposure
Iowa State University College of Veterinary Medicine
What is an “uncertain” shedding status
• An uncertain shedding status is used when diagnostic data is available to suggest shedding may have ceased in a herd but without sufficient confidence to support a negative shedding status
– Lack of power in the sampling and testing strategy!
• An uncertain shedding status is considered transitory in the progress towards eliminating PRRS virus from an infected herd
Iowa State University College of Veterinary Medicine
Classification system for breeding herdsHerd category Shedding status Exposure status
Positive Unstable (I)
Positive Positive
Positive Stable (II-A)
Uncertain Positive
Positive Stable (undergoing elimination)
(II-B)
Uncertain – undergoing elimination
Positive
Provisional Negative (III)
Negative Positive
Negative (IV)
Negative Negative
Roman numerals were assigned to each category to emphasize that the status of herds must follow a specific order as they progress from Positive Unstable to Negative
Iowa State University College of Veterinary Medicine
Classification of breeding herds – IPositive shedding and exposure status
Herd category CriteriaSupporting Evidence
Required
I “Positive Unstable”
• Herds that do not meet the criteria for any of the other categories (II through IV) are category I by default.
• None required.
Iowa State University College of Veterinary Medicine
Classification of breeding herds – IIUncertain shedding and positive exposure status
Herd category CriteriaSupporting Evidence
Required
II-A
“Positive Stable”
•Starts after a 90 day period of
sustained lack of viremia in
weaning age pigs and no clinical
signs of PRRS in the breeding
herd.
•Herd has not initiated an
elimination program.
•Test serum from weaning age pigs by PCR.
•No positive results over a 90 day period (4 consecutive negative herd tests sampling every 30 days or more frequently) and no clinical signs consistent with PRRS observed in breeding herd.
II-B
“Positive Stable”
(undergoing
elimination)
•Same as II-A except; Herd has
initiated an elimination program
and intends to become Negative.
•Same as II-A
Iowa State University College of Veterinary Medicine
Definition of elimination & when it beginsDefinition
Initiation of elimination Elimination by any method requires 1) cease entry of PRRS virus positive animals and 2) discontinue intentional exposure to live virus.
Elimination begins when last seropositive breeding replacements are introduced or when the last intentional exposure to any live PRRS virus occurs in the herd, whichever is later.
For the breeding herd all PRRS virus elimination procedures can be placed into two categories: depopulation / repopulation and herd rollovers
Iowa State University College of Veterinary Medicine
Classification of breeding herds – III Negative shedding and positive exposure status
Herd category CriteriaSupporting Evidence
Required
Provisional Negative(III)
• Only for herds that have completed a herd rollover
• Starts 60 days after “negative breeding replacements” are first introduced during a herd rollover with diagnostic evidence that they remain uninfected.
• If growing pigs are present at the same premises, a confirmation of negative exposure status in that sub-population is also required.
• Test serum from negative breeding replacements by ELISA .
• No positive results, after ruling out false positives, at least 60 days after the initial introduction of negative breeding replacements.
• Test serum from growing pigs by ELISA.
• No positive results, after ruling out false positives
Iowa State University College of Veterinary Medicine
Classification of breeding herds - IV Negative shedding and exposure status
For herds established Negative by “herd rollover”
Herd category CriteriaSupporting Evidence
Required
Negative (IV)
All breeding animals in the herd are seronegative by ELISA
• Test serum from adult breeding animals by ELISA.
• No positive results, after ruling out false positives, subsequent to completion of rollover -confirmed by breeding animal inventory lists from production records.
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
• Trying to “rule out” that virus is circulating in the herd
In other words we want to say with some confidence that PRRS virus is not being shed and transmitted to other pigs in the herd
• Need to choose the right test– Need a sensitive test
• Need to look in the right place• Need to carefully choose sample size and frequency
– Prevalence is likely low as we are transitioning from Category I to II so we need sufficient sample size to find the small percentage of animals that may still be shedding virus
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
This is the test of choice
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
We don’t have unlimited money available for diagnostics
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
This is where we expect to find virus
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
How many and how frequently to sample
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
Will save money and time - OK?
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category II-A or II-B breeding herds
Requirements
Test(s) performed PCR
Animals tested Weaning age pigs
Specimen(s) collected Serum (blood, notch/swab, tail/swab)
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled Light weight males from gilt litters may increase sensitivity (Optional)
Systematic sampling procedure One pig per litter, random
Minimum number of samples per herd test 30 samples - Determined by target prevalence to be detected of 10% and 95% confidence level for any population size
Pooling strategies (if any) Pools of 5
Procedures to rule out false positives None
Minimum number of periodic herd tests Minimum of 4 to account for variation in prevalence and increase confidence of finding positives if present
Frequency of herd tests (minimum frequency) Every 30 days or more frequently to confirm statusFrequency to reconfirm status after initial tests depends upon reason for classifying herd
Decision rules that classify the herd None positive over a 90 day period (4 consecutive negative herd tests if sampling every 30 days); No clinical signs in breeding herd
Target prevalence to be detected 10%
Confidence level (for any population size) > 95%
How do we use the individual animal test results to make a diagnosis for the herd (ie. classification)?
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category III breeding herds
Requirements
Test(s) performed ELISA
Animals tested Negative breeding replacements (negative when entered – serve as sentinels)
Specimen(s) collected Serum
Sampling or whole-herd testing (every animal in population) Sampling
If sampling;
Targeted subpopulation (if any) sampled None
Systematic sampling procedure Random sample from multiple ages of breeding replacements
Minimum number of samples per herd test 60 samples - Determined by target prevalence to be detected of 5% and 95% confidence level for any population size
Pooling strategies (if any) None
Procedures to rule out false positives For positive and suspect samples, retest by ELISA and perform IFA and PCR; resample population if still positive
Minimum number of periodic herd tests 1
Frequency of herd tests (minimum frequency) 1 at least 60 days after initial introduction of negative breeding replacements Frequency to reconfirm status after initial test depends upon reason for classifying herd
Decision rules that classify the herd No positive after ruling out false positives
Target prevalence to be detected 5%
Confidence level (for any population size) 95%
Iowa State University College of Veterinary Medicine
Recommended protocol to assess PRRS virus status of Category IV herds established by herd rollover
RequirementsTest(s) performed ELISAAnimals tested Adult breeding animals Specimen(s) collected Serum Sampling or whole-herd testing (every animal in population) Sampling
If sampling; Targeted subpopulation (if any) sampled NoneSystematic sampling procedure (if any) Random sample from multiple parities and stages of
gestation and boars Minimum number of samples per herd test 60 samples - Determined by target prevalence to be
detected of 5% and 95% confidence level for any population size
Pooling strategies (if any) None Procedures to rule out false positives For positive and suspect samples, retest by ELISA and
perform IFA and PCR; resample if still positive Minimum number of periodic herd tests 1Frequency of herd tests (minimum frequency) 1 test after the herd has been category III for a minimum
of 1 year Frequency to reassess status after initial test depends upon reason for classifying herd
Decision rules that classify the herd None positive after ruling out false positives Target prevalence to be detected 5%Confidence level (for any population size) 95%
Iowa State University College of Veterinary Medicine
Acknowledgements• American Association of Swine Veterinarians (AASV) • PRRS-CAP, USDA NIFA Award 2008-55620-19132• National Pork Board (NPB)