Effect of litter aggregation and pooling on detection of PRRSv in piglet processing fluids

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

The Science Page this week was written by the MSHMP team and covers pooling of processing fluids. A more detailed report can be found on the blog. Currently, a study to evaluate the number of negative processing fluids to declare a farm stable is ongoing. If you are interested in participating in this study, contact Dr. Juan Sanhueza (jsanhuez-at-umn-dot-edu).

Keypoints

  • Detection of PRRSV in processing fluids where one positive animal is mixed with large groups of negative animals is dependenton the level of viremia together with the sex of the positive pig.
  • Theoretically, low Ct values could be pooled/diluted more than 1,000 times and PRRSV could still be detected.
  • Processing fluids aggregation and pooling should be adapted according to the different prevalence scenarios and health goals (e.g eradication vs control).

Background

The use of processing fluids (PF) to monitor PRRSV in breeding herds has increased in the last two years. The processing fluids (e.g. tails and testicles) is a great cost-effective method that allows us to sample large numbers of animals without losing much of the individual sensitivity. In a previous study we evaluated the sensitivity of processing fluid PCR results at the litter level to detect at least one positive PCR piglet in a litter. Results from that study showed a sensitivity of 89% (95%CI: 66% to 97%). One of the interesting findings from that study was that we were able to detect PRRSV in seven out of eight litters that had at least one positive piglet. Sensitivity of the PF was influenced by number of positive piglets in the sample and viral quantity in the serum sample. This study lead us to the next study aiming at understanding the effect of aggregating litter processing fluids on PRRSV detection.

Objective

Current practices in the field involve aggregating more than one litter and pooling PF from different days or rooms. In this study we assessed the effect of both practices in the detection of PRRSV in PF.

Conclusions

The results of this study show that we can detect PRRSV after aggregating up to 50 litters, or above 1,000 dilutions when pooling. PRRSV detection with both methodologies is dependent on the level of positivity and the sex of the positive piglets since sensitivity is lower when a female pig was used as the source of virus in the aggregate sample. However, results of this study should be taken with caution and adapted to different prevalence scenarios and goals (eradication vs control). A more comprehensive study on the different strategies (number of litters to aggregate or number of PF to pool) that veterinarians and practitioners are currently using in the field and the time to declare a farm negative are needed.

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