Porcine reproductive and respiratory syndrome (PRRS) is an economically significant disease of swine that continues to frustrate pork producers and veterinarians alike, decimating pig populations and creating financial havoc for pork producers.
Clinical outbreaks of PRRS were first reported in the United States in the late 1980s; however, the etiology or cause of the disease remained unknown. Clinical signs of PRRS included severe reproductive failure, postweaning pneumonia, decreased performance and increased mortality.
Similar clinical outbreaks were reported in Germany in 1990 and were widespread throughout Europe by 1991. In 1991, the etiologic agent, PRRS virus, was identified by investigators in the Netherlands and the United States.
Today, PRRS is endemic in the global swine population; however, several countries, including Sweden, Switzerland, New Zealand and Australia, claim to be free of the disease.
The PRRS virus is an enveloped, single-stranded RNA virus that is classified in the order Nidovirales, family Arteriviridae, genus Arterivirus. There are two major prototypes of PRRS virus in the world: the European isolate (Lelystad virus) and the North American isolate (VR-2332).
Properties of PRRS virus include the ability to induce prolonged viremia, persistent infections and replication in macrophages, which function in the body as protection against infection. As an enveloped virus, PRRS survivability outside of the host is affected by temperature, pH and exposure to disinfectants.
Outbreaks of PRRS involve episodes of reproductive failure (third-trimester abortions; premature parturition and elevated levels of fetal mummies, stillbirths and neonatal deaths; and reduced growth performance and elevated mortality secondary to respiratory disease).
However, the intensity of the disease appears to vary with the isolate or strain, and in the pathogenicity (ability of the virus to produce disease) of PRRS virus virulence as observed in experimentally infected animals. The degree of clinical or outward signs of PRRS may be related to elevated viral concentration in blood and tissues, secondary to the ability of highly virulent strains to reproduce more efficiently in the pig.
Several other factors, such as animal age and bacterial co-infection, can influence virus replication and clinical signs. Studies comparing the effects of age determined that younger animals (4-8 weeks of age) infected with PRRS virus demonstrated a longer viremia (infection in the bloodstream) as well as higher excretion rates and replication rates in macrophages (cells that fight infection) when compared to older (16 to 24-week-old) pigs.
Additionally, certain bacterial agents, such as Mycoplasma pneumonia, appear to enhance the duration and severity of PRRS virus-induced pneumonia and lung lesions.
Furthermore, PRRS virus infection has been reported to increase the susceptibility of pigs to Streptococcus suis type 2 infection and escalate the severity of Salmonella cholerasuis infection.
PRRS virus is transmitted directly from infected pigs and contaminated semen within and between pig populations. PRRS virus has been recovered from many different porcine secretions and excretions, including blood, semen, saliva, feces, aerosols, milk and colostrum and meat juice.
Persistence is a characteristic of PRRS virus relating to direct transmission in that pigs can maintain infection for prolonged periods of time. Transmission to negative pigs has also been demonstrated via indirect routes, including needles, coveralls and boots, contaminated transport vehicles, insects and aerosols. Recent work has demonstrated the ability of certain strains of the virus to travel via aerosols out to 5-6 miles.
In contrast, while farm personnel do not harbor the virus in their respiratory tracts, the virus can be passed to susceptible pigs following contact with contaminated hands.
Since the virus was originally isolated decades ago, significant changes have occurred in the field of swine diagnostics. Assays with poor sensitivity for detection of virus, such as virus isolation in cell culture medium, have been supplemented with rapid, highly sensitive and specific polymerase chain reaction (PCR) testing. PCR testing can be done on a qualitative (positive vs. negative) basis, as well as a quantitative (how much virus is present in the sample) basis.
Laboratories are now equipped to run these tests in the same day they are collected, allowing relay of the results electronically within 24 hours or less. Samples that can be tested include tissue, blood, saliva, colostrum and air, for example.
In addition, nucleic acid sequencing or “virus fingerprinting” can also be done in order to track the virus and better understand the epidemiology or distribution in a population of pigs following an outbreak.
Antibodies to PRRS virus infection can be detected using enzyme-linked immunosorbent assays (ELISA) to better understand the prevalence of infection within a population or herd.
Finally, novel means of sampling, such as blood swabbing and oral fluid collection, have provided practitioners with valuable tools that require less labor and cost and can be used to detect both virus and antibodies.
Impact on Breeding Stock
As a result of its economic impact, PRRS has revolutionized the swine industry, promoting change across breeding stock health status, animal flow, diagnostic testing, herd biosecurity and regional control/elimination efforts.
It is virtually impossible to sell replacement breeding stock and semen from infected populations. Aggressive (weekly, even daily) testing of seedstock herds and artificial insemination centers (boar studs) has become routine in an effort to accurately characterize PRRS status.
In an effort to maximize biosecurity, replacement animals are now introduced to breeding herds in large batches, requiring fewer introductions per year. Replacement breeding animals are often delivered as weaned pigs and undergo extended periods of acclimation in designated gilt development units, where exposure to the farm-specific variant of the virus, along with vaccination, enhances the protective immune response.
Protocols for the introduction of supplies, personnel, sanitation of transport vehicles and filtration of incoming air are now routinely practiced to prevent the entry of new variants, particularly in swine-dense regions.
Commercially available vaccines have been available for PRRS virus since 1994. Killed-virus and modified-live virus products are available, although the efficacy differs significantly between types of vaccines. Due to the ability of the PRRS virus to undergo rapid and constant genetic change, the ability of vaccines to provide solid immunity across all strains has been impossible to demonstrate.
Significant, partial protection has been clearly demonstrated in sows and growing pigs when modified-live products have been evaluated. Due to the documented ability of modified-live virus vaccines to reduce the shedding of aerosolized virus from infected finishing sites to the surrounding environment, a potential application of these products may be mass vaccination of neighborhood finishing populations or “ring vaccination” around protected areas, which will likely be attempted in regional PRRS control projects.
Animal Flow Strategies
Due to the effect that concurrent infections with bacteria and other viruses can have on PRRS virus-infected pigs, animal flow strategies such as all-in all-out by room, barn and site, have been effective in reducing the clinical impact of the respiratory disease complex during the wean-to-finish period. Segregated production practices have also been effective in improving the health and performance of growing pigs.
It must be emphasized that since the PRRS virus can cross the placenta and infect piglets during gestation, the ability to eliminate the virus through the adoption of early weaning practices has not always been possible.
In an effort to support the position taken by the American Association of Swine Veterinarians, the National Pork Board and the National Pork Producers Council — stating that the elimination of PRRS virus from the North American swine industry is the long-term goal — are moving forward across North America.
Several voluntary, area-based control and elimination projects are underway. These projects have demonstrated the ability of producers and practitioners to collaborate in an effort to decrease the incidence and prevalence of PRRS virus in a region. Tools such as the Production Animal Risk Assessment Program (PADRAP) have provided practitioners with novel means to measure risk at the individual farm and regional level, as well as measure the impact of improved biosecurity practices.
These projects have advanced the formation of working groups; open sharing of diagnostic data; mapping of regions to describe the PRRS status of local farms; filtration of key sites, such as artificial insemination centers, gilt development units and daughter nucleus multiplication farms; and sharing of pigs and facilities in an effort to rid a neighborhood of the virus.
Because these are not government-regulated programs, 100% participation is not always possible. However, these projects have demonstrated that the ability to recruit a critical mass of producers can result in progress.
Despite all of the advances, PRRS continues to challenge the industry. Highly pathogenic strains with the ability for long-distance airborne spread have emerged. Commercial vaccines still offer limited results.
Research at the molecular level to identify the basis of virulence and immunity, along with continued efforts to explore novel vaccine development and genetic resistance, are very important.
The advancement of regional projects will be important to maintain the momentum of the last 2-3 years.
Other challenges include increasing producer participation in voluntary programs, seeking sources of funds to support area testing and communicating the successes and failures in an effort to educate and improve the chances of future success.
While we have accomplished a great deal in our ability to understand this complex disease, much more work is required. Collaboration between producers, practitioners and scientists will be critical to achieve the long-term goal of eradication of this challenging virus.
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