There was more than a day of PRRS discussion at this year’s American Association of Swine Veterinarians annual meeting in Toronto.

Swine veterinarian David Smidt, DVM,  Pro Pork Associates, Cherokee, Iowa, was among those presenting PRRS-related papers at the AASV meeting. He discussed experiences with mass vaccination for the control of  PRRS.

“Farms we manage have wrestled with PRRS virus control like nearly every production system in the U.S.,” says Smidt. “Part of our strategy for control of PRRSV is the strategic use of Ingelvac® PRRS MLV vaccine. The potential value an MLV appears to bring includes reducing shedding of resident virus to reduce risk of infection in gilt entries. While we would like to think use of MLV would magically control all PRRSV challenges, it is apparent that MLV is simply a tool to assist in control of PRRS in breeding herds.”

Smidt outlined the following management principles to aid in PRRSV control:

1. Avoid offering negative naive animals to a population potentially shedding virus; vaccinate all animals to reduce risk of shedding.

2. During times of high risk of virus shed, avoid introduction of animals.

3. Use rock solid biosecurity protocols to attempt to eliminate introduction of new virus strains.

Smidt concludes that the PRRS modified live virus has served as an effective tool in “homogenizing” the immunity in the majority of farms.

It also “appears to reduce risk of shedding resident field virus to MLV vaccinated, field-virus-negative gilts,” he says.

“Seasonal vaccination followed by 60-day closure in large sow farms lessens the risk of introduction of naive animals to those farms.


Strategic use of modified live vaccine has been an effective tool in reducing shedding of resident virus to reduce infection when adding gilts to a herd.

Less frequent PRRS
Further, “vaccinated farms receiving MLV vaccinated gilts from the gilt breeder have experienced least frequent PRRSV breaks.”

Smidt also says that “factors, not limited to herd size, have likely contributed to limiting the effectiveness of MLV in the largest farms.”

While the use of vaccine has been effective , it has been less successful  in protecting animals from infection from newly introduced field virus, thus heightening the need to practice strict biosecurity, according to Smidt..

Matthew B. Turner, DVM, Prestage Farms, Inc., Clinton, N.C., also discussed mass vaccination as it is applied to a production system.

Control of PRRS virus in a production system is different compared with control of the virus in a single farm, Turner explains. “Constraints on a production system, such as commingled flows, high regional pig density, large biological vector populations, lack of on-farm isolation/acclimation units, as well as centralized labor and transportation all combine to create many opportunities for lateral introduction of PRRS virus into farms and very few opportunities to acclimate breeding stock before entry to the breeding herd.”

Mass vaccination application
Turner says an approach described as “proactive double-mass vaccination,” was implemented in 2003. He describes the program as follows:

  • Sow herds: The mass vaccination program is characterized by a timed vaccination of the entire breeding population twice thirty days apart, with the second dose of vaccine administered roughly 30 days before the historical peak of the outbreak season. No routine PRRS virus vaccinations are administered to sow farms during the seasonal low periods, according to Turner. “However, replacement gilts continue to be double-vaccinated 30 days apart in isolation.”
  • Replacement gilts: “Control strategies were applied to reduce intro-duction of gilts shedding PRRS virus using the mass vaccination unidirectional flow theory where a closed population of PRRS-virus-positive animals are vaccinated twice 30 days apart to create a non‑shedding popu-lation of animals.

“The application of this concept was achieved by moving replacement gilts from continuous flow gilt finisher sites at 14 to 20 weeks of age to all-in-all-out isolation units where they are vaccinated at placement and four weeks post-placement with Ingelvac ATP to the entire population. Gilts were entered into sow farms no less than four weeks after the second mass vaccination. This vaccine strategy is utilized throughout the year on all replacement animals.”

Turner says that proactive double-mass vaccination has been more beneficial than reactive (after an outbreak) double-mass vaccination in preventing late-term abortions.

He emphasizes that vaccine, as with any other tool, cannot control all problems related to PRRS virus. At the same time, he adds, “modified live PRRS-virus vaccine can be a valuable tool in the management of PRRS virus, especially where eradication and improved biosecurity have failed to prevent new introductions.


Vaccine use has been less effective in protecting animals from infection by newly introduced field virus so strict biosecurity is vital.

“Although modified live vaccine virus has been shown to spread to susceptible animals, use on an entire population at a single time point has proven to reduce shedding of virus within the population.

“Interestingly, the vaccine virus is rarely isolated out of clinically ill pigs in our system. In addition to the reduced number of total abortions in the proactively vaccinated period of time, the timing of abortion events is altered, which significantly impacts the cost of an outbreak.”

PRRS planned exposure
Several swine veterinarians discussed “planned exposure” or “serum therapy” as a method to gain the upper hand on PRRS virus.

“In numerous situations serum exposure has worked where other methods have not in establishing full whole-herd exposure and immunity to the active strain on that same sow farm,” says Mark Wagner, DVM, Fairmont Veterinary Clinic, Fairmont, Minn. “The clinical disease presentation can worsen the first three weeks post-exposure but tends to improve every week after that,” Wagner says.  “This typically results in PRRS-virus, PCR-negative pigs being weaned 10 weeks post-exposure and fewer PRRS-related grow-finish problems in the groups weaned after that point.

“This management technique may have a place in some multiple site production systems to minimize PRRS subpopulations in the sow herd, improve herd stability and to lessen PRRS-related problems in the grow-finish system downstream.”

Wagner says “the dynamics of PRRS in a population continues to be poorly understood. Despite being a virus that tends to be very infectious and easily introduced into a population if pre-cautions are not taken, the ability for a population to become uniformly exposed and immunized can be difficult. If the goal is to produce non‑viremic pigs at weaning, uniformly exposing the population and developing immunity to that strain of PRRS becomes an even more critical issue.”


Seasonal vaccination followed by a 60-day closure in large sow farms has reduced the risk of introduction of naïve animals in some systems.

Wagner says that he has success-fully used “serum therapy” to achieve uniform acclimation/exposure and immunity.

“As one looks at breeding herd stability to PRRS virus, two areas need to be focused on. Establish good acclimation and immunity in the gilt to the known PRRS virus in the population, and achieve uniform exposure and immunity to sow population to the known PRRS virus in the population.”

This is especially true when most herds have replacement rates of 40 percent to 60 percent, according to Wagner. “Because of this, the potential of either active or naive animals existing in the current sow population due to inadequate exposure or immune development, the seemingly slow development of immunity to the virus, and the lack of cross protection all contribute to making stability a challenge.

“With the use of serum therapy, some of the uncertainty  and duration of exposure can be lessened.”

Wagner says you should remember the following points when considering serum therapy:

  • Eight percent to 20 percent of the population may abort. This will occur in a very short time period – often in three to four weeks.
  • Abortions generally start on day five to seven post-inoculation.
  • Average days post inoculation when able to consistently produce PRRS PCR- negative pigs is 68.

Some additional clinical outcomes to consider with serum therapy, include:

  • Lingering effects. Often, this procedure will not reduce potential abortions but will reduce the duration of time over which abortions occur in the herd. Also, the majority of abortions often occur during the first and second week post-serum therapy. By three weeks post-serum therapy very few abortions occur.
  • Pig quality. Many farms have consistently started producing PRRS PCR-negative nursing piglets between 60 to 70 days post-serum therapy. This has a significant impact on nursery and finisher performance.
  • Serology profiles. Populations tend to stabilize and PRRS titres trend downward over time. “By six months out from serum therapy, we often will see several PRRS ELISA negative animals in the population.”
  • Relapses of the same virus. “To date we have not re‑diagnosed the same known virus in a population if clinical signs reoccur.”

Mark FitzSimmons, DVM, MAF Vet Services, Mapleton, Minn., also discussed the use of planned exposure as a means of trying to control PRRS virus. “Years of unsuccessfully dealing with PRRS in sow herds has led us to try and understand the requirements for immunity,” says FitzSimmons.

Homologous protection
“The history of vaccine failures and previously infected sow herds going positive to different viruses brought us to the conclusion that we needed to develop homologous protection,” he says. “The only way we knew to produce homologous protection was to expose animals to the same virus.”

FitzSimmons says he has used the method to establish immediate sow herd stabilization. “By taking the virus strain that was currently circulating in the sow herd and injecting the entire sow population, we would attempt to create a 100 percent homologously protected population.”

He says the procedure has been used in the following situations:

1. A PRRS-virus-negative farm going positive.

2. A PRRS-virus-positive farm getting another virus strain.

3. When a positive farm is leaking its resident virus.

“In all three of these situations, the only way to assure 100 percent exposure is to give every sow the virus.”

FitzSimmons emphasizes “the entire basis of this program is that the only consistent and predictable protection is homologous.” Therefore, “every step is based on development of said homologous protection. Not only are the successes based on this premise but also the investigation of the apparent failures.”

He also points out that this way to try to control PRRS virus in sow herds obviously is not the ultimate solution. “It is difficult to maintain, but so far has been the best operational solution.”

Legal red flag for planned exposure

Legal red flag for PRRS planned exposure Tom Patterson, Sioux City, Iowa attorney who specializes in agricultural law, presented a paper at this year’s AASV annual meeting on “PRRS ‘planned exposure’ and ‘unplanned consequences.’”

Patterson cautioned that his comments should not be used or relied upon as definitive legal opinion. He suggested that swine veterinarians should seek their own legal advice as to the application of specific laws in their areas.

At the same time, he did caution if they use, or recommend use of, serum therapy, swine veterinarians need to be aware  there could be some unplanned legal consequences.

“Any veterinarian contemplating the employment of the injection of herd specific live virulent PRRS virus should be aware that even though their activities may be ‘exempt’ from the Code of Federal Regulations that might otherwise apply to the manufacture of vaccines, they nonetheless must be ready to respond with records of their PRRS amplification/serum harvest protocol and the actual steps taken by them in creating the vaccine.”

Patterson likened the treatment and control of economic losses that result from active field strain PRRS virus in client swineherds as a “commercial gambling enterprise of chance. The rules are less well defined than those agreed upon in the gambling houses of Las Vegas.

“As a veterinary practitioner, you are a player. Understand the hand you are playing. Remember some of the chips on the table are your clients that own the hogs and some might be yours.”