About one-third of all U.S. swine breeding herds experience a porcine reproductive and respiratory syndrome (PRRS) outbreak each year that results in reproductive failure, abortion and high pre-weaning mortality among piglets.
Tongue tips from dead animals, particularly piglets, are being considered as an alternative way to monitor PRRS during sow herd stabilization, reports the Swine Health Information Center (SHIC). But researchers want to know how to best process these samples to optimize sensitivity for virus detection?
A study led by Igor Paploski and Cesar Corzo at the University of Minnesota describes the impact of different tongue tip processing and testing protocols to optimize the sensitivity and specificity of PRRS virus detection in sow herds.
Searching for Answers
Samples were collected from seven sow farms between two to five months after the onset of a PRRSV virus outbreak. These samples included tongue tips from 20 piglet mortalities collected at each farm.
According to SHIC, the samples from five farms were used to test different pooling strategies (individual testing and pools of five and 20) and laboratory processing techniques (testing tongue tip fluids versus tongue tissue homogenate). Samples from the other two farms were used to simulate different storage and shipping conditions (frozen versus refrigerated) and test samples at different time points (1, 4, and 7 days). All samples were tested by PCR at the University of Minnesota Veterinary Diagnostic Laboratory.
What Did the Research Find?
In the end, 100 tongue tips were collected from the first five farm visits. Within-farm PRRS virus prevalence on dead piglets ranged from 0% to 100% when testing tongue tip fluid individually, and from 0% to 45% when testing tongue tissue homogenate individually, SHIC reports. The results of tongue tissue homogenate were compared to tongue tip fluid, which was considered the gold standard.
When testing tissue homogenate individually, the sensitivity was 36%, specificity was 100%, positive predictive value was 100%, and negative predictive value was 76%.
In addiiton, when testing tissue homogenate in pools of five, the sensitivity was 75%, specificity was 100%, positive predictive value was 100%, and negative predictive value was 86%. The authors say pooling samples reduced diagnostic accuracy but acknowledged that pooling may offer value to producers as it allows testing of more animals at a reduced cost. The study also assessed the effects of storage conditions and time elapsed between collection and testing on Ct values.
In addition, a total of 40 tongue tips were collected from two other farms. Results showed that Ct values from frozen samples were lower than those from refrigerated samples stored for the same length of time, SHIC reports. For each day that elapsed since collection, Ct values of the samples increased by 0.2 units on average.
“This reinforces the importance of keeping tongue tips frozen during shipping but also shows that refrigerated samples still yield positive results,” SHIC writes. “Minimizing delays in handling and shipment is crucial, as these factors impact the sample’s Ct values for each elapsed day since sample collection.”
Why Tongue Tips?
Tongue tips are easy to collect, SHIC points out. That’s why researchers are focusing on gathering more knowledge on this method’s potential. Tongue tip samples target animal mortalities, which may have a higher likelihood of infection, and diminishes welfare concerns during sample collection.
“This study provides valuable insights into how sample processing and submission circumstances impact PCR PRRSV testing results of tongue tips,” SHIC says. “The findings directly address the practical challenges faced by veterinarians and producers in managing PRRSV outbreaks.”
By quantifying the effects of various processing and storage factors, SHIC adds that the results provide actionable insights to improve diagnostic accuracy.
“The use of tongue tips, as a non-invasive sample source from dead animals, offers a valuable tool for PRRSV surveillance, provided the optimized protocols are implemented,” SHIC says.
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