Overview of Porcine Reproductive and Respiratory Syndrome
Porcine reproductive and respiratory syndrome (PRRS) was first reported in the USA in 1987. Since then, outbreaks of PRRS and successful isolation of the virus have been confirmed throughout North America and Europe.
The etiologic agent is a virus in the group Arteriviridae. The virus is enveloped and ranges in size from 45 to 80 mm. Inactivation is possible after treatment with ether or chloroform; however, the virus is very stable under freezing conditions, retaining its infectivity for 4 mo at −70°C (−94°F). As the temperature rises, infectivity is reduced (15–20 min at 56°C [132.8°F]).
After infection of a naive herd, exposure of all members of the breeding population is inconsistent, leading to development of naive, exposed, and persistently infected subpopulations of sows. This situation is exacerbated over time through the addition of improperly acclimated replacement gilts and leads to shedding of the virus from carrier animals to those that have not been previously exposed.
The primary vector for transmission of the virus is the infected pig. Contact transmission has been demonstrated experimentally, and spread of virus from infected seedstock originating from a single source has been described. Introduction of infected seedstock can lead to the introduction and coexistence of genetically diverse isolates of PRRS virus on the same farm. Controlled studies have indicated that infected swine may be longterm carriers, with adults able to shed PRRS virus for up to 86 days after infection, and weaned pigs able to harbor virus for 157 days. Experimentally infected boars can shed virus in the semen up to 93 days after infection.
Aerosol transmission of the virus has been confirmed as an indirect route of transmission and may depend on isolate pathogenicity. Highly virulent isolates that produce high titers of virus in blood and tissues have been shown to be spread via aerosols at a significantly higher frequency than less pathogenic isolates. Environmental factors, such as wind direction and velocity, significantly impact spread via this route as well. PRRS virus can also be transmitted by fomites, such as contaminated needles, boots, coveralls, transport vehicles, and shipping containers. Farm personnel are not a risk, unless hands are contaminated with blood from viremic pigs. Finally, transmission via certain species of insects (mosquitoes [Aedes vexans] and house flies [Musca domestica]) has been reported.
PRRS appears to have two distinct clinical phases: reproductive failure and postweaning respiratory diseases. The reproductive phase of the disease includes increases in the number of stillborn piglets, mummified fetuses, premature farrowings, and weak-born pigs. Stillbirths and mummies may increase up to 25%–35%, and abortions can be >10%. Anorexia and agalactia are evident in lactating sows and result in increased (30%–50%) preweaning mortality. Suckling piglets develop a characteristic thumping respiratory pattern, and histopathologic examination of lung tissue reveals a severe, necrotizing, interstitial pneumonia. PRRS is capable of crossing the placenta in the third and possibly second trimester of gestation. Piglets may also be born viremic and transmit the virus for 112 days after infection. Performance after weaning is also affected. Infection with PRRS virus results in destruction of mature alveolar macrophages, which has led to the hypothesis that infection results in immune suppression; however, controlled studies indicate that the virus may actually enhance specific parameters of the immune response.
Outbreaks of the reproductive form of PRRS have been reported to last 1–4 mo, depending on the facilities and initial health status of the pigs. In contrast, the postweaning pneumonic phase can become chronic, reducing daily gain by 85% and increasing mortality to 10%–25%. Numerous other pathogens are commonly isolated along with PRRS virus from affected nursery or finishing pigs. Other bacteria such as Streptococcus suis, Escherichia coli, Salmonella Choleraesuis, Haemophilus parasuis, and Mycoplasma hyopneumoniae have been reported, as well as viruses such as porcine respiratory coronavirus and swine influenza virus. Finally, differences in the clinical response to PRRS virus may also be due to strain variation. Studies have demonstrated the ability of different isolates to induce varying degrees of interstitial pneumonia in cesarean-derived/colostrum-deprived (CD/CD) piglets after intranasal inoculation.
The most commonly used serologic assay is the ELISA. It measures IgG antibodies to PRRS virus. It cannot measure the level of immunity in an animal or predict whether the animal is a carrier. Titers are detected within 7–10 days after infection and can persist for up to 144 days. Tests for PRRS virus include PCR, virus isolation, and immunohistochemistry. Nucleic acid sequencing of the open reading frame 5 region of the virus is an excellent tool for epidemiologic investigations in the field to confirm similarity between isolates recovered from different sites. Recently, oral fluid sampling has been widely applied as a means to sample a population of pigs. This method is cost effective and convenient and can be used for both virus and antibody detection at the pen level.
Currently, there are no effective treatment programs for acute PRRS. Attempts to reduce fever using NSAIDs (aspirin) or appetite stimulants (B vitamins) appear to have minimal benefit. The use of antibiotics or autogenous bacterins to reduce the effects of opportunistic bacterial pathogens has also been reported; however, results have been mixed.
Prevention of infection appears to be the primary means of control. Understanding the PRRS status of replacement gilts and boars, as well as proper isolation and acclimatization of incoming stock, are critical measures to prevent viral introduction. Pigs should be retested on arrival at the isolation facility and 45–60 days later, before entry to the herd. Elimination of existing infection by multisite production and segregated early weaning has also been described. Although these strategies have had some success, the longterm risks of reinfection appear high. Prevention of viral spread by nursery depopulation has been described. This is successful when virus transmission is not occurring in the sow herd (usually 12–18 mo after initial outbreak), but the nurseries and growing/finishing pigs are still infected. All nursery pigs are removed from the farm to be finished elsewhere. The nurseries are then aggressively washed and disinfected and left empty for 7–14 days, after which they can be used normally. The technique has successfully eliminated PRRS virus from several herds, and pigs have remained seronegative (for >1 yr) to market age; production in the nurseries has improved, both in growth rate and mortality.
Commercial modified-live vaccines have been licensed and have been effective in controlling outbreaks, reducing shedding, and preventing economic losses.
Elimination of PRRS virus has been demonstrated to be possible on an individual farm basis. Methods such as whole herd depopulation-repopulation, test and removal, and herd closure have been documented as effective methods to eliminate PRRS virus from endemically infected herds. Unfortunately, a number of eradication efforts have failed because of introduction of new isolates through unidentifiable routes. This has resulted in an increased level of biosecurity on farms. Strict quarantine and testing programs, the purchase of PRRS virus-naive breeding stock and semen, sanitation of transport vehicles, and strict protocols of fomite and personnel movement between farms are critical components of an effective program. Recent advances in monitoring the status of artificial insemination centers include PCR analysis of blood samples collected from the auricular vein (blood swab). In addition, the application of air filtration to artificial insemination centers and breeding herds has been shown to significantly reduce the risk of airborne entry of the virus.