Pooling udder skin wipes to detect influenza in preweaning pigs

Preweaning pigs

This is a recent publication from the Torremorell’s lab refining the use of udder wipes to detect influenza in preweaning pigs. It is available in the Journal of Veterinary Diagnostic Investigation.

Key points

  • Pooling of three, five, and ten udder wipes was evaluated.
  • Sensitivity decreased if the Ct count was above 31.5.
  • Pooling more than three udder wipes can affect the ability to detect influenza virus.



This study used previously collected udder wipes tested by RT-PCR for influenza A and with known Ct-values. 0.1 mL of each sample was mixed with 0.2mL, 0.4mL and 0.9mL of negative udder wipe samples to create the pools of 3, 5 and 10 respectively.


Sensitivity (%) and 95% CI in groups with low (<31.5) and high (≥31.5) initial cycle threshold (Ct) values, based on Ct values from undiluted samples (reference values).

Mixing the positive udder wipes in pools of 3, 5, and 10 resulted in sensitivities of 84.2%, 68.0%, and 63.0%, respectively and an increase in the Ct-values. While the sensitivity was not affected if the initial Ct-value was low (higher virus load), pooling significantly decreased the ability to detect influenza virus if the initial Ct-value was above 31.5 (low viral load). Polynomial regression showed that a 1:2 polling was enough to obtain a false-negative if the initial Ct-value is 34.

The full publication can be read on the journal’s website at https://doi.org/10.1177/10406387211039462


Influenza A virus (IAV) active surveillance in pigs prior to weaning is commonly conducted by collecting individual samples, mostly nasal swabs. Recently, the use of udder skin wipes collected from lactating sows was identified as an effective sampling method to indicate IAV status of suckling piglets prior to weaning. However, there is limited information on the effect of pooling multiple udder wipes on the ability to detect IAV. We evaluated the effect of pooling 3, 5, or 10 udder wipes on the sensitivity of detecting IAV and compared the results with testing the wipes individually. The likelihood of detecting positive udder wipes decreased with pooling when the initial positive cycle threshold value was ≥31.5; pooling of up to 3 samples could be performed without affecting sensitivity significantly. Our results support pooling of udder skin wipes to conduct surveillance of IAV in pigs prior to weaning.