Porcine reproductive and respiratory syndrome (PRRS) is a viral disease that causes reproductive failure in breeding pigs and respiratory illness in young pigs. It costs U.S. pork producers an estimated $1.2 billion per year in lost production, making it one of the most economically damaging diseases in the global swine industry.
What Causes PRRS
PRRS is caused by a small, enveloped RNA virus in the family Arteriviridae. The virus particles are tiny, only 40 to 60 nanometers across, but they pack a serious punch. Two distinct species circulate worldwide: PRRSV-1, originally identified in Europe, and PRRSV-2, first found in North America. Both species have since spread globally, and each contains multiple genetic lineages that continue to evolve.
This genetic diversity is a major reason PRRS remains so difficult to control. In the U.S., a variant called L1C 1-4-4 emerged in late 2020 and spread rapidly. In China, highly pathogenic strains have dominated since the mid-2000s. The virus mutates fast enough that immunity to one strain offers limited protection against others.
Signs in Breeding Herds
The “reproductive” part of the name refers to what happens when the virus hits a breeding herd. Pregnant sows may abort, deliver stillborn piglets, or give birth to mummified fetuses and weak piglets that die within days. In acute outbreaks, herds can average as few as one weaned piglet per litter, a devastating drop from the normal eight to twelve. Sows often go off feed and some die.
In a large Canadian study of PRRS-positive herds, about 44% of farms with sows reported abortions, 41% reported stillborn piglets, and nearly 49% reported weak-born piglets. Larger herds faced higher odds of these problems. Farrow-to-finish operations were hit hardest, with over 60% of those farms reporting abortions during active infection.
Signs in Growing Pigs
In nursery and finishing pigs, PRRS primarily attacks the lungs. The virus targets immune cells called macrophages in the respiratory tract, which weakens the pig’s defenses and opens the door to secondary bacterial infections. Affected piglets show labored breathing, sometimes called “thumping” because of the visible effort in their flanks. Growth slows, feed efficiency drops, and mortality rises. The combination of PRRS with bacterial pneumonia is one of the most common and costly disease complexes in pig production.
How the Virus Spreads
PRRS spreads through several routes, which makes containment challenging. Direct nose-to-nose contact between infected and healthy pigs is the most straightforward path. The virus also travels in semen, so breeding with an infected boar can introduce it to an entire sow herd. Infected sows pass it to piglets before or during birth.
Airborne transmission is a particular concern. Research from early outbreaks found that while half of initial cases in a region could be linked to animal movement, that proportion dropped to 18% as the virus established itself, with airborne local spread accounting for roughly 63% of subsequent cases. Investigators estimate the virus can travel through the air within about a 3-kilometer radius of an infected herd, especially in cold, humid conditions.
The virus also hitches rides on contaminated boots, clothing, equipment, and transport vehicles. Insects can carry it between farms. This web of transmission routes means that even farms with no direct pig-to-pig contact with infected herds remain at risk.
Cold Weather Keeps the Virus Alive Longer
PRRS virus survives much longer in cold environments. At 4°C (about 39°F, typical refrigerator temperature), the virus has a half-life of roughly 155 hours, meaning it takes nearly a week for half the virus to lose infectivity. At 20°C (68°F), that drops to about 27 hours. At 30°C (86°F), the half-life is only 1.6 hours. The virus is most stable in cold, dry air (around 5°C and 17% humidity) and least stable in warm, humid conditions (41°C and 73% humidity).
In manure, the virus persists at similar rates, with a half-life of about 113 hours at 4°C. This means contaminated manure, slurry, and equipment pose a real transmission risk during fall and winter months, which is when many of the worst outbreaks occur.
How PRRS Is Diagnosed
Veterinarians typically confirm PRRS through a combination of clinical signs and laboratory testing. The two main lab approaches are antibody detection and direct virus detection.
Antibody tests, most commonly ELISA, check whether a pig’s immune system has responded to the virus. These can be run on blood serum or even saliva samples, making herd-level screening practical. However, antibodies take time to develop after infection, so this method can miss early cases.
For faster results, RT-PCR detects the virus’s genetic material directly in blood, tissue, or secretions. PCR is especially useful because it produces results quickly and can distinguish between different strains, which helps veterinarians track where an outbreak originated and whether a new strain has entered a region. Genetic sequencing takes this a step further, classifying the exact lineage of the virus to inform control decisions.
Vaccines Help but Don’t Eliminate the Virus
Vaccination with modified live virus (MLV) vaccines is the primary tool for managing PRRS in areas where the virus circulates. These vaccines use a weakened form of the virus that stimulates immunity without causing full-blown disease. Killed (inactivated) vaccines also exist but produce a weaker immune response.
Neither type of vaccine provides complete protection. MLV vaccines are more effective at reducing clinical signs and losses, but they do not create sterilizing immunity, meaning vaccinated pigs can still become infected and shed the virus. Protection is strongest against strains closely related to the vaccine strain and weaker against the genetically diverse strains that keep emerging. This is the central frustration of PRRS control: the virus evolves faster than vaccines can keep up.
Biosecurity Measures That Reduce Risk
Because vaccines alone cannot eliminate PRRS, farms rely heavily on biosecurity to keep the virus out. Effective programs layer multiple strategies together.
- Herd closure: Stopping the introduction of new animals for a period allows existing infections to burn through and clear, eventually producing a negative herd.
- Quarantine and testing: Any replacement animals are isolated and tested before joining the main herd.
- Air filtration: Filtering incoming air through high-efficiency filters has proven effective at blocking airborne virus particles. These systems use positive-pressure ventilation to push clean, filtered air into barns, preventing contaminated outside air from entering. Research in Minnesota demonstrated that air filtration, combined with other measures, provided a high level of protection against airborne PRRS transmission.
- Strict personnel protocols: Shower-in, shower-out procedures, dedicated clothing and boots for each barn, and hand sanitation between animal groups all reduce the chance of carrying the virus from one site to another.
- Transport sanitation: Trucks and trailers are thoroughly cleaned and disinfected between loads, since contaminated vehicles are a well-documented source of new infections.
- Insect control: Flies and other insects can mechanically carry the virus, so screening and pest management are part of a comprehensive plan.
The Economic Toll
A 2024 analysis from Iowa State University estimated that PRRS caused $1.2 billion per year in lost production in the U.S. pork industry between 2016 and 2020. That figure represents an 80% increase from the $664 million estimated for 2006 to 2010. The losses come from dead piglets, aborted litters, slower growth in surviving pigs, increased feed costs, higher veterinary bills, and the expense of biosecurity and vaccination programs. After four decades of fighting the virus, the trend line is still going the wrong direction, driven by the emergence of new, more virulent strains and the continued expansion of the global pig industry.