Porcine Reproductive and Respiratory Syndrome (PRRS) is a significant viral disease that impacts swine production across the globe. The disease presents in two overlapping forms: one affecting the reproductive health of breeding animals and the other causing respiratory illness in pigs of all ages. This dual impact makes PRRS one of the most economically devastating diseases in the swine industry today. It is estimated to cost the United States pork industry over $660 million annually in losses due to reduced productivity.
The Specific Viral Agent
The cause of PRRS is the PRRS virus (PRRSV), an enveloped, single-stranded RNA virus that belongs to the genus Arterivirus within the family Arteriviridae. The virus is characterized by high genetic variability and a rapid mutation rate, making the development of universally effective, long-lasting vaccines challenging. Two main genotypes are recognized globally: Type 1 (European strain) and Type 2 (North American strain). These two genotypes are genetically distinct, sharing only about 60% nucleotide identity at the genome level.
The virus targets the pulmonary alveolar macrophages (PAMs) and pulmonary intravascular macrophages. These immune cells are responsible for clearing pathogens in the lungs, and the virus replicates extensively within them. By destroying up to 40% of these macrophages, PRRSV severely compromises the pig’s immune system. This destruction compromises immune defenses, leading to the disease’s severity and opening the door for secondary bacterial infections, which often cause high mortality.
Recognizing Symptoms in Different Age Groups
The clinical manifestations of PRRS vary significantly depending on the age of the animal and the virulence of the specific viral strain involved. In breeding stock, the disease primarily presents as reproductive failure. Sows and gilts may experience fever and anorexia, with one of the most noticeable signs being late-term abortions, typically occurring after 100 days of gestation. Litters show a significant increase in stillborn piglets and mummified fetuses, and piglets born alive are often weak and non-viable.
In nursery and grow-finish pigs, the disease manifests predominantly as respiratory illness. Affected young pigs exhibit general signs like fever, lethargy, and a reduced appetite. They often develop interstitial pneumonia, leading to labored breathing, sometimes described as “thumping,” and a rough hair coat. The immunosuppression caused by PRRSV allows co-infections with bacteria like Mycoplasma hyopneumoniae or Streptococcus suis, which greatly increase the severity and duration of the respiratory disease and drive up mortality rates.
How the Disease Spreads
Transmission of PRRSV occurs through a variety of direct and indirect pathways, allowing it to move rapidly within a herd and between different farms. Direct pig-to-pig contact is the most common route of spread. Infected pigs shed high levels of the virus in their nasal secretions, saliva, urine, and feces, which transmits through direct contact.
Indirect transmission occurs via fomites, which are contaminated inanimate objects. This includes equipment, clothing, footwear, and especially vehicles used for transporting pigs, feed, or rendering.
Aerosol transmission is another documented route, allowing the virus to spread through the air over distances, sometimes up to 9 kilometers (5.5 miles) in high-density swine areas. Within breeding herds, contaminated semen is a major transmission pathway, as boars can shed the virus in their semen for weeks after infection. Additionally, biological vectors such as insects have been implicated in carrying the virus over short distances.
Management and Prevention Strategies
Controlling and preventing PRRS relies on a multi-faceted approach centered on biosecurity, vaccination, and strategic herd management. Biosecurity protocols are the first line of defense, focusing on both external measures to prevent the virus from entering the farm and internal measures to prevent its spread once inside. External biosecurity includes rigorous vehicle disinfection, implementing “shower-in/shower-out” procedures for personnel, and isolating new replacement animals for an acclimation period. In dense swine regions, air filtration systems for barns have proven effective at reducing the risk of airborne infection.
Vaccination is a primary tool used to stabilize herds and manage the disease, although it does not guarantee elimination of the virus. Modified Live Virus (MLV) vaccines are commonly used to expose animals to a weakened form of the virus, reducing clinical signs and viral shedding. Killed (inactivated) vaccines are also available and are considered safer, but they generally provide less robust protection than MLVs.
Herd management strategies are often implemented to eliminate or control persistent virus circulation. The “Load/Close/Expose” method involves maximizing the breeding herd population, closing the herd to any new introductions for a set period (typically 200 days), and intentionally exposing all resident animals to the resident virus or an MLV vaccine. This aims to build uniform immunity so the virus “burns out” in the absence of new susceptible animals. Monitoring the herd’s status is accomplished through diagnostic testing, such as Polymerase Chain Reaction (PCR) to detect viral presence and ELISA to measure antibody levels. For herds seeking the highest health status, the most definitive, albeit costly, elimination method is whole-herd depopulation followed by repopulation with virus-negative stock.