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Understand the end goal

Step 1

Not all herds or pig flows have to ultimately eradicate PRRS virus to be considered a success. Successful control starts with understanding your end goal:

• Is it OK to stay positive, if stable?
• Do you need to eliminate PRRS virus?
• Are you trying to keep an already negative site negative?

The systematic approach to PRRS control uses a series of action-measure-reaction steps to set, monitor and achieve those goals:

• Start

  Decide which phase of the operation or production system you will target to stabilize, eliminate or prevent introduction of PRRS, based on the flow/site current status, clinical severity and production effects of the disease, as well as your ultimate goal. Elimination need not always be the end game.

• Stabilize

  PRRS tends to cause its most damage when naïve pigs are introduced into phases where the virus is circulating. So, where maintaining a PRRS-naïve status isn't feasible, the goal becomes to prevent development of those “subpopulations” of infection, stabilizing the immune patterns.

• Eliminate

  Once immune patterns can be held stable — in particular, once subpopulations contributing to persistent circulation and infection are eliminated-then the virus can be eliminated…if the chances of maintaining the virus-free state are good.

• Prevent

  After you succeed in eliminating PRRS virus — or if you enter the flowchart already PRRS-free — intervention steps should focus on preventing PRRS from entering and affecting clean herds, flows and areas.

Note that each step requires careful measurement tools, to assess success or failure of the step in real-time, allowing for flexible adjustments in the plan.

Determine your current status

Step 2

Next, determine the presence of the virus, and the way it spreads and maintains itself in the herd or pig flow. That diagnostic status will determine both the goals and the control options you choose.

Is the operation currently:

• PRRS-negative?
• PRRS-positive, but immunologically stable?
• Positive and immunologically unstable?

If positive, what are the viral circulation patterns?

• In the gilt pool only?
• In the wider breeding herd?
• In the semen source(s)?
• Nursery?
• Finisher?

Assess the risk factors

Step 3

Once you have the most cost-effective control goal in mind and, based on step 2, know where your operation falls on the PRRS-control goal path, it's critical to thoroughly assess all the risks that could potentially cause or contribute to a failure of the control efforts you implement.

Failure can be defined either as an outbreak of PRRS in a clean herd or as an increase in the severity of outbreaks in a currently infected herd. The thorough risk assessment is a focused attempt to identify and quantify all the factors that add risk that your stabilization or elimination program will fail.

Those risk factors can be internal or external, or a combination of both.

• Internal farm site risks are the factors that can contribute to increased risk that PRRS virus can circulate and be transmitted within the population. Typically, those internal risks may include:

  • Operation size
  • Type of operation
  • Flow patterns
  • Age of the sow herd
  • Condition of facilities and winter ventilation

• External farm site risks represent a list of factors that can increase the risk that PRRS virus can be brought in from outside the population. They may include:

  • Rodent, fly and wild animal entry controls
  • Visitor controls
  • Truck and other vehicle controls
  • Proximity to neighboring herds and roads hauling livestock, as well as the level of PRRS infection in neighboring herds

Also included in this risk assessment is a thorough evaluation of the status of gilt replacements and management protocols as well as the semen source, all in an effort to identify potential risk factors as it regards to development of a successful control program. The risk assessment often helps give an operation a reality check before embarking upon a potentially expensive control program, helping all participants better understand realistic expectations with any control strategy and the inherent risks and hurdles involved in achieving their goals. The risk assessment raises awareness of factors that may need to be addressed and managed if the PRRS control program is to be successful at achieving the goal set forth.

Choose a control option

Step 4

PRRS control is seldom an all-or-none proposition. Control methods need to be chosen based on the goals of the farm; the current PRRS infection status; the risk factors relevant to achieving the farm's goal; and the cost of disease in comparison to the cost of control measures.

Control Options

Natural exposure

Random or controlled

• Are based on a hypothesis of homologous sterilizing immunity, although not confirmed experimentally.

• Controlled exposure does cause initial production loss, i.e. increased abortion rate, increased pre and post weaning mortality, and even dead sows.

Killed vaccination

Costly, unpredictable

• No cell-mediated immunity, does not generate necessary cell-mediated immunity for protective immune response.

• Limited data demonstrating effective protective immune response.

• Provides limited efficacy.

Autogenous killed vaccines

Costly, unpredictable

• Same limitations as above.

Modified-live vaccine program (Phase-specific)

Ameliorates production losses

• Known stimulation of protective immunity.
• Documented heterologous protection.

Modified-live vaccine program (Entire population immune management)

Best option for population approach

• Minimizes field virus circulation, transmission.
• Known stimulation of protective immunity.
• Manageable prelude to virus elimination.

Monitor and measure progress

Step 5

Once you enter the control phase of step 4, follow up by continually re-evaluating the goals, risks and control options, to determine if you're using the “optimal” approach to fit the current situation. That monitoring stage involves diagnostic profiling, performance data analysis, and ongoing re-evaluation of the risk assessment from step 3.

More Info

As part of its recently funded PRRS -control initiative, The National Pork Board has updated and made available a PRRS research compendium online. The 142-page document reviews all aspects of the disease, including how the virus spreads and behaves in a population, effects on the pig, financial impact, diagnostics, prevention, control and elimination.

Zimmerman J.; Yoon K.J.; Neumann E. editors. 2003 PRRS Compendium. Producer edition. Des Moines: National Pork Board; 2003.