Swabs for Precision

You can use swabs for PRRSV surveillance in weaning-age pigs with precision.

by Onyekachukwu Henry Osemeke

how to collect samples from piglet
Figure 1. A pictorial summary of samples collected from each piglet. A: Serum collection B: Ear-vein blood swab collection C: Oral swab collection D: Nasal swab collection.

[Note that this article is a re-written and shortened version taken from its original published form in Frontiers in Veterinary Science by the original author.]

Despite advancements in understanding the porcine reproductive and respiratory syndrome virus and increased efforts to prevent, control, or eliminate it, the porcine reproductive and respiratory syndrome virus (PRRSV) still causes significant issues for the pig farming industry worldwide (Cheng et al., 2022; Holtkamp et al., 2013; Nathues et al., 2017).

Weaning-age pigs play a significant role in spreading PRRSV and are the subpopulation of choice for surveilling PRRSV in breeding herds (Holtkamp et al., 2021).

While traditional blood sampling is ideal for monitoring the virus, alternative sample types, such as swabs, are easier to collect and are frequently submitted to US veterinary diagnostic laboratories for PRRSV investigation.

However, there are no guidelines on how these swab samples could be used for PRRSV surveillance in specific scenarios.

This study sought to compare PRRSV detection rates by reverse transcription real-time polymerase chain reaction (RT-rtPCR) in swabs and serum obtained from weaning-age pigs with the end goal of determining comparable sample sizes.


Three eligible PRRSV-positive herds in the midwestern United States were selected for this study—666 pigs were sampled for serum, nasal swabs (NS), ear-vein blood swabs (ES), and oral swabs (OS).

All samples were tested for PRRSV RNA by RT-rtPCR at a National Animal Health Laboratory Network (NAHLN)-accredited veterinary diagnostic laboratory.

The binary outcome (RT-rtPCR positive or negative) was obtained for each sample tested, and the distribution of the cycle threshold (Ct) values was evaluated using box plots.

Cohen’s Kappa analysis (McHugh, 2012) was used to assess agreement in the RT-rtPCR results between all paired combinations of sample types.

Sensitivity and specificity values were obtained for each swab sample using serum as the reference. Appropriate sample sizes were then calculated using the obtained sensitivity and specificity values.


As anticipated, serum samples exhibited the highest RT-rtPCR positivity rate, with 96 pigs testing positive.

These serum samples also demonstrated the lowest median Ct value (Figure 2).

chart showing distribuition of cts by sample type
Figure 2. RT-rtPCR Ct distribution by sample type. (ES: ear-vein blood swabs; NS: nasal swabs; OS: oral swabs; Srm: serum). The red dashed line represents the RT-rtPCR Ct cut-off value for test positivity.

Of the 96 pigs testing positive by serum, 80 tested positive by ES and OS, while 72 tested positive by NS.

Among the swab sample types, ES showed the lowest median Ct value, whereas NS showed the highest.

It was also observed that the lower the RT-rtPCR Ct in serum samples, the higher the likelihood that any swab sample from that pig will be positive.

Cohen’s Kappa analysis revealed near-perfect agreement (≥ 0.81) between all paired combinations of sample types.

Notably, a small number of pigs tested positive in swab samples but negative in serum: seven by OS, two by ES, and four by NS.

The recommended sample sizes for each sample type are presented in Table 1 below.

chart showing recommended sample sizes
Table 1. Recommended sample size estimates for serum, ear-vein blood swabs, nasal swabs, and oral swabs.

For more sample size recommendations at different prevalence scenarios, please refer to the “Serum and swabs” tab on the free fieldepi sample size calculator for “PRRSV surveillance,” located at www.fieldepi.org/calc/.


When practitioners opt to use individual pig antemortem samples rather than the more cost-efficient population-based sampling methods, the information presented here serves as a guide for sampling weaning-age pigs with precision.

To review the full article, visit https://www.frontiersin.org/articles/10.3389/fvets.2023.1200376/full .

Research Team

Onyekachukwu H. Osemeke1, DVM, MS, PhD; Guilherme A. Cezar1, DVM, MS; Rodrigo C. Paiva1, DVM, MBA, MS; Daniel C. A. Moraes1, DVM, MBA, MS; Isadora F. Machado1, DVM, MS; Edison S. Magalhaes1, DVM, MS, PhD; Ana Paula S. Poeta Silva1, DVM, MS, PhD; Mafalda Mil-Homens1, DVM, MS; Li Peng, DVM, MS; Swaminathan Jayaraman, BEng, MS; Giovani Trevisan1, DVM, MBA, PhD; Gustavo S. Silva1, DVM, MS, PhD; Phillip C. Gauger2 DVM, MS, PhD; Daniel C. L. Linhares1, DVM, MBA, PhD