Swine Disease Eradication Center -- PRRSV Diagnostic Update

September 16, 2009 — 

Recently a PRRSV boar stud infection went undetected through the routine PCR monitoring protocol. The infection was later confirmed by seroconversion using the IDEXX ELISA test. As a result, two sow herds receiving semen from the infected stud became infected as well.

Results from the investigation suggest that a new PRRSV strain has emerged. This new strain has a new cut pattern defined as 3-4-2 and varies markedly from the other 22 standard reference strains, including vaccine strains, used for comparison purposes. The new strain variation to the closest and more distant reference strains is significant at 8.3% and 18% respectively.

This new strain was not detected by the NAEU Taqman PCR test at the University Of Minnesota Veterinary Diagnostic Laboratory but it was detected by the commercially available Ambion PCR test. This new strain was hit and missed by the Tetracore test suggesting lower test sensitivity in pooled samples.

As new and genetically diverse PRRSV strains emerge, it must be realized that PCR-based techniques will not always be able to detect every new strain 100% of the time. PCR-based techniques use very specific and sensitive primers that need to be adapted continuously to detect emerging strains. Unfortunately this is not an uncommon situation in PRRSV and modifications to existing PCR tests are needed from time to time. In addition, when monitoring herds consider including serological testing to strengthen the ability to detect new infections.

The U of MN VDL understands the need to respond rapidly to this situation and has taken steps to modify the Taqman technique to detect the new 3-4-2 strain on a timely manner. The updated NAEU PRRS PCR test is expected to be available on September 16, 2009.

For more information contact Dr. Kurt Rossow at the University of Minnesota Veterinary Diagnostic Laboratory at 1-800-605-8787 or by Email at: vdl@umn.edu