July and August, 1999

Update on the acute PRRS investigative study

Eric J Bush, DVM, MS; Barbara Corso, DVM, MS; Jeff Zimmerman, DVM, PhD; Sabrina Swenson, DVM, PhD; Dave Pyburn, DVM; Tom Burkgren, DVM, MBA

EJB, BC, SS: USDA:APHIS:VS (EB and BC at Centers for Epidemiology and Animal Health and SS at National Veterinary Services Laboratories.); JZ: Iowa State University; DP: NPPC; TB: AASP.

During the latter half of 1996, a series of abortion storms occurred, primarily in Southeast Iowa but in many other states as well. The outbreaks were characterized by abortions in 10%-50% of sows, regardless of parity, over a 3-6 week period, and high sow mortality in 5%-10% of inventory in a 1-5 week period. The clinical signs were compatible with losses previously reported in porcine reproduction and respiratory syndrome virus (PRRSV)-naïve herds (hence the name "acute PRRS"), but these herds had used vaccines for control of PRRSV.

There was a strong possibility that these outbreaks represented an emerging disease, with the potential to wreak havoc on domestic production and export markets. These concerns resulted in a series of actions on the part of practitioners, laboratory diagnosticians, the National Pork Producers Council (NPPC), the USDA's Animal and Plant Health Inspection Service (APHIS), and allied industry:

  • APHIS assembled an early response team to investigate herds in Southeast Iowa affected with the abortion storms, to quickly rule out the possibility of a foreign animal disease.
  • The AASP developed a case definition and used a mail questionnaire to query practitioners' knowledge of this syndrome.
  • In January 1997, a group of producers, practitioners, laboratory diagnosticians, pharmaceutical representatives, and animal health officials held two meetings to better understand the significance of these outbreaks and develop a hypothesis for their occurrence.

These abortion storms were widely attributed to a severe manifestation of PRRS; however, it was unclear whether the syndrome was due to changes in the virus and/or in management practices, or whether it was caused by the presence of another agent, known or unknown. A field investigation was implemented to address these questions. The goals of the study were to:

  • characterize the distribution of PRRSV infection within herds, and
  • identify risk factors associated with outbreaks of acute PRRS.

A series of case herds were selected by laboratory contacts who flagged accessions for abortion storms. The submitting practitioner was subsequently contacted for additional case history. Case herds were enrolled if they had at least 50 breeding sows, used a computerized record keeping system, and were still demonstrating signs of the outbreak of abortion storms in the breeding herd at the time of the survey. The minimum abortion rate for defining a herd varied between 3%-8%, depending on the length of the outbreak at the time of the interview.

Practitioners of case herds collaborated with their local APHIS Veterinary Medical Officer (VMO) to randomly select control herds from the practitioners' client list. The herd practitioner and VMO jointly conducted farm visits for both case and control herds. All case and control herds were assigned a numeric identification to maintain the confidentiality of the producer. Funding for practitioner participation was provided jointly by the NPPC and the AASP.

The VMO administered a management questionnaire and procured productivity records, which were collated at the Centers for Epidemiology and Animal Health. Blood and fetal tissues were collected from 30 randomly selected sows and gilts for serology and virus isolation at the time of the visit and paired blood samples were collected approximately 3 weeks later. All samples were submitted to the National Veterinary Services Laboratories.

Seventeen case herds were enrolled between June 25, 1997 and November 10, 1999, along with 33 control herds. Herds were located in Iowa (32), Illinois (4), Minnesota (4), Nebraska (4), and North Carolina (8). Participating breeding units were mostly farrow-to-finish herds (21) or producers of weaned pigs (21).

For the 17 case herds, the breeding inventory ranged from 105-2495 sows and gilts with a median of 610 breeding females. All herds except one were located on a single breeding site. Conventional PRRS had been diagnosed in 13 herds between September 1987 and January 1998. All but two herds had used PRRSV vaccine at one time, but at the time of the abortion outbreak, eight of the herds were no longer using PRRSV vaccine.

For the 33 control herds, the breeding inventory ranged from 65-3677 sows and gilts with a median of 556 breeding females. Five of the herds had multiple breeding sites while the other 28 were single-site operations. Twenty herds had been diagnosed with conventional PRRS with reported dates of first diagnosis between July 1990 and March 1998. PRRSV vaccine had been used in 23 of the herds, but at the time samples were collected, only 16 herds were using PRRSV vaccine.

The response rate for this study was much lower than originally anticipated: the expected study size fell short of the original goal of 50 case herds and 100 control herds. The poor response may reflect a true decrease in the incidence of acute PRRS, or perhaps an unwillingness to participate in the study for a variety of reasons. Lack of understanding about the various roles of producers, practitioners, and VMOs may have led to poor trust regarding the confidentiality of data.

The Acute PRRS Investigative Study represents a first attempt for a coordinated response to a possible emerging swine disease that occurred in multiple counties and states. Two lessons were immediately evident:

  • the pork industry needs to develop the capability to rapidly identify an emerging animal disease, and
  • the planned rapid response for foreign animal diseases is not an appropriate paradigm for responding to emerging diseases, which requires a more deliberate scientific approach to determine appropriate response. A foreign animal disease should initiate a more rapid military-style approach leading to disease erradication.

Attempts for a coordinated response were awkward, but the cooperative field investigative study was instructive for the different segments of the pork industry to learn how to jointly address an emerging disease and provides valuable lessons for improving our response capabilities.

Analysis of data from this study is currently in progress and results will be submitted for publication to Swine Health and Production.