Valuable diagnostic tools aid in PRRS control strategies.
Since diagnostic tests are performed on serum (the liquid portion of blood), samples can be easily obtained to determine the porcine reproductive and respiratory syndrome (PRRS) status of live pigs.
Serology testing consists mainly of: 1) those tests that detect the pig's response to exposure to PRRS (immunity) and 2) those tests that detect the presence of the actual virus circulating in the pig's bloodstream.
Several tests detect the pig's immune response to PRRS. The most common is the enzyme-linked immunosorbent assay (ELISA), sold commercially as HerdChek. As the name implies, this test determines the PRRS status of the herd. The veterinary community quickly accepted this test for both population-based testing and for examining individual animals.
The polymerase chain reaction (PCR) test detects the presence of PRRS virus in the serum or blood. Automated, computerized methods of PCR speed turn around time and have substantially lowered the cost of this test.
Few, if any, serological tests reach 100% accuracy, and the PRRS tests are no exception. Tests are rated to measure specificity and sensitivity. Specificity defines the test's ability to avoid “false positives,” while sensitivity measures how often the test would miss a true positive.
All serological tests have some degree of error. Some tests have a known false-positive error rate of perhaps 0.5 to 1%.
In populations of expected negative pigs, finding even 1% positive animals using the ELISA test can be cause for concern. Lesser-used “back-up” tests are initiated to confirm or deny the presence of virus in the herd.
At times, these back-up tests are only used to clear up unexpected results. But there are also testing protocols that take a “belt and suspenders” approach, whereby they utilize two tests simultaneously to help account for test discrepancies. The best example is requesting ELISA and PCR tests at the same time on the same set of sera. The negative results on the PCR test may help refute a single positive on the ELISA test.
Another commonly used back-up test for the ELISA is the indirect fluorescent antibody (IFA). The IFA also tests for the presence of specific antibody within the pig's system, but in a different way from the ELISA. It serves as a confirmatory test for an unexpected positive on ELISA. It is generally accepted as less specific and less sensitive than ELISA, but remains the best choice for confirming ELISA results.
It is very important to know the vaccination history of the pig population before testing for PRRS, since the tests that measure immunity will not differentiate between a field strain infection and that produced by the modified-live-virus vaccines. A sample that is PCR positive, however, can sometimes be differentiated by sequence analysis.
Another key aspect of understanding PRRS serology is the timing of the sample in relation to time of exposure to the virus. Realize there is normal variation among individual animals, and differences in how individuals of various ages respond to exposure to the PRRS virus.
Case Study No. 1
Client A maintains a PRRS-naïve herd and monitors a cross-section of 30 sows every quarter. On this quarterly test, two samples were positive on the PRRS ELISA test. The client called because he was concerned these positives signaled problems ahead.
We immediately requested IFA testing on the two samples that were ELISA positive, and happily confirmed the IFA tests were negative, indicating the two ELISA samples were indeed “false positives.”
Case Study No. 2
Client B purchased PRRS-naive replacement gilts from a multiplier four times/year. Each shipment of gilts was isolated for 45 days and serologically tested for PRRS to prevent the introduction of PRRS virus.
About two weeks after the last group of gilts arrived and were isolated, the client called to schedule testing of gilts.
Instead of testing all the gilts, we sampled a statistically valid subset of 30 animals, and requested both ELISA and PCR tests for PRRS from the diagnostic laboratory.
The lab results revealed two samples were positive for PRRS virus on the PCR test. The lab retested the results and I resampled the gilts. This time, I made certain to resample the positive gilts as well as several of their penmates in isolation, again requesting both ELISA and PCR tests.
Unfortunately, the follow-up samples were all PCR positive and one third of the samples were also positive or suspicious on the ELISA test. These gilts were likely exposed to PRRS in transit to the isolation barn or shortly after arrival. They were immediately removed from the premises, and fortunately for the client, PRRS virus was not introduced into his sow herd.