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How to manage PRRS-negative pigs

Pork producers who manage to produce pigs that test negative to the porcine reproductive and respiratory syndrome virus should be able to enjoy the fruits of their labor in the form of improved pig performance and better financial returns.

But increasingly, the ability to carry out that goal is being cut short in the hog-dense regions of the Corn Belt, says Tim Loula, DVM, Swine Vet Center, St. Peter, Minn.

PRRS was first diagnosed in the United States in 1987. Despite many and varied efforts to control the virus, it seems to grow worse every year, Loula expressed his lament in a talk at the American Association of Swine Veterinarians annual meeting in March in Phoenix, Ariz.

PRRS strains continue to grow in number, severity and transmissibility; the virus has evolved into an “aerosol transmission pathogen,” he says.

PRRS outbreaks are often complicated by swine influenza virus, Mycoplasma pneumonia and sometimes porcine circovirus type 2. The result is that the grow-finish disease complex has become much more challenging to manage, Loula observes.

Hog-dense dilemma

Clean pigs flowing to a hog-dense area will break with PRRS approximately 50% of the time, Loula attests, based on extensive studies conducted by the Swine Vet Center. When clean pigs flow to extremely hog-dense northwest Iowa, there is a 90% chance that they will break with PRRS.

Sometimes, the symptoms are minimal, including coughing, depression and decreased feed intake. Other times, sites will experience severe effects with 5-10% death loss in a few weeks, requiring the use of antibiotics and aspirin for the secondary diseases.

Vaccine recommendations

“We are more and more commonly vaccinating the pigs going to sites with a history of turning PRRS-positive, either prior to transport or shortly after transport,” Loula points out.

The goal, of course, is to prevent an infection, but sites still become infected. Therefore, the hope is that the vaccination will serve as a valuable tool to reduce clinical signs and improve performance. That has proven to be the case, Loula says.

Research studies have shown that there is a reduction in duration of viremia (infection in the bloodstream) in animals that have been vaccinated for PRRS and then challenged with field strains of the virus.

Loula speculates vaccination should also aid in reduction of shedding and area spread. “Experience seems to suggest that vaccination is not eliminating shedding, but perhaps reducing the amount of shedding and at least reducing the length of the shedding and spreading of the virus. But we know that production performance won’t be the same as it is in groups of pigs that don’t get PRRS,” he explains.

Loula offers these thoughts on choosing the most appropriate option for vaccination:

  • For a finishing site, if the pigs are from a sow farm and nursery that are clean, vaccinate pigs during the first week post-arrival in the finisher.
  • For a wean-to-finish site, if the sow farm is in a clean area, vaccinate after piglets are settled in and started, 2-4 weeks post-arrival.
  • For a nursery site, if the sow farm is in a clean area, follow the same procedure as in the wean-to-finish scenario.
  • For a sow herd that is protected by a modified-live-virus PRRS vaccine and producing PRRS-negative pigs, vaccinate piglets while on the sows at approximately 15 days of age.

Loula amplifies: “Every study I’ve ever done on performance benefits has shown that PRRS vaccination has paid for itself.”

Besides the standard commercial PRRS vaccines from Boehringer Ingelheim Vetmedica Inc., Loula says the company has developed a three-way vaccination (3-FLEX) for PRRS, mycoplasma and circovirus.

“Full doses of the vaccine offer the greatest opportunity for protection,” he says.

Studies have been conducted using aerosol vaccine delivery (spraying into the nose or fogging), but these methods have not been widely adopted, he adds.

Cardinal rules to avoid PRRS

Clean pigs face many obstacles to remain free of PRRS. Follow these three rules:

  1. Flow clean pigs to clean areas.
  2. Don’t mix clean pigs with PRRS-positive pigs.
  3. Vaccinate clean pigs when moving them to pig-dense regions where area spread is likely.