Illustration of a pig with a strong immune system fighting off a virus.

Beyond the Sniffles: Unmasking the Secrets of Pig Immunity to Secure Our Food Supply

Jordan Keane in Science & Nature September 2025 • 4 min read.

"A deep dive into how understanding porcine reproductive and respiratory syndrome virus (PRRSV) resistance can revolutionize swine health and global food security."

Porcine Reproductive and Respiratory Syndrome (PRRS) isn't just a mouthful—it's a multi-billion dollar problem for the global swine industry. As one of the most economically devastating diseases affecting pigs, PRRS poses a significant threat to animal welfare and food security. For over two decades, U.S. swine producers have battled this persistent virus, struggling to contain its spread despite numerous efforts.

The primary phase of PRRS is typically monitored by tracking serum viremia, which indicates the presence of the virus in the bloodstream. While this phase generally clears within a few weeks, the virus often finds refuge in the tonsils, establishing a persistent infection that can last for months. This prolonged presence in the tonsils makes it a critical site for understanding how the virus lingers and continues to impact the animal's health.

Recent research has focused on understanding the factors that influence the levels of the virus in the tonsils, aiming to unlock new strategies for combating PRRS. By identifying the genetic components that contribute to viral persistence, scientists hope to breed more resilient pigs, reducing the impact of PRRS and securing a more stable and sustainable food supply.

How Can Genetic Insights Help Us Fight PRRS?

Illustration of a pig with a strong immune system fighting off a virus.

A new study dug deep into the dynamics of PRRSV in pigs, focusing on virus levels found in tonsil samples. The research team conducted several trials, experimentally infecting pigs with different PRRSV type 2 isolates. Specifically, they used NVSL-97-7895 (NVSL) and KS-2006-72109 (KS06) isolates, tracking the viral load in the tonsils at 35 and 42 days post-infection (dpi).

The study had four primary objectives:

Estimate the heritability of tonsil virus levels at 35 or 42 dpi, determining the extent to which genetics plays a role.
Identify factors affecting tonsil virus levels, including the presence of serum viremia, to understand the interplay between different aspects of the infection.
Pinpoint specific genomic regions associated with tonsil virus levels, offering potential targets for genetic selection.
Compare results between the two PRRSV isolates to identify common and distinct factors influencing viral persistence.

The findings revealed that the level of virus in the tonsils was lowly heritable for both the NVSL (0.05±0.06) and KS06 (0.11±0.10) isolates, indicating a limited but present genetic component. Intriguingly, the level of virus in the tonsils was phenotypically linked to traits associated with viral clearance from the serum. Pigs with lower tonsil virus levels exhibited earlier and faster rates of maximal serum viral clearance, lower total serum viral load, and reduced viremia at 35 or 42 dpi.

What's Next for PRRS Research?

While the study didn't identify major genomic regions directly influencing tonsil virus levels, it did highlight several regions showing potential associations, containing genes like CCL1, CCL2, CCL8, and others involved in viral infiltration, immune cell migration, and viral clearance. These findings suggest that selecting for viral clearance traits in serum could indirectly reduce PRRSV persistence in the tonsils. However, further research is needed to fully understand the genetic correlations between these traits and to determine the long-term effectiveness of this approach.

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Everything You Need To Know

What is Porcine Reproductive and Respiratory Syndrome (PRRS) and why is it a major concern for swine producers?

Porcine Reproductive and Respiratory Syndrome (PRRS) is a devastating disease affecting pigs, posing a major threat to animal welfare and food security. It is economically significant, costing the global swine industry billions of dollars. The virus is difficult to control, and its persistence leads to significant losses for swine producers. It causes reproductive issues and respiratory problems in pigs, which result in decreased productivity, higher mortality rates, and increased costs for disease management.

How does the PRRS virus persist in pigs, and why is the tonsil a critical area of focus?

The PRRS virus often clears from the bloodstream (monitored through serum viremia) within weeks, but it can establish a persistent infection in the tonsils. This prolonged presence in the tonsils makes it a key area to understand how the virus lingers and continues to affect the pig's health. Researchers are focusing on the tonsils to find the factors influencing the virus levels there, with the goal of developing new strategies to combat PRRS. Targeting the tonsils is crucial because this is where the virus hides and continues to cause health problems, making it a critical site for viral persistence.

What specific PRRSV isolates were used in the study, and what were the primary objectives?

The study focused on two PRRSV type 2 isolates: NVSL-97-7895 (NVSL) and KS-2006-72109 (KS06). The primary objectives were to estimate the heritability of tonsil virus levels, identify factors affecting tonsil virus levels (including serum viremia), pinpoint genomic regions associated with tonsil virus levels, and compare results between the two isolates to identify common and distinct factors influencing viral persistence.

What were the key findings regarding the heritability of virus levels in the tonsils and its link to serum viral clearance?

The study revealed that the level of virus in the tonsils was lowly heritable for both the NVSL (0.05±0.06) and KS06 (0.11±0.10) isolates. However, the level of virus in the tonsils was phenotypically linked to traits associated with viral clearance from the serum. Pigs with lower tonsil virus levels showed earlier and faster rates of maximal serum viral clearance, lower total serum viral load, and reduced viremia at 35 or 42 dpi. This suggests that genetic selection for traits related to viral clearance in the serum might indirectly reduce PRRSV persistence in the tonsils.

What are the next steps for PRRS research, and which genes are of particular interest?

While the study didn't identify major genomic regions directly influencing tonsil virus levels, it highlighted several regions showing potential associations, containing genes like CCL1, CCL2, and CCL8. These genes are involved in viral infiltration, immune cell migration, and viral clearance. Further research is needed to fully understand the genetic correlations between these traits and to determine the long-term effectiveness of selecting for viral clearance in the serum to reduce PRRSV persistence in the tonsils. Understanding the genetic links between these traits and their impact on the long-term effectiveness of control measures is the focus of ongoing research.