Winter is prime time for PRRS virus transmission. The virus is not all that hardy and does not survive long in hot and dry conditions more typical of the summer months.
Thus, it is no secret why it is best to attempt elimination of the virus during a hot, dry summer vs. a cold, wet winter. While winter is the high-risk season for PRRS virus, it is also the time to start planning elimination projects, so that the critical end period corresponds with the favorable weather conditions of summer to optimize chances of success.
Virus Elimination Plan
The “load-close-homogenize” approach to PRRS virus elimination is gaining acceptance as it builds a positive track record in farrow-to-wean farms. The key seems to be the ability to close the herd to all live animal entries for at least 210 days. Loading the farm with gilts prior to closure reduces the inevitable drop in inventory of productive females during the seven months of closure.
Homogenize refers to the immune status of the animals on the farm. The goal is to manage the immunity to put all the animals on the farm at or near the same level of immunity at one point in time to eliminate so-called subpopulations.
Essentially, the field virus ends up with no place to hide within the farm, and seven months later, it is gone! Of course, this is an oversimplification, and there are many steps within a farm that help to assure success. Things like limiting cross-fostering as well as discontinuing moving pigs between litters and litters between rooms, weekly serological surveillance, improved hygiene, needle management, and age and parity segregation when possible are all management changes that aid in successful PRRS virus elimination.
The most critical consideration prior to attempting elimination is to evaluate all current biosecurity practices to prevent the reintroduction of PRRS virus once it has been eliminated. Should the risks of reintroduction be too great, use an alternative strategy to control the virus until the area becomes a better candidate for elimination.
Case Study No. 1
A weaned pig producer had been living with PRRS virus in his herd for over three years. While the impact on pig production appeared minimal, he was receiving a substantial discount on the PRRS-positive pigs he sold.
After assessing the risk of reintroduction and the likelihood of success with an elimination project, he chose the load-close-homogenize approach. He closed the sow farm to all new entries in December and utilized modified-live-virus vaccine to homogenize the herd immunity.
Three months later, he began breeding replacement gilts in an off-site gilt developer in anticipation of successful elimination after a herd closure of seven months.
Weekly serological monitoring of ready-to-wean piglets revealed that both the field strain and vaccine strains were no longer in the offspring at four months following herd closure. The herd remained closed to introductions of the bred gilts for another three months, then were moved in just ahead of farrowing in July.
Continued weekly monitoring of the offspring demonstrated the project was successful.
Case Study No. 2
A 250-sow, farrow-to-finish, single-site farm had been PRRS-positive, but hadn’t experienced a clinical outbreak for over three years. The surrounding area had once been populated with many small, farrow-to-finish farms, as well as some very large wean-to-finish sites. But attrition had occurred for various reasons, leaving the owner wondering if the time had come to attempt PRRS virus elimination.
A PRRS risk assessment was performed to characterize the level and type of risk, and serum samples were collected to determine the level of exposure of field virus within the population. Various scenarios and options were discussed regarding the best strategy to eliminate the virus. Each scenario interrupted the continuous pig flow of the farm along with a period of closure to incoming gilts.
After considering the risks presented by the large PRRS-positive growing pig populations within a five-mile radius, along with the success of the prior four years of control strategies, the owner decided not to attempt elimination at this time.
This scenario would be a great opportunity to initiate an area control and elimination discussion with the other producers in the area. Consult your swine veterinarian for control and elimination strategies customized to fit your specific situation.