Date Full Report Received02/04/2019
Date Abstract Report Received06/11/2019
InvestigationInstitution: Iowa State University College of Veterinary Medicine
Primary Investigator: Dr. Daniel Linhares DVM
Co-Investigators: Will Lopez, Jeffrey J. Zimmerman, Luis Gimenez-Lirola, Phillip Gauger
Funded ByNational Pork Board
The aim of this project was to assess the use of processing fluid samples in PRRSV monitoring and surveillance. For the purpose of this project, “processing fluid” (PF) was defined as an aggregate sample composed of the fluids recovered from the tissues (testicles and tails) collected from piglets at the time of processing (Lopez et al., 2017).1 Processing fluids are easily obtained by farm staff under field conditions and represent a unique opportunity to significantly improve current monitoring schemes.2 The use of processing fluids 1) reduces the cost of PRRSV surveillance, 2) increases the number of piglets sampled, and 3) increases the frequency of testing.
The overall objective of this study was to evaluate the use of processing fluids for PRRSV monitoring. Our specific aims were to 1) determine limit of PRRSV detection in processing fluids using a commercial quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay; 2) optimize serologic assays to detect anti-PRRSV antibodies in processing fluids, and 3) at low prevalence (<5%), describe the probability of PRRSV detection using processing fluids tested with qRT-PCR.
To achieve Aim 1, 6 replications of eight two-fold serial dilutions were done on a PRRSV-positive field sample with known qRT-PCR results using processing fluids from PRRS-naïve herds and the PRRSV limit of detection was assessed. For Aims 2 and 3, litter-matched processing fluids and individual pig blood samples were collected from 77 litters. Sample collection was carried out in two PRRSV-positive breeding herds at different sampling point in time following PRRS outbreaks and therefore, capture different PRRSV prevalence levels within the herds (from high to low prevalence), thus allowing for the assessment of the probability of PRRSV detection in processing fluids using the status of individual pig sera as the reference. Samples were tested for antibody using the IDEXX PRRS X3 Ab ELISA test: first using the kit’s manufacturer’s directions and secondly, after making adaptations. Processing fluids obtained from PRRS-naïve herds were used for comparison.
Based on the PRRSV detection probabilities derived from the qRT-PCR results from the serially diluted processing fluid field sample, the limit of PRRSV detection by qRT-PCR in processing fluids was 1 PRRSV-positive pig out 270 total pigs in the pool, with a 95% confidence level, given that there is only one viremic piglet with a qRT-PCR CT value of 29 in the sample.
The results obtained with a modified PRRS IgG antibody ELISA showed a better and clear discrimination between positive and negative processing fluid samples compared to the original ELISA kit, designed only for IgG antibody detection in serum samples. The IgA results showed good discrimination between positive and negative processing fluid samples overall; however, not all samples detected positive by PCR were IgA-positive. Contrary, IgM results showed poor discriminatory power to detect active or recent PRRSV infection in processing fluids.
The probability of PRRSV detection in processing fluids was 72.7% at the individual litter level, as compared with the blood serum sample qRT-PCR results used as the gold standard method for comparison and to define the true status of the litters regarding PRRSV. However, when pooled by farrowing room or by the whole batch of piglets processed during a day, processing fluids had a probability of PRRSV detection of 100%.
The work hereby presented addresses all of these points and provides the industry with a reliable, fast, practical, and affordable surveillance system to detect PRRSV at low prevalence in suckling pigs. It is our expectation that this will pave the road for disease elimination efforts at regional and/or national levels. Controlling and eliminating PRRS from the U.S. swine industry will significantly improve the economical sustainability and competitive advantage of pork production in this country.