Tue, 08 Aug 2017
Every year, porcine reproductive and respiratory syndrome (PRRS) infects an estimated 25% to 45% of sow herds in the US, according to Clayton Johnson, DVM, Carthage Veterinary Clinic, Carthage, Illinois.
No other hog disease today exerts a larger economic toll on hog farms than PRRS.
On farms that break with PRRS, producers should decide if they are going to control the virus and keep antibiotics available for secondary bacterial infections, or if they are going to eliminate it, Johnson told Pig Health Today.
“Elimination is very difficult,” he said. “And inappropriate attempts to do elimination may actually hinder PRRS management by creating naïve animals that won’t stay naïve. They will propagate the disease at a greater level than an immune animal.”
A herd’s “break rate” for PRRS can help determine if a herd is a candidate for elimination. The break rate should be less than once every 3 years to consider elimination, Johnson said. If it is very regular, the herd probably needs additional biosecurity work or technology to minimize outbreaks.
‘Load, close and expose’
Producers who experience PRRS outbreaks should strive to maintain immunity in the herd. Johnson suggested achieving immunity by first eliminating wild viruses with the “load, close and expose” methodology.
“Bring the population up to maximum capacity in the farm,” he said. “Then close the farm to new introductions. With time, the virus will burn out. Then the population moves from infected to resistant and builds up antibodies so there’s no place for a virus to hide in naïve animals and propagate.”
Immunity is maintained by continuing a vaccination program in the herd with, for example, modified-live vaccines, Johnson added.
PRRS toolbox growing
The toolbox for treating and eliminating PRRS is getting bigger. One cooperative tool — known as Area Regional Control Projects — shows promise in areas with heavy PRRS populations. In these situations, a group of producers works together and shares their information about PRRS, including status and strategies for control, Johnson explained. Data shows PRRS rates dropped for farms participating in the groups.
Genetic research offers even more potential for eliminating PRRS. A project at the University of Missouri produced pigs resistant to PRRS. Gene editing removed the pigs’ ability to produce a specific protein which pigs need to become infected with PRRS.
While a solution like gene editing is far in the future, Johnson said it is exciting to think there is a solution “that can take a big chunk out of PRRS infections and other diseases, too.”