First dubbed the mystery disease, porcine reproductive and respiratory syndrome remains a mystery on many levels. It is fast evolving into the pork industry’s biggest and most expensive swine health problem.

“There is no doubt that PRRS is the most expensive and difficult-to-combat disease ever to hit the U.S. hog industry,” contends veterinarian Tim Loula, Swine Veterinary Center, St. Peter, Minn.

“Management, epidemiology, pig-flow design, vaccination and isolation/acclimatization programs provide some of the weapons necessary to combat PRRS. Many veterinarians and producers are now convinced that herds need to move to a negative status in order to eliminate the costly effects of PRRS outbreaks and preventative programs,” Loula adds.

Scott Dee, with the University of Minnesota College of Veterinary Medicine, describes PRRS control in the United States today as in a “state of total chaos.”

Dee contends that the biology of the PRRS virus is winning the battle, and perhaps the war. Undetermined methods of virus entry into PRRS-negative herds still exist. Virus mutation, strain variation and recombination events are occurring. “Vaccine efficacy across farms can be inconsistent and the future of modified-live vaccines for PRRS control is being discussed. Some infected breeding stock companies still ignore the problem,” Dee says.

Nevertheless, there is a growing segment of practitioners, producers and seedstock companies who wish to eradicate the disease from the industry.

“Various groups have calculated the cost of the disease within their respective systems and now realize that they cannot afford to live with PRRS,” Dee notes. “Seedstock companies are feeling the pressure, on a national and international basis to produce non-infected pigs. Some veterinarians are presenting large-scale virus elimination projects.”

A leading PRRS researcher since he witnessed his first case in 1990, Dee continues to look for ways to control and eradicate the disease. Many of the protocols used today are based on his research. He offers three concepts that have emerged as a result of his studies.
1. The breeding herd’s PRRS status drives the rest of the system.
2. Gilt management should focus on introducing younger animals in fewer groups, less frequently throughout the year.
3. Every swine facility should design the ability to conduct regular, partial depopulations of the nursery and finisher into its system.

But before PRRS can be eradicated, there are still many questions that need to be answered. Some of those include:

  • How to determine the true prevalence of infected breeding swine.
  • How to identify and distinguish between “low-risk” and “high-risk” animals.
  • What is the true relationship of a positive polymerase chain reaction test on blood or tissue and the presence of viable, infectious virus?
  • What are the roles of external stimuli, risk factors and on-farm conditions that encourage virus replication?
  • What exactly is the amount of virus necessary for transmission and infection of neighboring swine?

More work needs to be done. Dee and his graduate student, Mike Bierke, are beginning research to address these and other questions.

Meanwhile, veterinarian Bill Hollis, Carthage Veterinary Service, Carthage, Ill., believes to better control and/or eliminate PRRS, practitioners need improved diagnostic tests and more answers about the persistence of the PRRS virus and “shedding” of serologically positive animals.

“We need a better test because polymerase chain reaction results on serum is not 100 percent accurate,” says Hollis, who acknowledges that PCR is a “great test for a small portion of serum.” However, the results don’t always give a completely accurate picture of the pig’s PRRS-virus status.

“My understanding is that a pig could have cleared the virus from its serum but might still have the virus in its tonsils or elsewhere. So the PCR serum test is not the ultimate test,” he says.

And so some of the PRRS mystery – and a lot of frustration – continues.

Decoding PRRS Status

What is a PRRS-negative herd? The label seems clear enough, but classifying a herd is not that simple.

Swine practitioners at the Carthage Veterinary Services, Carthage, Ill., use the following status to guide porcine reproductive and respiratory syndrome control and elimination plans. These classifications are based on PRRS researcher Scott Dee’s protocol:

  • Negative: This herd has not been infected with the PRRS virus and is serologically negative and clinically naive to the disease.
  • Stable inactive: Stable refers to the breeding herd’s PRRS status, while the term inactive refers to the weaned pig population.

    A stable breeding herd has been previously infected but production has returned to acceptable levels. Vertical transmission of PRRS is not detected. Serological profiling of a stable inactive herd indicates that while positive animals can be detected, titers or ELISA ratios are consistently low throughout the tested population.

    Inactive weaned pig populations demonstrate similar clinical and diagnostic profiles. Performance is similar to pre-infection levels and clinical signs of PRRS-related respiratory syndromes are not detectable. Serological profiles indicate the presence of colostral antibodies in recently weaned pigs but does not seroconvert through the post-weaning period.

  • Stable herd: A stable active farm has a similar breeding history and current status as a stable inactive farm; however there is evidence of active PRRS infection in post-weaning pigs. Serological profiles indicate seroconversion in the late nursery or finishing populations. This herd also presents a characteristic clinical picture with weaned pigs of excellent quality possessing high levels of colostral antibody.

    Two to three weeks after weaning, infection occurs as colostral immunity wanes. The virus source is generally older, previously infected nursery or finishing pigs or a mix of older pigs with younger pigs. Upper respiratory diseases (sneezing, edema, coughing) occur and are further impacted by bacterial infection such as Strep. suis, Mycoplasma pneumonia, Salmonella, Erysipelas and Hemophilus parasuis. Mortality picks up, average daily gain and feed conversion wane.

  • Unstable herd: This is a herd that has recently undergone acute infection or is experiencing chronic infection.

    PRRS symptoms surface. These would include: premature farrowing of weak pigs, abortions and anorexia/agalactia in breeding and lactating animals. Weaned pig quality is poor with several pigs showing respiratory distress with secondary infections. Influenza-like respiratory symptoms can be detected in finishing pigs along with secondary bacterial infections.

    Serological sampling indicates recent exposure to PRRS.

    A stable inactive herd is an immediate candidate for PRRS elimination. A stable herd can be a candidate for PRRS elimination if the nursery/finishing population is off-site or can be partially depopulated to achieve a stable status. An unstable herd must be stabilized before elimination can begin.