March 5, 2019
By David H. Baum, Luis Gimenez-Lirola and Jeff Zimmerman, Iowa State University Veterinary Diagnostic Laboratory
These two examples recently came to the Iowa State University Diagnostic Laboratory. They show antibody testing as a tool to monitor pig populations for porcine reproductive and respiratory syndrome virus.
Application No. 1: Population monitoring a.k.a. “Well, have they been exposed?”
A set of six serum samples was collected from a boar stud and sent to the Iowa State University Veterinary Diagnostic Laboratory. Our client requested PRRS antibody and polymerase chain reaction testing of the individual samples (antibody) and of a pool made up of all six samples. Those results are in Table 1.
Table 1: Summary of screening and confirmatory tests. The positive/negative cut-off value for the PRRSV X3 ELISA is 0.400; a sample is positive if the S/P ≥ 0.400. The positive/negative cut-off value for the PRRSV PCR is 40; a sample is positive if its Ct <40. The screening test results for PRRS antibody (by ELISA) and PRRS nucleic acid (by PCR) are under the columns “PRRSV X3” and PRRSV PCR. The confirmatory test results for PRRS antibody (by ELISA) and PRRS nucleic acid (by PCR) are under the columns “PRRSV X3 — Retest” and “PRRSV PCR — Retest”. The PRRSX3 — Retest was of the two positive samples. The PRRSV PCR — Retest was of the individual samples of the Pool (1-6).
This client knew how to use ante-mortem diagnostic tools and their results to understand the pathogen status of a population.
Application No. 2: Population monitoring a.k.a. “How well have they been exposed?
In another submission, one of our clients sent 110 serum samples from a population of growing pigs. This client wanted to know the samples’ PRRSV X3 results to assess if an injection crew properly administered a PRRS vaccine. Without showing all 110 results, while the client expected variation in results and understood not all individuals in a population respond identically when vaccinated there was concern about the negative values.
Then, the ISU VDL created a histogram of those results (Figure 1). A normal distribution of S/P values is expected. The ranges within each bar remind us not every pig responds identically to vaccine and that an S/P between 0.200-0.399 may well be a response to vaccine. Six pigs with results less than the positive-negative cutoff would award the crew a score of 94.5%. All might sleep better each night if you knew every injection crew’s results looked like this distribution. The client knew how to use this information to instruct his team.
Figure 1: Histogram of PRRSV X3 ELISA S/P values following administration of PRRS vaccine. The x-axis is the range of PRRSV X3 ELISA S/P values. The y-axis is the number of pigs with an S/P that is within the bar’s range of S/P values (X- axis). Each bar’s average S/P is above each bar.
Sources: David H. Baum, Luis Gimenez-Lirola and Jeff Zimmerman, Iowa State University, who are solely responsible for the information provided, and wholly owns the information. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.
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