iowa state university college of veterinary medicine diagnostic sampling strategies and...

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


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


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


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


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.


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


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


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


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


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.


Classification of breeding herds – IIUncertain shedding and positive exposure status


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


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


Classification of breeding herds – III Negative shedding and positive exposure status


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


Classification of breeding herds - IV Negative shedding and exposure status

For herds established Negative by “herd rollover”


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.


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



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



Requirements





If sampling;











We don’t have unlimited money available for diagnostics



Requirements





If sampling;











This is where we expect to find virus



Requirements





If sampling;











How many and how frequently to sample



Requirements





If sampling;











Will save money and time - OK?



Requirements





If sampling;











How do we use the individual animal test results to make a diagnosis for the herd (ie. classification)?


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


If sampling;

Targeted subpopulation (if any) sampled None

Systematic sampling procedure Random sample from multiple ages of breeding replacements


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


Confidence level (for any population size) 95%


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%


Acknowledgements• American Association of Swine Veterinarians (AASV) • PRRS-CAP, USDA NIFA Award 2008-55620-19132• National Pork Board (NPB)

iowa state university college of veterinary medicine diagnostic sampling strategies and...

Documents

herd shedding status

herd classification

prrs virus status of

uncertain shedding status

negative exposure status

positive slide

prrsv status of herds

aasv prrs task force