How Long PRRSV Persists in Feed: Detection vs. Infectious Risk

A positive PCR result on a feed sample can raise immediate concern, but it does not automatically mean the feed is capable of infecting pigs. As feed biosecurity has become a greater focus within swine production, advances in diagnostic testing have also made it easier to detect extremely small amounts of viral genetic material. The challenge is understanding what those results actually mean. 

Studies investigating porcine reproductive and respiratory syndrome virus (PRRSV) in feed may use standard PCR or quantitative PCR to detect viral RNA, viability PCR to provide additional information about particle integrity, cell culture to recover viable virus or animal bioassays to evaluate infectivity. Although these endpoints are often discussed together, they answer different scientific questions and should not be interpreted interchangeably.

Understanding those differences helps producers make feed biosecurity decisions based on evidence rather than assumption.

The goal of feed biosecurity is not to eliminate every theoretical risk. It is to understand which risks are credible enough to justify practical control measures. 

Myth 1: A standard PCR-positive feed sample means the feed is infectious

Standard PCR-based assays, including conventional PCR and quantitative PCR, can detect small quantities of viral RNA. However, they cannot distinguish between RNA associated with intact virus and residual genetic material from damaged or inactivated virus particles.

A positive result therefore confirms that viral RNA is present, but it does not demonstrate that viable virus remains or that pigs could become infected from the sampled feed.

Viability PCR, or vPCR, adds a sample-treatment step designed to reduce amplification from virus particles with damaged or compromised protective structures. This can provide a better indication of whether intact and potentially viable virus remains than standard qPCR alone. However, vPCR is still an indirect measure and should not be interpreted as definitive proof of infectivity. Assay performance can also be influenced by the virus, the method of inactivation and the sample matrix.

Stronger evidence of viability requires successful virus isolation in cell culture. The clearest evidence of infectious risk comes from animal bioassays or natural-feeding studies that demonstrate infection under defined experimental conditions.

This distinction matters because discussions about PRRSV persistence often combine RNA detection, particle integrity, viral viability and infectivity as though they describe the same outcome. They do not.

Myth 2: Dry feed cannot support PRRSV survival

Research indicates that PRRSV can remain viable in the environment and in certain feed ingredients under specific conditions. Survival is influenced by the ingredient matrix, virus strain, temperature, humidity and storage duration. In general, PRRSV has remained viable longer under cooler conditions, although outcomes vary considerably among studies and matrices.

Under a simulated 37-day transboundary shipping model, viable PRRSV was detected in conventional soybean meal and dried distillers grains with solubles under defined experimental conditions. Infectivity was confirmed through swine bioassay, although viable virus was not recovered through cell culture.

Commercial-transport studies provide additional evidence that the feed matrix matters. Viable PRRSV was detected in organic and conventional soybean meal following 21-day and 23-day transport models. In the larger-volume 23-day study, however, PRRSV was not detected in complete-feed samples and pigs consuming the complete feed did not become infected with PRRSV.These findings support a qualified conclusion, feed and feed ingredients may serve as potential vehicles for PRRSV under specific conditions, but risk cannot be generalized across every ingredient, feed formulation or commercial situation.

Rather than asking how long PRRSV survives in feed generally, a more useful question is whether a specific ingredient under specific environmental and storage conditions presents sufficient risk to justify additional biosecurity measures.

Myth 3: One negative sample proves an entire lot is clear

Negative laboratory results also require careful interpretation. Commercial feed batches are large, contamination may be concentrated in isolated hot spots and viral concentrations, when present, may be extremely low. A small number of samples may therefore fail to contain the contaminated portion of a batch.

Evidence from African swine fever virus investigations illustrates this broader sampling challenge. Although these studies evaluated ASFV rather than PRRSV, they found relatively few positive feed and ingredient samples while also detecting viral DNA on feed-truck cabs, wheels, worker clothing, feed-contact equipment and high-traffic environmental surfaces.

In one commercial-system investigation, only one of 142 finished-feed, ingredient and processing-water samples was PCR-positive. Additional positive samples were identified on feed-delivery vehicles and within the feed-mill environment, including truck cabs, worker clothing, feed lines and equipment surfaces.

A separate investigation involving three commercial feed mills detected ASFV DNA in only a small proportion of feed and ingredient sample pools. The researchers also acknowledged that low detection rates could reflect the true contamination level, sampling limitations or dilution caused by sample pooling.

These studies do not establish the distribution of PRRSV in commercial feed systems. They do demonstrate why a negative feed result cannot be treated as proof that an entire lot, mill or transportation system is free from contamination.

Feed testing is most useful when incorporated into a broader verification program that considers the places contamination may enter, accumulate or move through the supply chain.

Practical steps for managing feed biosecurity risk

Current evidence supports managing feed as one component of a layered biosecurity program rather than relying on any single test or intervention.

An effective feed biosecurity program should:

• Assess ingredient origin and potential supply-chain exposure before ingredients enter the mill.
• Establish approved supplier, receiving and ingredient-handling protocols.
• Use risk-based sampling across feed, ingredients, dust, vehicles, equipment and environmental surfaces rather than relying on a single grab sample.
• Interpret standard PCR, vPCR, virus-isolation and bioassay results according to what each method can demonstrate.
• Separate high-risk ingredient, vehicle and personnel traffic where practical.
• Apply scientifically supported holding times, temperature controls or feed-mitigation technologies when appropriate.
• Prevent recontamination during storage, transport and feed delivery.
• Document, monitor and periodically verify the effectiveness of the complete program.

No individual sample, laboratory method or mitigation measure provides a standalone guarantee. Their value comes from how they work together within a risk-based system.

Feed is one pathway among many

Feed is one possible PRRSV pathway within a broader biosecurity system, and current evidence does not establish it as the dominant route under commercial conditions.

Under defined experimental and transport conditions, however, viable PRRSV has persisted in specific feed ingredients long enough to support risk-based feed biosecurity measures. Research has also demonstrated that PRRSV viability in the broader environment is influenced by temperature, surface type, biological matrix and duration of exposure.

For that reason, feed should be managed as one component of a comprehensive PRRSV biosecurity program alongside animal movement, transportation, personnel, equipment, supplies and facility hygiene.

The strongest feed biosecurity programs do not rely on assumptions about risk or on a single negative result. They combine appropriate diagnostic tools with ingredient controls, environmental monitoring and practical process safeguards to protect feed from sourcing through manufacturing, transport and delivery. To learn more, contact your clean feed expert today.