Many improvements in biosecurity systems have evolved over the past 20 years and the breeding stock industry has established free herds and proven that they can remain free from PRRS when strict standards are applied and when the farms are isolated from other pig production facilities. The magic separation distance is unknown but more miles between pig sites is advantageous. From documented experience, it appears airborne PRRS introduction may occasionally occur from distances up to 2.5 miles. In areas where pig density is concentrated this distance may often be exceeded. Certainly large populations of PRRS positive growing pigs are a significant risk to any breeding herd whether that herd is currently PRRS positive or negative.
Often times when management strategies fail to contain and control a disease agent, numerous experimental schemes evolve. This has been witnessed repeatedly over the years with attempted PRRS control as the commercial pig industry toyed with many vaccine schemes, commercial and autogenous vaccines, serum inoculations, natural exposure, and a host of genetic introduction and other game plans. As with many other viral disease agents there is a natural passage to broad population immunity which in the case of PRRS, often eliminates the virus from small herds – even when the growing pigs remain on site. This same immunologic progression after a new isolate has entered a farm or system often confuses us into believing that the management interventions were successful when actually the population response to naturally occurring immunity is responsible for the observed improvement. Batch farrowing on a one or two times per year schedule is a natural elimination tool for the small producer providing the virus isn’t repeatedly “purchased-in” through semen or replacement stock.
The following table suggests that the value of eradicating PRRS from a country, system or farm has significant value compared to the eradication of Pseudorabies virus (PRV) and to the bacteria Mycoplasma hyopneumoniae. The significant difference when comparing the three economically important swine agents is that PRV had widespread support for eradication by the swine commodity organizations, state and local associations, and the federal government. PRV vaccines were highly effective and the allure of significant increases in exports facilitated this support. Although vaccination as an eradication tool is not currently feasible for PRRS economic hardships and a desire for improved productivity and lower production costs has rekindled the interest in PRRS eradication in the United States and Canada.
Table: A comparison of PRRS and Mycoplasma to PRV, exploring potential criteria for national PRRS elimination consideration.
|Criteria for targeting a disease for Eradication||Mycoplasma||PRRS||PRV|
|Biologic and Technical feasibility|
|Other species affected
|Effective intervention tools
|Simple and practical diagnostics
||Facility based||Area/facility based||National/differential|
|Understood area transmission modes
|Costs and Benefits|
|Estimated annual industry savings
||Chronic disease||Chronic/mortality||Facilitated exports|
||Pig welfare||Welfare/Morale||Other species|
||None||None/research funding||National program|
|Industry and Political Considerations|
|Swine industry commitment
|Core partnerships and advocates||None||AASV, NPB, B-stock industry, key universities||AASV, NPB, NPPC, State & Federal government, State associations|
Local and regional PRRS elimination
Elimination of PRRS from multi-site production has become a standard process, especially in the stand alone breed-to-wean unit. Non-clinical farms that have a good history and diagnostic monitoring can fast track through roll-over eliminations. In this situation the PRRS negative replacement stock can be placed into the site as sentinels after a 60-90 day closure to all additions. Once sentinels remain negative for 60 days the farm is re-opened to normal replacement stocking rates with negative replacements. Other variations of this basic plan have been successful and each depends on specific farm knowledge and producer commitment level.
Farms with active outbreaks must be closed to all introductions for longer periods before successful addition of sentinels. The exact closure time for success is dependent on the population size, piglet fostering strategy, parity distribution, management commitment, and perhaps the nebulous characteristics of the specific virus present. Closure for two hundred days from last clinical signs to first sentinel addition has been a good benchmark or target for roll-over elimination in active farms. Many variations of the roll-over PRRS elimination have been successfully applied and are best contrived by those who know the farm and its limitations.
Many of these various methodologies are published in proceedings and peer reviewed veterinary journals. Regional eliminations have been successful as well as a number of large and medium sized multisite operations. Many isolated single site breed-to-wean and continuous flow operations which have been closed to outside genetic introductions have remained PRRS negative for years.
National PRRS elimination
What is needed for a successful national eradication can be summarized within a few bullet points.
• First and foremost is a majority commitment by producers and their representative organizations.Although vaccine was essential in the PRV eradication program, it is not a necessity for PRRS. A vaccine that universally reduces shedding by providing cross protection between the numerous genetic combinations of virus would prove to be very helpful in control but is not needed for eradication. There is a big difference in the natural ecology of the two viruses. PRV was a very stable DNA virus which survived well both inside and outside the pig. Life carriers of the virus allowing periodic return to contagiousness and were common with PRV. Not so with PRRS where there are no true carriers. PRRS is highly unstable outside the pig and is quickly and permanently inactivated by sunlight, drying, heat, and most disinfectants. The virus relies on changing its antigenic face and finding new susceptible pigs in the interim. There are many example elimination successes in our industry yet there has been no national traction for eradication. In lieu of the current economic environment this should be re-considered as an opportunity to significantly improve efficiency, reduce inputs, and improve our global competitiveness and export potential post recession. There is a need for reducing the national sow herd size before regaining market profitability. The herd closure and roll-over elimination of PRRS can provide this while greatly improving farm production economics once the global market regains balance.
• PRRS negative boar/semen and sow replacement stock.
• An organized national plan and program run by producers which recognize zones (areas, states, split state, etc.) based on PRRS status and activity.
• Individual herd clean up plans and timelines followed by coordinated effort.
• Communication between the stakeholders
• Eventual pig movement restrictions based on the PRRS status of the zones.