Some producers are coping with persistent PRRS problems.

In many sow herds, once the initial outbreak of porcine reproductive and respiratory syndrome (PRRS) subsides, production often returns to normal. That's not so with chronic cases.

“Problem herds” may hold the key to advancing plans for long-term control and eradication of the PRRS virus, declares Minnesota veterinarian Gordon Spronk of the Pipestone (MN) Veterinary Clinic.

While control strategies for PRRS appear to work in many herds, other, similar herds sometimes fail to return to normal production, he reported at the Leman Swine Conference, St. Paul, MN.

These chronically infected herds require new strategies to address unique health issues and, in turn, to help chart a course to rid regions of PRRS virus, suggests Spronk.

PRRS Challenges

Hog density creates PRRS health challenges — and many areas of southern Minnesota and northern Iowa are prime examples, he explains.

For his talk, Spronk selected seven sow herds in the region and compared production data over the last six years. The herds range in size from 3,200 to 3,400 sows, operated as breed-to-wean sites.

During that production period, the clinical presentation of PRRS virus in those seven herds was characterized as:

  • Normal weaned pig production followed by episodes of increased rates of abortion (greater than 2%/week), and poor weaned pig performance (greater than 4% mortality eight weeks after weaning).

  • Repeated circulation of the same strain of PRRS virus at the sow site and in weaned pigs.

  • Variations in the signs of PRRS virus between these similar sow farms. Some of the farms experienced high abortion rates, while others had no clinical signs in sows, but suffered severe mortality problems in the nursery.

  • New PRRS isolates or strains were introduced to the sow farms by unknown sources.

  • Diagnostic tests were unable to predict weaned pig performance. Many times, workers in the nurseries were able to detect PRRS problems in the sow farm before clinical problems and diagnostic testing were able to confirm a problem.

All of the seven farms have their own on-site gilt developer units, and one farm has its own on-site composting facility to eliminate the rendering service from entering the site, says Spronk.

From clinical observations and diagnostic testing, Spronk concluded that 13 new PRRS viruses infected the seven sow farms over a four-year period.

Moreover, the viruses were different from farm to farm. “Our observation is we don't believe we are taking the same virus and moving it around between sow farms, because they are distinctly different as we can measure,” he adds.

Control Measures

Spronk provided details on a 3,400-sow herd, presenting the history of PRRS, the control measures and the results obtained.

Production was fairly normal in 2003, but tests revealed there was PRRS virus circulating in the herd. A commercial PRRS vaccine was used on a quarterly basis.

In early 2004, introduction of a new PRRS virus was confirmed, and use of the commercial vaccine was stopped. Serum inoculation using a PRRS strain isolate injected at mid-gestation was then adopted as a control measure. The serum strain was prepared on-farm from replacement gilts that were infected with a field strain of the PRRS virus.

After a time, the sow herd experienced a new PRRS outbreak. Testing revealed introduction of a new strain of PRRS virus, known as MN 184, says Spronk. Serum injection at mid-gestation was repeated to provide immunity for the herd from further breaks.

However, due to repeated reproductive problems and severe challenges in wean-to-finish performance, it was decided to close the farm. The sow farm is currently in the middle of a PRRS elimination program.

This farm highlights the perplexity of PRRS virus infections in dense production areas, says Spronk. In short, it takes a while for reproductive performance to improve.

Even so, for 2005, this farm achieved 25 pigs/sow/year (p/s/y), and was looking to reach 26 p/s/y for 2006.

But persistent poor pig performance in the wean-to-finish phase has everyone at the sow farm scratching their heads, he says.

PRRS Conclusions

That's led Spronk and associates to conclude that in this subset of seven sow farms, there is a 50% chance of a new PRRS infection occurring every two years.

“Depending on the clinical presentation of the PRRS isolate, herd elimination may be necessary in the most severe cases in order to remain profitable,” he explains.

Spronk's final four observations on problem PRRS herds include:

  • New PRRS viruses replace old ones. “Once a new PRRS virus comes in, we cannot isolate the old virus.”

  • PRRS virus varies in its initial impact on sow farms. In the sow herd described, the virus caused performance bad enough that the market would discriminate against hogs from this farm. “If the buyer knew hogs were coming from this farm, there was either a discount on them, or the market didn't want them at all,” says Spronk.

  • Clinical signs of PRRS may appear first in the nursery and not the sow farm. When this happens, current control procedures may not work, and a herd elimination plan is often implemented.

Tough PRRS Virus

University of Minnesota immunologist Michael Murtaugh confirms Spronk's observations — the PRRS virus is indeed changing in Minnesota.

Back in 2001, the University of Minnesota Veterinary Diagnostic Laboratory confirmed just one case of the PRRS strain known as MN 184, says Murtaugh. By 2005, the lab reported 235 cases of the PRRS strain.

This is a highly virulent strain of the PRRS virus, and the jump in reported cases is not good news for the Minnesota swine industry, he stresses.

PRRS Research Consortium Advances Knowledge Base

In order to expedite the growth of scientific knowledge about porcine reproductive and respiratory syndrome (PRRS), a broad coalition of PRRS researchers joined forces, forming what is known as the “Big Pig” Project. Bob Rowland of Kansas State University and Jeff Zimmerman, DVM, of Iowa State University were project leaders.

This group of scientists was assembled to coordinate research efforts to understand PRRS virus replication, persistence and immunity, according to Joan Lunney, research scientist in USDA's Agricultural Research Service (ARS), one of the eight coalition members.

“The Big Pig project represents the first comprehensive study of PRRS virus replication and immunity within a single, relatively large population of experimentally infected pigs,” she explains.

For this study, 109 pigs were infected with PRRS virus at Iowa State University (ISU) and sampled by Zimmerman's group every two weeks up to 203 days. There were 56 pigs from matched litters in the uninfected controls.

More than 20,000 pig samples were distributed to five institutions involved in the study: ISU, Kansas State University, South Dakota State University, IDEXX Laboratories and USDA-ARS. Samples were also sent to coalition members at the University of Minnesota, the University of Michigan and USDA's National Animal Disease Center at Ames, IA.

The results from this project will be compiled and deposited in a single Web-based database for use by all coalition members and other researchers.

As reported by Zimmerman at the Dec. 2 International PRRS Symposium in Chicago, six conclusions were reached:

  • Virus replication or growth is extensive during the grow-finish phase of production. When pigs reached market weight of 250 lb., 147 days after infection and 168 days after farrowing, nearly 60% of pigs remained positive for PRRS virus infection in tissues.

    “The virus load was much greater than predicted, and identifies a continuous source of virus that can break back into the nursery (intra-farm transmission) or spread to other farms (inter-farm transmission),” says Lunney.

  • In regards to growth performance, PRRS virus infection results in a stratified population. On one end of the spectrum were normal appearing but infected pigs that grew at the same rate as control pigs.

    At the other end of the spectrum were about 15% of infected pigs identified as “light.” These “outliers” are responsible for the approximately $500 million/year in losses to the swine industry, says Lunney.

    “This is the first clear demonstration that PRRS virus alone impacts growth performance,” she adds. Followup studies are underway to determine whether immune factors such as cytokines (substances that are secreted by cells of the immune system) or growth factors contributed to slow development.

    This research could lead to identifying pigs that are genetically resistant to PRRS virus, and focusing on treatment of light pigs to restore optimal growth performance in PRRS virus-infected herds.

  • There is no correlation between acquired immunity and virus load in serum or tissues. These results support previous work, and are important for the development of interventions that prevent infection vs. those that impact immunity in infected pigs.

  • Diagnostic tests used to detect PRRS virus sometimes provide inconclusive results. Bioassays are used as the gold standard to detect the ability of a pig exposed to the virus to transmit the virus.

    In this study, polymerase chain reaction-based assays identified PRRS virus nucleic acid in tissues; however, the bioassay results for the same samples tested negative for the presence of infectious virus. These data suggest that viral RNA may be present, but in a non-infectious form.

  • The IDEXX commercial ELISA (enzyme-linked immunosorbent assay) test for PRRS was able to confirm long-term viral antibody persistence in swine. The IDEXX Herdchek PRRS 2XR Antibody Test Kit detected PRRS activity 112 days post-exposure. The seropositive response persisted through day 182 in 26 out of 28 animals. These results prove that serum antibodies against PRRS virus can be detected well into late phases of infection, says Lunney.

  • Tests to predict persistence of PRRS virus are still unavailable. Serological and immunological tests have not identified any parameter indicative of viral clearance or continued persistence of the virus.

Overall, this set of carefully collected samples is just beginning to identify some key factors involved in the effect of PRRS virus on piglet growth and the development of disease, concludes Lunney.